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Dynamics of dissolved silicium and nitrogen-nutrients at low temperature in the Ems-dollard estuary
277
Netherlands Journal of Sea Research 20 (2/3): 277-284 (1986)
DYNAMICS OF DISSOLVED SILICIUM AND NITROGEN-NUTRIENTS AT LOW TEMPERATURE IN THE EMS-DOLLARD ESTUARY
W. HELDER and R.T.P. DE VRIES Netherlands Institute for Sea Research, P.O. Box 59, 1790 AB Den Burg, Texel, The Netherlands
ABSTRACT With salinity and natural fluorescence as conservative tracers to discriminate between fresh water contributions from two different fresh water sources (the river Ems and the river Westerwoldse Aa), deviations from conservative behaviour for silicate, ammonia and nitrate during winter conditions (4-6°C) are reported and discussed. For silicate a net removal from overlying water during transport through the estuary of 150/0 was found. Nitrification in overlying water (260 ~mol. m - 3. h - 1) was the only important process in nitrogen cycling; sediment-water exchange was of no importance during this period. 1. INTRODUCTION Regeneration of nutrients from organic matter is an important feature in many estuaries where allochtonous and autochtonous organic particulates are efficiently trapped (POSTMA, 1961, 1967, 1980; VAN STRAATEN, 1960). Due to the shallowness of these areas, benthic regeneration is important. Concentrations of dissolved inorganic compounds in the overlying water can be influenced by sediment-water exchange, depending on the rates of sedimentation and mineralization in the sediment, and the flushing time of the estuary. The production of ammonia from N-containing organic matter takes place both aerobically and anaerobically, but for the oxidation of ammonia to nitrite and nitrate (nitrification) by the chemoautotrophic bacteria Nitrosomonas and Nitrobacter respectively, aerobic conditions are required. Because in most estuarine and coastal sediments the penetration of oxygen is restricted to a narrow zone of 1 to 2 cm (REVSBECH et al., 1980a; 1980b; HELDER & BAKKER, 1985), nitrification occurs only in this restricted zone and nitrite and nitrate, after their for-
mation, will diffuse downward into the sediment or, depending on the concentration gradient across the sediment-water interface, upward into the overlying water. In deeper, anaerobic sediment, nitrite and nitrate act as electron acceptors for anaerobic organic matter degradation. Thus, nitrite and nitrate produced in the O2-containing overlying water and sediment layer will be reduced and end up as ammonia (dissimilative nitrate reduction) or as the gaseous end products N2 and N20 (denitrification). In the Ems-Dollard estuary, benthic nitrogen regeneration influences the concentrations of ammonia, nitrite and nitrate in the overlying water during its passage through the estuary, (RUTGERSVAN DER LOEFF et al., 1981; HELDERet al., 1983). This is due to a net denitrification (removal of nitrate) in the innermost part of the Dollard where both the overlying water and the sediment are completely devoid of oxygen because of high rates of sewage input (average rate of denitrification 2.0 mmol.m-2.d-1). In the outer part of the Dollard and adjacent Ems estuary, a net nitrification rate of 2.5 mmol.m-2.d -1 occurs. Silicate concentrations in the overlying water of the Dollard and the Ems estuary are also strongly influenced by sediment-water exchange. Averaged over the year a net production of 2.3 mmol-m-2.d -1, with a maximum of 9.2 mmol.m--2.d -1 in August and September, was measured (HELDER et al., 1983). In this paper we present results from a winter situation (4-6°C, January, 1975) when concentration changes of ammonia and nitrate can be attributed completely to nitrification in the water passing through the estuary. Under these conditions removal of silicate from overlying water was observed. 2. AREA OF INVESTIGATION The Ems-Dollard estuary is situated at the German-
278
W. HELDER & R.T.P. DE VRIES
Dutch border and is part of the Wadden Sea (Fig. 1), characterized by tidal channels and extensive tidal flats. The tidal prism of the estuary is about 1100.106m3; that of the Dollard separately 120.106m 3. The tide is semi-diurnal with a tidal range of 3.2 m. In the tidal inlet near the island of Borkum, current velocities reach 1.5 m.s -1. Main fresh water inputs are the river Ems with a mean discharge of 125 m 3. s - 1 and the river Westerwoldse Aa (WWA) with a mean discharge of 12.5 m3.s- 1. The waters of the rivers Ems and WWA have quite different characteristics. The river WWA introduces waste water from potato flour mills into the Dollard and is responsible for heavy organic pollution in the inner part of the Dollard. Daily organic matter discharge is highest in September-December. During this period in 1974, the discharge into the Dollard was about 200.103 kg C-d-1 (vAN ES, 1977). Carbon budget calculations indicate that about 45% of the total carbon input into the Dollard originates from the adjacent North Sea, while another 40% comes from the river WWA. Together these are responsible for a carbon input of 750 gC. m - 2. y - 1. The associated Ninput from the WWA is mainly in the form of ammonia with maximum concentrations up to 5 mmol. dm -3 in
40'
50'
September-December. During our survey period in January 1975, TOC in the river WWA was at a low level (35 mgC-dm-3), and as an exception oxygen was not totally absent in the river water (20 #mol- dm- 3). The river Eros, in contrast to the WWA, is well oxygenated (80-100% 02 saturation), organic matter is relatively low (<12 mg TOC.dm-3; LAANE, 1982; CADEE & LAANE, 1982), and the predominant form of nitrogen is nitrate, which reaches maximum concentrations of about 350 ymol.dm -3 in early spring (HELDER et al., 1983). Sediments in the Dollard are rich in organic matter, from 3% C in the innermost part to 0.4% C near the mouth (VANES et aL, 1980). The accumulation rate of sediment varies, depending on location, from 0.5 to 3.0 cm-y -1 (GERRITSEN, 1972). 3. METHODS a) Surveys and analysis: Surveys in the Ems-Dollard estuary were carried out between 14 and 23 January 1975, covering, along the main axis of the estuary, the area between the island of Borkum and the fresh water input of the river Ems (about 70 km) and the
7 °00,
lO'
T
30'-
30' B
Emden
20' N
53%'J L
3
40'
50'
7~O t
IO'
20'
Fig. 1. Ems-Dollard estuary with location of stations. (o) Sample station. (-!') Anchor station.
NUTRIENTS AT LOW TEMPERATURE IN THE EMS-DOLLARD ESTUARY
area between the mouth of the Dollard and the river WWA (about 17 kin). One survey consisted of 30 sampling points (Fig. 1). On 21 January an anchor station in the Dollard was occupied over one tidal cycle (Fig. 1). Surface-water samples were filtered through 0.45 /~m cellulose acetate filters and stored frozen until analysis within one week. The analyses of NH4 +, N O 2 - , NO3- and dissolved Si were carried out on an auto-analyzer (Technicon. AA-II) following the procedures of STRICKLAND & PARSONS (1972). For the ammonia determination in these low-salinity waters, a modified phenol-hypochlorite method (HELDER & DE VRIES, 1979) was used. Salinity was measured with a field probe (Electronic Switch-gear, T-S probe) and, more accurately, in the laboratory with an inductively coupled salinometer (Guildline), calibrated with Copenhagen standard sea water. Determination of oxygen followed the Winkler technique (CARRITT& CARPENTER, 1966). Natural fluorescence was measured at room temperature on a Turner-Ill fluorimeter equipped with a primary filter with a maximum transmission of 358 nm and a secondary filter combination with a maximum transmission at 457 nm according to DUURSMA & ROMMETS (1961). b) Calculation of deviations from conservative behaviour: Because the Ems-Dollard estuary receives fresh water from two rivers (Ems and WWA), which are different in chemical composition, two tracers are needed for calculation of water composition at the sampling points. In addition to salinity, we used natural fluorescence (caused by humic substances) to calculate water composition in volume percentages of the three mixing end-members (sea water, Ems water and WWA water) according to ZIMMERMAN & ROMMETS (1974): 3 ~
i=1
j ai Si = 100SJ
3 j ~1 ai fi--- 100fJ 3 ~
i=1
j a i = 100
279
in which a! is the volume percentage of water mass i, characteri~,ed by salinity si and fluorescence fl in water sample j, which has a salinity sJ and a fluorescence fJ. To be useful as a tracer, the natural fluorescence must be different in Ems and WWA and it must be a conservative property of the water mass, or in other words, its degradation or formation must be slow compared to the residence time of water in the area, and finally its concentration changes in the mixing end-members must be small at a time scale comparable to the residence time of water in the area (ZIMMERMAN & ROMMETS, 1974). From calculated water composition at sample points and concentrations of the compound of interest (silicate, ammonia, nitrite, and nitrate) in the mixing end-members, the conservative concentrations of these compounds can be calculated and compared to observed concentrations. As for the natural fluorescence, concentration variations in the end-members must be small during the period of interest (Liss, 1976; ASTON, 1985; LODER & REICHARD, 1981). 4. RESULTS Table 1 shows the concentrations of dissolved Si, fluorescence (mFI), NH4 +, N O 2 - , a n d NO3- of the mixing end-members together with the variation in these parameters during the survey period. In Fig. 2 (data of all surveys) and Fig. 3a (data of anchor station) the natural fluorescence is plotted as a function of salinity. The difference in fluorescence concentration between the two fresh- water sources (rivers Ems and WWA) is sufficiently large to enable calculation of volume percentages of the three mixing endmembers at sample points. In the Dollard, fresh water from the river WWA is obviously mixing with a mixture of mainly Ems water and sea water; the final mixture has a salinity of about 7 and enters the Dollard through the mouth (Fig. 2). In Fig. 3b the calculated water composition at the anchor station in the Dollard is shown. The strong variation in water composition over a tidal period is evident. Calculated water composition and the concentra-
TABLE 1 Composition of mixing end members. Indicated ranges reflect variation in concentrations over the 10-day period, based on 4 surveys. "Sea"-water values based on one survey.
River WWA River EMS "sea"-water near Borkum
natural fluorescence
dissolved Si
NH4 +
NO2-
NO3-
(mFI)
(#mol. dm ~ 3)
(#mol. dm - 3)
(#mol. dm 3)
(t~mol. dm - 3)
281 - 295 193 - 197 35
300 - 325 200 - 234 65
400 - 478 57 - 65 6.8
2-7 4-5 3.6
7 - 27 306 - 343 81.7
280
W. HELDER & R.T.P. DE VRIES
%
mFI
300-~o
100
50
0
5
lO
15
20
25 S
Fig. 2. Distribution of natural fluorescence (mFI) as a function of salinity. (o) Ems estuary, ( ) Dollard. tions of dissolved Si of the end-members were used to calculate the concentration determined only by mixing of the three sources. In Fig. 4 these calculated concentrations are compared with the observed values. Such calculations were also applied to assess the behaviour of inorganic N-compounds in the Dollard and adjacent Ems. Differences between calculated or conservative concentrations and the observed ones at sample points (A-values) are positive for nitrate (AN O 3 - ) in the whole area (Fig. 5a), indicating NO3production. Values of A-NH4 + are negative, however, (Fig. 5b), and the A-values, although of different sign, are of the same magnitude for NO3- and NH4 + (Fig. 6). Because ~-N concentrations (NH4 + + NO2- + NO3-) were a convervative property of Dollard water (Fig. 7), this suggests that nitrification (oxidation of ammonia to nitrate) is the most impor-
mFI 250
O"
me (hr)
Fig. 3b. Calculated water composition at anchor station in the Dollard over a tidal cycle. Water composition in volume percentages of mixing end-members.
tant process in nitrogen cycling in the Dollard in this period. Values of A-NO2- are low ( < 2 Hmol.dm -3) and therefore not shown. This last result differs from earlier reports, which indicated that in situ production of nitrite can be significant, leading to NO2- concentrations in overlying Dollard water of up to 30 /~mol-dm -3 (HELDER et aL, 1983).
Si (calc.] )umol -1 300-
200-
~++ • +
\ 1OO -
200-
O
I 100
i
i
200 Si (obs.)
15o-
a
o~\ 1() S
Fig. 3a. Distribution of natural fluorescence (mFI) as a function of salinity (S) over a tidal cycle at anchor station in the Dollard. Location of station see Fig. 1.
300 ju m o l . I-~
Fig. 4. Comparison of conservative dissolved-Si concentration as calculated by using salinity and natural fluorescence as mixing indicators (Si-calculated) and observed dissolvedSi concentrations (Si-observed). (e) Data from the Dollard. ( + ) Data from the adjacent Ems estuary. (A) Data from the inner Dollard area with dissolved 02 concentration <50 /~mol 02- I -
NUTRIENTS AT LOW TEMPERATURE IN THE EMS-DOLLARD ESTUARY
5. DISCUSSION The relationship between the concentration of a dissolved component and a conservative index of estuarine mixing has been applied by many workers to distinguish conservative and non-conservative behaviour of those components. The method and its problems have been reviewed and discussed (LODER & REICHARD, 1981; LISS, 1976; ASTON, 1985). Most often salinity or chlorinity is used as the conservative mixing indicator. In the case of the Ems-Dollard estuary, the use of natural fluorescence as an additional tracer allows us to discriminate between conANO~
Jmol-I "I
100-
./"
+50-
0
l j
-50-
281
tributions from the river Ems and the river WWA. Natural fluorescence in combination with salinity has been applied previously in Dutch coastal water (OTTO, 1967; ZIMMERMAN & ROMMETS, 1974), in waters off the coast of Virginia and South Carolina (WILLEY & ATKINSON, 1982) and in the Ems-Dollard estuary (LAANE, 1981; HELDER et al., 1983). In experiments with Dollard water, VAN ES & LAANE (1982) did not find significant changes in natural fluorescence over a period of 30 days, a relatively long period compared with the residence time of water in the Dollard (10 days; HELDER & RUARDIJ, 1982). The conservative behaviour of natural fluorescence is also indicated in Fig. 2, which shows straight-line relationships between salinity and natural fluorescence. A comparison of calculated and observed dissolved Si concentrations in Fig. 4 shows that, in the area as a whole, calculated values are 10 to 15% higher, which suggests a removal of dissolved Si of that amount. The highest degree of deficiency in dissolved Si occurs in the water in the innermost part of the Dollard, where dissolved Si concentrations are high and 02 concentrations are low. Removal (10 to 30%) of dissolved Si from estuarine waters has been observed previously in other estuaries, although the mechanism of the removal process is not well understood. There is some evidence that suspended particles play a role (Liss, 1976; ASTON, 1985). A mechanism for silicate removal was proposed by LODER et al. (1978), who -ANH 4 ./j mol.l -~
IO0ANH,~
JmOt.l -~
1OO-
0 2 N m o l • I -~ 400 -
3OO-
C
•
200-
oo,o I •
50-
o~o
•
100-
O
f O
5
i~
i;
+
s
Fig. 5. (a) Differences between calculated and observed concentrations of nitrate (A-NO3-) as a function of salinity (S); (b) Differences between calculated and observed concentrations of ammonia (A-NH4+) as a function of salinity (S); (c) Oxygen concentrations as a function of salinity (S). Data from a survey in the Dollard (16 January, 1985) covering the area between the entrance of the river WWA (-0.5 S) and the mouth ( - 1 5 S).
0
+
ee
•
5'o
1(~0 ANO~ jumol. 1-1
Fig. 6. Relation between production of nitrate (ANO3-) and consumption of ammonia (ANH4+) in the Dollard (0) and adjacent Ems (+). Data from all surveys and from anchor station in the Dollard.
282
W. HELDER & R.T.P. DE VRIES
found that in anoxic pore waters after exposure to air, together with oxidation of Fe (11), dissolved Si is removed by scavenging of amorphous Fe (Ill) hydroxides. In this respect it is interesting to observe (Fig. 4) that the highest degree of removal of dissolved Si occurs in the area where near-anoxic WWA water mixes with oxygen-containing Dollard water. Net removal of silicate from overlying water is a low temperature (winter) phenomenon in the Ems-Dollard estuary. During summer at high temperature, there is a considerable addition of dissolved silicate (up to 9.2 mmol.m-2.d-1 in August-September) despite enhanced biological uptake, indicating the importance of sediment water-exchange at an elevated temperature (HELDER et aL, 1983). Total inorganic nitrogen (XN) was conservative in the Ems-Dollard during our winter survey (Fig. 7), implying that there were no significant estuarine sources or sinks in this period (e.g. by sedimentwater exchange) of dissolved inorganic N compounds. The removal of ammonia is thus balanced by the addition of nitrate (nitrification) within the water column. Using a residence time of 8 days for the appropriate river discharge rate (HELDER & RUARDIJ, 1982), we estimate a nitrification rate of 260 #mol.m-3.hr -1, which compares with 60 to 105 /~mol.m-3.hr -~ observed in the Danish Wadden
~N
(calc,]
)u m o l . I -~ 500
-
400 -
500
.)/'
y
-
200-
200
15 0
%o
5~o YN
(obs.)
jumol.
I -~
Fig. 7. Relation between EN (NH4~ + NO2- + NO3-) calculated by using salinity and natural flurescence as conservative mixing indicators (~-Ncalc.) and ~-Nas observed at stations in the Dollard and Ems-estuary. (o) Dollard; (+) Ems-estuary.
Sea in winter (HENRIKSEN et al., 1984). Both these values from the Wadden Sea area are relatively high. In his review, KAPLAN (1983) gives a range of 0.7-100 /~mol. m - 3 . h r - 1 for nitrification rates in estuarine and coastal environments. In shallow estuaries where particulates alternate between accumulation and resuspension, a nitrifying bacterial population can resist being washed out from the area by being associated with these particulates (HELDER & DE VRIES, 1983; MORRISet al., 1985; KNOX et al., in press; OWENS, 1986). The local nitrifiers can respond to favourable 02 and NH4 + conditions. The first step of nitrification, the oxidation of ammonia to nitrite, is less sensitive to low oxygen concentration than the second step, the oxidation of nitrite to nitrate. In laboratory experiments with nitrifiers from the Dollard, we found that below 100 #mol O2.dm-3 the oxidation of nitrite to nitrate is inhibited, while the oxidation of ammonia to nitrite becomes limited at a concentration less than 30/~mol 02" dm-3 (HELDER & DEVRIES, 1983). This different sensitivity to oxygen of the two nitrification steps can explain why nitrite is usually accumulating at the low ambient O2 concentration in the Dollard (HELDERet al., 1983). It also explains the absence of a distinct NO2- maximum during this winter survey, when 02 concentrations in Dollard water were relatively high (Fig. 5c). The lack of importance of sediment-water exchange for dissolved Si, as well as for ammonia and nitrate, during this winter survey in the Ems-Dollard estuary, agrees with the observations of KLUMP (1980) and KLUMP & MARTENS (1981), who found that benthic fluxes from Cape Lookout (North Carolina) sediments are 2 to 3 orders of magnitude lower in winter than in summer. These authors calculated that 70% of the nitrogen and 90% of the phosphorus regenerated by the sediment on an annual basis are released during the period of June through October, when temperature is > 15°C. On the other hand, nitrification in the water column is apparently not much restricted by the low temperature. Although nitrification in laboratory cultures often seems to stop below 4-5°C (see for a review KAPLAN, 1983), it is well-established that nitrification can occur at temperatures even lower than 0°C (HORRIGAN, 1981). In moderate climates, the seasonal temperature selection on nitrifying bacterial populations can perhaps explain the relatively high nitrification rates at low temperature. The opposite effect of temperature on sedimentwater exchange and on nitrification in the water column can thus lead to a seasonal shift in the relative importance of sediment and water column in the cycle of nitrogen.
NUTRIENTS AT LOW TEMPERATURE IN THE EMS-DOLLARD ESTUARY
6. R E F E R E N C E S ASTON, S.R., 1985. Estuarine chemistry. In: J.P. RILEY & R. CHESTER. Chemical Oceanography. 2nd ed. Vol. 7. Academic Press, London, New York, San Francisco: 362-440. CADEE, G.C. & R.W.PM. LAANE. 1982. Behaviour of POC, DOC and fluorescence in the fresh water tidal compartment of the river Ems.--Mitt. GeoI-Pal&ont. Inst. Uniw Hamburg 55: 331-342. CARRITT, D.E. & J.H. CARPENTER, 1966. Comparison and evaluation of currently employed modifications of the Winkler method for determining dissolved oxygen in seawater; a NASCO report.--J. Mar. Res. 24:286-318. DUURSMA, E.K. & J.W. ROMMETS, 1961. Interpretation mathematique de la fluorescense des eaux douces, saumatres et marines.--Neth. J. Sea Res. 1 : 391-405. Es, F.B. VAN, 1977. A preliminary Carbon budget for a part of the Ems estuary: the Dollard.--Helgol&nder wiss. Meeresunters. 36: 283-294. Es, F.B. VAN & R.W.P.M. LAANE, 1982. The utility of organic matter in the Ems-Dollard estuary.--Neth. J. Sea Res. 16:300-314. Es, F.B. VAN, M.A. VAN ARKEL, L.A. BOUWMAN & H.G.J. SCRR(~DER, 1980. Influence of organic pollution on bacterial, macrobenthic and meiobenthic populations in intertidal flats of the Dollard.--Neth. J. Sea Res. 14: 288-304. GERRITSEN, F., 1972. De ontwikkeling van de Dollard over de periode 1952-1969/'70. Rijkswaterstaat, Directie Groningen, afd. Studiedienst Delfzijl. Nota 72-1 : 1-23. HELDER, W. & J.F. BAKKER, 1985. Shipboard comparison of micro and mini electrodes for measuring oxygen distribution in marine sediments.--Limmol.Oceanogr. 30: 1106-1109. HELDER, W. & P. RUARDIJ, 1982. A one-dimensionel mixing and flushing model of the Ems-Dollard estuary: Calculation of time scales at different river discharges.--Neth. J. Sea Res. 15: 293-312. HELDER, W. & R.T.P. DE VRIES, 1979. An automatic phenolhypochlorite method for the determination of ammonia in sea- and brackish waters.--Neth. J. Sea Res. 13: 154-160. - - - - , 1983. Estuarine nitrite maxima and nitrifying bacteria (Ems-Dollard estuary)--Neth. J. Sea Res. 17: 1-18. HELDER, W., R.T.P. DE VRIES& M.M. RUTGERSVANDERLOEFF, 1983. Behaviour of nitrogen nutrients and dissolved silica in the Ems-Dollard estuary.--Can. J. Fish. Aquat. Sci. 40 (Suppl.1): 188-200. HENRIKSEN, K., A. JENSEN& M.B. RASMUSSEN,1984. Aspects of nitrogen and phosphorus mineralization and recycling in the northern part of the Danish Wadden Sea.-Neth. Inst. for Sea Res. Publ. Series 10: 51-71. RORRIGAN, S.C., 1981. Primary production under the Ross Ice Shelf, Antarctica.--Limnol. Oceanogr. 26: 378-382. KAPLAN, W.A., 1983. Nitrification. In: E.D. CARPENTER& D.C. CAPONE. Nitrogen in the marine environment. Academic Press, New York: 139-191. KLUMP, J.V., 1980. Benthic nutrient regeneration and the mechanisms of chemical sediment-water exchange in
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an organic-rich coastal marine sediment. Ph.D. Thesis, University of North Carolina, Chapel Hill. KLUMP, J.V. & C.S. MARTENS, 1981. Biogeochemical cycling in an organic-rich coastal marine basin. II. Nutrient sediment-water exchange processes.--Geochim Cosmochim. Acta 45: 101-121. KNOX, S., M. WRITFIELD,D.R. TURNER& M.I. LIDDICOAT, 1986. Statistical analysis of estuarine profiles: II1.Application to nitrate, nitrite and ammonium in the Tamar Estuary,--Estuar. coast, mar. Sci.: in press. LAANE, R.W.P.M., 1981. Composition and distribution of dissolved fluorescent substances in the Ems-Dollard Estuary.--Neth. J. Sea Res. 15: 88-99. - - - - , 1982. Sources of dissolved organic carbon in the EmsDollard Estuary: The rivers and phytoplankton.--Neth. J. Sea Res. 15: 331-339. LIss, P.S., 1967. Conservative and non-conservative benaviour of dissolved constituents during estuarine mixing. In: J.D. BURTON & P.S. LISS. Estuarine Chemistry. Academic Press, London and New York: 1-229. LODER, T.C. & R.P. REICHARD, 1981. The dynamics of conservative mixing in estuaries.--Estuaries 4: 64-69. LODER, T.C., W.B. LYONS, S. MURRAY & H.D. McGuINESS, 1978. Silicate in anoxic pore waters and oxidation effects during sampling.--Nature, London 273: 373-374. MORRIS, A.W., R.J.M. HOWLAND, E.MS. WOODWARD & R.C.F. MANTOURA, 1985. Nitrite and ammonia in the Tamar Estuary.--Neth. J. Sea Res. 19: 217-222. OTTO, L., 1967. Investigations on optical properties and watermasses of the southern North Sea.--Neth. J. Sea Res. 3: 532-552. OWENS, N.J.P., 1986. Estuarine nitrification: A naturally occurring fluidized bed reaction?--Est, coast. Shelf Sci. 12: 205-216. POSTMA, H., 1961. Suspended matter and Secchi disc visibility in coastal waters.--Neth. J. Sea Res. 1: 359-390. - - - - , 1967. Sediment transport and sedimentation in the marine environment. In: G.H. LAUFF. Estuaries. Publ. 83 A.A.A.S.: 158-179. - - - - , 1980. Sediment transport and sedimentation. In: E. OLAUSSON & I. CATO. Chemistry and biogeochemistry of estuaries. John Wiley & Sons, Chichester and New York: 153-186. REVSBECH, N.P., J. SORENSEN, T.H. BLACKBURN & J.P. LOMROLT, 1980a. Distribution of oxygen in marine sediments measured with microelectrodes.--Limnol. Oceanogr. 25:403-411. REVSBECH, N.P., B.B. JC)RGENSEN& T.H. BLACKBURN, 1980b. Oxygen in the sea bottom measured with a micro electrode.--Science, N.Y. 207: 1355-1356. RUTGERS VAN DER LOEFF, M.M., F.B, VAN ES, W. HELDER & R.T.P. DE VRIES,1981. Sediment-water exchanges of nutrients and oxygen on tidal flats in the Ems-Dollard Estuary.--Neth. J. Sea Res. 15: 113-129. STRAATENVAN, L.M.J.U., 1960. Transport and composition of sediments.--Verh. K. Ned. Geol. Mijnb. Genoot. 19: 279-292. STRICKLAND,J.D.H. & TR. PARSONS, 1972. A practical handbook of seawater analysis. (2nd ed.)--Bull. Fish. Res. Bd Can. 167: 1-310.
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WILLEY,J.C. & L.P. ATKINSON, 1982. Natural fluorescense as a tracer for distinguishing between Piedmont and coastal plain riverwater in the nearshore waters off Georgia and North Carolina. --Estuar. coast. Shelf Sci. 14: 49-49.
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Netherlands Journal of Sea Research 20 (2/3): 277-284 (1986)
DYNAMICS OF DISSOLVED SILICIUM AND NITROGEN-NUTRIENTS AT LOW TEMPERATURE IN THE EMS-DOLLARD ESTUARY
W. HELDER and R.T.P. DE VRIES Netherlands Institute for Sea Research, P.O. Box 59, 1790 AB Den Burg, Texel, The Netherlands
ABSTRACT With salinity and natural fluorescence as conservative tracers to discriminate between fresh water contributions from two different fresh water sources (the river Ems and the river Westerwoldse Aa), deviations from conservative behaviour for silicate, ammonia and nitrate during winter conditions (4-6°C) are reported and discussed. For silicate a net removal from overlying water during transport through the estuary of 150/0 was found. Nitrification in overlying water (260 ~mol. m - 3. h - 1) was the only important process in nitrogen cycling; sediment-water exchange was of no importance during this period. 1. INTRODUCTION Regeneration of nutrients from organic matter is an important feature in many estuaries where allochtonous and autochtonous organic particulates are efficiently trapped (POSTMA, 1961, 1967, 1980; VAN STRAATEN, 1960). Due to the shallowness of these areas, benthic regeneration is important. Concentrations of dissolved inorganic compounds in the overlying water can be influenced by sediment-water exchange, depending on the rates of sedimentation and mineralization in the sediment, and the flushing time of the estuary. The production of ammonia from N-containing organic matter takes place both aerobically and anaerobically, but for the oxidation of ammonia to nitrite and nitrate (nitrification) by the chemoautotrophic bacteria Nitrosomonas and Nitrobacter respectively, aerobic conditions are required. Because in most estuarine and coastal sediments the penetration of oxygen is restricted to a narrow zone of 1 to 2 cm (REVSBECH et al., 1980a; 1980b; HELDER & BAKKER, 1985), nitrification occurs only in this restricted zone and nitrite and nitrate, after their for-
mation, will diffuse downward into the sediment or, depending on the concentration gradient across the sediment-water interface, upward into the overlying water. In deeper, anaerobic sediment, nitrite and nitrate act as electron acceptors for anaerobic organic matter degradation. Thus, nitrite and nitrate produced in the O2-containing overlying water and sediment layer will be reduced and end up as ammonia (dissimilative nitrate reduction) or as the gaseous end products N2 and N20 (denitrification). In the Ems-Dollard estuary, benthic nitrogen regeneration influences the concentrations of ammonia, nitrite and nitrate in the overlying water during its passage through the estuary, (RUTGERSVAN DER LOEFF et al., 1981; HELDERet al., 1983). This is due to a net denitrification (removal of nitrate) in the innermost part of the Dollard where both the overlying water and the sediment are completely devoid of oxygen because of high rates of sewage input (average rate of denitrification 2.0 mmol.m-2.d-1). In the outer part of the Dollard and adjacent Ems estuary, a net nitrification rate of 2.5 mmol.m-2.d -1 occurs. Silicate concentrations in the overlying water of the Dollard and the Ems estuary are also strongly influenced by sediment-water exchange. Averaged over the year a net production of 2.3 mmol-m-2.d -1, with a maximum of 9.2 mmol.m--2.d -1 in August and September, was measured (HELDER et al., 1983). In this paper we present results from a winter situation (4-6°C, January, 1975) when concentration changes of ammonia and nitrate can be attributed completely to nitrification in the water passing through the estuary. Under these conditions removal of silicate from overlying water was observed. 2. AREA OF INVESTIGATION The Ems-Dollard estuary is situated at the German-
278
W. HELDER & R.T.P. DE VRIES
Dutch border and is part of the Wadden Sea (Fig. 1), characterized by tidal channels and extensive tidal flats. The tidal prism of the estuary is about 1100.106m3; that of the Dollard separately 120.106m 3. The tide is semi-diurnal with a tidal range of 3.2 m. In the tidal inlet near the island of Borkum, current velocities reach 1.5 m.s -1. Main fresh water inputs are the river Ems with a mean discharge of 125 m 3. s - 1 and the river Westerwoldse Aa (WWA) with a mean discharge of 12.5 m3.s- 1. The waters of the rivers Ems and WWA have quite different characteristics. The river WWA introduces waste water from potato flour mills into the Dollard and is responsible for heavy organic pollution in the inner part of the Dollard. Daily organic matter discharge is highest in September-December. During this period in 1974, the discharge into the Dollard was about 200.103 kg C-d-1 (vAN ES, 1977). Carbon budget calculations indicate that about 45% of the total carbon input into the Dollard originates from the adjacent North Sea, while another 40% comes from the river WWA. Together these are responsible for a carbon input of 750 gC. m - 2. y - 1. The associated Ninput from the WWA is mainly in the form of ammonia with maximum concentrations up to 5 mmol. dm -3 in
40'
50'
September-December. During our survey period in January 1975, TOC in the river WWA was at a low level (35 mgC-dm-3), and as an exception oxygen was not totally absent in the river water (20 #mol- dm- 3). The river Eros, in contrast to the WWA, is well oxygenated (80-100% 02 saturation), organic matter is relatively low (<12 mg TOC.dm-3; LAANE, 1982; CADEE & LAANE, 1982), and the predominant form of nitrogen is nitrate, which reaches maximum concentrations of about 350 ymol.dm -3 in early spring (HELDER et al., 1983). Sediments in the Dollard are rich in organic matter, from 3% C in the innermost part to 0.4% C near the mouth (VANES et aL, 1980). The accumulation rate of sediment varies, depending on location, from 0.5 to 3.0 cm-y -1 (GERRITSEN, 1972). 3. METHODS a) Surveys and analysis: Surveys in the Ems-Dollard estuary were carried out between 14 and 23 January 1975, covering, along the main axis of the estuary, the area between the island of Borkum and the fresh water input of the river Ems (about 70 km) and the
7 °00,
lO'
T
30'-
30' B
Emden
20' N
53%'J L
3
40'
50'
7~O t
IO'
20'
Fig. 1. Ems-Dollard estuary with location of stations. (o) Sample station. (-!') Anchor station.
NUTRIENTS AT LOW TEMPERATURE IN THE EMS-DOLLARD ESTUARY
area between the mouth of the Dollard and the river WWA (about 17 kin). One survey consisted of 30 sampling points (Fig. 1). On 21 January an anchor station in the Dollard was occupied over one tidal cycle (Fig. 1). Surface-water samples were filtered through 0.45 /~m cellulose acetate filters and stored frozen until analysis within one week. The analyses of NH4 +, N O 2 - , NO3- and dissolved Si were carried out on an auto-analyzer (Technicon. AA-II) following the procedures of STRICKLAND & PARSONS (1972). For the ammonia determination in these low-salinity waters, a modified phenol-hypochlorite method (HELDER & DE VRIES, 1979) was used. Salinity was measured with a field probe (Electronic Switch-gear, T-S probe) and, more accurately, in the laboratory with an inductively coupled salinometer (Guildline), calibrated with Copenhagen standard sea water. Determination of oxygen followed the Winkler technique (CARRITT& CARPENTER, 1966). Natural fluorescence was measured at room temperature on a Turner-Ill fluorimeter equipped with a primary filter with a maximum transmission of 358 nm and a secondary filter combination with a maximum transmission at 457 nm according to DUURSMA & ROMMETS (1961). b) Calculation of deviations from conservative behaviour: Because the Ems-Dollard estuary receives fresh water from two rivers (Ems and WWA), which are different in chemical composition, two tracers are needed for calculation of water composition at the sampling points. In addition to salinity, we used natural fluorescence (caused by humic substances) to calculate water composition in volume percentages of the three mixing end-members (sea water, Ems water and WWA water) according to ZIMMERMAN & ROMMETS (1974): 3 ~
i=1
j ai Si = 100SJ
3 j ~1 ai fi--- 100fJ 3 ~
i=1
j a i = 100
279
in which a! is the volume percentage of water mass i, characteri~,ed by salinity si and fluorescence fl in water sample j, which has a salinity sJ and a fluorescence fJ. To be useful as a tracer, the natural fluorescence must be different in Ems and WWA and it must be a conservative property of the water mass, or in other words, its degradation or formation must be slow compared to the residence time of water in the area, and finally its concentration changes in the mixing end-members must be small at a time scale comparable to the residence time of water in the area (ZIMMERMAN & ROMMETS, 1974). From calculated water composition at sample points and concentrations of the compound of interest (silicate, ammonia, nitrite, and nitrate) in the mixing end-members, the conservative concentrations of these compounds can be calculated and compared to observed concentrations. As for the natural fluorescence, concentration variations in the end-members must be small during the period of interest (Liss, 1976; ASTON, 1985; LODER & REICHARD, 1981). 4. RESULTS Table 1 shows the concentrations of dissolved Si, fluorescence (mFI), NH4 +, N O 2 - , a n d NO3- of the mixing end-members together with the variation in these parameters during the survey period. In Fig. 2 (data of all surveys) and Fig. 3a (data of anchor station) the natural fluorescence is plotted as a function of salinity. The difference in fluorescence concentration between the two fresh- water sources (rivers Ems and WWA) is sufficiently large to enable calculation of volume percentages of the three mixing endmembers at sample points. In the Dollard, fresh water from the river WWA is obviously mixing with a mixture of mainly Ems water and sea water; the final mixture has a salinity of about 7 and enters the Dollard through the mouth (Fig. 2). In Fig. 3b the calculated water composition at the anchor station in the Dollard is shown. The strong variation in water composition over a tidal period is evident. Calculated water composition and the concentra-
TABLE 1 Composition of mixing end members. Indicated ranges reflect variation in concentrations over the 10-day period, based on 4 surveys. "Sea"-water values based on one survey.
River WWA River EMS "sea"-water near Borkum
natural fluorescence
dissolved Si
NH4 +
NO2-
NO3-
(mFI)
(#mol. dm ~ 3)
(#mol. dm - 3)
(#mol. dm 3)
(t~mol. dm - 3)
281 - 295 193 - 197 35
300 - 325 200 - 234 65
400 - 478 57 - 65 6.8
2-7 4-5 3.6
7 - 27 306 - 343 81.7
280
W. HELDER & R.T.P. DE VRIES
%
mFI
300-~o
100
50
0
5
lO
15
20
25 S
Fig. 2. Distribution of natural fluorescence (mFI) as a function of salinity. (o) Ems estuary, ( ) Dollard. tions of dissolved Si of the end-members were used to calculate the concentration determined only by mixing of the three sources. In Fig. 4 these calculated concentrations are compared with the observed values. Such calculations were also applied to assess the behaviour of inorganic N-compounds in the Dollard and adjacent Ems. Differences between calculated or conservative concentrations and the observed ones at sample points (A-values) are positive for nitrate (AN O 3 - ) in the whole area (Fig. 5a), indicating NO3production. Values of A-NH4 + are negative, however, (Fig. 5b), and the A-values, although of different sign, are of the same magnitude for NO3- and NH4 + (Fig. 6). Because ~-N concentrations (NH4 + + NO2- + NO3-) were a convervative property of Dollard water (Fig. 7), this suggests that nitrification (oxidation of ammonia to nitrate) is the most impor-
mFI 250
O"
me (hr)
Fig. 3b. Calculated water composition at anchor station in the Dollard over a tidal cycle. Water composition in volume percentages of mixing end-members.
tant process in nitrogen cycling in the Dollard in this period. Values of A-NO2- are low ( < 2 Hmol.dm -3) and therefore not shown. This last result differs from earlier reports, which indicated that in situ production of nitrite can be significant, leading to NO2- concentrations in overlying Dollard water of up to 30 /~mol-dm -3 (HELDER et aL, 1983).
Si (calc.] )umol -1 300-
200-
~++ • +
\ 1OO -
200-
O
I 100
i
i
200 Si (obs.)
15o-
a
o~\ 1() S
Fig. 3a. Distribution of natural fluorescence (mFI) as a function of salinity (S) over a tidal cycle at anchor station in the Dollard. Location of station see Fig. 1.
300 ju m o l . I-~
Fig. 4. Comparison of conservative dissolved-Si concentration as calculated by using salinity and natural fluorescence as mixing indicators (Si-calculated) and observed dissolvedSi concentrations (Si-observed). (e) Data from the Dollard. ( + ) Data from the adjacent Ems estuary. (A) Data from the inner Dollard area with dissolved 02 concentration <50 /~mol 02- I -
NUTRIENTS AT LOW TEMPERATURE IN THE EMS-DOLLARD ESTUARY
5. DISCUSSION The relationship between the concentration of a dissolved component and a conservative index of estuarine mixing has been applied by many workers to distinguish conservative and non-conservative behaviour of those components. The method and its problems have been reviewed and discussed (LODER & REICHARD, 1981; LISS, 1976; ASTON, 1985). Most often salinity or chlorinity is used as the conservative mixing indicator. In the case of the Ems-Dollard estuary, the use of natural fluorescence as an additional tracer allows us to discriminate between conANO~
Jmol-I "I
100-
./"
+50-
0
l j
-50-
281
tributions from the river Ems and the river WWA. Natural fluorescence in combination with salinity has been applied previously in Dutch coastal water (OTTO, 1967; ZIMMERMAN & ROMMETS, 1974), in waters off the coast of Virginia and South Carolina (WILLEY & ATKINSON, 1982) and in the Ems-Dollard estuary (LAANE, 1981; HELDER et al., 1983). In experiments with Dollard water, VAN ES & LAANE (1982) did not find significant changes in natural fluorescence over a period of 30 days, a relatively long period compared with the residence time of water in the Dollard (10 days; HELDER & RUARDIJ, 1982). The conservative behaviour of natural fluorescence is also indicated in Fig. 2, which shows straight-line relationships between salinity and natural fluorescence. A comparison of calculated and observed dissolved Si concentrations in Fig. 4 shows that, in the area as a whole, calculated values are 10 to 15% higher, which suggests a removal of dissolved Si of that amount. The highest degree of deficiency in dissolved Si occurs in the water in the innermost part of the Dollard, where dissolved Si concentrations are high and 02 concentrations are low. Removal (10 to 30%) of dissolved Si from estuarine waters has been observed previously in other estuaries, although the mechanism of the removal process is not well understood. There is some evidence that suspended particles play a role (Liss, 1976; ASTON, 1985). A mechanism for silicate removal was proposed by LODER et al. (1978), who -ANH 4 ./j mol.l -~
IO0ANH,~
JmOt.l -~
1OO-
0 2 N m o l • I -~ 400 -
3OO-
C
•
200-
oo,o I •
50-
o~o
•
100-
O
f O
5
i~
i;
+
s
Fig. 5. (a) Differences between calculated and observed concentrations of nitrate (A-NO3-) as a function of salinity (S); (b) Differences between calculated and observed concentrations of ammonia (A-NH4+) as a function of salinity (S); (c) Oxygen concentrations as a function of salinity (S). Data from a survey in the Dollard (16 January, 1985) covering the area between the entrance of the river WWA (-0.5 S) and the mouth ( - 1 5 S).
0
+
ee
•
5'o
1(~0 ANO~ jumol. 1-1
Fig. 6. Relation between production of nitrate (ANO3-) and consumption of ammonia (ANH4+) in the Dollard (0) and adjacent Ems (+). Data from all surveys and from anchor station in the Dollard.
282
W. HELDER & R.T.P. DE VRIES
found that in anoxic pore waters after exposure to air, together with oxidation of Fe (11), dissolved Si is removed by scavenging of amorphous Fe (Ill) hydroxides. In this respect it is interesting to observe (Fig. 4) that the highest degree of removal of dissolved Si occurs in the area where near-anoxic WWA water mixes with oxygen-containing Dollard water. Net removal of silicate from overlying water is a low temperature (winter) phenomenon in the Ems-Dollard estuary. During summer at high temperature, there is a considerable addition of dissolved silicate (up to 9.2 mmol.m-2.d-1 in August-September) despite enhanced biological uptake, indicating the importance of sediment water-exchange at an elevated temperature (HELDER et aL, 1983). Total inorganic nitrogen (XN) was conservative in the Ems-Dollard during our winter survey (Fig. 7), implying that there were no significant estuarine sources or sinks in this period (e.g. by sedimentwater exchange) of dissolved inorganic N compounds. The removal of ammonia is thus balanced by the addition of nitrate (nitrification) within the water column. Using a residence time of 8 days for the appropriate river discharge rate (HELDER & RUARDIJ, 1982), we estimate a nitrification rate of 260 #mol.m-3.hr -1, which compares with 60 to 105 /~mol.m-3.hr -~ observed in the Danish Wadden
~N
(calc,]
)u m o l . I -~ 500
-
400 -
500
.)/'
y
-
200-
200
15 0
%o
5~o YN
(obs.)
jumol.
I -~
Fig. 7. Relation between EN (NH4~ + NO2- + NO3-) calculated by using salinity and natural flurescence as conservative mixing indicators (~-Ncalc.) and ~-Nas observed at stations in the Dollard and Ems-estuary. (o) Dollard; (+) Ems-estuary.
Sea in winter (HENRIKSEN et al., 1984). Both these values from the Wadden Sea area are relatively high. In his review, KAPLAN (1983) gives a range of 0.7-100 /~mol. m - 3 . h r - 1 for nitrification rates in estuarine and coastal environments. In shallow estuaries where particulates alternate between accumulation and resuspension, a nitrifying bacterial population can resist being washed out from the area by being associated with these particulates (HELDER & DE VRIES, 1983; MORRISet al., 1985; KNOX et al., in press; OWENS, 1986). The local nitrifiers can respond to favourable 02 and NH4 + conditions. The first step of nitrification, the oxidation of ammonia to nitrite, is less sensitive to low oxygen concentration than the second step, the oxidation of nitrite to nitrate. In laboratory experiments with nitrifiers from the Dollard, we found that below 100 #mol O2.dm-3 the oxidation of nitrite to nitrate is inhibited, while the oxidation of ammonia to nitrite becomes limited at a concentration less than 30/~mol 02" dm-3 (HELDER & DEVRIES, 1983). This different sensitivity to oxygen of the two nitrification steps can explain why nitrite is usually accumulating at the low ambient O2 concentration in the Dollard (HELDERet al., 1983). It also explains the absence of a distinct NO2- maximum during this winter survey, when 02 concentrations in Dollard water were relatively high (Fig. 5c). The lack of importance of sediment-water exchange for dissolved Si, as well as for ammonia and nitrate, during this winter survey in the Ems-Dollard estuary, agrees with the observations of KLUMP (1980) and KLUMP & MARTENS (1981), who found that benthic fluxes from Cape Lookout (North Carolina) sediments are 2 to 3 orders of magnitude lower in winter than in summer. These authors calculated that 70% of the nitrogen and 90% of the phosphorus regenerated by the sediment on an annual basis are released during the period of June through October, when temperature is > 15°C. On the other hand, nitrification in the water column is apparently not much restricted by the low temperature. Although nitrification in laboratory cultures often seems to stop below 4-5°C (see for a review KAPLAN, 1983), it is well-established that nitrification can occur at temperatures even lower than 0°C (HORRIGAN, 1981). In moderate climates, the seasonal temperature selection on nitrifying bacterial populations can perhaps explain the relatively high nitrification rates at low temperature. The opposite effect of temperature on sedimentwater exchange and on nitrification in the water column can thus lead to a seasonal shift in the relative importance of sediment and water column in the cycle of nitrogen.
NUTRIENTS AT LOW TEMPERATURE IN THE EMS-DOLLARD ESTUARY
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WILLEY,J.C. & L.P. ATKINSON, 1982. Natural fluorescense as a tracer for distinguishing between Piedmont and coastal plain riverwater in the nearshore waters off Georgia and North Carolina. --Estuar. coast. Shelf Sci. 14: 49-49.
ZrMMERMAN, J.T.F.
& J.W. ROMMETS, 1974. Natural fluorescense as a tracer in the Dutch Wadden Sea and adjacent North Sea.--Neth. J. Sea Res. 8: 117-125.