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The distribution and geochemistry of some trace metals in the Bermuda coastal environment
Estuarine, Coastal and Shelf Science (1984) 18,245-262
The Distribution and Geochemistry of some Trace Metals in the Bermuda Coastal Environment
T. D. Jickells
and A. H. Knap
Marine and Atmospheric Programme, Bermuda Biological Station, Ferry Road, Bermuda Received 30 December I982 and in revised form 8 July 1983
Keywords: islands
metals;
water analysis (chemical);
lagoons; sediments;
Bermuda
Results of the analyses of the water column and sediments of the Bermuda Platform for Cd, Cu, Fe, Mn, Ni, Pb and Zn are presented. The major process controlling the water column concentrations is physical mixing of open ocean waters with inshore waters, which are polluted by a wide range of diffuse inputs. Sedimentation within the inshore waters plays a lesser, but significant, role as do fluxes from the sediments of Fe and Mn and possibly phytoplankton uptake of Zn. Concentrations within the sediments are controlled by the formation of trace metal enriched clay/organic particles in the inshore areas and their subsequent redistribution by sediment resuspension, except for Fe and Mn which are largely associated with clay lattices.
Introduction data are available on trace metal concentrations in polluted marine environments (Leatherland & Mackay, 1975; Forstner & Wittman, 1979; Klinkhammer 81Bender, 1981; Duinker & Nolting, 1982). However, it is only recently that advances in sampling and analytical techniques have allowed the accurate determinations of trace metal concentrations in open ocean waters (Boyle et al., 1981; Bruland & Franks, 1983; Moore, 1981). These advancesare now being applied to continental shelf areasand marginal seassubject to relatively small anthropogenic impact (Wallace et al., 1981; Campbell i? Yeats, 1982; Magnussen & Rasmussen,1982). There appear to be, however, no data available on trace metal concentrations around oceanic islandswhere oceanic water spills directly into inshore environments without the intermediate continental slope environment. This paper describesthe results of a two-year survey of trace metal concentrations in the waters and sedimentsof such an environment, the island of Bermuda, and attempts to analyse the processescontrolling them. Bermuda is located about 1000km off the coast of North America and consistsof a series of interconnected islands of aeolian limestone overlying a volcanic seamount.The islands are surrounded by coral reefs and a platform of biogenic calcareous sands.Bermuda is densely populated but has no heavy industry and no rivers, due to the porous nature of the rock. Extensive
245 0272-7714/84/030245+18803.00/0
0
1984
Academic
Press
Inc.
(London)
Limited
246
T. D. Jickells & A. H. Knap
Methods Waters Surface sampleswere collected by hand, directly into sample storage bottles from slowmoving non-metallic boats, all possibleprecautions being taken to minimize contamination and contact with surface lilms. Subsurface water sampleswere collected in acid-leached Niskin sampling bottles. Sampleswere collected and stored in rigorously acid precleaned polyethylene bottles and preserved by acidification to about pH 2.2. Preconcentration of samplesfor analysisfor cadmium (Cd), copper (Cu), iron (Fe), nickel (Ni) and zinc (Zn) was by APDC chelation and solvent extraction closely following the method of Magnussen and Westerlund (1981) with chloroform as the solvent. The only modification was the use of an evaporation step rather than back extraction. Manganese (Mn) was determined by oxine chelation and solvent extraction using the method of Bewers et al. (1976) with the addition of a back extraction step rather than direct analysisof the solvent. Lead (Pb) was determined using a chelating resin by the method of Sturgeon et al. (1980). Final determination of all metals was by graphite furnace atomic absorption spectrophotometry (AAS) using a Perkin Elmer 373 with background correction and a HGA 400 furnace. Recoveries were determined by standard additions on samples,and blanks by re-extraction of previously extracted samples,after addition of a further aliquot of the preserving acid, each run in triplicate. From the standard deviation of these blanks we have calculated limits of detrction as three times the standard deviation (Strickland & Parsons, 1972). Precision of the analyses(-t 1 S.D.) hasbeen determined by a seriesof triplicate analysesof samples from the middle of the observed concentration range. The averageof all recoveries, blanks, precision and limits of detection of the methods are listed in Table 1. Nutrient concentrations and salinities were determined by standardmethods (Strickland & Parsons,1972). Filtration of samplesgave rise to contamination problems and consequently this was not done on a routine basisso the data presented are for total leachable trace metals at pH 2.2. On several occasionsparticulate trace metal concentrations were measured by filtration through acid precleaned 0.4 ,trm nucleopore titers and subsequentleaching of the filters with 1% nitric acid. By comparing these data with total metal concentrations, as determined on duplicate samples,the relative importance of the two phaseswas assessed. Blank problems prevented the measurementsof particulate cadmium concentrations. Sediments Surface sediment and sediment core sampleswere collected by divers, and then stored frozen prior to analysis. Samplesfor trace metal analyseswere first dried at 80 “C, then between 1 and 5 g were digested in boiling nitric acid, made up to 25 ml in 1%nitric acid and analysedby flame AAS. Blanks and spiked sampleswere run with the normal samples.
TABLE
1. Recoveries,
Recoveries (%) Blanks (pg I-‘) Precision (pg 1-l) Limits of detection
blanks,
(pg I-‘)
precision
and limits of detection
of trace metal methods
Cd
CU
Fe
Mn
Ni
Zn
101 3 x 10-T &3.6x 10-3 4 x 10-3
83 0.08
84 0.19
100
87 0.02 kO.02 0.03
103 0.07 f0.16 0.07
to.03
0.04
kO.15 0.15
0.01 kO.02 0.04
248
T. D. Jickells & A. H. Knap
Results Water There was no systematic temporal variability in the data, therefore data for each station have been averaged over all samplescollected (Table 2). The results falling outside three times the standard deviation of the remaining data were considered aberrant and were excluded from the data set (Magnussen & Westerlund, 1980). This process removes 6% or lessof the data for each parameter. Sediments Sediment sampleswere collected in June and July 1982. The results of the trace metal analysesand the other available data on the sedimentsare shownin Table 3. To characterize the grain size, only the percentage of the sample smaller than 63 pm (as determined by particle size analysis) is listed. Discussion waters Dissolvedvs. particulate distribtion: Throughout this report dissolved trace metalswill be defined asthose that passthrough a 0.4 pm filter. Basedon comparisonof total and particulate concentrations, Cu, Ni and Mn are present in the Bermuda inshore waters dominantly (> 80%) in the dissolved phase while Fe is dominantly (> 80%) particulate. For Zn, relatively high filter blanks cause some uncertainty but it too appears to be predominantly dissolved. By analogy to the other metals it is suggestedthat Cd is dominantly dissolved. This dominance of the dissolved phase for all but Fe is in keeping with the findings of other workers in coastal and marginal sea environments (Murray & Gill, 1978; Boyle et al., 1981; Wallace et al., 1981; Magnussen& Rasmussen,1982; Campbell &Yeats, 1982). SargassoSea: During May and June 1982 seven sampleswere collected in the Sargasso Sea 5-12 miles off Bermuda. The results of the analysesof these samplesalong with some comparative data from other workers in the SargassoSea are presented in Table 4(a). The data for Cu, Ni and Mn are similar to other reported data for the SargassoSea, while for Zn and Cd the techniques usedhere are insufficiently sensitive to detect concentrations as low as those reported by Bruland & Franks (1983). This indicates that the SargassoSeaaround Bermuda is a true oceanic environment and that the data can be used for comparisonwith the inshore waters values. For Fe the results are erratic and probably represent contamination of some of the samples. If three outliers are rejected, the average concentration is 0.33 + 0.21 lrg 1-i. There are very little published data on open ocean Fe concentrations largely becauseof the extreme contamination problems (Gordon et al., 1982). These latter authors reported dissolved Fe concentrations in the Pacific of 8-17 ng l-1 with particulate Fe the dominant form. Wallace et al. (1977) reported particulate Fe concentrations in the SargassoSea of 0.05-0.008 pg 1-i and Krishnaswami and Sarin (1976) reported 0.09-O. 15 pg 1-l sothe values found in this study are somewhat higher than in these other studies. North Lagoon and Great Sound: North Lagoon is a large area of mixed coral reefs and sand flats surrounded by coral reefs with rapid water exchange with the open ocean. The surface waters of the SargassoSea impinge on the platform without upwelling (Bodungen
Trace metals in the Bermuda coastal environment
247
Bermuda sediments are dominantly calcareous (95-1000/ocarbonate) and therefore the digestion was considered to be essentially complete and to result in total metal concentrations for the sediment. Results with other sediment samplesindicate this digestion to be comparableto hydrofluoric acid (Jickells, unpublished data). Particle size determinations were carried out on dried sedimentsusing standard sieves, and organic content was determined by measuring weight lossafter heating at 450-500 “C. Noncalcareousresidue wasdetermined by weighing the residue after dissolution in 10%hydrochloric acid. Samplingsites Sampling sites were designed to give good hydrographic, geographic and environmental coverage and are shown in Figures 1, 3 and 5.
North
Lagoon
Figure 1. (a) Map of Bermuda showing showing sediment sampling stations.
water
sampling
stations.
(b) Map
of Bermuda
‘cl S
ii S
icl S
LI S
1;; S
Castle Harbour
St George Harbour
Harrington Sound
Great Sound
Hamilton Harbour
2.
36 54 0.15 15 36.52 0.17 11 36.47 0.18 12 36.22 0.20 16 36.42 0.17 14 36.31 0.17 15 36.19 0.31 14
Salinity (WO) 7.3 1.8 15 6.0 2.4 11 4.8 1.1 12 9.0 2.6 16 6.9 0.9 14 5.9 1.3 15 5.0 1.0 14
Secchi disk (m depth) 0.41 0.18 16 0.55 0.27 12 0.67 0.32 12 1.01 0.50 15 0.74 0.39 14 1.84 1.07 15 1.80 0.78 13
a
in surface
Chlorophyll (WA 1-1)
Trace metal concentrations
0.04 0.02 15 0.05 0.05 12 0.10 0.11 12 0.54 0.33 16 0.10 0.07 12 1.57 0.84 15 2.54 1.30 14
4.7 3.6 16 10.5 4.9 12 10.2 6.7 11 6.8 4.0 15 4.7 2.0 12 12.7 9.3 15 11.4 2.8 12
from Bermuda
Nitrate + nitrite (PM 1-I)
samples
0.20 0.09 15 0.23 0.07 12 0.49 0.22 12 0.40 0.14 14 0.42 0.10 14 1.27 0.39 14 1.70 0.57 14
inshore
water@
1.07 0.38 15 2.05 1 .oo 12 3.95 1.36 11 1.67 0.66 16 1.78 0.38 13 4.22 1.34 15 6.1 1.9 13
0.19 0.13 5 0.46 0.17 5 0.59 0.29 6 0.52 0.23 7 0.44 0.18 5 0.90 0.38 6 1.36 1.18 6
0.14 0.04 16 0.16 0.04 12 0.19 0.04 11 0.16 0.04 16 0.17 0.07 14 0.22 0.05 14 0.23 0.09 14
0.15 1
0.29 0.21 14 1.3 0.5 12 1.1 0.5 12 0.6 0.4 14 0.7 0.4 12 2.6 1.5 14 2.5 0.9 13
‘Sampling began in September 1980 at all but St Georges and Castle Harbours where sampling began in February 1981. Sampling was carried out monthly until January 1982 and there is thus between 12 and 16 months of data for all stations for all metals except: (i) Mn for which analyses began in July 1981; (ii) Pb for which only one survey was carried out in October 1981. bM-mean; S-standard deviation; n-number of samples.
rc1 S n
M S
Statisticsb
1A
Station
TABLE
250
T. D. J’ickells & A. H. Knap
TABLE
3. Trace metal concentrations
Station
%orgb
%resc
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
6.4 3.3 6.5 3.8 7.5 3.6 3.3 3.6 4.6 3.8 5.8 11.7 4.5 6.8 6.7 5.8 13.7 31.4 10.5 14.9 15.9 5.8 5.8 2.7 8.1 9.7 6.0 9.0
5.3 1.6 5.2 3.0 6.0 2.0 1.6 1.7 2.2 1.5 4.2 0.9 1.8 2.7 2.2 2.8 5.7 8.1 5.9 7.5 1.5 0.6 1.7 1.1 3.2 7.2 0.5 0.4
in Bermuda
$6~63P’ 8.1 2.6 3.9 8.4 8.5 0 5.8 5.8 11.9 14.4 3.6 20.1 15.0 8.2 19.1 15.5 9.0 12.1 2.5 14.0 9.1 1.1 0.7 0.1 7.5 12.1 0.1 0
surface
sedimentsa
Cu
Fe
Mn
(PPm)
(PP~)
(PPm)
20.0 4.4 28.0 15.0 40.0 5.2 <2.5 <2.5 <2,5 <2.5 8.0 <2,5 3.0 5.8 10.0 13.0 545.0 46.0 23.0 50.0 <2,5 <2,5 <2.5 <2.5 4.0 12.0 <2.5 <2,5
1.40 1.20 I.20 1.60 3.40 0.40 0.40 0.50 0.30 0.20 0.70 0.10 0.65 0.80 0.95 1.05 4.10 3.50 3.60 3.40 0.45 0.15 0.90 0.20 1.70 2.50 0.10 0.10
39 26 43 33 37 <7 <7 <7 <7 <7 19 11 16 14 14 15 45 39 38 38 <7 <7 <7 <7 24 34 <7 <7
Pb (w-4
57 14 65 62 100 20 <5 <5 <5 <5 19 5 21 26 38 38 110 80 46 101 11 <5 6 <5 13 31 <5 <5
50.0 9.6 60.0 41 .o 70.0 31.0 10.0 11.0 Il.0 5.5 250.0 2.2 13.0 19.0 29.0 23.0 83.0 62.5 47.0 72.0 8.0 2.9 15.5 5.6 14.0 37.5 1.4 4.2
‘Ni was below the limit of detection ( < Sppm) except stations 17-20 where the result was < 12.5 ppm at all stations. Cd was below the limit of detection (< 1.2 ppm) except stations 17-20 where the result was ~3 ppm at all stations except station 11 where 1 ‘8 ppm was measured. b%org refers to the percentage organic content of the sediments. ‘%res refers to the percentage non-calcareous residue in the sediments. d% < 63 pm refers to the percentage of the sediment less than 63 pm in size.
et al., 1982). The processescontrolling primary production and nutrient cycling on the Bermuda platform have been extensively studied (Bodungen et al., 1982) and will not be
discussedhere. Trace metal concentrations increase l-lo-fold in North Lagoon over the SargassoSea and most of the metal concentrations double again in Great Sound. A comparison of the concentrations in North Lagoon with data from other coastal waters and marginal seas (Table 4(b)) showsBermuda levels to be comparableor lower than most other areasthough there is a wide range of values. Coastal waters are subject to a range of anthropogenic and geochemicalprocessesincluding fluxes from the sediment to the water column and removal of metals by adsorption or biological uptake. Sediment fluxes usually only arise from clay sediments (Boyle et al., 1981) and consequently may not be significant for Bermuda’s calcareousenvironment. Similarly the relatively low suspendedsolid loads and primary productivity of Bermuda waters may make these processesless important than in other areas.A major potential input to the North Lagoon/Great Sound system is water
Trace metals in the Bermuda coastal environment
TABLE
4. Trace metal concentrations
(a) Sargasso Sea
Cd (ng 1-l)
This study Bruland and Franks (1983) Boyle et al. (1981) Spencer et al. (1982) Yeats (unpublished)
<4 0.2
(b) Marginal
Cd (ng 1-I)
seas and coastal waters
Bermuda; this work (Station 1A) Bafhn Bay; Campbell and Yeats (1982) Gulf Stream; Wallace et al. (1981) Gulf Stream; Spencer et al. (1982) Gulf Stream; Boyle et al. (1981) Scotian Shelf; Bewers et al. (1976) North Sea; Preston et al. (1972) Danish Sound and Kattegat; Magnussen and Rasmussen (1982) U.S. Northeast Shelf Waters; Bruland and Franks (1983)
4.7 20-66 0.3-0’9 4.5 17 36 -
reported
for various
251
marine
CU (Pg 1-l)
Mn (1.18 I-1)
Ni (Pi3 1-f)
0.05 0.08 0.08 0.11 0.10
0.11 0.13 0.13
0.16 0.12 0.12 0.14
<0.07 0.004 0.01 <0.06
0.2
1.07
0.19
0.14
0.18-0.36 O-17 O-16 O-56 0.9
0.5-2.5 3.2 1
25
0.45
0.2-7.0
22
0.25
O-06
-
0.14-0.38 0.47 0.37
locations
Zn @g 1-Y
0.17-0.29 0.23 0.25 0.40
0.15
0.3
0.03 -
0.02 0.01 1.1 3.9
-
0.48
0.04
0.80
1.15
0.33
-
0.16
exchange with the more polluted waters of Hamilton Harbour. Water exchange between North Lagoon and Harrington Sound or Castle and St George Harbours is relatively much smaller. If water exchange with Hamilton is dominant, then a plot of trace metals against a parameter which measuresmixing proportion should be linear. Unfortunately, unlike an estuary, salinity is not a useful indicator in this system since the salinity gradient is too small and the fresh water input is diffuse rather than a point source. Similarly, nutrient and phytoplankton parameters are non-conservative and of no use as indicators due to the residence time of the water massesrelative to biological productivity. However, if the average trace metal concentrations (Table 2) in the inner Hamilton Harbour/Hamilton Harbour/Great Sound/North Lagoon/SargassoSeasystemare plotted against one another then approximately linear relationships result (Figure 2). All correlations are significant at the 99% level. Pb and Cd correlate better with Zn than Cu due to differences in their behaviour within the harbour itself, though whether these differencesare real is uncertain since many of the Cd data are close to the detection limit and there is only one data set for Pb. The linear interrelation of these elements suggeststhat either all these metals are subject to non-conservative processesof equal magnitude or that the behaviour of all these elementsis dominanted by physical mixing in this system. The latter seemsmore likely and this will be considered further in a later section. The similar behaviour of Fe, though dominantly particulate, and the other dominantly dissolved trace metals is in agreement with the observation of Mayer (1982) that while Fe may rapidly flocculate in seawater, the settlement of someof these floes may be a slow processrelative to water exchange.
252
T. D. Jickells & A. H. Knap
7 : IO . . 3 . :E . I-0
3 d 2.0 cu +g
0.4
7 .’ s ’ . s / Id ../’ I.0 2-o
7 7!A
2 El
l
/ .
/* I-0
cu (pg L-‘)
2.0
I- = 0.907 /
0”
I
2.0
cu (pg I-‘)
y
2.0
r =0.985
T
i
IO
.
2
I: 7
i-0
2.0 1-‘I r=0.971
l 0
p
I.0 cu (pg
4
3
a”
7
1.. i@
l
’
20
l H
0.2
7
.
5
r = 0.959 7J y
cu (pig 1. ’ I I I r = 0.964
2
I-‘)
/ I.0
a”
. /
El I-0 Zn t/&i
2.0
cl
. I-0
1-1)
Figure 2. The interrelationship note different scales.
./
Zn (pg
of average
water column
2.0 1-l)
trace metal concentrations.
Hamilton Harbour: While dilution appearsto be the dominant processcontrolling the distribution of trace metalsin the North Lagoon/Great Sound system, the distribution of trace metals and nutrients within the Hamilton Harbour area is complex. The harbour is subject to street runoff and cooling water dischargesfrom the city of Hamilton but not direct sewageinputs, these being piped offshore. In addition, recent measurementsof atmospheric particulate trace metal concentrations in Bermuda (Plumb, unpublished data) show high levels of Pb and Cd in urban compared to ocean air with smaller enhancementsfor Cu and Mn. Direct fallout of this material may be a significant source of those metals. Trace metal distributions (Figure 3) are similar to one another but different from the groundwater derived inputs (nitrate, salinity and silicate) or chlorophyll. Elevated trace metal levels occur along the north shore and at the eastern end of the harbour which is used as a yacht marina implying that street runoff and atmospheric fallout from the city of Hamilton are primarily responsible for the elevated trace metal concentrations in the harbour. Corrosion of moorings, boats and metal structures, boatyard operations (Young
Trace metals in the Bermuda coastal environment
NO2 + NO3 (pm
(-‘I
Zn (pg
I-‘)
253
Chlorophyll
o (pg
(-‘I
+g
\-‘I
c
Figure 3. The distribution of nitrite plus nitrate (NO, +NO,), chlorophyll in the surface waters of Hamilton Harbour and Great Sound, 29 September
Ni
a, Zn and Ni 1981.
et al., 1979), sewer overflow and fluxes of Fe and Mn from anoxic sedimentsare probably
alsoimportant sources.The restricted water exchange between the harbour area and Great Sound (due to the lines of islands and sills acrossthe mouth of the harbour) allows the build up of relatively high concentrations of metals and nutrients. The differences between the nutrient and trace metal distributions in the harbour are consistent with the relatively low trace metal concentrations and the relatively high nitrate concentrations found in the ground water (Jickells, unpublished data). Sound: This body of water has an extremely long residence time (140 days or more, Barnes & Bodungen, 1978) and essentially no direct trace metal inputs apart from atmospheric fallout, land runoff and in situ corrosion from a few pleasure boats. As would be expected under these conditions trace metal concentrations are low. The increasesin Cd, Cu, Ni, Pb and Zn that are found over North Lagoon can be attributed largely tc land runoff, while the increasesin Mn and Fe may be due to fluxes from an anoxic deer water area in the sound. Devil’s Hole, in the southern corner of Harrington Sound, is the major part of Harrington Sound subject to such seasonalanoxia. It is considerably deeper than the rest of the Sound (24 m compared to 5-15 m) and the formation of a seasonalthermocline gives rise to anoxic conditions in the deep water which have been extensively studied (Thorstenson & Mackenzie, 1974; Balzer & Wefer, 1981; Bodungen et al., 1982). The water column in Devil’s Hole was sampledat five depths monthly throughout 1981 and three more times in 1982. The water column is vertically homogeneousin winter but, Hurrington
254
T. D. Jickells & A. H. hap
00 dlil
00 r
--
00 -; 22
T
Trace metals in the Bermuda coastal environment
Figure 5. The distribution 14 April 1982.
of Cd, Cu, Fe and Zn in the surface
255
waters of Castle Harbour,
linear increasesin DIN (dissolved inorganic nitrogen- ammoniaplus nitrite plus nitrate), silicate, Fe and Mn with decreasingoxygen concentrations are seenafter the development of the thermocline (examples of each situation are shown in Figure 4). There is some scatter due to the continuation of primary production below the thermocline (Bodungen et al., 1982), sediment resuspensionand the probable settling of Fe after it reprecipitates in oxic waters. Other trace metals do not show systematic variability. During the period 4 June-l 1 August 1982, there appearsto have been little disturbance of the subthermocline waters (basedon salinity data), and it is therefore possibleto calculate the flux rates of these various ions to and from the sedimentsby assumingthat the only site of releaseand consumption is the sedimentsand that no water exchange occurs. The calculated rates are 220 ml 0, m-2 day-l (consumption), 410 pg at. N rnM2day-l, 470 pg at. Si m-2 day-i, 220 pg Fe m-2 day-i and 160 pg Mn m-2 day-i (all released). These estimatesare necessarilycrude but do allow an initial estimation of the significance of fluxes from the sediments.The oxygen consumption rate is within the range previously reported for Devil’s Hole while the DIN and silicate fluxes calculated here are two-to-fou times lower than those found using flux chambers (Balzer, 1981; Bodungen et al., 1982, Figure 4. Vertical profiles of temperature (t), oxygen (O,), ammonia (NH,), Cd, Cu, Fe. Mn, Ni and Zn in the water column of Devil’s Hole Harrington Sound under stratified conditions. (a), (b) and (c)-samples taken on 7 July 1981, and well mixed conditions (d), (e and (f)-samples taken on 8 October 1981.
256
T. D. Jickells & A. H. Knap
possibly due to biological uptake. The fluxes of Mn calculated here are considerably lower than those reported for other areas where clay sedimentspredominate (Wilke & Dayal, 1982). Some additional sampleswere collected from an area in Hamilton Harbour subject to similar seasonalanoxia to that in Harrington Sound and the rates of increase of DIN, silicate, Fe and Mn with decreasingoxygen in these two areasappear to be very similar. St Georgeand Castle Harbours: The majority of the water exchange from these two areas occurs directly with the open oceanand accordingly these have been consideredasa separate system with little impact on North Lagoon. St George’s Harbour has somesimilarities to Hamilton with no direct sewageinput and slow water exchange and thus acts asa natural sediment trap. The resuspensionof the fine sedimentsby ship movements gives rise to elevated Fe levels while street runoff, atmospheric fallout and boatyard operations probably cause the increase in the other metals, though fluxes of iron and manganesefrom anoxic sedimentsmay alsobe important. Castle Harbour is an open body of water with good water exchange and little development along the shoreline apart from the airport along the north shore. In the northern corner of the area, land reclamation by scrap metal dumping (mainly motor vehicles) followed by rockfill has continued for several years. A plume of elevated trace metal water (mainly Cd, Cu, Fe and Zn) can be traced acrossthe harbour (see Figure 5), though its precise distribution is wind and tide dependent. Metal levels immediately off the dumpsite are generally high but variable, so dumping history is clearly important. The pattern of enrichment, Cd and Zn> Fe and Cu, suggeststhat electronegativity rather than the quantities dumped is the factor controlling the input. Sediments The interpretation of sediment trace metal levels is always complicated by their dependence on sediment type. Inspection of the data suggeststhat the best characterization parameter is the percentage residue of non-calcareous (presumably clays and organics) material in the sediments.Thus in Table 5 the results of linear correlations of all the metals against percentage residue are presented and the relationship for Zn shown in Figure 6 as an example. A few data points have been excluded, two becausethey are high (Cu at one site in Hamilton Harbour and Zn off the dump) and one low Cu value in Devil’s Hole (Harrington Sound). It should alsobe noted that the only site at which Cd wasabove detection limit wasimmediately off the dumpsite in Castle Harbour. The data clustering below the detection limit (treated as being half the detection limit) tends to causethe graph not
TABLE
5. Best fit data for sediment
Parameter’ %res Mres %res %res %res
7x Cu VS. Fe vs. hIn vs. Pb vs. Zn
O%res = percentage
Correlation 0.935 0.883 0.872 0,821 0,890 residue.
results vs. percentage (r)
residue
Slope
Intercept
6.34 0.48 5.84 12.05 9.46
-7.2 -0.25 0.3i -6.38 -2.04
Number
of samples 26 28 28 28 27
(n)
257
Trace metals in the Bermuda coastal environment
Percentage
Figure
6. Plot of Zn vs. percentage
resldue
non-calcareous
residue in Bermuda
6. A comparison of trace metal (TM) concentrations that in a standard clay (Forstner & Wittman, 1979)
TABLE
TM in residual fraction (R) TM in standard clay (S) Enrichment factor R/S
0.3
-
Cu
Fe
634 45
48 100 46 700
14
1
sediments.
in the residual
fraction
with
Mn
584
-
850 0.7
68 -
1205
946
20 60
95
10
to pass through the origin. For metals with few points below the limit of detection the best fit line appearsto go through the origin, and all correlations are highly significant. These correlations suggestthat most of the trace metalsin thesesedimentsare associated with the non-calcareous fraction either as lattice-bound material within the clays or adsorbedto them. In Table 6 the concentrations in the residual fraction (derived by extrapolation) are compared to the trace metal content of standard clays (Forstner & Wittman, 1979). It appears that Fe and Mn have dominantly lattice sources(the enrichment of < 1 for Mn may be due to mobilization from reducing sediments) while Cu, Pb and Zn do not. These latter three are probably derived anthropogenically and become adsorbedto the non-calcareous particles. Katz and Kaplan (1981) described a similar mechanismfor the dispersal of sedimentary trace metalsoff the Californian coast. In Figure 7 the profiles of trace metal concentrations in a sediment core from Devil’s Hole, Harrington Sound are shown. The core was collected from a water depth of 24 m in an area of little natural sediment resuspensionand a sedimentation rate of about 1 mm year-i (Erlenkeuser, 1981). Fe, Mn and percentage residue are relatively constant with depth, suggestinglittle post-depositional migration in these anoxic sediments, while Cu, Pb and Zn are markedly enriched in surface sediments,again probably due to anthropogenit activities. Using the results from the deeper parts of the core to calculate enrichment factors results in values close to unity for Cu and Zn and about 10 for lead.
258
T. D. Jickells & A. H. Knap
Cu hmi
Fe (ppt)
IO
Mn (ppm)
20 I
I
20
4c-
I
t
!’
!I i
i .A / i
) I
I
Pb (ppm)
I
I
Zn (ppm)
30:
Figure 7. Vertical profiles Harrington Sound.
of Cu, Fe, Ah,
Pb and Zn in a sediment
core from Devil’s
Hole
Sediment cores collected from St George and Hamilton Harbours had constant trace metal concentrations with depth, presumably due to sediment resuspensionand bioturbation. Lyons et al. (1982) reported elevated trace metal levels in the surface layers of a sediment core from the extreme eastern end of Hamilton Harbour. Sediment concentrations of Fe, Pb and Cu in this area (which corresponds to the area of maximum water column concentrations) are higher than those reported here. Removal processes
Sedimentation rates (corrected for resuspension) have been measured in Hamilton Harbour, Harrington Sound and North Lagoon (Bodungen et al., 1982). By using these sedimentation rates in associationwith sediment trace metal concentrations from nearby sites it is possibleto calculate the rate of removal of metals to the sediments.This calculation assumesthat the sedimented material has a similar composition to the sediments themselves.If there is a significant flux of metalsout of the sedimentsthen this assumption may break down. As seenearlier, the only metal for which measurableflux rates from the sedimentshave been observed are Fe and Mn under anoxic conditions. Calculations suggest that the percentage of sedimentedFe and Mn recycled back to the water column is small (< lo%), though for Fe the release of even 1% of the sedimenting material back to the water column could have a significant effect on the water column concentrations, since so high a percentage of the Fe is sedimented. Knowing the areas, volumes and flushing times of the basins(Morris et cd., 1977; Barnes
Trace metals in the Bermuda coastal environment
259
& Bodungen, 1978) and the average water column concentrations (in Hamilton Harbour this is the average of both stations) the rate of removal by water exchangecan be calculated. Both sets of calculations are crude, largely becauseof uncertainties in flushing times and sedimentation rates plus the assumedhomogeneity within the bodies of water. They do, however, provide a useful tist order approach and in Table 7 the percentage of the total removal (sedimentation + flushing) by sedimentation is presented. A comparison of sedimentation OS.water exchange requires that the measurementsof water column and sediment concentrations be total metal concentration. This is indeed the case for the sediments and those metals which are dominantly dissolved in the water column. For Fe, however, the caseis different and Duinker and Nolting (1977) have shown that dilute acid leaching, such as occurs during storage at pH 2 ‘2, recovers only 15-40% of the total particulate iron. Despite reservations regarding the crudenessof the approach, inspection of Table 7 reveals several interesting points. In the North Lagoon area with its rapid water exchange, coarse sedimentsand low primary productivity, essentially all trace metal removal is by water exchange. In Hamilton Harbour flushing is again the dominant mechanismof removal as predicted earlier from the relationships in Figure 2, though sedimentation does play a role in trace metal removal as may be expected in an inshore area with higher productivity, turbid water and relatively restricted water exchange. Fe clearly behaves quite differently; the calculations suggest that sedimentation is the dominant removal route while the relationships in Figure 2 imply that this cannot be the case. Part of the solution to this paradox probably lies in the problem of incomplete leaching of particulate iron during storage, and if the assumption is made that only 25% of the iron is leached from the particulates (Duinker & Nolting, 1977) then the percentage of iron removal by sedimentation falls to 61%. In addition, the relationships in Figure 2 concern the water in the centre of the harbour and some of the iron inputs to the edges of the harbour (in situ corrosion and street runoff) may be rapidly sedimentedand then distributed throughout the sediments. The order of preferential sedimentationin Hamilton Harbour (Fe > Pb > h4n > Cu > Zn) is consistent with the known geochemistry of these elements (Bruland & Franks, 1981) and the results of recent laboratory simulationsof coastalecosystems(Santschi et al., 1980). TABLE
7. Percentage
of trace metal removal Exchange
Metal
North Lagoon and the Sargasso Sea
Cd CU Fe Mn Ni Pb Zn
< 17
that is by sedimentation
in the various
basins
between
Harrington Sound and North Lagoon <35 37 98 50 <71 67 54
Hamilton Harbour and Great Sound <75 22 86 30 <52 40 19
The offshore value of Bruland and Franks (1983) has been used for Cd and that of Schaule and Patterson (1983) for Pb. Concentrations in the water column at the two Hamilton Harbour stations have been averaged.
260
T. D. Jickells & A. H. Knap
It also suggeststhat the pattern of enrichment seenin the Hamilton Harbour sediments (Pb > Cu>Zn, Table 6) may reflect the geochemistry of the elements more than the pattern of pollution. In Harrington Sound the proportion of trace metals removed by sedimentationis higher than in other areasasmight be expected with the long residencetime of this body of water, though this may also reflect the extrapolation of the Devil’s Hole sedimentation rate to the whole of this area. One interesting feature though, is the relatively high proportion of Zn removed by sedimentation.A possibleexplanation is that phytoplankton uptake may be significant. Phytoplankton uptake will not be distinguished from inorganic sedimentation processes in these calculations since trace metalsincorporated in the plankton will be leached during storage of water samplesand during digestion of sediments. However, by using data abstracted from Martin and Knauer’s studies (1973) of Pacific plankton and the annual primary production rates of Bodungen et al. (1982), it is possibleto calculate the annual uptake of trace metals by the phytoplankton. Bodungen et al. (1982) calculated that 65% of the primary production in Harrington Sound was recycled within the water column so this figure can be used to allow for grazing. Since the only calculated figures for North Lagoon are ‘ lessthan ’ figures it is impossibleto quantify the significanceof phytoplankton uptake to sedimentation though it is apparent that it could be significant. The other two inshore basinsshow quite different characteristics. In Hamilton Harbour, the maximum potential contribution of phytoplankton uptake to the sedimentation removal route is 7% for Zn (after correcting the 65% recycling within the water column) and for the other elementsthe contribution is much smaller.By contrast, in Harrington Sound this maximum contribution of phytoplankton uptake to sedimentationranges from 1l-12% (Mn, Fe and Pb) to 30% for Cu and 60% for Zn. This distribution is again consistent with the known oceanographicbehaviour of these elements(Bruland & Franks, 1983), Zn (and Cd) showing the greatest phytoplankton uptake while Cu, Mn and Pb removal is dominated by inorganic removal processes.This alsoexplains the increasedproportion of Zn in the sediments of Harrington Sound. Clearly the type of removal route which operates can change from the turbid waters of Hamilton Harbour (which resemble the system of Santschi et al., 1980) to the clear waters of Harrington Sound.
Conclusions Cd, Cu, Fe, Mn, Ni, Pb and Zn inputs to the Bermuda inshore waters are generally in the form of diffuse inputs from land runoff, atmospheric deposition and in situ corrosion. These inputs are concentrated in Hamilton and St George’s Harbours where restricted water exchange leads to increased levels in the water column. In addition a few discrete sourcescan be identified, noteably scrap metal dumping. Trace metals in the water column are dominantly in the dissolved form (co.4 pm) except for Fe which is dominantly particulate. The distribution of trace metals in much of the water column of the Bermuda platform can be explained by physical mixing of contaminated inshore waters with clean open ocean waters. The distribution of trace metals in the sedimentsfollows a similar pattern to those in the water column, and is controlled by the formation of trace metal enriched clay and organic particles in the inshore waters and their subsequent redistribution throughout the Bermuda platform by water movements.
Trace metals in the Bermuda coastal environment
261
The overall dominanceof water exchange over sedimentation asa removal route is supported for all metals (except Fe) by calculations of the rates of removal by the two routes. In the turbid waters of Hamilton Harbour phytoplankton uptake appears to play a minor role but this situation changesin the lessturbid waters of the other parts of the Bermuda platform and phytoplankton-mediated sedimentationmay be the dominant mechanismof Zn sedimentation in Harrington Sound. Mn is the only metal for which fluxes out of the sediments are important compared to the sedimentation rate, though fluxes of Fe may also be an important source of water column Fe. Acknowledgements This work was funded by the Bermuda Government. We thank S. R. Smith for help with sampling and nutrient analysesand other colleaguesin the M.A.P. for help with the sediment survey. We thank T. M. Church, C. I. Measures and P. A. Yeats for their helpful comments on this manuscript; J. Cadwallader, S. M. Jickells and W. E. Sterrer for editorial assistanceand Zina Francis and Margaret Emmott for typing the manuscript. The use of unpublished data from P. A. Yeats and D. Plumb is also gratefully acknowledged. References Balzer, W. 1981 Oxygen consumption and nutrient release from the bottom of Devil’s Hole Bermuda. In Harrington Sound Project Special Publication 19. Bermuda Biological Station, Bermuda, 94 pp. BaIzer, W. & Wefer, G. 1981 Dissolution of carbonate minerals in a subtropical shallow marine environment. Marine Chemistry 10,545-558. Barnes, J. A. & Bodungen, B. v. 1978 The Bermuda Marine Environment, Vol. 2. Special Publicarion 17. Bermuda Biological Station, Bermuda, 190 pp. Bewers, J. M., Sundby, B. & Yeats, I’. A. 1976 The distribution of trace metals in the western North Atlantic off Nova Scotia. Geochimica et Cosmochimica Acta 40,687496. Bodungen, B. v., Jickells, T. D., Smith, S. R., Ward, J. A. D. & Hillier, G. B. 1982 The Bermuda Mawze Environment, Vol. 3. Special Publication 18. Bermuda Biological Station, Bermuda, 123 pp. Boyle, E. A., Huested, S. S. & Jones, S. I’. 1981 On the distribution of copper, nickel and cadmium in the surface waters of the north Atlantic and north Pacific ocean. Journal of Geophysical Research 86, 8048-8066. Brown, I. F. 1980 The nitrogen cycle and heat budget of a subtropical lagoon, Devil’s Hole, Harrington Sound, Bermuda: Implications for nitrous oxide production and consumption in marine environments. Ph.D. Thesis, Northwestern University, Evanston, Illinois, U.S.A., 317 pp. Bruland, K. W. & Frankie, R. I’. 1983 Mn, Ni, Cu, Zn and Cd in the western North Atlantic. NATO Advanced Research Institute Conference ‘ Trace Metals in Seawater ‘. Erice, Italy, March-April 1981. Plenum Press, New York. Campbell, J. A. & Yeats, P. A. 1982 The distribution of iron, manganese, nickel, copper and cadmium in the waters of Baffin Bay and the Canadian Arctic archipelago. Oceanologica Acta 5, 161-167. Duinker, J. C. & Nolting, R. F. 1977 Dissolved and particulate trace metals in the Rhine Estuary and the Southern Bight. Marine Pollution Bulletin 8, 65-71. Duinker, J. C. & Nolting, R. F. 1982 Dissolved copper, zinc and cadmium in the Southern Bight of the North Sea. Marine Pollution Bulletin 13, 93-96. Erlenkeuser, H. 1981 Fossil carbonates in the recent sediments of Harrington Sound, Bermuda. In Hartington Sound Project, Special Publication 19. Bermuda Biological Station, Bermuda, 94 pp. Forstner, U. & Wittman, G. T. W. 1979 Metal Pollution in the Aquatic Environment. Springer Verlag, Berlin, 486 pp. Gordon, R. M., Martin, J. H. & Knauer, G. A. 1982 Iron in northeast Pacific waters. Nature 299, 611612. Katz, A. & Kaplan, I. R. 1981 Heavy metal behaviour in coastal sediments of southern California: A critical review and synthesis. Marine Chemistry 10, 261-299. Klinkhammer, G. I’. & Bender, M. I. 1981 Trace metal distribution in the Hudson River Estuary. Estuarine, Coastal and Shelf Science 12, 629-643. Krishnaswami, S. & Sarin, M. M. 1976 Atlantic surface particulates: composition, settling rates and dissolution in the deep sea. Earth and Planetary Science Letters 32, 43-40. Leatherland, T. M. & Mackay, D. W. 1975 Chemical processes in an estuary receiving major inputs of industrial and domestic wastes. In Estuarine Chemistry (Burton, J. D. & Liss, I’. E., eds). Academic Press, London, 229 pp.
262
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& A. H. Knap
Lyons, Wm B., Armstrong, P. B. & Gaudette, H. E. 1982 Trace metal concentration and fluxes in Bermuda sediments. Marine Pollution Bulletin 14, 65-68. Magnusson, B. & Rasmussen, L. 1982 Trace metal levels in coastal seawater. Investigations of Danish waters. Marine Pollution Bulletin 13, 81-84 Magnusson, B. & Westerlund, S. 1981 Solvent extraction procedures combined with back-extraction for trace metal determinations by atomic absorption spectrometry. Analytica Chimica Acta 131, 63-72. Martin, J. B. & Knauer, G. A. 1973 The elemental composition of plankton. Geochimica et Cosmochimica Acta 37, 1639-1653. Mayer, L. M. 1982 Retention of riverine iron in estuaries. Geochimica et Cosmochimica Acta 46, 1003-1011. Moore, R. M. 1981 Oceanographic distribution of zinc, cadmium, copper and aluminium in waters of the central Arctic. Geochimica et Cosmochimica Acta 45, 2475-2482. Morris, B., Barnes, J., Brown, F. & Markham J. 1977 The Bermuda Marine Environment, Vol. 1 Special Publication 15. Bermuda Biological Station, Bermuda, 120 pp. Murray, J. W. & Gill, G. 1978 The geochemistry of iron in Puget Sound. Geochimica et Cosmochimica Acta 42, 9-19. Preston, A., Jeffries, D. F., Dutton, J. W. R., Harvey, B. R. & Steele, A. K. 1972 British Isles coastal waters: The concentrations of selected heavy metals in seawater, suspended matter and biological indicators-a pilot survey. Environmental Pollution 3, 69-82. Santschi, I’. H., Li, Y. H. & Carson, S. R. 1980 The fate of trace metals in Narraganset Bay, Rhode Island: Radiotracer experiments in microcosm. Estuarine, Coastal and Shelf Science 10,635-654. Schaule, B. K. & Patterson, C. C. 1983 Perturbations of the natural lead profile in the Sargasso Sea by industrial lead. NATO Advanced Research Institute Conference ‘ Trace Metals in Seawater ‘. Erice, Italy, March-April, 1981. Plenum Press, New York. Spencer, M. J., Piotrowicz, S. R. & Betzer, I’. R. 1982 Concentrations of cadmium, copper, lead and zinc in surface waters of the northeast Atlantic oceancomparison of Go-flo and teflon water samplers. Marine Chemistry 11, 403-411. Strickland, J. D. H. & Parsons, T. R. 1972 A Practical Handbook of Seawater Analysis. Fisheries Research Board of Canada, Ottawa, 310 pp. Sturgeon, R. E., Berman, S. S., Desaulniers, A. St Russell, D. S. 1980 Pre-concentration of trace metals from seawater for determination by graphite-furnace atomic-absorption spectrometry. Talanta 27, 85-94. Thorstenson, D. C. & Mackenzie, F. T. 1974 Time variability of pore water chemistry in recent carbonate sediments, Devil’s Hole, Harrington Sound, Bermuda. Geochimica et Cosmochimica Acra 38, 1-19. Wallace, G. T., Hoffman, G. L. & Duce, R. A. 1977 The influence of organic matter and atmospheric deposition on the particulate trace metal concentration of northwest Atlantic seawater. Marine chemistry 5, 143-170. Wallace, G. T., Mahoney, 0. M., Dulmage, R., Sorti, F. & Dudek, N. 1981 First order removal of particulate aluminium in oceanic surface layers. Nature 293, 729. Wilke, R. J. & Dayal, R. 1982 The behaviour of iron manganese and silicon in the Peconic River estuary, New York. Estuarine, Coastal and Shelf Science 15, 577-586. Young, D. R., Alexander, G. V. & McDermont-Ehrlich, D. 1979 Vessel related contamination of southern California harbours by copper and other metals. Marine Pollution Bulletin 10, 50-56.
The Distribution and Geochemistry of some Trace Metals in the Bermuda Coastal Environment
T. D. Jickells
and A. H. Knap
Marine and Atmospheric Programme, Bermuda Biological Station, Ferry Road, Bermuda Received 30 December I982 and in revised form 8 July 1983
Keywords: islands
metals;
water analysis (chemical);
lagoons; sediments;
Bermuda
Results of the analyses of the water column and sediments of the Bermuda Platform for Cd, Cu, Fe, Mn, Ni, Pb and Zn are presented. The major process controlling the water column concentrations is physical mixing of open ocean waters with inshore waters, which are polluted by a wide range of diffuse inputs. Sedimentation within the inshore waters plays a lesser, but significant, role as do fluxes from the sediments of Fe and Mn and possibly phytoplankton uptake of Zn. Concentrations within the sediments are controlled by the formation of trace metal enriched clay/organic particles in the inshore areas and their subsequent redistribution by sediment resuspension, except for Fe and Mn which are largely associated with clay lattices.
Introduction data are available on trace metal concentrations in polluted marine environments (Leatherland & Mackay, 1975; Forstner & Wittman, 1979; Klinkhammer 81Bender, 1981; Duinker & Nolting, 1982). However, it is only recently that advances in sampling and analytical techniques have allowed the accurate determinations of trace metal concentrations in open ocean waters (Boyle et al., 1981; Bruland & Franks, 1983; Moore, 1981). These advancesare now being applied to continental shelf areasand marginal seassubject to relatively small anthropogenic impact (Wallace et al., 1981; Campbell i? Yeats, 1982; Magnussen & Rasmussen,1982). There appear to be, however, no data available on trace metal concentrations around oceanic islandswhere oceanic water spills directly into inshore environments without the intermediate continental slope environment. This paper describesthe results of a two-year survey of trace metal concentrations in the waters and sedimentsof such an environment, the island of Bermuda, and attempts to analyse the processescontrolling them. Bermuda is located about 1000km off the coast of North America and consistsof a series of interconnected islands of aeolian limestone overlying a volcanic seamount.The islands are surrounded by coral reefs and a platform of biogenic calcareous sands.Bermuda is densely populated but has no heavy industry and no rivers, due to the porous nature of the rock. Extensive
245 0272-7714/84/030245+18803.00/0
0
1984
Academic
Press
Inc.
(London)
Limited
246
T. D. Jickells & A. H. Knap
Methods Waters Surface sampleswere collected by hand, directly into sample storage bottles from slowmoving non-metallic boats, all possibleprecautions being taken to minimize contamination and contact with surface lilms. Subsurface water sampleswere collected in acid-leached Niskin sampling bottles. Sampleswere collected and stored in rigorously acid precleaned polyethylene bottles and preserved by acidification to about pH 2.2. Preconcentration of samplesfor analysisfor cadmium (Cd), copper (Cu), iron (Fe), nickel (Ni) and zinc (Zn) was by APDC chelation and solvent extraction closely following the method of Magnussen and Westerlund (1981) with chloroform as the solvent. The only modification was the use of an evaporation step rather than back extraction. Manganese (Mn) was determined by oxine chelation and solvent extraction using the method of Bewers et al. (1976) with the addition of a back extraction step rather than direct analysisof the solvent. Lead (Pb) was determined using a chelating resin by the method of Sturgeon et al. (1980). Final determination of all metals was by graphite furnace atomic absorption spectrophotometry (AAS) using a Perkin Elmer 373 with background correction and a HGA 400 furnace. Recoveries were determined by standard additions on samples,and blanks by re-extraction of previously extracted samples,after addition of a further aliquot of the preserving acid, each run in triplicate. From the standard deviation of these blanks we have calculated limits of detrction as three times the standard deviation (Strickland & Parsons, 1972). Precision of the analyses(-t 1 S.D.) hasbeen determined by a seriesof triplicate analysesof samples from the middle of the observed concentration range. The averageof all recoveries, blanks, precision and limits of detection of the methods are listed in Table 1. Nutrient concentrations and salinities were determined by standardmethods (Strickland & Parsons,1972). Filtration of samplesgave rise to contamination problems and consequently this was not done on a routine basisso the data presented are for total leachable trace metals at pH 2.2. On several occasionsparticulate trace metal concentrations were measured by filtration through acid precleaned 0.4 ,trm nucleopore titers and subsequentleaching of the filters with 1% nitric acid. By comparing these data with total metal concentrations, as determined on duplicate samples,the relative importance of the two phaseswas assessed. Blank problems prevented the measurementsof particulate cadmium concentrations. Sediments Surface sediment and sediment core sampleswere collected by divers, and then stored frozen prior to analysis. Samplesfor trace metal analyseswere first dried at 80 “C, then between 1 and 5 g were digested in boiling nitric acid, made up to 25 ml in 1%nitric acid and analysedby flame AAS. Blanks and spiked sampleswere run with the normal samples.
TABLE
1. Recoveries,
Recoveries (%) Blanks (pg I-‘) Precision (pg 1-l) Limits of detection
blanks,
(pg I-‘)
precision
and limits of detection
of trace metal methods
Cd
CU
Fe
Mn
Ni
Zn
101 3 x 10-T &3.6x 10-3 4 x 10-3
83 0.08
84 0.19
100
87 0.02 kO.02 0.03
103 0.07 f0.16 0.07
to.03
0.04
kO.15 0.15
0.01 kO.02 0.04
248
T. D. Jickells & A. H. Knap
Results Water There was no systematic temporal variability in the data, therefore data for each station have been averaged over all samplescollected (Table 2). The results falling outside three times the standard deviation of the remaining data were considered aberrant and were excluded from the data set (Magnussen & Westerlund, 1980). This process removes 6% or lessof the data for each parameter. Sediments Sediment sampleswere collected in June and July 1982. The results of the trace metal analysesand the other available data on the sedimentsare shownin Table 3. To characterize the grain size, only the percentage of the sample smaller than 63 pm (as determined by particle size analysis) is listed. Discussion waters Dissolvedvs. particulate distribtion: Throughout this report dissolved trace metalswill be defined asthose that passthrough a 0.4 pm filter. Basedon comparisonof total and particulate concentrations, Cu, Ni and Mn are present in the Bermuda inshore waters dominantly (> 80%) in the dissolved phase while Fe is dominantly (> 80%) particulate. For Zn, relatively high filter blanks cause some uncertainty but it too appears to be predominantly dissolved. By analogy to the other metals it is suggestedthat Cd is dominantly dissolved. This dominance of the dissolved phase for all but Fe is in keeping with the findings of other workers in coastal and marginal sea environments (Murray & Gill, 1978; Boyle et al., 1981; Wallace et al., 1981; Magnussen& Rasmussen,1982; Campbell &Yeats, 1982). SargassoSea: During May and June 1982 seven sampleswere collected in the Sargasso Sea 5-12 miles off Bermuda. The results of the analysesof these samplesalong with some comparative data from other workers in the SargassoSea are presented in Table 4(a). The data for Cu, Ni and Mn are similar to other reported data for the SargassoSea, while for Zn and Cd the techniques usedhere are insufficiently sensitive to detect concentrations as low as those reported by Bruland & Franks (1983). This indicates that the SargassoSeaaround Bermuda is a true oceanic environment and that the data can be used for comparisonwith the inshore waters values. For Fe the results are erratic and probably represent contamination of some of the samples. If three outliers are rejected, the average concentration is 0.33 + 0.21 lrg 1-i. There are very little published data on open ocean Fe concentrations largely becauseof the extreme contamination problems (Gordon et al., 1982). These latter authors reported dissolved Fe concentrations in the Pacific of 8-17 ng l-1 with particulate Fe the dominant form. Wallace et al. (1977) reported particulate Fe concentrations in the SargassoSea of 0.05-0.008 pg 1-i and Krishnaswami and Sarin (1976) reported 0.09-O. 15 pg 1-l sothe values found in this study are somewhat higher than in these other studies. North Lagoon and Great Sound: North Lagoon is a large area of mixed coral reefs and sand flats surrounded by coral reefs with rapid water exchange with the open ocean. The surface waters of the SargassoSea impinge on the platform without upwelling (Bodungen
Trace metals in the Bermuda coastal environment
247
Bermuda sediments are dominantly calcareous (95-1000/ocarbonate) and therefore the digestion was considered to be essentially complete and to result in total metal concentrations for the sediment. Results with other sediment samplesindicate this digestion to be comparableto hydrofluoric acid (Jickells, unpublished data). Particle size determinations were carried out on dried sedimentsusing standard sieves, and organic content was determined by measuring weight lossafter heating at 450-500 “C. Noncalcareousresidue wasdetermined by weighing the residue after dissolution in 10%hydrochloric acid. Samplingsites Sampling sites were designed to give good hydrographic, geographic and environmental coverage and are shown in Figures 1, 3 and 5.
North
Lagoon
Figure 1. (a) Map of Bermuda showing showing sediment sampling stations.
water
sampling
stations.
(b) Map
of Bermuda
‘cl S
ii S
icl S
LI S
1;; S
Castle Harbour
St George Harbour
Harrington Sound
Great Sound
Hamilton Harbour
2.
36 54 0.15 15 36.52 0.17 11 36.47 0.18 12 36.22 0.20 16 36.42 0.17 14 36.31 0.17 15 36.19 0.31 14
Salinity (WO) 7.3 1.8 15 6.0 2.4 11 4.8 1.1 12 9.0 2.6 16 6.9 0.9 14 5.9 1.3 15 5.0 1.0 14
Secchi disk (m depth) 0.41 0.18 16 0.55 0.27 12 0.67 0.32 12 1.01 0.50 15 0.74 0.39 14 1.84 1.07 15 1.80 0.78 13
a
in surface
Chlorophyll (WA 1-1)
Trace metal concentrations
0.04 0.02 15 0.05 0.05 12 0.10 0.11 12 0.54 0.33 16 0.10 0.07 12 1.57 0.84 15 2.54 1.30 14
4.7 3.6 16 10.5 4.9 12 10.2 6.7 11 6.8 4.0 15 4.7 2.0 12 12.7 9.3 15 11.4 2.8 12
from Bermuda
Nitrate + nitrite (PM 1-I)
samples
0.20 0.09 15 0.23 0.07 12 0.49 0.22 12 0.40 0.14 14 0.42 0.10 14 1.27 0.39 14 1.70 0.57 14
inshore
water@
1.07 0.38 15 2.05 1 .oo 12 3.95 1.36 11 1.67 0.66 16 1.78 0.38 13 4.22 1.34 15 6.1 1.9 13
0.19 0.13 5 0.46 0.17 5 0.59 0.29 6 0.52 0.23 7 0.44 0.18 5 0.90 0.38 6 1.36 1.18 6
0.14 0.04 16 0.16 0.04 12 0.19 0.04 11 0.16 0.04 16 0.17 0.07 14 0.22 0.05 14 0.23 0.09 14
0.15 1
0.29 0.21 14 1.3 0.5 12 1.1 0.5 12 0.6 0.4 14 0.7 0.4 12 2.6 1.5 14 2.5 0.9 13
‘Sampling began in September 1980 at all but St Georges and Castle Harbours where sampling began in February 1981. Sampling was carried out monthly until January 1982 and there is thus between 12 and 16 months of data for all stations for all metals except: (i) Mn for which analyses began in July 1981; (ii) Pb for which only one survey was carried out in October 1981. bM-mean; S-standard deviation; n-number of samples.
rc1 S n
M S
Statisticsb
1A
Station
TABLE
250
T. D. J’ickells & A. H. Knap
TABLE
3. Trace metal concentrations
Station
%orgb
%resc
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
6.4 3.3 6.5 3.8 7.5 3.6 3.3 3.6 4.6 3.8 5.8 11.7 4.5 6.8 6.7 5.8 13.7 31.4 10.5 14.9 15.9 5.8 5.8 2.7 8.1 9.7 6.0 9.0
5.3 1.6 5.2 3.0 6.0 2.0 1.6 1.7 2.2 1.5 4.2 0.9 1.8 2.7 2.2 2.8 5.7 8.1 5.9 7.5 1.5 0.6 1.7 1.1 3.2 7.2 0.5 0.4
in Bermuda
$6~63P’ 8.1 2.6 3.9 8.4 8.5 0 5.8 5.8 11.9 14.4 3.6 20.1 15.0 8.2 19.1 15.5 9.0 12.1 2.5 14.0 9.1 1.1 0.7 0.1 7.5 12.1 0.1 0
surface
sedimentsa
Cu
Fe
Mn
(PPm)
(PP~)
(PPm)
20.0 4.4 28.0 15.0 40.0 5.2 <2.5 <2.5 <2,5 <2.5 8.0 <2,5 3.0 5.8 10.0 13.0 545.0 46.0 23.0 50.0 <2,5 <2,5 <2.5 <2.5 4.0 12.0 <2.5 <2,5
1.40 1.20 I.20 1.60 3.40 0.40 0.40 0.50 0.30 0.20 0.70 0.10 0.65 0.80 0.95 1.05 4.10 3.50 3.60 3.40 0.45 0.15 0.90 0.20 1.70 2.50 0.10 0.10
39 26 43 33 37 <7 <7 <7 <7 <7 19 11 16 14 14 15 45 39 38 38 <7 <7 <7 <7 24 34 <7 <7
Pb (w-4
57 14 65 62 100 20 <5 <5 <5 <5 19 5 21 26 38 38 110 80 46 101 11 <5 6 <5 13 31 <5 <5
50.0 9.6 60.0 41 .o 70.0 31.0 10.0 11.0 Il.0 5.5 250.0 2.2 13.0 19.0 29.0 23.0 83.0 62.5 47.0 72.0 8.0 2.9 15.5 5.6 14.0 37.5 1.4 4.2
‘Ni was below the limit of detection ( < Sppm) except stations 17-20 where the result was < 12.5 ppm at all stations. Cd was below the limit of detection (< 1.2 ppm) except stations 17-20 where the result was ~3 ppm at all stations except station 11 where 1 ‘8 ppm was measured. b%org refers to the percentage organic content of the sediments. ‘%res refers to the percentage non-calcareous residue in the sediments. d% < 63 pm refers to the percentage of the sediment less than 63 pm in size.
et al., 1982). The processescontrolling primary production and nutrient cycling on the Bermuda platform have been extensively studied (Bodungen et al., 1982) and will not be
discussedhere. Trace metal concentrations increase l-lo-fold in North Lagoon over the SargassoSea and most of the metal concentrations double again in Great Sound. A comparison of the concentrations in North Lagoon with data from other coastal waters and marginal seas (Table 4(b)) showsBermuda levels to be comparableor lower than most other areasthough there is a wide range of values. Coastal waters are subject to a range of anthropogenic and geochemicalprocessesincluding fluxes from the sediment to the water column and removal of metals by adsorption or biological uptake. Sediment fluxes usually only arise from clay sediments (Boyle et al., 1981) and consequently may not be significant for Bermuda’s calcareousenvironment. Similarly the relatively low suspendedsolid loads and primary productivity of Bermuda waters may make these processesless important than in other areas.A major potential input to the North Lagoon/Great Sound system is water
Trace metals in the Bermuda coastal environment
TABLE
4. Trace metal concentrations
(a) Sargasso Sea
Cd (ng 1-l)
This study Bruland and Franks (1983) Boyle et al. (1981) Spencer et al. (1982) Yeats (unpublished)
<4 0.2
(b) Marginal
Cd (ng 1-I)
seas and coastal waters
Bermuda; this work (Station 1A) Bafhn Bay; Campbell and Yeats (1982) Gulf Stream; Wallace et al. (1981) Gulf Stream; Spencer et al. (1982) Gulf Stream; Boyle et al. (1981) Scotian Shelf; Bewers et al. (1976) North Sea; Preston et al. (1972) Danish Sound and Kattegat; Magnussen and Rasmussen (1982) U.S. Northeast Shelf Waters; Bruland and Franks (1983)
4.7 20-66 0.3-0’9 4.5 17 36 -
reported
for various
251
marine
CU (Pg 1-l)
Mn (1.18 I-1)
Ni (Pi3 1-f)
0.05 0.08 0.08 0.11 0.10
0.11 0.13 0.13
0.16 0.12 0.12 0.14
<0.07 0.004 0.01 <0.06
0.2
1.07
0.19
0.14
0.18-0.36 O-17 O-16 O-56 0.9
0.5-2.5 3.2 1
25
0.45
0.2-7.0
22
0.25
O-06
-
0.14-0.38 0.47 0.37
locations
Zn @g 1-Y
0.17-0.29 0.23 0.25 0.40
0.15
0.3
0.03 -
0.02 0.01 1.1 3.9
-
0.48
0.04
0.80
1.15
0.33
-
0.16
exchange with the more polluted waters of Hamilton Harbour. Water exchange between North Lagoon and Harrington Sound or Castle and St George Harbours is relatively much smaller. If water exchange with Hamilton is dominant, then a plot of trace metals against a parameter which measuresmixing proportion should be linear. Unfortunately, unlike an estuary, salinity is not a useful indicator in this system since the salinity gradient is too small and the fresh water input is diffuse rather than a point source. Similarly, nutrient and phytoplankton parameters are non-conservative and of no use as indicators due to the residence time of the water massesrelative to biological productivity. However, if the average trace metal concentrations (Table 2) in the inner Hamilton Harbour/Hamilton Harbour/Great Sound/North Lagoon/SargassoSeasystemare plotted against one another then approximately linear relationships result (Figure 2). All correlations are significant at the 99% level. Pb and Cd correlate better with Zn than Cu due to differences in their behaviour within the harbour itself, though whether these differencesare real is uncertain since many of the Cd data are close to the detection limit and there is only one data set for Pb. The linear interrelation of these elements suggeststhat either all these metals are subject to non-conservative processesof equal magnitude or that the behaviour of all these elementsis dominanted by physical mixing in this system. The latter seemsmore likely and this will be considered further in a later section. The similar behaviour of Fe, though dominantly particulate, and the other dominantly dissolved trace metals is in agreement with the observation of Mayer (1982) that while Fe may rapidly flocculate in seawater, the settlement of someof these floes may be a slow processrelative to water exchange.
252
T. D. Jickells & A. H. Knap
7 : IO . . 3 . :E . I-0
3 d 2.0 cu +g
0.4
7 .’ s ’ . s / Id ../’ I.0 2-o
7 7!A
2 El
l
/ .
/* I-0
cu (pg L-‘)
2.0
I- = 0.907 /
0”
I
2.0
cu (pg I-‘)
y
2.0
r =0.985
T
i
IO
.
2
I: 7
i-0
2.0 1-‘I r=0.971
l 0
p
I.0 cu (pg
4
3
a”
7
1.. i@
l
’
20
l H
0.2
7
.
5
r = 0.959 7J y
cu (pig 1. ’ I I I r = 0.964
2
I-‘)
/ I.0
a”
. /
El I-0 Zn t/&i
2.0
cl
. I-0
1-1)
Figure 2. The interrelationship note different scales.
./
Zn (pg
of average
water column
2.0 1-l)
trace metal concentrations.
Hamilton Harbour: While dilution appearsto be the dominant processcontrolling the distribution of trace metalsin the North Lagoon/Great Sound system, the distribution of trace metals and nutrients within the Hamilton Harbour area is complex. The harbour is subject to street runoff and cooling water dischargesfrom the city of Hamilton but not direct sewageinputs, these being piped offshore. In addition, recent measurementsof atmospheric particulate trace metal concentrations in Bermuda (Plumb, unpublished data) show high levels of Pb and Cd in urban compared to ocean air with smaller enhancementsfor Cu and Mn. Direct fallout of this material may be a significant source of those metals. Trace metal distributions (Figure 3) are similar to one another but different from the groundwater derived inputs (nitrate, salinity and silicate) or chlorophyll. Elevated trace metal levels occur along the north shore and at the eastern end of the harbour which is used as a yacht marina implying that street runoff and atmospheric fallout from the city of Hamilton are primarily responsible for the elevated trace metal concentrations in the harbour. Corrosion of moorings, boats and metal structures, boatyard operations (Young
Trace metals in the Bermuda coastal environment
NO2 + NO3 (pm
(-‘I
Zn (pg
I-‘)
253
Chlorophyll
o (pg
(-‘I
+g
\-‘I
c
Figure 3. The distribution of nitrite plus nitrate (NO, +NO,), chlorophyll in the surface waters of Hamilton Harbour and Great Sound, 29 September
Ni
a, Zn and Ni 1981.
et al., 1979), sewer overflow and fluxes of Fe and Mn from anoxic sedimentsare probably
alsoimportant sources.The restricted water exchange between the harbour area and Great Sound (due to the lines of islands and sills acrossthe mouth of the harbour) allows the build up of relatively high concentrations of metals and nutrients. The differences between the nutrient and trace metal distributions in the harbour are consistent with the relatively low trace metal concentrations and the relatively high nitrate concentrations found in the ground water (Jickells, unpublished data). Sound: This body of water has an extremely long residence time (140 days or more, Barnes & Bodungen, 1978) and essentially no direct trace metal inputs apart from atmospheric fallout, land runoff and in situ corrosion from a few pleasure boats. As would be expected under these conditions trace metal concentrations are low. The increasesin Cd, Cu, Ni, Pb and Zn that are found over North Lagoon can be attributed largely tc land runoff, while the increasesin Mn and Fe may be due to fluxes from an anoxic deer water area in the sound. Devil’s Hole, in the southern corner of Harrington Sound, is the major part of Harrington Sound subject to such seasonalanoxia. It is considerably deeper than the rest of the Sound (24 m compared to 5-15 m) and the formation of a seasonalthermocline gives rise to anoxic conditions in the deep water which have been extensively studied (Thorstenson & Mackenzie, 1974; Balzer & Wefer, 1981; Bodungen et al., 1982). The water column in Devil’s Hole was sampledat five depths monthly throughout 1981 and three more times in 1982. The water column is vertically homogeneousin winter but, Hurrington
254
T. D. Jickells & A. H. hap
00 dlil
00 r
--
00 -; 22
T
Trace metals in the Bermuda coastal environment
Figure 5. The distribution 14 April 1982.
of Cd, Cu, Fe and Zn in the surface
255
waters of Castle Harbour,
linear increasesin DIN (dissolved inorganic nitrogen- ammoniaplus nitrite plus nitrate), silicate, Fe and Mn with decreasingoxygen concentrations are seenafter the development of the thermocline (examples of each situation are shown in Figure 4). There is some scatter due to the continuation of primary production below the thermocline (Bodungen et al., 1982), sediment resuspensionand the probable settling of Fe after it reprecipitates in oxic waters. Other trace metals do not show systematic variability. During the period 4 June-l 1 August 1982, there appearsto have been little disturbance of the subthermocline waters (basedon salinity data), and it is therefore possibleto calculate the flux rates of these various ions to and from the sedimentsby assumingthat the only site of releaseand consumption is the sedimentsand that no water exchange occurs. The calculated rates are 220 ml 0, m-2 day-l (consumption), 410 pg at. N rnM2day-l, 470 pg at. Si m-2 day-i, 220 pg Fe m-2 day-i and 160 pg Mn m-2 day-i (all released). These estimatesare necessarilycrude but do allow an initial estimation of the significance of fluxes from the sediments.The oxygen consumption rate is within the range previously reported for Devil’s Hole while the DIN and silicate fluxes calculated here are two-to-fou times lower than those found using flux chambers (Balzer, 1981; Bodungen et al., 1982, Figure 4. Vertical profiles of temperature (t), oxygen (O,), ammonia (NH,), Cd, Cu, Fe. Mn, Ni and Zn in the water column of Devil’s Hole Harrington Sound under stratified conditions. (a), (b) and (c)-samples taken on 7 July 1981, and well mixed conditions (d), (e and (f)-samples taken on 8 October 1981.
256
T. D. Jickells & A. H. Knap
possibly due to biological uptake. The fluxes of Mn calculated here are considerably lower than those reported for other areas where clay sedimentspredominate (Wilke & Dayal, 1982). Some additional sampleswere collected from an area in Hamilton Harbour subject to similar seasonalanoxia to that in Harrington Sound and the rates of increase of DIN, silicate, Fe and Mn with decreasingoxygen in these two areasappear to be very similar. St Georgeand Castle Harbours: The majority of the water exchange from these two areas occurs directly with the open oceanand accordingly these have been consideredasa separate system with little impact on North Lagoon. St George’s Harbour has somesimilarities to Hamilton with no direct sewageinput and slow water exchange and thus acts asa natural sediment trap. The resuspensionof the fine sedimentsby ship movements gives rise to elevated Fe levels while street runoff, atmospheric fallout and boatyard operations probably cause the increase in the other metals, though fluxes of iron and manganesefrom anoxic sedimentsmay alsobe important. Castle Harbour is an open body of water with good water exchange and little development along the shoreline apart from the airport along the north shore. In the northern corner of the area, land reclamation by scrap metal dumping (mainly motor vehicles) followed by rockfill has continued for several years. A plume of elevated trace metal water (mainly Cd, Cu, Fe and Zn) can be traced acrossthe harbour (see Figure 5), though its precise distribution is wind and tide dependent. Metal levels immediately off the dumpsite are generally high but variable, so dumping history is clearly important. The pattern of enrichment, Cd and Zn> Fe and Cu, suggeststhat electronegativity rather than the quantities dumped is the factor controlling the input. Sediments The interpretation of sediment trace metal levels is always complicated by their dependence on sediment type. Inspection of the data suggeststhat the best characterization parameter is the percentage residue of non-calcareous (presumably clays and organics) material in the sediments.Thus in Table 5 the results of linear correlations of all the metals against percentage residue are presented and the relationship for Zn shown in Figure 6 as an example. A few data points have been excluded, two becausethey are high (Cu at one site in Hamilton Harbour and Zn off the dump) and one low Cu value in Devil’s Hole (Harrington Sound). It should alsobe noted that the only site at which Cd wasabove detection limit wasimmediately off the dumpsite in Castle Harbour. The data clustering below the detection limit (treated as being half the detection limit) tends to causethe graph not
TABLE
5. Best fit data for sediment
Parameter’ %res Mres %res %res %res
7x Cu VS. Fe vs. hIn vs. Pb vs. Zn
O%res = percentage
Correlation 0.935 0.883 0.872 0,821 0,890 residue.
results vs. percentage (r)
residue
Slope
Intercept
6.34 0.48 5.84 12.05 9.46
-7.2 -0.25 0.3i -6.38 -2.04
Number
of samples 26 28 28 28 27
(n)
257
Trace metals in the Bermuda coastal environment
Percentage
Figure
6. Plot of Zn vs. percentage
resldue
non-calcareous
residue in Bermuda
6. A comparison of trace metal (TM) concentrations that in a standard clay (Forstner & Wittman, 1979)
TABLE
TM in residual fraction (R) TM in standard clay (S) Enrichment factor R/S
0.3
-
Cu
Fe
634 45
48 100 46 700
14
1
sediments.
in the residual
fraction
with
Mn
584
-
850 0.7
68 -
1205
946
20 60
95
10
to pass through the origin. For metals with few points below the limit of detection the best fit line appearsto go through the origin, and all correlations are highly significant. These correlations suggestthat most of the trace metalsin thesesedimentsare associated with the non-calcareous fraction either as lattice-bound material within the clays or adsorbedto them. In Table 6 the concentrations in the residual fraction (derived by extrapolation) are compared to the trace metal content of standard clays (Forstner & Wittman, 1979). It appears that Fe and Mn have dominantly lattice sources(the enrichment of < 1 for Mn may be due to mobilization from reducing sediments) while Cu, Pb and Zn do not. These latter three are probably derived anthropogenically and become adsorbedto the non-calcareous particles. Katz and Kaplan (1981) described a similar mechanismfor the dispersal of sedimentary trace metalsoff the Californian coast. In Figure 7 the profiles of trace metal concentrations in a sediment core from Devil’s Hole, Harrington Sound are shown. The core was collected from a water depth of 24 m in an area of little natural sediment resuspensionand a sedimentation rate of about 1 mm year-i (Erlenkeuser, 1981). Fe, Mn and percentage residue are relatively constant with depth, suggestinglittle post-depositional migration in these anoxic sediments, while Cu, Pb and Zn are markedly enriched in surface sediments,again probably due to anthropogenit activities. Using the results from the deeper parts of the core to calculate enrichment factors results in values close to unity for Cu and Zn and about 10 for lead.
258
T. D. Jickells & A. H. Knap
Cu hmi
Fe (ppt)
IO
Mn (ppm)
20 I
I
20
4c-
I
t
!’
!I i
i .A / i
) I
I
Pb (ppm)
I
I
Zn (ppm)
30:
Figure 7. Vertical profiles Harrington Sound.
of Cu, Fe, Ah,
Pb and Zn in a sediment
core from Devil’s
Hole
Sediment cores collected from St George and Hamilton Harbours had constant trace metal concentrations with depth, presumably due to sediment resuspensionand bioturbation. Lyons et al. (1982) reported elevated trace metal levels in the surface layers of a sediment core from the extreme eastern end of Hamilton Harbour. Sediment concentrations of Fe, Pb and Cu in this area (which corresponds to the area of maximum water column concentrations) are higher than those reported here. Removal processes
Sedimentation rates (corrected for resuspension) have been measured in Hamilton Harbour, Harrington Sound and North Lagoon (Bodungen et al., 1982). By using these sedimentation rates in associationwith sediment trace metal concentrations from nearby sites it is possibleto calculate the rate of removal of metals to the sediments.This calculation assumesthat the sedimented material has a similar composition to the sediments themselves.If there is a significant flux of metalsout of the sedimentsthen this assumption may break down. As seenearlier, the only metal for which measurableflux rates from the sedimentshave been observed are Fe and Mn under anoxic conditions. Calculations suggest that the percentage of sedimentedFe and Mn recycled back to the water column is small (< lo%), though for Fe the release of even 1% of the sedimenting material back to the water column could have a significant effect on the water column concentrations, since so high a percentage of the Fe is sedimented. Knowing the areas, volumes and flushing times of the basins(Morris et cd., 1977; Barnes
Trace metals in the Bermuda coastal environment
259
& Bodungen, 1978) and the average water column concentrations (in Hamilton Harbour this is the average of both stations) the rate of removal by water exchangecan be calculated. Both sets of calculations are crude, largely becauseof uncertainties in flushing times and sedimentation rates plus the assumedhomogeneity within the bodies of water. They do, however, provide a useful tist order approach and in Table 7 the percentage of the total removal (sedimentation + flushing) by sedimentation is presented. A comparison of sedimentation OS.water exchange requires that the measurementsof water column and sediment concentrations be total metal concentration. This is indeed the case for the sediments and those metals which are dominantly dissolved in the water column. For Fe, however, the caseis different and Duinker and Nolting (1977) have shown that dilute acid leaching, such as occurs during storage at pH 2 ‘2, recovers only 15-40% of the total particulate iron. Despite reservations regarding the crudenessof the approach, inspection of Table 7 reveals several interesting points. In the North Lagoon area with its rapid water exchange, coarse sedimentsand low primary productivity, essentially all trace metal removal is by water exchange. In Hamilton Harbour flushing is again the dominant mechanismof removal as predicted earlier from the relationships in Figure 2, though sedimentation does play a role in trace metal removal as may be expected in an inshore area with higher productivity, turbid water and relatively restricted water exchange. Fe clearly behaves quite differently; the calculations suggest that sedimentation is the dominant removal route while the relationships in Figure 2 imply that this cannot be the case. Part of the solution to this paradox probably lies in the problem of incomplete leaching of particulate iron during storage, and if the assumption is made that only 25% of the iron is leached from the particulates (Duinker & Nolting, 1977) then the percentage of iron removal by sedimentation falls to 61%. In addition, the relationships in Figure 2 concern the water in the centre of the harbour and some of the iron inputs to the edges of the harbour (in situ corrosion and street runoff) may be rapidly sedimentedand then distributed throughout the sediments. The order of preferential sedimentationin Hamilton Harbour (Fe > Pb > h4n > Cu > Zn) is consistent with the known geochemistry of these elements (Bruland & Franks, 1981) and the results of recent laboratory simulationsof coastalecosystems(Santschi et al., 1980). TABLE
7. Percentage
of trace metal removal Exchange
Metal
North Lagoon and the Sargasso Sea
Cd CU Fe Mn Ni Pb Zn
< 17
that is by sedimentation
in the various
basins
between
Harrington Sound and North Lagoon <35 37 98 50 <71 67 54
Hamilton Harbour and Great Sound <75 22 86 30 <52 40 19
The offshore value of Bruland and Franks (1983) has been used for Cd and that of Schaule and Patterson (1983) for Pb. Concentrations in the water column at the two Hamilton Harbour stations have been averaged.
260
T. D. Jickells & A. H. Knap
It also suggeststhat the pattern of enrichment seenin the Hamilton Harbour sediments (Pb > Cu>Zn, Table 6) may reflect the geochemistry of the elements more than the pattern of pollution. In Harrington Sound the proportion of trace metals removed by sedimentationis higher than in other areasasmight be expected with the long residencetime of this body of water, though this may also reflect the extrapolation of the Devil’s Hole sedimentation rate to the whole of this area. One interesting feature though, is the relatively high proportion of Zn removed by sedimentation.A possibleexplanation is that phytoplankton uptake may be significant. Phytoplankton uptake will not be distinguished from inorganic sedimentation processes in these calculations since trace metalsincorporated in the plankton will be leached during storage of water samplesand during digestion of sediments. However, by using data abstracted from Martin and Knauer’s studies (1973) of Pacific plankton and the annual primary production rates of Bodungen et al. (1982), it is possibleto calculate the annual uptake of trace metals by the phytoplankton. Bodungen et al. (1982) calculated that 65% of the primary production in Harrington Sound was recycled within the water column so this figure can be used to allow for grazing. Since the only calculated figures for North Lagoon are ‘ lessthan ’ figures it is impossibleto quantify the significanceof phytoplankton uptake to sedimentation though it is apparent that it could be significant. The other two inshore basinsshow quite different characteristics. In Hamilton Harbour, the maximum potential contribution of phytoplankton uptake to the sedimentation removal route is 7% for Zn (after correcting the 65% recycling within the water column) and for the other elementsthe contribution is much smaller.By contrast, in Harrington Sound this maximum contribution of phytoplankton uptake to sedimentationranges from 1l-12% (Mn, Fe and Pb) to 30% for Cu and 60% for Zn. This distribution is again consistent with the known oceanographicbehaviour of these elements(Bruland & Franks, 1983), Zn (and Cd) showing the greatest phytoplankton uptake while Cu, Mn and Pb removal is dominated by inorganic removal processes.This alsoexplains the increasedproportion of Zn in the sediments of Harrington Sound. Clearly the type of removal route which operates can change from the turbid waters of Hamilton Harbour (which resemble the system of Santschi et al., 1980) to the clear waters of Harrington Sound.
Conclusions Cd, Cu, Fe, Mn, Ni, Pb and Zn inputs to the Bermuda inshore waters are generally in the form of diffuse inputs from land runoff, atmospheric deposition and in situ corrosion. These inputs are concentrated in Hamilton and St George’s Harbours where restricted water exchange leads to increased levels in the water column. In addition a few discrete sourcescan be identified, noteably scrap metal dumping. Trace metals in the water column are dominantly in the dissolved form (co.4 pm) except for Fe which is dominantly particulate. The distribution of trace metals in much of the water column of the Bermuda platform can be explained by physical mixing of contaminated inshore waters with clean open ocean waters. The distribution of trace metals in the sedimentsfollows a similar pattern to those in the water column, and is controlled by the formation of trace metal enriched clay and organic particles in the inshore waters and their subsequent redistribution throughout the Bermuda platform by water movements.
Trace metals in the Bermuda coastal environment
261
The overall dominanceof water exchange over sedimentation asa removal route is supported for all metals (except Fe) by calculations of the rates of removal by the two routes. In the turbid waters of Hamilton Harbour phytoplankton uptake appears to play a minor role but this situation changesin the lessturbid waters of the other parts of the Bermuda platform and phytoplankton-mediated sedimentationmay be the dominant mechanismof Zn sedimentation in Harrington Sound. Mn is the only metal for which fluxes out of the sediments are important compared to the sedimentation rate, though fluxes of Fe may also be an important source of water column Fe. Acknowledgements This work was funded by the Bermuda Government. We thank S. R. Smith for help with sampling and nutrient analysesand other colleaguesin the M.A.P. for help with the sediment survey. We thank T. M. Church, C. I. Measures and P. A. Yeats for their helpful comments on this manuscript; J. Cadwallader, S. M. Jickells and W. E. Sterrer for editorial assistanceand Zina Francis and Margaret Emmott for typing the manuscript. The use of unpublished data from P. A. Yeats and D. Plumb is also gratefully acknowledged. References Balzer, W. 1981 Oxygen consumption and nutrient release from the bottom of Devil’s Hole Bermuda. In Harrington Sound Project Special Publication 19. Bermuda Biological Station, Bermuda, 94 pp. BaIzer, W. & Wefer, G. 1981 Dissolution of carbonate minerals in a subtropical shallow marine environment. Marine Chemistry 10,545-558. Barnes, J. A. & Bodungen, B. v. 1978 The Bermuda Marine Environment, Vol. 2. Special Publicarion 17. Bermuda Biological Station, Bermuda, 190 pp. Bewers, J. M., Sundby, B. & Yeats, I’. A. 1976 The distribution of trace metals in the western North Atlantic off Nova Scotia. Geochimica et Cosmochimica Acta 40,687496. Bodungen, B. v., Jickells, T. D., Smith, S. R., Ward, J. A. D. & Hillier, G. B. 1982 The Bermuda Mawze Environment, Vol. 3. Special Publication 18. Bermuda Biological Station, Bermuda, 123 pp. Boyle, E. A., Huested, S. S. & Jones, S. I’. 1981 On the distribution of copper, nickel and cadmium in the surface waters of the north Atlantic and north Pacific ocean. Journal of Geophysical Research 86, 8048-8066. Brown, I. F. 1980 The nitrogen cycle and heat budget of a subtropical lagoon, Devil’s Hole, Harrington Sound, Bermuda: Implications for nitrous oxide production and consumption in marine environments. Ph.D. Thesis, Northwestern University, Evanston, Illinois, U.S.A., 317 pp. Bruland, K. W. & Frankie, R. I’. 1983 Mn, Ni, Cu, Zn and Cd in the western North Atlantic. NATO Advanced Research Institute Conference ‘ Trace Metals in Seawater ‘. Erice, Italy, March-April 1981. Plenum Press, New York. Campbell, J. A. & Yeats, P. A. 1982 The distribution of iron, manganese, nickel, copper and cadmium in the waters of Baffin Bay and the Canadian Arctic archipelago. Oceanologica Acta 5, 161-167. Duinker, J. C. & Nolting, R. F. 1977 Dissolved and particulate trace metals in the Rhine Estuary and the Southern Bight. Marine Pollution Bulletin 8, 65-71. Duinker, J. C. & Nolting, R. F. 1982 Dissolved copper, zinc and cadmium in the Southern Bight of the North Sea. Marine Pollution Bulletin 13, 93-96. Erlenkeuser, H. 1981 Fossil carbonates in the recent sediments of Harrington Sound, Bermuda. In Hartington Sound Project, Special Publication 19. Bermuda Biological Station, Bermuda, 94 pp. Forstner, U. & Wittman, G. T. W. 1979 Metal Pollution in the Aquatic Environment. Springer Verlag, Berlin, 486 pp. Gordon, R. M., Martin, J. H. & Knauer, G. A. 1982 Iron in northeast Pacific waters. Nature 299, 611612. Katz, A. & Kaplan, I. R. 1981 Heavy metal behaviour in coastal sediments of southern California: A critical review and synthesis. Marine Chemistry 10, 261-299. Klinkhammer, G. I’. & Bender, M. I. 1981 Trace metal distribution in the Hudson River Estuary. Estuarine, Coastal and Shelf Science 12, 629-643. Krishnaswami, S. & Sarin, M. M. 1976 Atlantic surface particulates: composition, settling rates and dissolution in the deep sea. Earth and Planetary Science Letters 32, 43-40. Leatherland, T. M. & Mackay, D. W. 1975 Chemical processes in an estuary receiving major inputs of industrial and domestic wastes. In Estuarine Chemistry (Burton, J. D. & Liss, I’. E., eds). Academic Press, London, 229 pp.
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T. D. Jickells
& A. H. Knap
Lyons, Wm B., Armstrong, P. B. & Gaudette, H. E. 1982 Trace metal concentration and fluxes in Bermuda sediments. Marine Pollution Bulletin 14, 65-68. Magnusson, B. & Rasmussen, L. 1982 Trace metal levels in coastal seawater. Investigations of Danish waters. Marine Pollution Bulletin 13, 81-84 Magnusson, B. & Westerlund, S. 1981 Solvent extraction procedures combined with back-extraction for trace metal determinations by atomic absorption spectrometry. Analytica Chimica Acta 131, 63-72. Martin, J. B. & Knauer, G. A. 1973 The elemental composition of plankton. Geochimica et Cosmochimica Acta 37, 1639-1653. Mayer, L. M. 1982 Retention of riverine iron in estuaries. Geochimica et Cosmochimica Acta 46, 1003-1011. Moore, R. M. 1981 Oceanographic distribution of zinc, cadmium, copper and aluminium in waters of the central Arctic. Geochimica et Cosmochimica Acta 45, 2475-2482. Morris, B., Barnes, J., Brown, F. & Markham J. 1977 The Bermuda Marine Environment, Vol. 1 Special Publication 15. Bermuda Biological Station, Bermuda, 120 pp. Murray, J. W. & Gill, G. 1978 The geochemistry of iron in Puget Sound. Geochimica et Cosmochimica Acta 42, 9-19. Preston, A., Jeffries, D. F., Dutton, J. W. R., Harvey, B. R. & Steele, A. K. 1972 British Isles coastal waters: The concentrations of selected heavy metals in seawater, suspended matter and biological indicators-a pilot survey. Environmental Pollution 3, 69-82. Santschi, I’. H., Li, Y. H. & Carson, S. R. 1980 The fate of trace metals in Narraganset Bay, Rhode Island: Radiotracer experiments in microcosm. Estuarine, Coastal and Shelf Science 10,635-654. Schaule, B. K. & Patterson, C. C. 1983 Perturbations of the natural lead profile in the Sargasso Sea by industrial lead. NATO Advanced Research Institute Conference ‘ Trace Metals in Seawater ‘. Erice, Italy, March-April, 1981. Plenum Press, New York. Spencer, M. J., Piotrowicz, S. R. & Betzer, I’. R. 1982 Concentrations of cadmium, copper, lead and zinc in surface waters of the northeast Atlantic oceancomparison of Go-flo and teflon water samplers. Marine Chemistry 11, 403-411. Strickland, J. D. H. & Parsons, T. R. 1972 A Practical Handbook of Seawater Analysis. Fisheries Research Board of Canada, Ottawa, 310 pp. Sturgeon, R. E., Berman, S. S., Desaulniers, A. St Russell, D. S. 1980 Pre-concentration of trace metals from seawater for determination by graphite-furnace atomic-absorption spectrometry. Talanta 27, 85-94. Thorstenson, D. C. & Mackenzie, F. T. 1974 Time variability of pore water chemistry in recent carbonate sediments, Devil’s Hole, Harrington Sound, Bermuda. Geochimica et Cosmochimica Acra 38, 1-19. Wallace, G. T., Hoffman, G. L. & Duce, R. A. 1977 The influence of organic matter and atmospheric deposition on the particulate trace metal concentration of northwest Atlantic seawater. Marine chemistry 5, 143-170. Wallace, G. T., Mahoney, 0. M., Dulmage, R., Sorti, F. & Dudek, N. 1981 First order removal of particulate aluminium in oceanic surface layers. Nature 293, 729. Wilke, R. J. & Dayal, R. 1982 The behaviour of iron manganese and silicon in the Peconic River estuary, New York. Estuarine, Coastal and Shelf Science 15, 577-586. Young, D. R., Alexander, G. V. & McDermont-Ehrlich, D. 1979 Vessel related contamination of southern California harbours by copper and other metals. Marine Pollution Bulletin 10, 50-56.