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The distribution of dissolved iodine in hebridean waters during mid-winter
Marine Environmenlal
Research. Vol 40, No. 3, pp. 277-288, 1995 Elsevier Science Ltd Printed in Great Britain 0141-I 136/95 $9.50+0.00
0141-1136(94)00147-2
The Distribution of Dissolved Iodine in Hebridean Waters During Mid-Winter Victor W. Truesdale 36 Ladycroft (Received
Park, Blewbury,
Oxon OX1 1 9QW, UK
29 July 1994; revised version accepted 7 October
received 1994)
2 October
1994;
ABSTRACT The distribution of dissolved total iodine and iodate in the waters ofl the Scottish coast around the Outer Hebrides in late January 1977 is described. Three types of water are delineated by the iodine results, open ocean proper, outer continental-shelf water, and coastal current water. The results confirm the sluggishness of iodide oxidation in seawater. The region between the ocean proper and the western edge of the coastal current is characterised by interconversion of iodate and iodide with little concomitant loss of total iodine. This behaviour is attributed to biological eflects during the previous summer and autumn, and is seen as a general phenomenon extending along the continental margin. Implications are discussed of the finding that some of the Scottish coastal currents contained much lower concentrations of total iodine and iodate concentrations than those observed in the Irish Sea during the previous autumn.
INTRODUCTION Dissolved iodine occurs in the oceans mainly as iodate and iodide, and at a total concentration of approximately 0.45 ,uM. In oceanic surface waters there appears to be a general zonal pattern such that depletions of total and iodate iodine are greatest in equatorial regions (e.g. Tsunogai & Henmi, 197 1; Wong, 1977; Elderfield & Truesdale, 1980). Iodide follows an opposing trend to iodate showing that there is interconversion of the two principal forms. Iodine’s presence in plants, animals and sediments has led workers to favour ‘biophilic’ explanations for its behaviour in open ocean waters. Support for this comes from a unique study of the 277
278
V. W. Truesdale
distribution of particulate iodine in the Atlantic (Wong et al., 1976) and from correlations of iodine species with nutrients (Wong & Brewer, 1974; Elderfield & Truesdale, 1980; Truesdale, 1995). In coastal waters iodine behaves in a similar manner but the changes are generally more pronounced (Truesdale, 1978a; Jickells et al., 1988; Wong & Zhang, 1992; Truesdale, 1994). This paper describes the distribution of dissolved iodine in the waters off the west coast of Scotland during the winter of 1977/78. While it satisfies a need for greater knowledge of the spatial variation of iodine, more importantly, it provides information about the iodine system during the period when regenerative processes prevail. Very little information of this kind is available, Truesdale’s (1978a) 18-month temporal study of iodine in the Menai Straits making up most of it. Whereas that body of water is very shallow and of small scale, those studied here cover a significant part of a continental shelf-sea. It is also important to appreciate that the information gained in this study is less readily available from open-ocean locations. This is because the ideal mid-winter sampling time also coincides with maximal vertical mixing in the water column, and this tends to dissipate accumulations of iodine species which would be of interest (Jickells et al., 1988).
THE HYDROGRAPHIC
SYSTEM STUDIED
The study applies to the North Atlantic Ocean between the Scottish Mainland, Rockall Bank and Ireland (Fig. l), and includes the Minch, the channel between the Outer Hebrides and the Scottish Mainland. It is supported by information collected farther south on a SW transect between the British mainland and the Azores (Fig. 1). Ellett & Edwards (1983) have described the T/S distribution prevailing at the time of the Hebridean survey. Surface salinity was approximately 35.3 and 35.0 over the 200m, 1OOmdepth contours, respectively, and 34.5 down the middle of the Minch. The coastal waters off the west coast of Scotland are a part of a much larger shelf system that extends along the entire western coast of Britain, and includes the Celtic and Irish Seas. On leaving the Irish Sea through its North Channel, water progresses northwards along the west coast of Scotland, receiving further run-off from the Clyde and other Scottish rivers (Ellett, 1979; Ellett & Edwards, 1983). Some of the coastal water flows through the Minch but a part separates and circulates around the western coast of the Outer Hebrides. Towards the outer shelf this coastal current is bounded by Atlantic water brought from the south in a persistent current
Distribution of dissolved iodine in Hebridean waters
279
-500 N
(b)
,’ %
I,
ATLANTIC :- OCEAN
59
x .^__
58 ON
57 I..
56
150 w
Fig. 1. (a) The sampling line gives the approximate
100
50 w
areas in relation to the European continental shelf (the broken position of the 1OOOmisobath). (b)The sampling positions in the Hebridean waters.
by vestiges of the North Atlantic Current passing north-eastwards towards the Norwegian Sea. Lower salinity coastal water thereby extends from the Irish Sea and close to the Scottish coast, and eventually round the north of Scotland into the North Sea (Folkard, 1981).
V. W. Truesdale
280
MATERIALS
AND METHODS
Samples were obtained between 30 January and 13 February 1978, on cruise 2 (1978) of R.R.S. Challenger at the 53 stations shown in Fig. 1. Fifty-two surface samples were also taken during R.R.S. Discovery’s cruise 89 (1977) along the cruise track shown in Fig. 1, between 24 November and 18 December. Whereas surface samples were taken from the ship’s continuously-running clean seawater supply, deeper samples were taken by sampling bottle at fewer stations. All samples were stored in glass bottles, in the dark at 4-10°C until analysis which was carried out within two weeks (Truesdale, 1968). Salinity was determined by an inductivelycoupled salinometer. Total iodine (iodate plus iodide) was determined catalytically by the CerV-As”’ reaction using a Technicon Autoanalyser I system, optimised according to the approach described by Truesdale & Smith, 1975. Iodateiodine was determined as the tri-iodium ion using a Technicon Auto-analyser II system (Truesdale, 1978b). The precision of the analytical methods is extremely high, with a coefficient of variation of less than 1% being reported during analysis (Truesdale, 1995). Unfiltered samples were analysed in both cases.
RESULTS To eliminate the effects due to simple dilution, iodine concentrations have been rationalised to 35 salinity (eg., RTI = *x35), as is common with iodine studies. Accordingly, the variables RIOs- and RTI are reported for corresponding measured concentrations, of iodate and total iodine. Note that rationalisation does not imply any environmental behaviour, as might be deduced, for example, from a graph of iodine concentration versus salinity for samples between open ocean water and riverine end-members; it merely provides a measure of the abundance of iodine relative to that of all the dissolved salts. The Challenger cruise The iodine distribution in the surface samples (Fig. 2) is perhaps best viewed as involving three water-types: oceanic waters to the west of the Outer Hebrides, outer-shelf water on the western shore of the Outer Hebrides, and Minch waters (Table 1). The highest values of salinity, RIOs- and RTI are associated with the ocean proper. The waters of the
281
Distribution of dissolved iodine in Hebridean waters
TABLE 1 of Salinity, Iodine and Nitrate in the Waters under Study
Mean Concentrations
cOnCentratiOn(pM)
Salinity
RI03-
RTI
N03--N‘=
Oceanic West coast, Outer Hebrides
35.4 34.6
0.43 0.34
0.44 0.43
12 9
The Minch: South Middle North
34.4 34.5 34.5
0.26 0.24 0.20
0.37 0.30 0.25
10 8 8
Location
a Taken from Houston,
1992.
west coast of the Outer Hebrides are characterised by essentially the same RTI as the oceanic waters, but a significantly lower RIOs-. The waters of the Minch contained less again of both. The broken lines of Fig. 2 depict the change in RIOs- and RTI for the transition from the ocean proper, to the west of the Outer Hebrides. The concentrations of both RIOs- and RTI decreased toward the north in the Minch, as shown by the three envelopes of points in Fig. 2. The trends within the data set also show up clearly on the plot of RIOsversus RTI (Fig. 3). The upper broken line represents the condition in which, from the oceanic end-member down, all the iodine would be - -+-t
f
3
s
;
x
0.30
ii 3
N
2 g
__++z- +d
:r .'
0
+++
+ Q
+ +
,+A-
t+/+
l
0*
0.20
34.0
d
+
0.40
IODATE
TOTAL L
I
34.5
35.0
SALINITY
i 35.5
34.0
34.5
36.0
36.5
SALINITY
Fig. 2. Variation of RTI and RIOs- with salinity in the Hebridean waters. The upper, middle and lower envelopes delineate the waters of the Minch according to their southern, middle and northern locations, respectively. The broken lines indicate shorewards transition from the Atlantic Ocean waters to those at the SW tip of the Outer Hebrides.
V. W. Truesdale
282
0.20
0.30
0.40
RTI OJMl Fig. 3. The graph of RIOs- versus RTI for the area under study. The points represent actual analyses. Those in the three envelopes refer, in ascending RTI concentration, to north, middle and south Minch samples. The vertical broken line on the right indicates interconversion of iodate to iodide at constant RTI, corresponding with movement inshore from the ocean toward the SW tip of the Outer Hebrides. The lower, intersecting broken line expresses broadly the inshore trend from the western coast of the Outer Hebrides.The upper broken line indicates the theoretical dilution of oceanic water in which the iodine is all iodate. The trapezoid and the circle within it represent, respectively, the condition of the Irish Sea and the Minch in the previous autumn.
present as iodate, i.e. RIOs- = 1. x RTI. As all the points lie below this, transformations of iodate to iodide occur within this system. The trend between the oceanic waters proper and those off the west coast of the outer Hebrides is represented as a vertical assemblage of points at approximately 0.+M RTI. The trend between the waters immediately to the west of the outer Hebrides and those within the Minch follows a sloping line toward the origin. The sampling frequency was too sparse to support any closer interpretation of this; for example, using lines of different gradient to depict various ratios of iodide and iodate uptake. A collation of deep-water samples taken at a variety of the stations within the sampling grid, all to the west of the Outer Hebrides, shows that RTI was essentially constant at 0.44 PM in the immediate underlying waters (Fig. 4). In contrast, RIOs- was approximately 0.42 ,UM between 100 and 1000 m, but 0.44 PM in the deepest water, below 1000 m. The Discovery cruise
The surface distribution of RIOs- and RTI on the SW transect (Fig. 1) between Land’s End and a position close to the Azores (41” 3’N, 24” 48’W) is shown in Fig. 5. Along this line RTI remained relatively constant
Distribution of dissolved iodine in Hebridean waters
RI03
SALINITY 34.5 a
35.0
35.5
.35 .40
283
RTI .a
.45
.45
5oa
2000
Fig. 4.
Variation of salinity, RI03- and RTI with depth at the stations to the west of the Outer Hebrides.
x
x
x
0.35 _
*x
AZORES =
0.30 _
9 0
0.25 _
4
xx 2;
LAND’S END
I 25
I 20
1
LO:GIT”DE
1 10
4 5
0
Fig. 5.
The distribution
of RTI (+ ) and RIOi (x) in surface waters between Land’s End and the Azores in December 1977.
at 0.44 ,uM. Working inwards from the RI03- also remained relatively constant ever, east of about 10’ W on the transect, End and at the continental edge, RIOs-
Azores, along 80% of this line at approximately 0.39 PM. Howi.e. some 250 miles from Land’s dropped progressively to about
284
Fig. 6.
V. W. Truesdale
The plot of RIOj- against RTI for surface samples between Land’s End and the Azores.
0.34 ,uM off Land’s End, itself. When presented as a plot of iodate against total iodine concentrations (Fig. 6), these results form an essentially vertical assemblage of points similar to that found off the Scottish coast (Fig. 3).
DISCUSSION This study demonstrates that appreciable amounts (20 to 30%) of iodide can persist in coastal waters during mid-winter when seasonal regeneration of plant nutrients is maximal. At the 58”N latitude of this survey area, mid-winter growth conditions for phytoplankton, particularly illumination, are sub-optimal. Primary productivity would have been at approximately 10% of its yearly maximum (Joint, Pers. Comm.) and would not have increased significantly until the spring bloom, some two months later. Concurrently, NOs--N concentrations would have peaked and nitrite-and ammonium-N would be negligible (Houston, 1992). Iodine’s behaviour in these waters is similar in principle to that recorded for the Menai Straits during winter (Truesdale, 1978a), where up to 39% of the iodine remained as iodide. Therefore, the results confirm that the final stage of regeneration, oxidation of iodide to iodate, is slower than that observed with comparable nutrient species. The finding ethos Jickells’ and co-workers’ comment (Jickells et al., 1988) that iodide which works its way into the deeper part of the oceanic water column must have persisted
Distribution of dissolved iodine in Hebridean waters
285
for periods of months, perhaps years. Indeed, it adds to a gradually increasing awareness of the sluggishness of this oxidation step (Luther et al., 1995). It also follows that in appropriate circumstances iodide should be useful as a hydrographic tracer. It is generally accepted that during growth promoting periods of the year a shorewards trend of decreasing RIOs- and RTI exists in temperate shelf-seas (Truesdale, 1978a; Jickells et al., 1988; Wong & Zhang, 1992; Truesdale, 1994). In determining how a similar trend could persist in the Hebridean Seas during mid-winter it has to be recognised that the minimum RIOs- and RTI concentrations observed in the Scottish waters are lower than those observed in the same shelf-sea system during the previous autumn. On the basis of a simple nutrient-like, uptake-regeneration model for iodine, the opposite would have been expected. This anomaly is shown in Fig. 3. where the trapezoid in the centre encompasses 95% of the results reported by Truesdale (1994) for the Irish Sea, some 4” latitude further south. Moreover, the circle within the trapezoid represents surface samples from 8 equally-spaced stations down the centre of the Minch collected and analysed with those from the Irish Sea (Truesdale, 1994). RTI and RIOs- was then 0.38 & 0.01 and 0.32 f 0.01 pM, respectively. The Scottish results reported here also cover a greater concentration range than the winter concentrations reported by Truesdale (1978a) in his 18-month study of the temporal variation of iodine in the Menai Straits. There, RTI and RI03- were 0.41& 0.01 and 0.25 f 0.01 PM, respectively, once again falling within the trapezoid of Fig. 3. Whatever the cause of the above anomaly it is nevertheless likely that the northwards advection of coastal water will be partly responsible for the shorewards trends described above. Thus, taking the mean velocity of the coastal current to be approximately 3 km day-’ (Ellett & Edwards, 1983), the journey from the North Channel of the Irish Sea to the Minch takes about 4 months. Moreover, as Houston’s (1992) comprehensive study of nutrient behaviour on the shelf shows, the integrity of the coastal current is maintained even though seasonal mixing occurs on the more seaward part of the shelf. She observed (Table 1) that peak winter NOs-N concentrations vary from 12 ,UM in the surface waters of the ocean proper (e.g. west of the 1OOOmline), to 10 PM--N over most of the shelf area between Ireland and the Outer Hebrides, and finally down to 8 FM-N in the Minch. (In summer these are all typically < 2 pM N03--N.) Therefore, winter N03--N in the Minch is similar to that in the Irish Sea to the south, but different to that on the open shelf because of winter incursion of oceanic water onto the shelf. Of course, nitrate in these winter surface waters arises from re-mineralisation of N as well as upward advection of nitrate as the deeper waters are mixed into the surface (Ellett,
286
V. W. Truesdale
1979; Wafar et al., 1983). The shallowness and density structure of the coastal current limits the second of these effects, and Minch water is able to retain the same 7-8 ,uM N03--N concentration as seen generally in winter in both the Celtic and Irish Seas, and parts of the English Channel which are similarly protected from upward advection (Slinn, 1974; Butler et al., 1979; Joint & Williams, 1983; Foster, 1984). The corollary also explains why at the deep, oceanic stations (Fig. 4.) iodine speciation was essentially the same at the surface as at 100-1000 m; as Ellett (1979) states, in winter these waters can be mixed to depths which exceed 400 m. The decrease of RIOs- at essentially constant RTI in the boundary area between the ocean proper and the western shore of the Outer Hebrides is interesting because it seems to represent a different set of processes to those occurring within the coastal current. Thus, it is not as easily explained as the mixing of the extreme end-member waters, i.e. oceanic and coastal waters. This is because the part of any mixture contributed by the coastal water end-member, while lowering the RIOs- concentration, must also lower its RTI concentration. Therefore, it is reasonable to infer that the water located against the western shore of the Outer Hebrides is a third end-member of the system. This interpretation is supported by the finding of the same effect (Fig. 6) in the Discovery observations in the Atlantic Ocean, south of Ireland in late November and early December, i.e. two months earlier. While in terms of seasons the Discovery sampling period is marginal, the sampling locations are much farther south where the climate favours continuation of growth-promoting conditions rather than the marked bias toward regeneration in the Scottish waters. It is probable, therefore, that during summer the non-assimilatory reduction of iodate (Tsunogai_& Sase, 1969) is a significant hydrographic feature along the whole of the Continental edge bordering the British Isles. Then, a combination of the sluggish iodide oxidation and the general northwestwards drift of the waters enables its influence to be detected in Hebridean waters during winter. This study is based on the approximations, firstly, that the main features of the iodine distribution can be mapped using only iodate and iodide species, and secondly, that the CerV-As”’ method for total iodine responds essentially only to these. Since the iodine cycle involves organically-bound iodine which could be ‘reactive’ or ‘unreactive’ to the total iodine method (Truesdale, 1975; Truesdale, 1978b), these assumptions must always be reviewed. In this case of oxic, open-sea waters, the approximations are reasonable, most complications of this type occurring instead in anoxic basins (Luther, 1991; Luther et al., 1991). The approximations are also supported by studies of the effect of UV photo-oxidation of samples prior to their analysis by various analytical
Distribution of dissolved iodine in Hebridean waters
287
methods (Truesdale, 1975; Butler & Smith, 1980). Moreover, the accuracy (specificity) of the analytical methods used here have been subject to the most rigorous scrutiny yet devised for any iodine methods (Truesdale, 1978b; Truesdale & Smith, 1979) and no untoward effects were observed.
ACKNOWLEDGEMENTS The author wishes to thank Anton Edwards and David Ellett of SMBA, Dunstaffnage, both for their kindness in supplying samples and supporting information from the Challenger cruise, and for their endless patience! Thanks also go to John Gould of IOS, Wormley, and Bob Dickson of MAFF, Lowestoft, for similar support from the Discovery and Ciroluna cruises. The iodine analyses were performed at the Institute of Hydrology, and thanks go to Dr J.S.G. McCulloch, then Director, for supporting the work. I am also grateful to Lucinda Houston and Grahame Savidge, both of Queen’s University, Belfast, for providing access to Lucinda’s thesis at the earliest possible time, and for interesting discussions. REFERENCES Butler, E. C. V. & Smith, J. D. (1980). Iodine speciation in seawaters -the anaand differential pulse lytical use of ultra-violet photo-oxidation polarography. Deep-Sea Res., 27A, 489-93. Butler, E. I., Knox, S. & Liddicoat, M. I. (1979). The relationship between inorganic and organic nutrients in seawater. J. Mar. Biol. Assoc. UK, 59,239-50. Elderfield, H. & Truesdale, V. W. (1980). On the biophilic nature of iodine in seawater. Earth Planet. Sci. Lett., 50, 105-14. Ellett, D. J. (1979). Some Oceanographic features of Hebridean waters. Proc. Royal Sot. Edinburgh,
77B, 61-74.
Ellett, D. J. & Edwards, A. (1983). Oceanography and inshore hydrography of the Inner Hebrides. Proc. Royal Sot. Edinburgh, 83B, 143-60. Folkard, A. R. (1981). Atlas of the Seas around the British Isles. Directory of Fishery Research, Ministry of Agriculture, Fisheries and Food, pp. 2.06, 2.07, 2.23. Foster, P. (1984). Nutrient distributions in the winter regime of the northern Irish Sea. Mar. Environ. Res., 13, 81-95. Houston, L. M. L. (1992). Distribution and Dynamics of Nutrients on the Scottish North West Shelf Region. Unpublished Ph.D. Thesis, The Queen’s University of Belfast, pp. 158. Jickells, T. D., Boyd, S. S. & Knap, A. H. (1988). Iodine cycling in the Sargasso Sea and the Bermuda Inshore waters. Mar. Chem., 24,61-82. Joint, I. R. & Williams, R. (1983). In: Annual Report (1983) of the Institute of Marine Environment Research, Plymouth, UK. Natural Environment Research Council, Swindon, UK.
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Luther, G. W. III (1991). Sulphur and iodine speciation in the water column of the Black Sea. In: Black Sea Oceanography, eds E. Izdar & J.W. Murray. Kluwer Academic Publishers, The Netherlands, pp. 187-204. Luther, G. W. III, Ferdelman, T., Culberson, C. H., Kostka, J. & Wu, J. (1991). Iodine chemistry in the water column of the Chesapeake Bay: Evidence for organic iodine forms. Est., Coastal and Shelf Sci., 32, 267-79. Luther, G. W. III, Wu, J. & Cullen, J. B. (1995). Redox chemistry of iodine in seawater: Frontier molecular orbital theory considerations. Amer. Chem. Sot., Advances in Chemistry Series No. 244. Aquatic Chemistry: Interfacial and Interspecies Processes, eds Chen Pao Huang, C. R. G’Metia &J. J. Morgan. Slinn, D. J. (1974). Water circulation and nutrients in the North-west Irish Sea. Est. Coastal Mar. Sci., 2, l-25. Truesdale, V. W. (1968). Studies on the Chemistry of Iodine in Seawater. Doctoral thesis; University of Wales, Aberystwyth. Truesdale, V. W. (1975). Reactive and unreactive iodine in seawater -a possible indication of an organically bound iodine fraction. Mar. Chem., 3, 111-19. Truesdale, V. W. (1978a). Iodine in inshore and offshore marine waters. Mar. Chem., 6, 1-13. Truesdale, V. W. (1978b). The automatic determination of iodate and total iodine in seawater. Mar. Chem., 6, 253-73. Truesdale, V. W. (1994). Distribution of dissolved iodine in the Irish Sea, a temperate shelf sea. Est. Coastal Mar. Sci., 38,43546. Truesdale, V. W. (1995). A re-assessment of Redfield correlations between dissolved iodine and nutrients in oceanic waters and a strategy for further investigations of iodine. Mar. Chem., 48, 43-56. Truesdale, V. W. & Smith, P. J. (1975). The automatic determination of iodide or iodate in solution by catalytic spectrophotometry, with particular reference to river water. Analyst, 100,11 l-23. Truesdale, V. W. & Smith, C. J. (1979). A comparative study of three methods for the determination of iodate in seawater. Mar. Chem., 7, 133-9. Tsunogai, S. & Sase, T. (1969). Formation of iodide-iodine in the ocean. DeepSea Res., 16, 489-96. Tsunogai, S. & Henmi, T. (1971). Iodine in the surface water of the ocean. J. Oceanography Sot. Japan, 27,67-72. Wafar, M. V. M., Le Corre, P. & Birrien, J. L. (1983). Nutrients and primary production in permanently well-mixed temperate coastal waters. Est., Coastal and Sherf Sci., 17,43 l-46. Wong, G. T. F. (1977). The distribution of iodine in the upper layers of the equatorial Atlantic. Deep-Sea Res., 24, 115-25. Wong, G. T. F. & Brewer, P. G. (1974). The determination and distribution of iodate in South Atlantic waters. J. Mar. Res., 32,25-36. Wong, G. T. F. & Zhang, L. (1992). Changes in iodine speciation across coastal hydrographic fronts in southeastern United States continental shelf waters. Cont. ShelfRes., 12, 717-33. Wong, G. T. F., Brewer, P. G. & Spencer, D. W. (1976). The distribution of particulate iodine in the Atlantic Ocean. Earth and Planet. Sci. Lett., 32, 441-50.
Research. Vol 40, No. 3, pp. 277-288, 1995 Elsevier Science Ltd Printed in Great Britain 0141-I 136/95 $9.50+0.00
0141-1136(94)00147-2
The Distribution of Dissolved Iodine in Hebridean Waters During Mid-Winter Victor W. Truesdale 36 Ladycroft (Received
Park, Blewbury,
Oxon OX1 1 9QW, UK
29 July 1994; revised version accepted 7 October
received 1994)
2 October
1994;
ABSTRACT The distribution of dissolved total iodine and iodate in the waters ofl the Scottish coast around the Outer Hebrides in late January 1977 is described. Three types of water are delineated by the iodine results, open ocean proper, outer continental-shelf water, and coastal current water. The results confirm the sluggishness of iodide oxidation in seawater. The region between the ocean proper and the western edge of the coastal current is characterised by interconversion of iodate and iodide with little concomitant loss of total iodine. This behaviour is attributed to biological eflects during the previous summer and autumn, and is seen as a general phenomenon extending along the continental margin. Implications are discussed of the finding that some of the Scottish coastal currents contained much lower concentrations of total iodine and iodate concentrations than those observed in the Irish Sea during the previous autumn.
INTRODUCTION Dissolved iodine occurs in the oceans mainly as iodate and iodide, and at a total concentration of approximately 0.45 ,uM. In oceanic surface waters there appears to be a general zonal pattern such that depletions of total and iodate iodine are greatest in equatorial regions (e.g. Tsunogai & Henmi, 197 1; Wong, 1977; Elderfield & Truesdale, 1980). Iodide follows an opposing trend to iodate showing that there is interconversion of the two principal forms. Iodine’s presence in plants, animals and sediments has led workers to favour ‘biophilic’ explanations for its behaviour in open ocean waters. Support for this comes from a unique study of the 277
278
V. W. Truesdale
distribution of particulate iodine in the Atlantic (Wong et al., 1976) and from correlations of iodine species with nutrients (Wong & Brewer, 1974; Elderfield & Truesdale, 1980; Truesdale, 1995). In coastal waters iodine behaves in a similar manner but the changes are generally more pronounced (Truesdale, 1978a; Jickells et al., 1988; Wong & Zhang, 1992; Truesdale, 1994). This paper describes the distribution of dissolved iodine in the waters off the west coast of Scotland during the winter of 1977/78. While it satisfies a need for greater knowledge of the spatial variation of iodine, more importantly, it provides information about the iodine system during the period when regenerative processes prevail. Very little information of this kind is available, Truesdale’s (1978a) 18-month temporal study of iodine in the Menai Straits making up most of it. Whereas that body of water is very shallow and of small scale, those studied here cover a significant part of a continental shelf-sea. It is also important to appreciate that the information gained in this study is less readily available from open-ocean locations. This is because the ideal mid-winter sampling time also coincides with maximal vertical mixing in the water column, and this tends to dissipate accumulations of iodine species which would be of interest (Jickells et al., 1988).
THE HYDROGRAPHIC
SYSTEM STUDIED
The study applies to the North Atlantic Ocean between the Scottish Mainland, Rockall Bank and Ireland (Fig. l), and includes the Minch, the channel between the Outer Hebrides and the Scottish Mainland. It is supported by information collected farther south on a SW transect between the British mainland and the Azores (Fig. 1). Ellett & Edwards (1983) have described the T/S distribution prevailing at the time of the Hebridean survey. Surface salinity was approximately 35.3 and 35.0 over the 200m, 1OOmdepth contours, respectively, and 34.5 down the middle of the Minch. The coastal waters off the west coast of Scotland are a part of a much larger shelf system that extends along the entire western coast of Britain, and includes the Celtic and Irish Seas. On leaving the Irish Sea through its North Channel, water progresses northwards along the west coast of Scotland, receiving further run-off from the Clyde and other Scottish rivers (Ellett, 1979; Ellett & Edwards, 1983). Some of the coastal water flows through the Minch but a part separates and circulates around the western coast of the Outer Hebrides. Towards the outer shelf this coastal current is bounded by Atlantic water brought from the south in a persistent current
Distribution of dissolved iodine in Hebridean waters
279
-500 N
(b)
,’ %
I,
ATLANTIC :- OCEAN
59
x .^__
58 ON
57 I..
56
150 w
Fig. 1. (a) The sampling line gives the approximate
100
50 w
areas in relation to the European continental shelf (the broken position of the 1OOOmisobath). (b)The sampling positions in the Hebridean waters.
by vestiges of the North Atlantic Current passing north-eastwards towards the Norwegian Sea. Lower salinity coastal water thereby extends from the Irish Sea and close to the Scottish coast, and eventually round the north of Scotland into the North Sea (Folkard, 1981).
V. W. Truesdale
280
MATERIALS
AND METHODS
Samples were obtained between 30 January and 13 February 1978, on cruise 2 (1978) of R.R.S. Challenger at the 53 stations shown in Fig. 1. Fifty-two surface samples were also taken during R.R.S. Discovery’s cruise 89 (1977) along the cruise track shown in Fig. 1, between 24 November and 18 December. Whereas surface samples were taken from the ship’s continuously-running clean seawater supply, deeper samples were taken by sampling bottle at fewer stations. All samples were stored in glass bottles, in the dark at 4-10°C until analysis which was carried out within two weeks (Truesdale, 1968). Salinity was determined by an inductivelycoupled salinometer. Total iodine (iodate plus iodide) was determined catalytically by the CerV-As”’ reaction using a Technicon Autoanalyser I system, optimised according to the approach described by Truesdale & Smith, 1975. Iodateiodine was determined as the tri-iodium ion using a Technicon Auto-analyser II system (Truesdale, 1978b). The precision of the analytical methods is extremely high, with a coefficient of variation of less than 1% being reported during analysis (Truesdale, 1995). Unfiltered samples were analysed in both cases.
RESULTS To eliminate the effects due to simple dilution, iodine concentrations have been rationalised to 35 salinity (eg., RTI = *x35), as is common with iodine studies. Accordingly, the variables RIOs- and RTI are reported for corresponding measured concentrations, of iodate and total iodine. Note that rationalisation does not imply any environmental behaviour, as might be deduced, for example, from a graph of iodine concentration versus salinity for samples between open ocean water and riverine end-members; it merely provides a measure of the abundance of iodine relative to that of all the dissolved salts. The Challenger cruise The iodine distribution in the surface samples (Fig. 2) is perhaps best viewed as involving three water-types: oceanic waters to the west of the Outer Hebrides, outer-shelf water on the western shore of the Outer Hebrides, and Minch waters (Table 1). The highest values of salinity, RIOs- and RTI are associated with the ocean proper. The waters of the
281
Distribution of dissolved iodine in Hebridean waters
TABLE 1 of Salinity, Iodine and Nitrate in the Waters under Study
Mean Concentrations
cOnCentratiOn(pM)
Salinity
RI03-
RTI
N03--N‘=
Oceanic West coast, Outer Hebrides
35.4 34.6
0.43 0.34
0.44 0.43
12 9
The Minch: South Middle North
34.4 34.5 34.5
0.26 0.24 0.20
0.37 0.30 0.25
10 8 8
Location
a Taken from Houston,
1992.
west coast of the Outer Hebrides are characterised by essentially the same RTI as the oceanic waters, but a significantly lower RIOs-. The waters of the Minch contained less again of both. The broken lines of Fig. 2 depict the change in RIOs- and RTI for the transition from the ocean proper, to the west of the Outer Hebrides. The concentrations of both RIOs- and RTI decreased toward the north in the Minch, as shown by the three envelopes of points in Fig. 2. The trends within the data set also show up clearly on the plot of RIOsversus RTI (Fig. 3). The upper broken line represents the condition in which, from the oceanic end-member down, all the iodine would be - -+-t
f
3
s
;
x
0.30
ii 3
N
2 g
__++z- +d
:r .'
0
+++
+ Q
+ +
,+A-
t+/+
l
0*
0.20
34.0
d
+
0.40
IODATE
TOTAL L
I
34.5
35.0
SALINITY
i 35.5
34.0
34.5
36.0
36.5
SALINITY
Fig. 2. Variation of RTI and RIOs- with salinity in the Hebridean waters. The upper, middle and lower envelopes delineate the waters of the Minch according to their southern, middle and northern locations, respectively. The broken lines indicate shorewards transition from the Atlantic Ocean waters to those at the SW tip of the Outer Hebrides.
V. W. Truesdale
282
0.20
0.30
0.40
RTI OJMl Fig. 3. The graph of RIOs- versus RTI for the area under study. The points represent actual analyses. Those in the three envelopes refer, in ascending RTI concentration, to north, middle and south Minch samples. The vertical broken line on the right indicates interconversion of iodate to iodide at constant RTI, corresponding with movement inshore from the ocean toward the SW tip of the Outer Hebrides. The lower, intersecting broken line expresses broadly the inshore trend from the western coast of the Outer Hebrides.The upper broken line indicates the theoretical dilution of oceanic water in which the iodine is all iodate. The trapezoid and the circle within it represent, respectively, the condition of the Irish Sea and the Minch in the previous autumn.
present as iodate, i.e. RIOs- = 1. x RTI. As all the points lie below this, transformations of iodate to iodide occur within this system. The trend between the oceanic waters proper and those off the west coast of the outer Hebrides is represented as a vertical assemblage of points at approximately 0.+M RTI. The trend between the waters immediately to the west of the outer Hebrides and those within the Minch follows a sloping line toward the origin. The sampling frequency was too sparse to support any closer interpretation of this; for example, using lines of different gradient to depict various ratios of iodide and iodate uptake. A collation of deep-water samples taken at a variety of the stations within the sampling grid, all to the west of the Outer Hebrides, shows that RTI was essentially constant at 0.44 PM in the immediate underlying waters (Fig. 4). In contrast, RIOs- was approximately 0.42 ,UM between 100 and 1000 m, but 0.44 PM in the deepest water, below 1000 m. The Discovery cruise
The surface distribution of RIOs- and RTI on the SW transect (Fig. 1) between Land’s End and a position close to the Azores (41” 3’N, 24” 48’W) is shown in Fig. 5. Along this line RTI remained relatively constant
Distribution of dissolved iodine in Hebridean waters
RI03
SALINITY 34.5 a
35.0
35.5
.35 .40
283
RTI .a
.45
.45
5oa
2000
Fig. 4.
Variation of salinity, RI03- and RTI with depth at the stations to the west of the Outer Hebrides.
x
x
x
0.35 _
*x
AZORES =
0.30 _
9 0
0.25 _
4
xx 2;
LAND’S END
I 25
I 20
1
LO:GIT”DE
1 10
4 5
0
Fig. 5.
The distribution
of RTI (+ ) and RIOi (x) in surface waters between Land’s End and the Azores in December 1977.
at 0.44 ,uM. Working inwards from the RI03- also remained relatively constant ever, east of about 10’ W on the transect, End and at the continental edge, RIOs-
Azores, along 80% of this line at approximately 0.39 PM. Howi.e. some 250 miles from Land’s dropped progressively to about
284
Fig. 6.
V. W. Truesdale
The plot of RIOj- against RTI for surface samples between Land’s End and the Azores.
0.34 ,uM off Land’s End, itself. When presented as a plot of iodate against total iodine concentrations (Fig. 6), these results form an essentially vertical assemblage of points similar to that found off the Scottish coast (Fig. 3).
DISCUSSION This study demonstrates that appreciable amounts (20 to 30%) of iodide can persist in coastal waters during mid-winter when seasonal regeneration of plant nutrients is maximal. At the 58”N latitude of this survey area, mid-winter growth conditions for phytoplankton, particularly illumination, are sub-optimal. Primary productivity would have been at approximately 10% of its yearly maximum (Joint, Pers. Comm.) and would not have increased significantly until the spring bloom, some two months later. Concurrently, NOs--N concentrations would have peaked and nitrite-and ammonium-N would be negligible (Houston, 1992). Iodine’s behaviour in these waters is similar in principle to that recorded for the Menai Straits during winter (Truesdale, 1978a), where up to 39% of the iodine remained as iodide. Therefore, the results confirm that the final stage of regeneration, oxidation of iodide to iodate, is slower than that observed with comparable nutrient species. The finding ethos Jickells’ and co-workers’ comment (Jickells et al., 1988) that iodide which works its way into the deeper part of the oceanic water column must have persisted
Distribution of dissolved iodine in Hebridean waters
285
for periods of months, perhaps years. Indeed, it adds to a gradually increasing awareness of the sluggishness of this oxidation step (Luther et al., 1995). It also follows that in appropriate circumstances iodide should be useful as a hydrographic tracer. It is generally accepted that during growth promoting periods of the year a shorewards trend of decreasing RIOs- and RTI exists in temperate shelf-seas (Truesdale, 1978a; Jickells et al., 1988; Wong & Zhang, 1992; Truesdale, 1994). In determining how a similar trend could persist in the Hebridean Seas during mid-winter it has to be recognised that the minimum RIOs- and RTI concentrations observed in the Scottish waters are lower than those observed in the same shelf-sea system during the previous autumn. On the basis of a simple nutrient-like, uptake-regeneration model for iodine, the opposite would have been expected. This anomaly is shown in Fig. 3. where the trapezoid in the centre encompasses 95% of the results reported by Truesdale (1994) for the Irish Sea, some 4” latitude further south. Moreover, the circle within the trapezoid represents surface samples from 8 equally-spaced stations down the centre of the Minch collected and analysed with those from the Irish Sea (Truesdale, 1994). RTI and RIOs- was then 0.38 & 0.01 and 0.32 f 0.01 pM, respectively. The Scottish results reported here also cover a greater concentration range than the winter concentrations reported by Truesdale (1978a) in his 18-month study of the temporal variation of iodine in the Menai Straits. There, RTI and RI03- were 0.41& 0.01 and 0.25 f 0.01 PM, respectively, once again falling within the trapezoid of Fig. 3. Whatever the cause of the above anomaly it is nevertheless likely that the northwards advection of coastal water will be partly responsible for the shorewards trends described above. Thus, taking the mean velocity of the coastal current to be approximately 3 km day-’ (Ellett & Edwards, 1983), the journey from the North Channel of the Irish Sea to the Minch takes about 4 months. Moreover, as Houston’s (1992) comprehensive study of nutrient behaviour on the shelf shows, the integrity of the coastal current is maintained even though seasonal mixing occurs on the more seaward part of the shelf. She observed (Table 1) that peak winter NOs-N concentrations vary from 12 ,UM in the surface waters of the ocean proper (e.g. west of the 1OOOmline), to 10 PM--N over most of the shelf area between Ireland and the Outer Hebrides, and finally down to 8 FM-N in the Minch. (In summer these are all typically < 2 pM N03--N.) Therefore, winter N03--N in the Minch is similar to that in the Irish Sea to the south, but different to that on the open shelf because of winter incursion of oceanic water onto the shelf. Of course, nitrate in these winter surface waters arises from re-mineralisation of N as well as upward advection of nitrate as the deeper waters are mixed into the surface (Ellett,
286
V. W. Truesdale
1979; Wafar et al., 1983). The shallowness and density structure of the coastal current limits the second of these effects, and Minch water is able to retain the same 7-8 ,uM N03--N concentration as seen generally in winter in both the Celtic and Irish Seas, and parts of the English Channel which are similarly protected from upward advection (Slinn, 1974; Butler et al., 1979; Joint & Williams, 1983; Foster, 1984). The corollary also explains why at the deep, oceanic stations (Fig. 4.) iodine speciation was essentially the same at the surface as at 100-1000 m; as Ellett (1979) states, in winter these waters can be mixed to depths which exceed 400 m. The decrease of RIOs- at essentially constant RTI in the boundary area between the ocean proper and the western shore of the Outer Hebrides is interesting because it seems to represent a different set of processes to those occurring within the coastal current. Thus, it is not as easily explained as the mixing of the extreme end-member waters, i.e. oceanic and coastal waters. This is because the part of any mixture contributed by the coastal water end-member, while lowering the RIOs- concentration, must also lower its RTI concentration. Therefore, it is reasonable to infer that the water located against the western shore of the Outer Hebrides is a third end-member of the system. This interpretation is supported by the finding of the same effect (Fig. 6) in the Discovery observations in the Atlantic Ocean, south of Ireland in late November and early December, i.e. two months earlier. While in terms of seasons the Discovery sampling period is marginal, the sampling locations are much farther south where the climate favours continuation of growth-promoting conditions rather than the marked bias toward regeneration in the Scottish waters. It is probable, therefore, that during summer the non-assimilatory reduction of iodate (Tsunogai_& Sase, 1969) is a significant hydrographic feature along the whole of the Continental edge bordering the British Isles. Then, a combination of the sluggish iodide oxidation and the general northwestwards drift of the waters enables its influence to be detected in Hebridean waters during winter. This study is based on the approximations, firstly, that the main features of the iodine distribution can be mapped using only iodate and iodide species, and secondly, that the CerV-As”’ method for total iodine responds essentially only to these. Since the iodine cycle involves organically-bound iodine which could be ‘reactive’ or ‘unreactive’ to the total iodine method (Truesdale, 1975; Truesdale, 1978b), these assumptions must always be reviewed. In this case of oxic, open-sea waters, the approximations are reasonable, most complications of this type occurring instead in anoxic basins (Luther, 1991; Luther et al., 1991). The approximations are also supported by studies of the effect of UV photo-oxidation of samples prior to their analysis by various analytical
Distribution of dissolved iodine in Hebridean waters
287
methods (Truesdale, 1975; Butler & Smith, 1980). Moreover, the accuracy (specificity) of the analytical methods used here have been subject to the most rigorous scrutiny yet devised for any iodine methods (Truesdale, 1978b; Truesdale & Smith, 1979) and no untoward effects were observed.
ACKNOWLEDGEMENTS The author wishes to thank Anton Edwards and David Ellett of SMBA, Dunstaffnage, both for their kindness in supplying samples and supporting information from the Challenger cruise, and for their endless patience! Thanks also go to John Gould of IOS, Wormley, and Bob Dickson of MAFF, Lowestoft, for similar support from the Discovery and Ciroluna cruises. The iodine analyses were performed at the Institute of Hydrology, and thanks go to Dr J.S.G. McCulloch, then Director, for supporting the work. I am also grateful to Lucinda Houston and Grahame Savidge, both of Queen’s University, Belfast, for providing access to Lucinda’s thesis at the earliest possible time, and for interesting discussions. REFERENCES Butler, E. C. V. & Smith, J. D. (1980). Iodine speciation in seawaters -the anaand differential pulse lytical use of ultra-violet photo-oxidation polarography. Deep-Sea Res., 27A, 489-93. Butler, E. I., Knox, S. & Liddicoat, M. I. (1979). The relationship between inorganic and organic nutrients in seawater. J. Mar. Biol. Assoc. UK, 59,239-50. Elderfield, H. & Truesdale, V. W. (1980). On the biophilic nature of iodine in seawater. Earth Planet. Sci. Lett., 50, 105-14. Ellett, D. J. (1979). Some Oceanographic features of Hebridean waters. Proc. Royal Sot. Edinburgh,
77B, 61-74.
Ellett, D. J. & Edwards, A. (1983). Oceanography and inshore hydrography of the Inner Hebrides. Proc. Royal Sot. Edinburgh, 83B, 143-60. Folkard, A. R. (1981). Atlas of the Seas around the British Isles. Directory of Fishery Research, Ministry of Agriculture, Fisheries and Food, pp. 2.06, 2.07, 2.23. Foster, P. (1984). Nutrient distributions in the winter regime of the northern Irish Sea. Mar. Environ. Res., 13, 81-95. Houston, L. M. L. (1992). Distribution and Dynamics of Nutrients on the Scottish North West Shelf Region. Unpublished Ph.D. Thesis, The Queen’s University of Belfast, pp. 158. Jickells, T. D., Boyd, S. S. & Knap, A. H. (1988). Iodine cycling in the Sargasso Sea and the Bermuda Inshore waters. Mar. Chem., 24,61-82. Joint, I. R. & Williams, R. (1983). In: Annual Report (1983) of the Institute of Marine Environment Research, Plymouth, UK. Natural Environment Research Council, Swindon, UK.
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Luther, G. W. III (1991). Sulphur and iodine speciation in the water column of the Black Sea. In: Black Sea Oceanography, eds E. Izdar & J.W. Murray. Kluwer Academic Publishers, The Netherlands, pp. 187-204. Luther, G. W. III, Ferdelman, T., Culberson, C. H., Kostka, J. & Wu, J. (1991). Iodine chemistry in the water column of the Chesapeake Bay: Evidence for organic iodine forms. Est., Coastal and Shelf Sci., 32, 267-79. Luther, G. W. III, Wu, J. & Cullen, J. B. (1995). Redox chemistry of iodine in seawater: Frontier molecular orbital theory considerations. Amer. Chem. Sot., Advances in Chemistry Series No. 244. Aquatic Chemistry: Interfacial and Interspecies Processes, eds Chen Pao Huang, C. R. G’Metia &J. J. Morgan. Slinn, D. J. (1974). Water circulation and nutrients in the North-west Irish Sea. Est. Coastal Mar. Sci., 2, l-25. Truesdale, V. W. (1968). Studies on the Chemistry of Iodine in Seawater. Doctoral thesis; University of Wales, Aberystwyth. Truesdale, V. W. (1975). Reactive and unreactive iodine in seawater -a possible indication of an organically bound iodine fraction. Mar. Chem., 3, 111-19. Truesdale, V. W. (1978a). Iodine in inshore and offshore marine waters. Mar. Chem., 6, 1-13. Truesdale, V. W. (1978b). The automatic determination of iodate and total iodine in seawater. Mar. Chem., 6, 253-73. Truesdale, V. W. (1994). Distribution of dissolved iodine in the Irish Sea, a temperate shelf sea. Est. Coastal Mar. Sci., 38,43546. Truesdale, V. W. (1995). A re-assessment of Redfield correlations between dissolved iodine and nutrients in oceanic waters and a strategy for further investigations of iodine. Mar. Chem., 48, 43-56. Truesdale, V. W. & Smith, P. J. (1975). The automatic determination of iodide or iodate in solution by catalytic spectrophotometry, with particular reference to river water. Analyst, 100,11 l-23. Truesdale, V. W. & Smith, C. J. (1979). A comparative study of three methods for the determination of iodate in seawater. Mar. Chem., 7, 133-9. Tsunogai, S. & Sase, T. (1969). Formation of iodide-iodine in the ocean. DeepSea Res., 16, 489-96. Tsunogai, S. & Henmi, T. (1971). Iodine in the surface water of the ocean. J. Oceanography Sot. Japan, 27,67-72. Wafar, M. V. M., Le Corre, P. & Birrien, J. L. (1983). Nutrients and primary production in permanently well-mixed temperate coastal waters. Est., Coastal and Sherf Sci., 17,43 l-46. Wong, G. T. F. (1977). The distribution of iodine in the upper layers of the equatorial Atlantic. Deep-Sea Res., 24, 115-25. Wong, G. T. F. & Brewer, P. G. (1974). The determination and distribution of iodate in South Atlantic waters. J. Mar. Res., 32,25-36. Wong, G. T. F. & Zhang, L. (1992). Changes in iodine speciation across coastal hydrographic fronts in southeastern United States continental shelf waters. Cont. ShelfRes., 12, 717-33. Wong, G. T. F., Brewer, P. G. & Spencer, D. W. (1976). The distribution of particulate iodine in the Atlantic Ocean. Earth and Planet. Sci. Lett., 32, 441-50.