Changes in iodine speciation across coastal hydrographic fronts in southeastern United States continental shelf waters

Continental Shelf Research, Vol. 12, No. 5/6, pp. 717-733, I992.

0278-4343/92 $5.00 + 0.00 (~ 1992 Pergamon Press Ltd

Printed in Great Britain.

Changes in iodine speciation across coastal hydrographic fronts in southeastern United States continental shelf waters GEORGET. F. WONG* and LINGSU ZHANG* (Received 15 October 1990; accepted 26 April 1991) Abstract--The hydrographic front that separates the turbid inner shelf water from the "clearer" midshelf water on the southeastern U.S. continental shelf is accompanied by dramatic changes in the speciation of iodine. The total iodine to salinity ratio, or specificiodine, in the inner shelf was slightly lower, usually about 10% lower, than that in the midshelf, which was in turn slightly lower than those observed in the open oceans. This suggests that the efficiency for the removal of dissolved iodine to the particulate phase may increase progressively from the open ocean to the midshelf water to the inner shelf water. On the other hand, the average concentration of iodate increased by a factor of about three from the inner shelf to the midshelf water while the concentration of iodide decreased by about 30%. Above a concentration of iodate of about 0.05 /~M, the concentration of iodide was inversely related to that of iodate with a ratio of about - 1 suggesting that the in situ reduction of iodate to iodide may play an important role in determining the distribution and speciation of iodine in these waters. The relationship between the concentration ratio of iodate to iodide and the concentration of nitrate plus nitrite also falls into a definable pattern. Waters with high concentration ratios of iodate to iodide (>0.5) were found mostly when the concentration of nitrate plus nitrite fell below 0.5 #M. Above this concentration of nitrate plus nitrite, the iodate to iodide ratio was almost invariably below 0.5.

INTRODUCTION HYDROGRAPHIC fronts are f r e q u e n t l y f o u n d in coastal waters. Mixing b e t w e e n the freshw a t e r runoffs f r o m l a n d a n d the saline offshore waters m a y set u p sharp salinity gradients. T h e h e a t i n g a n d c o o l i n g processes of the shallower coastal w a t e r a n d the d e e p e r offshore w a t e r m a y b e d i f f e r e n t e n o u g h to give rise to sharp t e m p e r a t u r e gradients. T h e processes that g o v e r n the chemical c o m p o s i t i o n of the coastal w a t e r a n d the offshore w a t e r c a n also be different. T h u s , these coastal h y d r o g r a p h i c fronts m a y be associated with sharp contrasts in their c h e m i c a l c o m p o s i t i o n or " c h e m i c a l " fronts. L a r g e v a r i a t i o n s in the c o n c e n t r a t i o n s of n u t r i e n t s a n d trace m e t a l s (KREMLING, 1983; BALLS, 1985) a n d biological activities (JACOBSEN et al., 1983) h a v e b e e n o b s e r v e d across such h y d r o g r a p h i c fronts. It is also c o n c e i v a b l e that extensive o x i d a t i o n - r e d u c t i o n processes m a y p r e f e r e n t i a l l y occur in the shallow coastal waters since they are f r e q u e n t l y c h a r a c t e r i z e d by high biological activities, extensive abiological p h o t o c h e m i c a l r e a c t i o n s a n d diffusional exchanges with the p o r e w a t e r in the u n d e r l y i n g s e d i m e n t , a n d all these processes m a y directly or indirectly i n v o l v e r e d o x r e a c t i o n s (BRooKs, 1979; SXUMM a n d MORGAN, 1981; ZAFIRIOU et al., 1984; MARTIN et al., 1985). O n c e a n u n s t a b l e r e d u c i n g species is a d d e d to the w a t e r *Department of Oceanography, Old Dominion University, Norfolk, VA 23529-0276, U.S.A. 717

718

G.T.F. WON6and LINGSUZHANG

either by in situ formation or through an external source, it may undergo further redox reactions with the "indigenous" chemical species. The possibility of an enhanced external input of reduced species to coastal waters has been demonstrated in a number of cases such as for manganese (MARTINet al., 1985) along the Californian coast and for methane in the northwestern Gulf of Mexico (BRoo~:s, 1979). The importance of in situ reduction has not been shown as definitively. LUTHERand COLE (1988) suggested that, in the semi-enclosed Chesapeake Bay, the reduction of iodate to iodide within the water column may be an important control of the speciation of iodine even in the surface waters. Iodine is a well known biophilic, redox-sensitive element. The vertical distribution of total dissolved iodine and particulate iodine in the oceans are characteristic of those of a bio-intermediate element. A slight depletion of a few per cent in dissolved iodine and a concomitant maximum in the concentration of particulate iodine are frequently observed in the surface waters (ELDERFIELDand TRUESDALE,1980; WON6 et al., 1976). Dissolved iodine exists in an oxidized form, as iodate, and a reduced form, as iodide, in seawater (TSUNOGAIand HENMI,1971; WON6 and BREWER,1974; 1977; WONG et al., 1976; 1985; ELDEREIELD and TRUESDALE,1980). Organic iodine has also been found in low concentrations (TRUESDALE~1975). Although iodide is thermodynamically unstable relative to iodate under oxic conditions (WONO, 1980; 1982), it co-exists with iodate even in oxygenated seawater as a result of a balance between the rate of in situ formation and/or external input and the kinetic stability of the reduced species (WONO, 1991). While the concentration of total dissolved iodine in the open oceans, which is about 0.45 pM (BRULAND,1983), does not vary greatly, the ratio of iodide to iodate may vary with depth and with geographical location as the concentration of iodide and iodate may range from <0.01 to 0.2 pM and 0.2 to 0.5 pM, respectively (SuGAWARAand TERADA,1957; TSUNOGAI, 1971 ; TSUNOGAIand HENMI, 1971; LISS et al., 1973; WON6 and BREWER, 1977; TRUESDALE, 1978; CHAPMAN, 1983; WON6 et al., 1985; JICKELLSet al., 1988; LUTHERand COLE, 1988). High concentrations of iodide are found primarily in the surface waters. Below the euphotic zone, the concentration of iodide frequently decreases to below 0.01 uM while that of iodate increases to an approximately constant level. TSUNOGAIand SASE (1969) suggested that iodide may be formed in surface waters by a biologically mediated reduction of iodate by using the enzyme nitrate reductase. Low but significant concentrations of iodide have been found in bottom waters (TsuNOGM, 1971) possibly as a result of a diffusive input from the sediments. However, in semi-enclosed bays and estuaries such as Chesapeake Bay, it has been reported that iodine may exist exclusively as iodide (LUTHERand COLE, 1988). We shall report here the distribution of iodide and iodate across the hydrographic fronts that separate the turbid inner shelf water from the "clearer" midshelf water on the southeastern United States continental shelf, commonly known as the South Atlantic Bight. These hydrographic fronts are evidently accompanied by a dramatic change in the redox speciation of iodine resulting in "redox-speciation" fronts. ENVIRONMENTAL SETTING The South Atlantic Bight extends from Cape Hatteras, North Carolina, at about 35°N to West Palm Beach, Florida, at 27°N (Fig. 1) (ATKINSONand MENZEL, 1985). It is bound by land to the west and the Gulf Stream to the east. A number of rivers empty into the Bight from the west. They are sufficiently evenly distributed along the coast to form approximately a line source of freshwater to the Bight (ATKINSON et al., 1983). The major

Changes in iodine speciation across hydrographic fronts

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contributors are the Cape Fear River, the Pee Dee River, the Cooper River, the Savannah River and the Altamaha River. The total river discharge is large enough to cause a significant freshening of the western part of the Bight. This input of freshwater varies during the year. Peak discharge occurs between February and April while the lowest discharge is observed between July and September (BLANTONand ATKINSON, 1983). Two of the major contributors of freshwater to the South Atlantic Bight, the Savannah River and the Altamaha River, discharge into the northern part of the study area. They bring in about 16 and 19% of the total river runoff to the Bight, respectively (ATKINSON and MENZEL, 1985; ATKINSON, 1985; WINDOMand SMITH, 1985). The South Atlantic Bight can be subdivided into three zones, as the dominating oceanographic process in each zone is different: the turbid inner shelf which is usually confined within the 20-m isobath, the "clearer" midshelf and the outer shelf where the Gulf Stream front often lies (ATKINSONet al., 1983). The circulation within the Bight and the exchanges of water among these three zones, the rivers and the Gulf Stream are spatially and temporally variable and may be affected by some combination of local winds, seasonal atmospheric changes and/or topographical features (BLANTON et al., 1981; HOFMANN et al., 1981; LEE et al., 1981; ATKINSONetal., 1983; BLANTONand ATKINSON,1983; LEE and ATKINSON,1983; ATKINSON, 1985; LEE et al., 1985; PIETRAFESAet al., 1985; WON~, 1988). METHODS

Stations were occupied in three transects normal to the coast off the mouth of the Savannah River to the north (the Savannah transect), off Brunswick (the Brunswick

720

G . T . F . WONG and LINGSU ZHANG

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Water depth and the distribution of salinity, temperature, phosphate, silicate and nitrate in surface water (O) and bottom water (+) along the Savannah transect.

transect) and off St Augustine to the south (the St Augustine transect) between 31 October and 5 November 1987 during FLEX (Fall Low salinity EXperiment) on board R/V Iselin (Fig. 1). Stations 107-117 in the Savannah transect, 128-135 in the Brunswick transect and 152-162 in the St Augustine transect were located in the turbid inner shelf where the water depth was less than 20 m (Figs 2-4) and the circulation and hydrography were most strongly influenced by density forcing and atmospheric forcing. These stations were respectively about 3, 5 and 2 km apart in the three transects. The rest of the stations in the transects were located in the midshelf. Data in the midshelf were obtained from only one station in the St Augustine transect. No data was obtained from the outer shelf. At each station, discrete surface and bottom water samples were collected with Niskin bottles mounted on a rosette. Sub-samples for the determination of dissolved iodine

721

Changes in iodine speciation across hydrographic fronts STATION NUMBER (BRUNSWICK) t'O

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DISTANCE FROM SHORE (Kin) Fig. 3.

Water depth and the distribution of salinity, temperature, phosphate, silicate and nitrate in surface water (O) and bottom water (+) along the Brunswick transect.

species were drawn and stored frozen in polythene bottles (Wor~6, 1973) and then returned to our shore-based laboratory for analyses. Iodate was analysed directly by differential pulse polarography with a PAR-174A polarographic analyser (HERRINGand LISS, 1974). Total dissolved inorganic iodine was determined by the same method after iodide had been oxidized to iodate with sodium hypochlorite and the excess oxidizing agent was destroyed with sodium sulfite (TAKAYANA6Iand WON6, 1986; LUTHERet al., 1988). The precision for the determination of iodate and total iodine were both about +5% and the detection limit was about 0.02/~M. The difference in the concentration between total iodine and iodate was considered as the concentration of iodide. Other subsamples were obtained for the determination of salinity, phosphate, nitrate plus nitrite and

722

G . T . F . WONG and L1NGSUZHANG STATION NUMBER (ST. AUGUSTINE) o

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DISTANCE FROM SHORE (Kin) Water depth and the distribution of salinity, temperature, phosphate, silicate and nitrate in surface water (O) and bottom water (+) along the St Augustine transect.

silicate by using established standard methods (STRICKLAND and PARSONS, 1972; CHANDLER et al., 1988). RESULTS AND DISCUSSIONS The distributions of salinity, temperature, phosphate, nitrate and silicate in the three transects are shown in Figs 2-4. The ranges of salinity were from 31.7 to 36.3, 32.6 to 36.3 and 33.8 to 36.2 psu in the Savannah, Brunswick and St Augustine transect, respectively. These relatively narrow salinity ranges reflect the small discharge of freshwater to the region during this time of the year. The major inputs of freshwater to the study area are

Changes in iodine speciation across hydrographicfronts

723

located in the northern part of the study area and fresher waters were found in that region. In the Savannah transect, two fronts can be readily identified in the inner shelf from the salinity distribution. The first front, an inshore front, was observed in the surface waters about 10-12 km offshore between stations 108 and 109. The surface salinity increased by 1.7 psu from 31.75 to 33.46 psu in 2.5 km while the temperature stayed relatively constant. This front probably represents the local influence of the Savannah River. The salinity of the surface and bottom water differed significantly in these inshore waters indicating that the water column was stratified at these most landward stations. The second front, or the main front, was found about 20-33 km offshore between Stations 112 and 117. In these 13 kin, salinity and temperature increased by 2.2 psu from 34.08 to 36.28 psu and 2.4°C from 18.48 to 20,92°C, respectively. The salinity and temperature in the surface and bottom water fell on almost identical trends beyond Station 109 indicating that the water column was vertically well mixed at these stations. This second front represents the boundary between the fresher turbid inner shelf water and the "clearer" midshelf water. In the Brunswick transect, only one front, which extended from the most shoreward station to about 15 km offshore between station 135 and 132, was observed. Within these 15 kin, salinity increased by 3.6 psu from 32.58 to 36.16 psu while temperature increased by 1.2°C from 18.4 to 19.6°C. This front represents the demarcation between the inner shelf and midshelf water. Salinity and temperature in the surface and bottom water were almost identical throughout the entire transect. In the St Augustine transect, the front occurred between stations 155 and 161 at about 2-15 km offshore. Within this distance, surface salinity increased by 1.9 psu from 33.79 to 35.73 psu while temperature increased from 20.1 to 21.7°C. The bottom salinity in the inner shelf in this transect was consistently slightly higher than the surface salinity. The strong hydrographic fronts found in these three transects are characteristic of the study area during our sampling period in the mariners' Fall (September to October) probably as a result of wind forcing. The mean wind during this period of time in the year is from the northeast. Indeed, episodes of strong northeasterly wind were observed during the cruise (CHANDLERet al., 1988). The associated onshore transport retains low-salinity water in the coastal zone and advects these waters southwards resulting in strong hydrographic fronts between the inner shelf and midshelf waters. These fronts may dissipate when the wind relaxes usually in a time scale of days to weeks (ATKINSONet al., 1983). The concentrations of phosphate, nitrate and silicate hovered around their detection limits in the midshelf waters, being below 0.1, 0.5 and 0.5 pM, respectively, in all cases (Figs 2-4). In the inner shelf, significantly higher concentrations might be found as would be expected as a result of inputs from land or the "land mass" effect. In some cases, the concentration changes within the inner shelf were systematic, resulting in distinct nutrientfronts which may or may not coincide with the hydrographic fronts. Furthermore, a nutrient-front may not be found simultaneously for the three nutrients. For example, in the Savannah transect, a nutrient-front was found at the inshore front where the concentrations of phosphate, nitrate and silicate dropped precipitously between Stations 108 and 110. However, similar phosphate- and silicate-fronts were not evident in the second front that separated the inner shelf from midshelf water. In fact, the concentrations of phosphate and silicate were ubiquitously low across this front. In the case of nitrate, the concentration increased slightly beyond the inshore front before it dropped to undetectable levels again at Station 116. Thus, in this transect, whereas the nutrients could be used effectively as an indicator of the inshore front, unlike that reported in the Scottish coastal

724

G.T.F. WONGand LINGSUZHANG

waters (KREMLING, 1983), they were much less useful as a tracer for distinguishing the water masses from each other further offshore. In the Brunswick transect, there was also a rise in the concentration of phosphate, nitrate and silicate shoreward between stations 135 and 133. However, the resulting nutrient-fronts did not coincide with the hydrographic fronts which were found further offshore between stations 132 and 135. In the St Augustine transect, there was again little correlation between the nutrient-fronts and the hydrographic fronts in the inner shelf waters. In fact, the distribution of phosphate and silicate seem erratic. These distributions indicate that, as expected, the concentration of nutrients was not controlled by hydrography alone. Biological processes, which are patchy and variable in these waters (JACOBSEN et al., 1983), also play an important role in determining the distributions of the nutrients. The distributions of total iodine to salinity ratio, or specific iodine, iodate, iodide and iodate to iodide ratio in the Savannah, Brunswick and St Augustine transects are shown in Figs 5-7, respectively. The average concentration of iodide, iodate, iodate to iodide ratio and specific iodine in the surface and bottom inner shelf and midshelf water in each transect are tabulated in Table 1. The average specific iodine in the midshelf waters ranged from 8.5 to 9.9 nM psu -1. These values are lower than those (about 10 to 14 nM psu -1) reported for open ocean waters (TsuNOGAI and HENMI, 1971). Since changes in specific iodine are caused by the interconversion of iodine between the dissolved and the particulate phases, most likely as a result of biological activities (TsuNOCAI and HENMI, 1971; WONG and BREWER, 1974; 1977; WONG et al., 1976; 1985; ELDERFIELDand TRUESDALE, 1980), the lower average specific iodine in the midshelf water relative to the open ocean waters suggests a more effective removal of dissolved iodine to the particulate phase in the midshelf water possibly by elevated biological activities. In the inner shelf, the average specific iodine was systematically lower still, ranging from 8.0 to 8.8 nM psu 1. The ratio of average specific iodine in the midshelf water to that in the inner shelf water ranged from 1.0 to 1.2 (Table 1). This indicates that not only was there no evidence of an enhanced input of dissolved iodine to the inner shelf water through river runoff or diffusion from the sediments, but that there might have been a slight preferential removal of dissolved iodine to the particulate form. The difference in average specific iodine between the surface and bottom waters was small, being less than 5% in all cases. A small depletion in the surface waters was observed in most cases. In a finer scale, within a given transect, specific iodine was quite uniform in the midshelf waters while it could fluctuate rather widely in the inner shelf water in the frontal zone (Figs 5-7). This might have been the result of a combination of the enhanced biological activities at these frontal regions (FLOODGATE et al., 1981) and the patchiness and variability of the biological removal of dissolved iodine in the inner shelf waters. In our study area, it has been reported that the frontal zone is an area of high phytoplankton and bacterioplankton abundance and activity (JACOBSEN et al., 1983). Dramatic changes in the speciation of dissolved iodine were observed across the hydrographic fronts from the inner shelf to the midshelf waters. The average concentration of iodate increased by a factor of 3 from around 0.06/~M in the inner shelf water to about 0.17/~M in the midshelf water (Table 1). Concomitantly, the concentration of iodide decreased by 30% from about 0.23/zM in the innershelf water to 0.16 p M in the midshelf water. These reciprocal changes in the concentrations of iodate and iodide led to a large increase in the iodate to iodide ratio from the inner shelf to the midshelf water by a factor of 2-7 (Table 1). The ratio remained rather constant at about 0.2 in the inner shelf water. It

725

Changes in iodine speciation across hydrographic fronts

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IODATE(gM)

+ IODIDE(gM)

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IODATE/ IODIDE

I

I

10

I

I

30

I

I

50

I

I

70

I

I

90

DISTANCEFROMSHORE(Km) Fig. 5.

The distribution of specific iodine, iodate, iodide and iodate to iodide ratio in surface water (O) and bottom water (+) along the Savannah transect.

increased to 0.9 or higher in the midshelf water. The value of this ratio in the inner shelf is one of the lowest observed in the open sea. Within each transect, the changes in the concentration of iodate and iodide a m o n g closeby stations in the frontal zone could be even more striking. This is especially true in the case of iodate whose concentration was close to the detection limit in the inner shelf water. For example, in the surface waters in the Savannah transect, across the front, the concentration could increase by a factor of 10 from 0.02/~M at Station 113 to 0.21 # M at Station 114 in a distance of about 3 kin. Concomitantly, the concentration of iodide decreased from 0.26 ~ M at station 113 to 0.24/~M at station 114 and then to 0.13 # M at station 115 in a distance of 6 km. The influence of the inshore front in the Savannah

726

G.T.F.

WONG and LINGSU ZHANG

STATIONNUMBER(BRUNSWICK) O3

15

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i

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i

i

i

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I

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10

I

I

30

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50

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90

DISTANCEFROMSHORE(Km) Fig. 6.

The distribution of specific iodine, iodate, iodide and iodate to iodide ratio in surface water ( 0 ) and bottom water ( + ) along the Brunswick transect.

transect on the speciation of dissolved iodine was not as dramatic as that of the main front which separated the inner shelf from midshelf water. H o w e v e r , there were still indications of an increase in the concentration of iodate and a concomitant decrease in the concentration of iodide of about 0.06~tM across this front. In general, in the inner shelf water, the change in the concentration of iodate across the frontal z o n e was less eratic than that of iodide, probably as a result of the more complex biogeochemistry of iodide. Whereas the concentration of iodate is affected primarily by the interconversion between iodate and iodide, the concentration of iodide may also be influenced by its biological uptake and the remineralization of biogenic particles. The concentration of both iodate and iodide remained rather uniform in the midshelf waters.

727

Changes in iodine speciation across hydrographic fronts

STATIONNUMBER(ST.AUGUSTINE) I

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II

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SPECIFICIODINE(nM/% °)

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................................ -~

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IODIDE(,u.i) .... . ..............

+

0.1

IODATE/ IODIDE I

I

10

I

I

30

I

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50

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70

DISTANCEFROMSHORE(Kin) Fig. 7.

The distribution of specific iodine, iodate, iodide and iodate to iodide ratio in surface water (©) and bottom water (+) along the St Augustine transect.

Although, in general, the distribution of iodate and iodide in the surface and bottom waters followed the same trend, the distributions in the bottom water seemed more erratic on occasions. Several factors can contribute to these erratic behaviors. For example, since the samples of bottom water were collected at different depths and the mixing processes in the bottom water in the study area may be temporally and spatially variable, water types with different geochemical history might have been sampled. The type and intensity of biological activities in the surface and bottom water may be different. The composition of the bottom water may be more prone to influence by the exchanges of iodine species between the bottom water and its underlying sediments and the input of reducing substances from the sediments and these processes may be temporally and spatially variable.

728

G . T . F . WONG and LIN6SV ZHAN6 Table 1.

Savannah transect Inner shelf (Sta. 107-113) Midshelf (Sta. 114-122) Midshelf/inner shelf

Brunswick transect Inner shelf (Sta. 132-135) Midshelf (Sta. 123-131) Midshelf/inner shelf

St Augustine transect Inner shelf (Sta. 156-162) Midshelf (Sta. 152-155) Midshelf/inner shelf

The average concentration of iodine species in the transects

Depth*

Iodide ~M)

Iodate ~M)

Iodate/Iodide

S.I. (nM psu -1)

S B S B

0.24 0.20 0.15 0.17

0.05 0.10 0.19 0.18

0.19 0.51 1.30 1.05

8.7 8.7 9.4 9.9

S B

0.60 0.88

4.2 1.8

7.0 2.0

1.1 1.1

S B S B

0.23 0.24 0.14 0.14

0.05 0.05 0.17 0.16

0.20 0.22 1.22 1.18

8.0 8.5 8.8 8.5

S B

0.63 0.59

3.8 3.1

6.1 5.3

1.1 1.0

S B S B

0.22 0.26 0.18 0.18

0.06 0.05 0.17 0.16

0.25 0.20 0.95 0.90

8.1 8.8 9.9 9.4

S B

0.82 0.70

3.1 3.2

3.8 4.6

1.2 1.1

*S--Surface water; B--bottom water. S.I.--Specific iodine = total iodine/salinity.

A number of processes may operate individually or interactively to give rise to the general trend of an increase in the average concentration of iodide and a concomitant decrease in the average concentration of iodate from the midshelf to the inner shelf waters across the hydrographic fronts. Given that the inner shelf water was midshelf water that had been modified slightly by mixing with fresh water, these processes may be: (1) an external input of iodide; and/or (2) in situ reduction of iodate to iodide in the inner shelf water. Iodide may be preferentially added to the inner shelf water by diffusion from the bottom sediment or by mixing with river water which has a much higher iodide to salinity ratio. However, since specific iodine decreased from the midshelf to the inner shelf water while the salinity decreased by about 8% at the same time, there should be a removal of iodine rather than an input of any iodine species. An input of iodide from any source should increase the specific iodine. Furthermore, these processes should not have any impact on the concentration of iodate aside from a dilution effect of 8% as the salinity decreased, and yet the iodate concentration decreased by a factor of 2-4. Thus, if an external input of iodide is to be invoked, it must be accompanied by an extensive removal of iodate and the occurrence of such a process has not been demonstrated. Alternatively, the data can be explained by a combination of the in situ reduction of iodate to iodide, the conversion of dissolved iodine to the particulate phase and an input of fresh water,

Changesin iodinespeciationacross hydrographicfronts

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Fig. 8. The relationship between iodate and iodide in surface (ll) and bottom (O) waters. Solid lines at concentrations of iodate above 0.1/~M denote a linear band, with a slope of - 1 , that contains most of the data points.

whereby, salinity was decreased by the input of flesh water, specific iodine was reduced by the conversion of iodine to the particulate form and the iodate to iodide ratio was decreased by the reduction of iodate to iodide. A number of processes can lead to the reduction of iodate to iodide. The biologically mediated enzymatic reduction by marine bacteria and extract of bacterial nitrate reductase have been demonstrated in the laboratory (TsuNOGAI and SASE, 1969). Laboratory evidence for the reduction of iodate to iodide by phytoplankton is less conclusive (SuGAWARAand TERADA,1967; BUTLERet al., 1981). The diffusion of sulfide from the sediments and its reduction of iodate to iodide, probably within the water column, has been proposed to explain the speciation of iodine in Chesapeake Bay (ZHANG and WHITFIELD, 1986; LUTHERand COLE, 1988). However, neither the detailed mechanisms nor the rates of these reactions are known. With our data, it is not possible to rule out any one of these possibilities. Nonetheless, whether iodate was reduced to iodide directly by biological processes or chemically by a reducing agent, the mole ratio of iodate reduced and iodide formed should be -1:1. The relationship between iodate and iodide in the surface and bottom waters is shown in Fig. 8. Above an iodate concentration of 0.05-0.1 pM, the data points can be confined within a broad linear band with a slope of - 1 . This relationship suggests that the interconversion between iodate and iodide might have been the dominating process controlling the speciation of iodine in these waters since the variations in the concentration of total iodine had remained relatively small. The fact that a broad band rather than a tight linear line was observed indicates that secondary processes such as the biological conversion of iodide to the particulate form may also have an effect on the distribution and speciation of iodine. At concentrations of iodate below 0.05/~M, which were observed exclusively in the inner shelf water, the relationship broke down. The concentration of iodide did not increase any further with decreasing iodate concentration. In fact, there might have been a reverse trend of decreasing iodide concentration with decreasing iodate concentration. This trend indicates that the secondary processes might have taken over as the controlling processes of the speciation and distribution of dissolved iodine in these productive inner shelf waters.

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Fig. 9. The relationship between iodate to iodide ratio and the concentration of nitrate plus nitrite in surface ( i ) and bottom (©) waters. I, II, III and IV denote combinations of the two parameters that may represent the four types of water as discussed in the text.

If iodate reduction is linked to nitrate reductase activity (TsuNOGAI and SASE,1969) and the concentration of total dissolved iodine remains approximately constant, there may be four possible relationships between the concentration ratio of iodate to iodide and the concentration of nitrate. In the first case, type I water, the nitrate concentration is high and nitrate reduction is active. As a result, iodate reduction is also active, resulting in low iodate to iodide ratio. In the second kind of water, type II, the concentration of nitrate is low. However, the capability for nitrate reduction is present and will be activated if nitrate is added. In this case, iodate reduction may still occur, resulting in low iodate to iodide ratio. In the third case, type III water, the concentration of nitrate is low and nitrate reductase activity is also low. Thus, iodate reduction is suppressed and the iodate to iodide ratio will be high. In the fourth kind of water, type IV, the concentration of nitrate is high. However, nitrate reductase activity may have been suppressed, for instance, by the availability of other forms of combined reduced nitrogen. Thus, iodate reduction will also be suppressed and the iodate to iodide ratio may remain high. Since the data for nitrate alone are not available, we have examined the relationship between iodate to iodide ratio and nitrate plus nitrite by assuming that nitrate represents the bulk of the nitrate plus nitrite and/or that nitrate reduction and nitrite reduction are coupled together (Fig. 9). The data points seem to fall into distinct clusters. There is apparently a critical concentration of nitrate plus nitrite at about 0.5 ktM and a critical iodate to iodide ratio of 0.5. Above 0.5/~M of nitrate plus nitrite, the ratio of iodate to iodide were mostly below 0.5. This may represent the water with high concentration of nitrate and high nitrate reductase activity, or type I water, which gives rise to low ratios of the concentration of iodate to iodide. Below this concentration, the ratio of iodate to iodide rarely dropped below 0.5. This would represent the type IlI water with low concentration of nitrate, low nitrate reductase activity and high iodate to iodide concentration ratio. Thus, if the four types of water may be defined by these two critical values, our data indicate that only waters of type I and type III were found in the study area. The rate of reduction of iodate to iodide in the oceans by biological or abioiogical processes is presently unknown. Our data place a constraint on the time scale of these

Changes in iodine speciation across hydrographicfronts

73 ]

processes. If the inner shelf water is slightly modified midshelf water, and, these frontal features are controlled by wind forcing and they dissipate in a time scale of days to weeks, the reduction of iodate to iodide must also occur within this time scale. During this period of time, as indicated by the difference in the concentrations of iodate and iodide at the two sides of the front, about 0.1~).2/~M of iodate has been reduced to iodide. CONCLUSIONS The concentration of iodate and iodide changed markedly from the inner shelf to the midshelf water across the hydrographic front that separates these two kinds of water in the South Atlantic Bight. The average concentration of iodate increased by a factor of about 3 from about 0.06/~M in the inner shelf water to about 0.17/~M in the midshelf water while the concentration of iodide decreased by about 30% from 0.23 to 0.16/~M. Specific iodine also increased slightly by 10% or less in most cases. Since the inner shelf water can be considered as midshelf water which has been modified as it is trapped along the coast, these distributions of iodide and iodate were probably caused primarily by an in situ reduction of iodate to iodide. The biological removal of dissolved iodine to the particulate phase played a secondary role. The effects of riverine input and diffusion of iodide from the sediment to the water column were not clearly evident. The relationship between the concentration of iodate and iodide at concentrations of iodate above 0.05 # M fell on a broad linear band with a slope of about - 1 . This is consistent with an in situ reduction of iodate to iodide. Below a concentration of iodate of 0.05 #M, the relationship broke down as other processes dominate such as biological uptake into a particulate phase. It is unclear from this data set whether iodate was reduced to iodide biologically or chemically. However, the relationship between iodate to iodide ratio and the concentration of nitrate plus nitrite does fall on a definitive pattern where high concentration ratios of iodate to iodide (>0.5) were found mostly when the concentration of nitrate plus nitrite fell below 0.05 ~M. A b o v e this concentration of nitrate plus nitrite, the iodate to iodide ratio was almost invariably below 0.5. This is consistent with a link between nitrate reduction and iodate reduction. Specific iodine decreased progressively from the open ocean to the midshelf to the inner shelf waters suggesting that the efficiency for the removal of dissolved iodine to the particulate phase may increase from the open ocean to the midshelf water to the inner shelf water. Acknowledgements--We thank L. Atkinson for reviewing a draft of this manuscript, for our participation in FLEX and for the use of his hydrographicdata obtained from this cruise. The work of L. Atkinsonwas supported in part by the Department of Energy under grant number DE-FG05-85ER60348.This research was supported in part by the National Science Foundation under grant number OCE-8910956to G. Wong.

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