Summer and winter distributions of dissolved iodine in the Skagerrak

Estuarine, Coastal and Shelf Science 57 (2003) 701–713

Summer and winter distributions of dissolved iodine in the Skagerrak V.W. Truesdalea,*, Didrik S. Danielssenb, Tim J. Waitea a

School of Biological and Molecular Sciences, Oxford Brookes University, Gipsy Lane Campus, Oxford OX3 0BP, UK b Institute of Marine Research, Flødevigen Marine Research Station, N-4817 His, Norway Received 12 June 2002; accepted 18 November 2002

Abstract The distribution of dissolved iodate, iodide and total iodine at 12 stations across the Skagerrak between Denmark and Norway in August 2000 and February 2001 is described. Overall, close agreement exists between the graphs of either iodate or total iodine versus salinity, despite hydrographic and nutrient observations demonstrating strong seasonal change in this temperate zone shelfsea system. The study, therefore, demonstrates for a third time that iodine interconversion in temperate shelf-seas is not related simply to either phytoplankton growth or photochemistry. Iodine–salinity graphs consist of a steep part between salinities 35.2 and 34.5, concerned with mixing between surface, intermediate and deep Atlantic water, and a near horizontal part between salinities of 23 and 34.5 concerned with the mixing of surface water from the North Sea and the Baltic Sea, and river run-off in the Skagerrak/ Kattegat area. It is concluded that the iodine system in the Skagerrak is controlled principally by advection in and mixing of waters of pre-formed iodine chemistries. A mechanism in which iodate is reduced during irrigation of coastal anoxic sediment is proposed to account for the lack of seasonal behaviour yet sustained iodate reduction in the water column. The problem of confusing actual chemical reduction of iodate with changes imposed by advection is discussed.
2003 Elsevier Science B.V. All rights reserved. Keywords: iodine; iodide; iodate; redox speciation; Skagerrak; North Sea

1. Introduction Iodine exists in oxic seawater in dissolved (Barkley & Thompson, 1960; Sugawara & Terada, 1967) and particulate forms (Wong, Brewer, & Spencer, 1976). It is a bio-intermediate element (Broecker & Peng, 1982) taken up by organisms in near-surface waters (Moisan, Dunstan, Udomkit, & Wong, 1994; Shaw, 1962; Vinogradov, 1953) and re-mineralised diagenetically (Kennedy & Elderfield, 1987; Price & Calvert, 1973). It is present in a total concentration of approximately 0.45 lM in seawater (Campos, Farrenkopf, Jickells, & Luther, 1996; Chapman, 1983; Elderfield & Truesdale, 1980; Farrenkopf, Luther, Truesdale, & van der Weijden, 1997; Jickells, Boyd, & Knap, 1988; Nakayama, Kimoto, Isskiki, Sohrin, & Okazaki, 1989;

* Corresponding author. E-mail addresses: [email protected] (V.W. Truesdale).

Truesdale, 1994a; Wong, 1977, 1991). Iodate predominates in deep waters where iodide concentrations are similar to those predicted by an extended Redfield remineralisation model for the decomposition of sinking organic material (Redfield, Ketchum, & Richards, 1963, Chap. 2; Truesdale, 1994a). In the permanently stratified surface waters of the tropics and sub-tropics, there is a pronounced interconversion of iodate to iodide (Raju, Rajendran, & Reddy, 1983; Tian et al., 1996; Truesdale, Bale, & Woodward, 2000; Tsunogai & Henmi, 1971). Biotic (Tsunogai & Sase, 1969) and abiotic (Spokes & Liss, 1996) mechanisms have been suggested for this reduction. If a biotic mechanism were applied, it would be interesting to know if the interconversion to iodide could be accounted for simply by uptake (Sugawara & Terada, 1957) and regeneration, with iodide representing the first re-mineralised form of iodine (assimilatory reduction). The alternative is to invoke a separate non-assimilatory (dissimilatory) reduction (Tsunogai & Sase, 1969). A second issue concerns the site and rate

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2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0272-7714(02)00412-2

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of iodide oxidation. At present, there is no clear mechanism for iodide oxidation (Edwards & Truesdale, 1997; Luther, Wu, & Cullen, 1995), although predominance of iodate in deep ocean waters shows that it must occur. High concentrations of iodide are also found in coastal waters of temperate zones (Truesdale, 1978a, 1994b, 1995; Wong, 1995; Wong & Zhang, 1992). Whether these are formed by the same mechanism as that in the tropical and sub-tropical surface waters is not known, and there is, therefore, a continuing need to investigate iodine in temperate shelf-seas to elucidate the mechanisms in both domains. This study focuses on the Skagerrak, a dynamic part of the North Sea shelf system, where deep as well as shallow water can be accessed. Together with earlier ones off the British west and east coasts (Truesdale, 1978a, 1994b, 1995; Truesdale & Jones, 2000; Truesdale & Upstill-Goddard, 2003), it is part of a larger programme covering much of the European temperate shelf-seas, including the enclosed Kattegat and Baltic Seas (Truesdale, Nausch, & Baker, 2001). A continuing objective is to define the relationship between iodate concentration and salinity, as closely as possible, and to test Truesdale and Jones’ (2000) hypothesis that iodine is in a steady state on the European shelf. 2. Hydrographic environment studied The Skagerrak (Fig. 1) is the area of sea between the northern shore of Denmark, and Norway and Sweden. Together with the Kattegat, it forms the transition between the Baltic and the North Seas. In the context of the North Sea, the Skagerrak forms the eastern extremity of a deep cleft, the Norwegian Trench, which

attains 700 m depth within the Skagerrak, but runs along the Norwegian coast, with a sill at about 270 m west of Utsira (North Sea Task Force, 1993a). The much greater area of the North Sea, to the south and west, is relatively shallow (30–200 m) and is an extension of the more general North Atlantic shelf, which turns into the North Sea around Shetland and the Scottish east coast. The fresh water supply to the Skagerrak originates mainly from the Baltic Sea and the Kattegat, with a smaller component from local rivers (Gustafsson and Stigebrandt, 1996). Given this juxtaposition of both the general Atlantic shelf and the Baltic Sea as a catchment, about 70% of the water entering the North Sea, whether fresh or saline, is assumed to pass through the Skagerrak. The general cyclonic circulation (Fig. 1) and the distribution of water masses in the Skagerrak (Aure, Danielssen, & Svendsen, 1998; Danielssen, Svendsen, & Ostrowski, 1996; Danielssen et al., 1997; Gustafsson & Stigebrandt, 1996) are mainly regulated by the water flow into and out of the North Sea and the steep local bottom topography. The latter is especially important in the Skagerrak as the tendency for current to flow along the contours means that a number of streamlines converge. Inshore, Jutland coastal water (JCW) delivers water from farther south, in the German Bight. With a shift offshore, and concomitant depth increase, contributions to intermediate water occur from southern North Sea water (SNSW; 30–40 m), central North Sea water (CNSW; 50–70 m), and to deep water from Atlantic water (AW; >100 m). The final mixture at any point is determined by competition and mixing of water from these various sources (Aure et al., 1998). The distribution of the relatively fresh surface water in the Skagerrak is strongly influenced by varying weather conditions; and during strong local winds, the circulation may not follow the general pattern (Fig. 1). Under such conditions, the outflowing Norwegian Coastal Water (NCW) may even be blocked and transported back across the Skagerrak as far as the Danish coast. The fresher, surface layer thins (10–20 m) toward the centre of the Skagerrak and the deeper AW forms a dome or ridge (Danielssen et al., 1997.)

3. Sampling and methods

Fig. 1. The hydrographic section (stations 21 to 32) across the Skagerrak between Hirtshals (H) and Arendal (A) in relation to the North Sea. The arrows depict the general circulation.

Samples were collected across the Skagerrak during cruises of the R.V. G. M. Dannevig in August 2000 and February 2001 at stations 21–32 (Fig. 1). Vertical profiles were obtained using a standard Neil Brown CTD rosette at standard depths below 5 m. Surface samples (0 m) were taken by bucket. Iodine samples were unfiltered and stored in sealed polypropylene sample tubes in a refrigerator at 4  C prior to analysis. The maximum storage time was 1 month. Nutrient analysis was conducted by Chem-lab autoanalyser (Føyn, Magnussen,

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& Seglem, 1981). Analytical precisions (coefficient of variation) and detection limits were typically 1% and 0.01 lM, respectively. Total iodine and iodate-iodine were determined automatically by the Ce(IV)–As(III) catalytic method (Truesdale & Smith, 1975) and the iodometric method (Truesdale, 1978b), respectively. The standard deviation for analytical control standards carried out through the entire process (usually about 10 replicates per day) is typically 0.006 and 0.013 lM, respectively. The accuracy of each analysis is probably within 5%. Iodide was determined by Cathodic Stripping Square Wave Voltammetry according to Luther, Swartz, and Ullman (1988), using a PC-controlled Princeton Applied Research 384B polarogram with a Model 303A mercury drop electrode with 0.10 V deposition voltage and 30 s deposition time. Each sample was analysed in duplicate, with triplicate scans; the standard deviation obtained from 30 replicate analyses of a seawater containing 0.067 lM iodide was 0.002 lM.

4. Results To provide essential physical and biological contexts, hydrographic and nutrient results are presented before those for iodine. Following the iodine results, iodine– salinity relationships are reported. To avoid confusion, the term iodate reduction is retained specifically to mean chemical reduction, and expressions such as reduced concentration of iodate are avoided. 4.1. Salinity and temperature During February 2001 (Fig. 2a), high salinity (Atlantic) water (>35) was found at depths between about 100 and 150 m and at the bottom at about 630 m (Fig. 2a), where salinity only increased from 35 to 35.2. Meanwhile, the salinity of intermediate waters (20 and 80 m) increased from 34 to 35. Near-surface JCW, inputting into the section just off Hirtshals, was of about salinity 33. In accordance with the input of freshwater from the Kattegat and Norwegian rivers (Aure et al., 1998), surface salinity decreased to about 22.6 across the survey line toward Arendal. During the previous August, the boundary between the surface and intermediate waters, as marked by the 34 isohaline, lay at about the same depth of 10–40 m. In contrast, the 35 isohaline was 40 m shallower, at about 40 m across most of the section. This change is a longterm feature of these waters (Danielssen et al., 1996) in which a greater proportion of AW (and conversely, a smaller proportion of water from the central and northern North Sea) is present during summer. During August, surface salinity decreased from 31.0 off Hirtshals to 27.7 off Arendal. Therefore, the salinity of

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the inputting JCW off Hirtshals was about 2 units less than it would be in February. Conversely, the low salinity off Arendal was about 5 units greater than would be observed in February. The cross-over in behaviour occurred in the middle of the Skagerrak, where the surface salinity was almost the same during both cruises. As the temperature of surface waters is high in summer and low in winter, the cumulative T–S plots for the two seasons studied (Fig. 3a) form mirror images. They diverge from the deep-water characteristic of about 6.9  C, and salinity 35.22 retained throughout the year, toward surface temperatures of about 16 and 1  C for summer and winter seasons, respectively. Fig. 3b shows the fine structure for the T–S relationship at the deepest location (630 m), station 26. In both seasons, as well as the bottom water, there appear to be separate water masses at about 100 and 250 m. The temperature of these intermediate waters is higher in winter than in summer, suggesting a lag in fjordic water exchange (Danielssen et al., 1996). The low salinity surface waters, as shown in Fig. 3a, lie above 100 m. During August, in the Skerries off Arendal (Fig. 3a), water of salinity about 34.2 presented unusually low temperatures; this was a fjordic stagnation from the previous winter. During February, a more or less gradual 4  C decrease in surface temperature occurred across the section, toward the Norwegian coast. This is consistent with the general downstream dilution of water input into the section from the Kattegat and adjacent fjords, which in winter is cold from enhanced winter run-off (Aure et al., 1998). In contrast, in August, surface temperature across the section was more or less uniform. 4.2. Nutrients All the nutrient–salinity graphs (nitrate, nitrite, phosphate and silicate) showed essentially the same features of a reversed-L shape, exemplified here by those for phosphate (Fig. 4; the lower limb of the silicate–salinity graph for February sloped upward toward low salinities). Of course, in this temperate system, the main change in nutrient distribution is the seasonal one in near-surface waters, controlled by the opposing effects of primary productivity and regeneration. Whereas in summer, phosphate concentrations in the first 30 m were typically less than 0.2 lM, in winter they were about 0.6 lM. This contrasts markedly with an absence of seasonality between 200 and 630 m, where phosphate concentrations increased gradually with depth from about 0.8 to 0.9 lM, respectively, during both cruises. In the intervening depths, between 30 and 200 m, a marked nutricline was evident in both seasons, with the summer one being the more intense. Further detail of nutrient distributions are available elsewhere (Aure et al., 1998; Dahl & Danielssen, 1981).

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Fig. 3. The combined T–S distributions for August (¤) and February ( ). (a) The results overall. The broken line marked ÔAÕ refers to mixing at the station adjacent to Arendel. (b) The results for deep water at the deepest station (26).

Apart from reflecting the differences in phosphate concentration between surface and deep-waters already mentioned, the plot of phosphate concentration versus salinity (Fig. 4) draws out the surface samples according to their dilution with fresher water. Incidentally, given that in the winter plot the line between salinity units 25 and 35 is almost horizontal, the concentration of phosphate in the diluting water appears to have been very close to that in the JCW itself. The plot for August (Fig. 4b) identifies a second water mass in the Skerries close to Arendal, at the slightly lower salinity of 34.2, consistent with that found in the T–S diagram. This corresponds with regeneration of phosphate in the stagnating water, and is echoed in the results for nitrate and silicate. 4.3. Total iodine During the winter cruise (Fig. 2b), total iodine concentration only varied between 0.40 and 0.44 lM in samples taken from between about 20 m and the botb Fig. 2. Vertical sections across the Skagerrak between Hirtshals (H) and Arendel (A) in February 2001 for (a) salinity, (b) total iodine, (c) iodate, (d) iodide. Iodine concentrations in lM.

Fig. 4. Phosphate–salinity graphs for (a) February and (b) August. The straight lines are fitted by-eye. The extra vertical line in (b) represents the condition of water adjacent to Arendel in August.

tom (630 m), respectively. Generally, between the surface and 20 m depth, total iodine concentration was about 0.40 lM in the JCW, decreasing across the section toward Arendal, to as low as about 0.20 lM. Unusually high concentrations of 0.58 and 0.40 lM were found in surface samples at stations 31 and 23. Overall, the distribution in summer was similar to that in winter. Indeed, below 20 m, it seemed to be identical. On the surface, across the section toward Arendal, there was, again, a general decrease of total iodine, this time from 0.40 lM to only about 0.35 lM. Several examples of exceptionally high total iodine concentrations (up to 1.0 lM) were witnessed at the surface. 4.4. Iodate-iodine During the winter cruise, iodate concentration (Fig. 2c) increased from 0.40 to 0.44 lM between about 150 and 630 m, and from 0.25 to 0.40 lM between 20 and 150 m. Iodate concentrations decreased across the

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section from about 0.30 lM in the JCW to about 0.15 lM off Arendal. During the summer cruise, a similar distribution was seen except that water relatively high in iodate concentration penetrated upwards in the water column (0.30 lM at 10 m, 0.40 lM at 50 m) at the deepest stations (stations 24–29). Also, as with total iodine, during the summer cruise, the decrease in iodate concentration across the section toward Arendal was not as marked as in winter. 4.5. Iodide-iodine During the winter cruise, iodide-iodine, estimated as the difference between total and iodate-iodine, was close to zero below 100 m (Fig. 2d). A difference of about 0.02 lM is insignificant as iodide is estimated as the difference between independent measurements of much higher concentrations of total and iodate-iodine, with any error in these passing into the difference. Nonetheless, at about 10 m, significant concentrations of iodide of about 0.15 lM existed. The unusually high total iodine concentrations at stations 31 and 23 were caused by unusually high iodide concentrations. In summer, essentially the same distribution was found except that iodide was less than 0.05 lM at all depths greater than 20 m. In the near-surface layers (5 m), iodide concentrations were similar to the value 0.15 lM, found during winter. Very high iodide concentrations (0.32–0.85 lM) were found in some surface waters during summer. A more precise determination of iodide by voltammetry at the deepest station confirmed the above trends. At depth greater than 200 m (Fig. 5), iodide concentration was about 0.025 lM in both seasons. In summer, this low iodide concentration extended upwards to about 50 m, whereas in winter, iodide concentrations were much greater. (Incidentally, this appearance of higher iodide concentrations during winter runs counter to any suggestion that primary production is the cause.) The iodide concentration just under the surface during both seasons was similar (0.12–0.15 lM). On the surface, during summer, it was often extremely high (Table 1), whereas only one such occurrence was observed during winter. 4.6. Iodine–salinity relationships Overall, the trends in the total iodine–salinity graph resemble those in the iodate–salinity graph, but are much subdued. Accordingly, to aid our understanding, the iodate graph is considered first. Overall, the two data sets combine (Fig. 6a) to give a tightly defined co-variation between iodate concentration and salinity, with a small degree of scatter suggesting more localised variation of iodate in some samples. The trends consist of a rapid drop in iodate concentration (0.15 lM) with a small decrease of salinity (35.2–34.6) followed by a second, smaller decrease in iodate con-

Fig. 5. The variation of iodide concentration with depth at the deepest station, during August and February.

centration (0.05 lM) over much larger salinity decreases (10; comparison of Fig. 6a with Fig. 2a, c shows that the steepest trend of the curve relates to the waters deeper than about 30 m, whereas the flatter trend relates to the much lower salinities of the near-surface (0–30 m) in waters across the Skagerrak. In accordance with the salinity distribution, the overall span of points for the August cruise can be seen to be slightly less than that for February. The anomalies evident in August in the T–S and nutrient characteristics of the stagnating water in the Skerries off Arendal were not reflected in the iodate– salinity plot. Examination of the iodate–salinity graph at salinities above about 34.7 (Fig. 7) suggests that the intermediate Table 1 The enhancement of iodide concentrations in some surface samples (0 m) during August and February Station 32 31 30 29 28 27 26 25 24 23 22 21 a

Summer [Iÿ in lM]

Winter [Iÿ in lM]

0.40 0.28

0.19 0.40a 0.19 0.24 0.14 0.18 0.22 0.21 0.23 0.28 0.16

0.24 0.33 0.45 0.36 0.70 0.17 Exceptionally high iodide during winter. Iodide is determined by voltammetry.

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Fig. 7. The co-variation of iodate concentration with salinity at high salinity. n, February 2001; d, August 2000, but without stations 25 and 26; ¤, August 2000, stations 25 and 26 only. The broken steep line refers to February 2001, while the solid steep line refers to August 2000. The much shallower solid line is the right-hand extremity of the trend line for low salinities from Fig. 6a.

Fig. 6. The co-variation of iodine concentrations and salinity. (a) Combined data sets for iodate—n and represent August and February samplings, respectively. The upper (dashed) line represents the conservative dilution line for iodate in the near-surface waters. The lower line approximates the trend. (b) Combined data sets for total iodine. M identifies surface samples for total iodine. The lines approximate the trends.

than about 40 m in which total iodine decreases from about 0.42 to 0.40 lM, as marked by the upper dashed line in Fig. 6b. Further interpretation of Fig. 6b is complicated by the presence of the unusually high concentrations of total iodine encountered at the surface, principally during August, and on two occasions during February. However, when these are disregarded the data for the shallow samples are seen to lie close to the line for the theoretical dilution, with river water, of the intermediate water at about 40 m (34.5, 0.39 lM). Detailed examination of the data suggests that the data from the August cruise lie closer to the two lines imposed in Fig. 6, than do those for February; the latter appearing to follow an arc inside the intersection. This difference is too small to warrant further investigation here. Nevertheless, it deserves mention because it replicates the behaviour already seen in the iodate–salinity graphs. As with iodate, total iodine concentrations in August did not display any anomalous behaviour in the stagnating water of the Skerries, off Arendal. 5. Discussion

and deep-water system can be characterised by two different mixing patterns. In February, only one was evident, the points conforming tightly (Fig. 7) to an imposed straight line (Table 2). During August, there was more scatter, which reflects stations 25 and 26 retaining their February-like behaviour (Fig. 7), but the remainder of the deep stations following a steeper line (Table 2). This retention of mixing behaviour at the deepest station (station 26) is independently corroborated by the iodide versus salinity plot (not shown) derived from the results in Fig. 5, in which the points for the intermediate and deeper waters superimpose. The co-variation of total iodine with salinity is shown in Fig. 6b, where total iodine is distributed between 0.2 and 1.0 lM across a salinity band between 22 and 35.2. Comparison with Fig. 2a, b shows that the points between salinity 34.5 and 35.2 represent waters deeper

5.1. The hydrographic and biological context All our measurements for the Skagerrak, whether for nutrients, temperature or iodine species, when plotted Table 2 Linear regression parameters for iodate concentration on salinity for mixing between intermediate and deep waters Sample selection Gradient  standard error (lM) Intercept  standard error (lM) r2

February cruise 0.31  0.01

August cruise without stations 25 and 26 0.38  0.05

Stations 25 and 26 0.24  0.03

ÿ10.3  0.4

ÿ13.0  1.6

ÿ8.0  0.9

0.963

0.697

0.818

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against salinity yield a composite graph in the shape of a or a , or a close approximation thereto. In each case, the steep limb of the data represents the mixing between deep and intermediate waters (34.5 < salinity < 35.2), while the near-horizontal limb represents the lateral mixing between near-surface JCW (0–30 m) inputting in the southern part of the section and Kattegat and local river water, which forms NCW. The phosphate results (Fig. 3) demonstrate this best as they are well-represented in either season by two intersecting straight lines, one depicting the estuarine dilution the other the vertical mixing. Moreover, the data for deep-water mixing in winter (Fig. 3a) superimpose upon that for the summer (Fig. 3b), showing that the mixing pattern at intermediate depths changes within a season. Although the T–S plots are not quite so simple, they support the latter deduction, showing how all but the deepest water is affected markedly by seasonal heat exchange. Finally, the nutrient results delineate the two quite different biological regimes expected in these summer and winter temperate waters. The results for both total and iodate-iodine (Fig. 6) broadly follow this analysis except that there is an absence of a major seasonal difference. This absence was accompanied by a lack, in August, of any anomaly in total and iodate-iodine concentrations in the stagnating water in the Skerries, off Arendal. 5.2. Overall, iodate reduction predominates As in earlier oceanographic studies, iodate reduction (chemical) predominates with just a small amount of iodine loss. Thus, the difference in concentration between AW at depth and surface water entering the Skagerrak from the west is 0.03 lM for total inorganic iodine and 0.15 lM for iodate. These differences are comparable with those of 0.02 and 0.20 lM, respectively, reported between offshore and onshore waters of the eastern North Sea (Truesdale & Upstill-Goddard, 2003). This assessment of iodate reduction was corroborated when, at the deepest station, iodide profiles obtained by voltammetry (Fig. 5) were found to mirror the iodate profiles obtained by iodometry. As previously reported (Truesdale, 1995), the major issue is to identify where the reduction occurs, and whether it is contemporaneous with sampling or occurs earlier, outside the sampling area. 5.3. The implications of insignificant seasonal change in the iodine–salinity graph A notable feature of the present results is the overall similarity of the iodate–salinity graphs for summer and winter cruises (Fig. 6a). This similarity is much more striking than any subtle differences existing between

them (Fig. 7). These surveys, therefore, present a second, full and independent corroboration of the effect first observed in the Menai Straits (Truesdale, 1978a), of pronounced and sustained iodate reduction unaccompanied by marked seasonal variation; the first corroboration related to the Hebridean shelf (Truesdale & Jones, 2000). The lack of anomalous concentrations of total and iodate-iodine in August in the Skerries, off Arendal are consistent with the lack of seasonal change in iodine chemistry, as indeed they are with earlier studies of stored water in Loch Etive (Edwards & Truesdale, 1997) and the Irish Sea (Truesdale, 1994b). The co-existence of pronounced iodate reduction with an overall lack of seasonal behaviour is indeed perplexing and has severe ramifications to the study of iodine. When viewed against the conspicuous and concomitant changes observed in nutrient chemistry (Fig. 4), and hence, by implication, the primary production regime in the near-surface waters, it suggests that in temperate waters, iodate reduction is not linked directly to either primary production or abiotic photochemistry (Spokes & Liss, 1996). The first of these deductions conflicts with earlier historical studies of tropical and sub-tropical waters, which link iodate reduction to nutrient loss, and hence implicitly to primary productivity, in the broadest sense (e.g., Elderfield & Truesdale, 1980). This dilemma has been avoided in three ways. Firstly, it has been conjectured that conditions during the spring growth period in temperate waters are especially inconducive to iodate reduction, whereas those in tropical waters promote it. Secondly, that the co-variation between iodate and nutrients in the tropics actually indicates a link between iodate reduction and something more generally a function of productivity, e.g. regeneration. Thirdly, that primary production and iodate reduction are not linked. The first approach is well exemplified by the idea that nitrate and iodate compete in nitratereductase facilitated reduction, and that because of high spring nitrate concentrations, iodate reduction is not observed in temperate waters. In contrast, in tropical waters, what production there is will be related to re-cycled nitrogen except at the base of the photic zone, where again nitrate-promoted growth can occur. In turn, this has spawned the idea that it might be regenerated primary productivity, which is responsible for iodate reduction (Tian et al., 1996). The authors are currently drawn to the second approach, which leads to a consideration of sediment interactions as a possible reason for iodate reduction. The problem of conspicuous iodate reduction coexisting with a lack of seasonal change suggests additionally that iodide oxidation, which some workers (e.g. Campos et al., 1996) seem to believe is in dynamic opposition to iodate reduction, is unlikely to contribute in a major way to the iodine chemistry of shelf-seas. As it is likely to be biologically mediated (Luther et al.,

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1995), it could reasonably be expected to show a seasonal change in its rate. However, in a system where iodate and iodide are in a steady state (Truesdale & Jones, 2000), this would imply an antipathetic seasonal change in iodate-reduction rate. Although possible, this seems overly restrictive as a preliminary assumption. 5.4. Iodate distribution and advection The presence in the Skagerrak of an essentially constant 0.25 lM span in iodate concentration alongside major seasonal changes in biology (Fig. 6) is perhaps most easily explained by advection of pre-formed iodine chemistry. Thus, the iodate distribution reflects the iodine chemistry of the deep AW, the shallower waters of the German Bight and the central North Sea, and the Kattegat. In the case of the latter three waters, the length of time they are in the Skagerrak prior to mixing with each other and the AW is almost certain to be much less than the time in which they have had to form. For example, the coastal water off Hirtshals, which is derived from the German Bight can be considered as having drifted for several months (North Sea Task Force, 1993b) along the similar environment of the north European and British coastline before entering the Skagerrak. Meanwhile, the bottom water and significant portions of the intermediate water (say >100 m) represent AW of much greater age. Indeed, T–S characteristics (Fig. 3b) demonstrate that the bottom water masses are from different stagnations, yet possess the same iodine chemistry. 5.5. Wider implications of an interpretation relying on advection The idea that the distribution of iodine is largely determined by advection and mixing of pre-formed chemistries has severe implications on the further investigation of iodine in temperate waters, and perhaps elsewhere. It suggests a greater likelihood of mistakenly attributing iodate concentration to variation to biology. In turn, this means there is a continuing need to reassess whether changes in iodine concentrations have been attributed to biological effects when, in fact, an advective event had been observed. Of course, this is a particular example of the general risk invoked by not being able to take a Lagrangian approach to sampling. The authors suspect that Tian et al.’s (1996) contention that iodide production in the autumn is due to regenerated production offers a good example of this. Their argument is that an increase in mean iodide concentration of about 0.060 lM in the first 50 m of water column of the Mediterranean Sea between three samplings, in September, November and December, and the rest of the year is attributable to regenerated (primary) production. There is no reason to suspect the

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differences in mean iodide concentration, especially as they rely on many individual water samples. However, their results show that between September and November, the thermocline deepened markedly such that the area under the temperature depth profile representing warm, stratified water (>13  C) increased by a factor of about 1.66. Such warmings, especially when accompanied by surface cooling more reasonably expected at that late stage of the year, suggest a concomitant change in lateral advection, with the possible entry of different water masses. This alternative explanation for the changes in mean iodide concentration needs detailed consideration before the biological reduction can be accepted. In contrast, it is noted that Campos et al. (1996) suspected a similar problem in their time-series analysis of stations off Hawaii and Bermuda. They opted for little evidence of seasonality in iodate reduction, but only after rejecting one of the eight monthly sets of measurements made at Bermuda. They suggested that these measurements represented the passage of a self-contained, warm, downwelling core eddy. 5.6. A possible seasonal change in iodate behaviour Given that the existence of a marked seasonal change in iodate behaviour would offer us the best chance of deducing as to what causes iodate reduction in the oceans, the change in the mixing regime for iodate at some of the deeper stations (Fig. 7) is potentially important. It seems reasonable to assume that this change is related to the tendency, confirmed in this study, for a higher proportion of AW to appear in this Skagerrak section during summer (Danielssen et al., 1996). However, the link is by no means straightforward. Incursion of AW proper, at intermediate depths (40 to 80 m), will not automatically impose an increase in the gradient of the steepest part of the iodate versus salinity plot. The shift to higher salinity would ordinarily be accompanied by a higher iodate concentration (Truesdale & Upstill-Goddard, 2003), merely shifting observations up the steepest part of the curve in Fig. 7. Indeed, this was observed at the deepest station for both iodate and iodide. The steepening of the mixing line at all but the deepest stations, therefore, implies that the iodate concentration remained low while the salinity increased. Such an occurrence is consistent with the in situ reduction of iodate during the incursion. The extent of the iodate reduction associated with incursion can be estimated from the shift in the point of intersection of the mixing lines in Fig. 7. In February and August, intersection occurs at salinity 34.75 and 35.04, respectively. A simple replacement by AW would be expected to raise the iodate concentration to 0.4 lM on the February line. Therefore, a difference of approximately 0.09 lM exists between this and what was actually observed in August. It is intended to investigate

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this promising phenomenon further, with appropriate caution as mentioned earlier. 5.7. The unusually high iodide concentrations The high iodide concentrations found in surface waters, particularly during the summer cruise in the Skagerrak, are unusual, but not unique. Chapman (1983) reported similar concentrations in South African waters, albeit in mid-water samples. Alternative reasons have been searched for the measurements, and apart from a remote possibility of contamination from iodide used in oxygen determination, none has been identified. They are not the result of an analytical interference as they were detected independently both as a difference between total iodine and iodate concentrations, and as iodide directly using voltammetry. Moreover, there is no reason to suspect a storage problem, as all other samples received the same treatment. Fig. 6b shows distinctly that if the measurements are reliable, the high iodide concentrations identify a surface phenomenon. 5.8. Generalising the iodate–salinity graph The general shape of iodate–salinity distributions obtained from the North Sea and adjacent Hebridean shelf-seas (Truesdale & Jones, 2000) are shown in Fig. 8. Each curve represents an average behaviour obtained by an Ôeye-fittingÕ of a sum of exponentials model (Truesdale & Jones, 2000). Note that the model has no mathematical or oceanographic significance other than to provide the trend-lines, which summarise over 200 measurements. For a given fitting, most points would lie within an envelope of about 0.015 lM of the line. The Skagerrak is represented by separate curves for the two mixing regimes observed there. Fig. 8 shows that given

the 0.015 lM envelope for data scatter, significant differences in the distributions are few. These relate to the end-members and to the differences between the North Sea and other stations, at salinities less than 34. Over much of the salinity range shown then, Truesdale and Upstill-Goddard’s (2003) results for winter off the British east coast lie relatively close to those reported in this study for the Skagerrak. This is not surprising when the North Sea is viewed as a kink in the north European shelf system, as the British waters are then seen to be feeding into the Skagerrak. This adds support to the idea of there being a general similarity for iodate behaviour in the North Sea across the 32–34.5 salinity range. However, at salinities higher than 34.5, it appears that the Skagerrak results do not share the end-member encountered in the British east coast at 35.4 salinity. The 35.4 salinity end-member (Fig. 8) identifies nearsurface water of the North Atlantic Current, which flows north-eastwards across the north of the British Isles, to the Norwegian Sea. The 35.2 end-member on the bottom of the Norwegian trench represents this same water, although slightly diluted. The difference between the end-members is therefore real. Uncertainty in their position on the iodate–salinity graph is due mainly to uncertainty in the measurement of iodate concentration. The Skagarrak end-member would be a minimum of about 0.003 lM lower than the Atlantic end-member if only pure dilution of the oceanic water from salinity 35.4 to 35.2 had taken place. Equally, if the diluent were the 32 salinity water flowing out from the Skagerrak, the lowering of iodate concentration would be about 0.014 lM (Fig. 8). In fact, as the AW feeds down the Norwegian trench and over the 270 m sill, it is likely to be diluted by North Sea water richer in iodate than the outflowing Skagerrak water, and hence the 0.014 lM lowering is an overestimate. The likely change is therefore comparable with the possible survey-to-survey analytical error in iodate. As a result, it can reasonably be inferred that the bottom waters of the Norwegian trench will be represented in Fig. 8 by the arrow between the two end-members. This is a condition not anticipated by Truesdale and Upstill-Goddard (2003) in their attempt to define the iodate–salinity relationship for the North Sea. 5.9. Sandy sediment irrigation—a possible explanation for iodate reduction

Fig. 8. The shape of the iodate versus salinity graphs at various positions on the European shelf. (a) Hebridean Sea (Truesdale & Jones, 2000), (b) North Sea (Truesdale & Upstill-Goddard, 2003), (c) and (d) Skagerrak, this study, (e) conservative dilution of the oceanic end-member with river water, (f ) conservative dilution of the oceanic end-member with Skagerrak surface water.

The present explanation of the Skagerrak iodine distributions relies heavily on the advection of waters with pre-formed iodine chemistries, with little chemical reduction of iodate actually in the Skagerrak. This seems justifiable, given both the long periods in which the waters have to form prior to their relatively rapid transit through the Skagerrak, and the lack therein of

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a large seasonal change in iodine chemistry. Ultimately, this explanation is unsatisfying as it does not explain as to how iodate is actually reduced in upstream shelfsea waters. So far, various reasons have been proposed ranging from dissimilatory reduction during phytoplankton growth, bacterial action and even abiotic, photochemical reduction. All could be relevant to the shelf waters of the North Sea, but there are serious limitations with all of them as discussed in detail by Truesdale and Bailey (2002). Given the doubt surrounding iodate reduction mechanisms (Truesdale & Bailey, 2002) and its connection to the insignificant seasonal signal on the otherwise marked reduction of iodate in shelf-seas (Truesdale, 1978a, 1994b, 1995; Truesdale & Jones, 2000; Truesdale & Upstill-Goddard, 2003), the authors are continually drawn to the possibility of interaction between dissolved iodate and anoxia in the uppermost layers of sediment. In effect, the sediment would act as a giant Jones reductor commonly used in analytical chemistry to prepare reduced chemical species (Skoog, West, & Holler, 1996); seawater containing iodate is taken through the reductor and emerges with a higher proportion of iodide. The idea is consistent with the shallow areas of shelf-seas being the Ôactive sitesÕ of iodate reduction (Truesdale, 1994b), as the shallow water column of coastal waters enhances the relative contact between the water and sediment. It is also more consistent with the lack of major seasonal variation in iodate reduction observed in this study and generally (Truesdale, 1978b, 1994b, 1995; Truesdale & Jones, 2000) since diagenesis can continue throughout the year. Finally, anoxia is known within sediments of the North Sea. Indeed, it is particularly apparent in areas such as the German Bight, where, under appropriate physical and biological conditions, the anoxic interface enters the water column (Gerlach, 1990). The proposed sedimentary involvement does not conflict with existing knowledge of iodine diagenesis (Kennedy & Elderfield, 1987), once it is accepted that the steady state and diffusion hypothesis associated with deep-sea sediment will not apply. Much North Sea sediment consists of sandy and gravelly debris above till (Folkard, 1981) laid down during the ice-age when much of the North Sea bottom was above sea-level. This was subsequently modified by the addition of marine organic debris and estuarine mud as, with global warming, the sea encroached. Heuttel, Ziebis, Forster, and Luther (1998) explained as to how advective transport can become a major transport mechanism for solutes in such sandy beds. In these coastal sediments, permeability is higher than in muddy cohesive ones, and the combination of high flow velocities and surface wave orbitals reaching the sea floor can force water several centimetres into the sediment. Indeed, Lohse, Epping, Helder, and van Raaphorst (1996) provided direct evidence of this in the southern North Sea. Flume experiments (Heuttel

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et al., 1998) show that even small pressure gradients generated by topographically deflected bottom-flow are sufficient. Moreover, such forcing can lead to upwelling of reduced, metal-rich water downstream of mounds and other surface features of the sediment. Additional bio-irrigation is possible from worms and burrowing shrimps as studied by Luther et al. (1998). Hence, the ordered, steady-state layering associated with the diffusion-controlled system of pelagic sediments will be replaced by a much more random, heterogeneous system in the upper layers of coastal sediment. Nevertheless, once bottom water is able to penetrate the sediment, the iodate dissolved in it will encounter the reducing environment, and just as in the deep-sea sediment, MnII will reduce iodate (Luther, Sunby, Lewis, Brendel, & Silverberg, 1997; Anschutz et al., 2000; Truesdale, Watts, & Rendell, 2001b). It is also possible that a small proportion of the iodate may sorb to iron oxides in the oxic sediment (Luther et al., 1998), thereby slightly diminishing the total iodine content of the water. It is difficult to attach a precise figure to the iodate concentration of coastal waters because, no doubt, they are the end-members of estuarine mixing systems, as Truesdale and Upstill-Goddard (2003) have discussed in detail. Sampling somewhat farther into an estuary on one occasion thereby returns a lower figure than that obtained on another, where sampling stops farther out. Nevertheless, with certain exceptions, the iodate concentration in shallow temperate coastal waters is surprisingly constant at around 0.2 lM (Truesdale, 1978a, 1994a,b; Truesdale & Jones, 2000; Wong, 1995; Wong & Zhang, 1992). The exceptions relate to water columns of less than 20 m depth where iodate is very nearly absent (Wong & Zhang, 1992). Given the possibility of a sediment/water interaction being the cause of iodate reduction in shelf waters, it seems reasonable to ask whether the wide-spread 0.2 lM iodate concentration might actually reflect an equilibrium condition. This would be between the iodine system and another within the sediment, which leads to an iodate to iodide ratio close to 1. This, of course, implies an iodide to iodate oxidation. Truesdale (1994b), and Truesdale and UpstillGoddard (2003) suggested earlier that a reactor model based upon horizontal advection–diffusion with iodate consumption might well be used for iodine in coastal waters. Equally, the box-model approach used by Wong (1995) for offshore waters, might well apply. In both these approaches, constancy in iodate concentration in coastal waters would be arrived at quite differently from the chemical equilibrium under discussion in this study. As Truesdale and Upstill-Goddard (2003) have argued, the advection–diffusion approach would be expected to allow for variation in iodate concentration as the extent of exchange with more offshore water varies. In contrast, the inclusion of an equilibrium, as envisaged earlier, would be expected to smooth out such variation.

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The fact that Truesdale and Upstill-Goddard’s (2003) prediction (based on qualitative use of the advection– diffusion model) of low iodate concentrations in the JCW is not borne out in practice, seems to add some credibility to the equilibrium idea. Our future work will be directed toward testing these hypotheses.

Acknowledgements We thank Terje Ja˚vold, Svein Enersen and the crew of the R.V. G. M. Dannevig for the sampling.

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