Iodate and Iodide Distributions in the Waters of a Stratified Estuary

CROATICA CHEMICA ACTA CCACAA 79 (1) 143¿153 (2006) ISSN-0011-1643 CCA-3075 Original Scientific Paper

Vesna @ic and Marko Branica

Division for Marine and Environmental Research, Ru|er Bo{kovi} Institute, 10000 Zagreb, Croatia (E-mail: [email protected])

RECEIVED JUNE 9, 2005; REVISED OCTOBER 3, 2005; ACCEPTED JANUARY 13, 2006

Speciation and distribution of inorganic iodine in the highly stratified Krka estuary (east coast of the Adriatic Sea) were investigated in the period from December 2000 to April 2001. Con- servative behavior of total inorganic iodine was confirmed during the investigated period, but significant temporal and longitudinal variations were also observed. Concentrations of iodate and iodide were less variable in the more saline layers (S ³ 10) than in the upper brackish and interfacial layers, indicating lower dynamics and less pronounced influence of secondary pro- Keywords cesses on the inorganic iodine distribution. An increase in iodate and iodide concentrations to- iodate ward the estuary mouth was determined in the upper layers. Seasonal variations in iodate and iodide iodide distributions in the brackish layer were indicative of the important role of the freshwater Krka River Estuary phytoplankton and organic matter for the inorganic iodine cycle in the estuary.

INTRODUCTION ganic compounds. Concentrations of organic iodine vary considerably between different estuaries but also within The estuarine chemistry of iodine is very complex, and a single estuary. It can represent a significant proportion it has been widely investigated in numerous studies, of the total iodine in some estuarine systems.4,7,8 Depth which contributed useful information about the biophilic profiles of organic iodine differ from those of iodate and and redox sensitive nature of iodine.1–5 In contrast to open iodide. The lowest concentrations of organic iodine were oceans, estuaries are highly influenced by human activities via municipal wastewater discharges and spills, which observed below the halocline, where the total iodine con- may cause an increase in the concentration of nutritious centrations are similar to those detected in seawater. Some- salts, organic matter and other pollutants. Concentrations what higher values of dissolved iodine near the bottom and relative abundance of iodate and iodide in estuaries are a result of the diffusion and re-mineralization proces- vary significantly due to the influence of various factors, ses. Reduced iodine forms (iodide and organically bound such as geomorphology and hydrology of the estuary, in- iodine) prevail in the estuaries that are affected by pro- 7,9,10 tensity of the primary production, the content and com- longed periods of oxygen deficiency. position of dissolved and particulate organic matter, sed- The objective of this work was to explain the pro- iment composition, as well as human activities.6 cesses that affect the concentrations and distribution of Both conservative and non-conservative behaviors of inorganic iodine in the Krka river estuary, which has spe- iodate and iodide in the estuaries have been established. cific hydrological, chemical and biological characteristics. Non-conservativity of iodine in the water column can be It gave us the opportunity to study elemental speciation attributed to the biological cycle of the phytoplankton and in two fundamentally different compartments, the upper the formation of organic iodine in the reactions between fresh/brackish and the underlying marine layer, as well highly reactive molecular iodine and hypoiodite with or- as in the narrow boundary layer between them. 144 V. @IC AND M. BRANICA

Scope of Study source of freshwater is the Krka River, with the average input of 55 m3 s–1, but significant monthly and seasonal The Krka River estuary is situated in the eastern part of variations have been observed (from 5 m3 s–1 to 440 the Adriatic coast. This karst river forms a 25 km long m3 s–1). The estuary is highly stratified as a result of its estuary that spreads from the Skradinski Buk waterfalls sheltered position and low tidal movements (0.2–0.5 m).12 (43°49.3' N, 15°56.7' E) to the [ibenik Channel (43°43.3' N, The upper brackish current flows seaward, while the 15°51.4' E) (Figure 1). The estuary is preceded by a se- deeper seawater current has the opposite flow direction. ries of waterfalls and moss-covered tufa barriers which The boundary layer is characterized by a steep halocline retain sediment load from the upper region, the Visovac that varies both in thickness and depth in dependence on Lake. Within the estuary, both the input of suspended freshwater inflow and wind. material (mainly via the Gudu~a Creek) and the sedi- Variations in the exchange time of both freshwater mentation rate are low. Sedimentation material is mostly and seawater depend on the season. For freshwater, the terrigenious in the upper part of the estuary, while in the variations range from 20 days in winter to 80 days in lower part it mainly consists of the authigenic (biogenic) summer, while for seawater from 50–100 days in winter fraction.11 up to 250 days in July and August.13 As a result of the The depth gradually increases from 1 m below the stabile stratification of the estuary, a water temperature waterfalls to 42 m in the [ibenik Channel. The main inversion was recorded during the winter. The tempera-

Figure 1. The Krka river estuary with locations of the sampling stations.

Croat. Chem. Acta 79 (1) 143¿153 (2006) IODATE AND IODIDE DISTRIBUTIONS IN THE WATERS OF A STRATIFIED ESTUARY 145 ture maximum around the halocline occurred from spring of the method for iodate determination was typically ±4 % at to autumn,14,15 with the highest value of 34 °C recorded higher iodate concentrations and up to 15 % at concentra- on August 01, 1998.16 tions below 0.05 µmol dm–3. The precision in iodide determination was typically bellow6%atiodide concentrations above 0.01 µmol dm–3 and ±15 % at lower concentrations. EXPERIMENTAL Water samples were collected in the period from December RESULTS AND DISCUSSION 2000 and April 2001 at six estuarine stations (E2, E9, E3, E4a, L1, E5) and two referent stations for freshwater (E0) Sigma-t, Salinity, Temperature and pH and seawater (C1) (Figure 1). Stations E0 and C1 represent During the investigated period, the stabile vertical strati- two closest end-member locations where no mixing pro- fication at all estuarine sampling stations was character- cesses between the river water and seawater were observed. ized by a sharp density gradient in the interfacial layer, At the estuarine stations E3, E4a and E5, samples were which divided the upper-brackish and the deeper-saline collected with 5 L Niskin samplers, while at stations E2, water. Due to the high salinity range along the depth E9, L1 and C1 samples were taken by a scuba diver, facing profiles, temperature had only a minor influence on the the direction of the current.17 Owing to good visibility and hydrostatic stability of water layers, and it was only in a veil-like appearance of the narrow intermediate layer, this December 2000 that isohaline conditions coincided with sampling procedure enables collection of discrete samples Ds D around it. The temperature was determined in situ with an a slight temperature increase with depth ( t / z= –3 –1 Hg-thermometer, the salinity with a refractometer (Atago, –0.04 kg m cm ). Density gradients both in the brack- Japan) and pH with a manual pH-meter MP120 with an ish and saline layers were rather small (up to 0.06 kg –3 –1 InLab 427 electrode (Mettler-Toledo AG) immediately af- m cm ) when compared to the values obtained in the ter sample collection. Samples for the determination of io- interfacial layer (up to 2.21 kg m–3 cm–1). dine species were stored unfiltered in polypropylene bottles The depth and longitudinal variations in salinity in dark at 4 °C until analyses, which were completed within were in accord with the geomorphologic and hydrologic two weeks. Voltammetric measurements were performed description of the Krka estuary. Salinity at the surface with a BAS-100A Electrochemical analyzer (Bioanalytical varied between 1.0 and 3.5 in the upper part of the estuary Systems, West Lafayette, Indiana, USA). An EA 290 HMDE (stations E2 and E9), while the variations in the lower electrode (Metrohm, Switzerland) combined with a Pt-coun- estuarine section (E3–E5) were more pronounced (be- ter electrode and an Ag/AgCl as a referent electrode were tween 2.3 and 10). Salinity of 38 was recorded in deeper used. Iodate and iodide were determined directly by differ- layers. Lower salinity of the near-bottom layer was ob- ential pulse voltammetry18 and cathodic stripping square served only in the most shallow part on December 1, wave voltammetry,19 respectively. Oxygen was removed 2000 (S = 32). Within only a few days, however, the sa- from the samples by bubbling with nitrogen and by addi- linity at this station increased to 38, and the halocline tion of sodium sulphite.20 Necessary precautions were taken to avoid conversion of dissolved organic iodine to io- was shifted toward the surface by almost 1 m (Figure 2). dide.21 Concentrations of iodate and iodide were deter- These data suggested that a high freshwater flow was mined by the standard addition method and the results are compensated by a counter-flux of marine water. expressed as the arithmetic mean and the respective stan- The water temperature profiles showed a seasonal dard deviation of at least duplicate analyses. The precision pattern. During winter and early spring, the temperatures

Figure 2. Salinity, temperature and pH depth profiles at station E2 obtained on December 1 (white filled symbols) and December 7 (black symbols) 2000.

Croat. Chem. Acta 79 (1) 143¿153 (2006) 146 V. @IC AND M. BRANICA

(a) S, t/°C c /mmol dm–3 0 10203040 0.0 0. 1 0.2 0.3 0.4 0.5 0 1 2 m

/ 3 4 depth 5 6 7 8.1 8.2 8.3 8. 4 8.5 0102030 –3 pH st /gdm

S, t/°C c /mmol dm–3 (b) 0 10203040 0.0 0. 1 0.2 0.3 0.4 0.5 0 2 4 6 8 10 depth / m 12 14 16 8. 1 8. 2 8. 3 8. 4 8.5 0102030

–3 pH st /g dm

Figure 3. Depth profiles of salinity (o), temperature (l), pH (+), iodate (¯), iodide (l), iodate+iodide (p) and st (___) at stations: a) E4a (February 15, 2001), b) L1 (April 30, 2001). in the marine layer were always higher than those in the cesses around the intermediate layer. These pH varia- upper brackish layer, varying between »20 °C in Decem- tions could be caused by the increased photosynthetic ber and »15 °C in February. An apparent diurnal vari- activity,22 with the maximum appearing as a result of the ability of temperatures in the upper layers, caused by in- production phase, whereas the minimum reflects the res- tensive solar irradiation, was registered by the end of the piration phase. April. At the lower edge of the interfacial layer, within the salinity range between 25 and 38, a slight tempera- Iodate and Iodide ture maximum (up to 1.0 °C) was occasionally observed From the hydrological point of view, the physical and from December to April. According to the literature data, chemical properties of estuarine waters result from the this maximum is most pronounced in the spring/summer interaction between the seaward flow of the Krka River season and appears as a result of intensive solar irradia- freshwater and the opposite flow of marine water. It tion through the transparent brackish layer and slow would therefore be appropriate to present iodate and io- mixing processes between the brackish and seawater.15 dide concentrations in both original waters first. At the pH varied between 8.5 (in upper layers) and 8.1 (in riverine end, only iodide was detected in a low concen- saline water), with the exception of December 2000 tration (4.1 ± 0.39 nmol dm–3). As stated by Truesdale and when the pH range ranged between 8.1 and 7.7. The spe- Upstill-Goddard,23 and Truesdale and Jones,24 the distri- cific Z-shaped pH vertical profiles (as in Figure 3b) with bution of iodine species within the estuaries, unlike that higher values in the upper part of the steep halocline and of nutrients, is primarily determined by the coastal end- lower values below it were especially pronounced in the member, since the iodine concentrations in river water as spring season, indicating more intensive biological pro- well in freshwater of anthropogenic source are far lower

Croat. Chem. Acta 79 (1) 143¿153 (2006) IODATE AND IODIDE DISTRIBUTIONS IN THE WATERS OF A STRATIFIED ESTUARY 147 than in seawater. Thus, more attention will be paid to the Within the estuarine area, according to these marked coastal end-member, where iodate, iodide and total inor- concentration differences between freshwater and seawater, ganic iodine concentrations were in ranges from 0.27– wide concentration ranges of both iodate (<0.015–0.39 0.34 mmol dm–3, 0.07–0.11 mmol dm–3 and 0.35–0.45 mmol dm–3) and iodide (0.003–0.29 mmol dm–3) were mmol dm–3, respectively. Assuming higher salinity of these found. samples (S = 38), the concentrations of total inorganic iodine are lower than those reported for ocean waters, where Depth Distributions. – Iodate and iodide depth distribu- concentrations are around 0.45 mmol dm–3. However, tions at estuarine stations showed a similar pattern dur- both found concentrations of total inorganic iodine and ing the investigated period; low concentrations in the the wide range of concentration at the reference coastal upper layer, a sharp concentration gradient at the fresh- station are comparable to literature data for the Adriatic water/seawater interface and relatively high concentrations waters25,26 and the Mediterranean high salinity waters in the saline layer, indicating that dilution processes were (0.34–0.48 mmol dm–3).8 Depth profiles at station C1 in- the primary processes that affected iodate and iodide con- dicate a slight enrichment of both iodate and iodide in centrations. Superimposed on this general pattern, the the near-bottom area. These high iodide concentrations subsurface iodide concentration maximum (station E4a, and low iodate to iodide mole ratios (between 3.0 and Figure 3a) and multiple iodate and iodide peaks at the 4.0) are common for the euphotic zone, due to the bio- lower edge of the halocline (station L1, Figure 3b) were philic nature of iodine.27 indicators of the specific conditions within the water

(a) S, t/°C c /mmol dm–3 (b) S, t/°C c /mmol dm–3 020400.0 0.2 0.4 0.6 020400.0 0.2 0.4 0.6 0 0 1 1 2 2 3 3 4 4 depth / m depth / m 5 5 6 6 7 7 8 8 7.6 8.1 8.6 02040 7.6 8.1 8.6 02040 –3 –3 pH st /g dm pH st /g dm

4 4

8 8 depth / m depth / m

12 12

16 16 7.6 8.1 8.6 02040 7.6 8.1 8.6 02040 –3 –3 pH st /g dm pH st /g dm

Figure 4. Depth distributions of salinity (o), temperature (l), pH (+), iodate (¯), iodide (l), iodate+iodide (p) and st (___) at station E2: a) 7 Dec. 2000 and b) 24 Mar. 2001 and at station E9: c) 8 Dec. 2000 and d) 25 Mar. 2001.

Croat. Chem. Acta 79 (1) 143¿153 (2006) 148 V. @IC AND M. BRANICA column in the [ibenik Bay and of high biogeochemical TABLE I. Linear regression parameters between iodine species and –3 reactivity of iodine. According to the other studies con- salinity. The units for both slopes and intercepts are nmol dm ducted in the Krka estuary, the narrow interfacial layer ± ± 2 between the low salinity brackish water and more saline Slope Interccept r N std. err. std. err. marine water is enriched both in particulate and in dissolved organic matter,28–30 phytoplankton,31–33 as well as Dec. 2000 the bacterial population.34,35 The main sources of dis- Iodate 4.14 ± 0.31 6.48 ± 7.15 0.87 29 solved organic matter are the degradation products of the Iodide 7.15 ± 0.25 –1.14 ± 5.69 0.97 29 freshwater phytoplankton. The upper and interfacial lay- Iodate + iodide 11.29 ± 0.31 5.34 ± 7.01 0.98 29 ers in the [ibenik Bay were found to be particularly en- Feb.–Apr. 2001 vironmentally disturbed due to additional nutrient input, Iodate 7.46 ± 0.23 –9.64 ± 5.85 0.92 101 so the enhanced phytoplankton activity and high primary ± ± production were confirmed in this lower part of the estu- Iodide 2.85 0.16 0.59 4.11 0.76 101 ary. Assuming the biophilic nature of iodine and as its Iodate + iodide 10.31 ± 0.21 –9.05 ± 5.36 0.96 101 bioconcentration in some marine organisms has been confirmed, the peak concentration at the lower edge of were linearly related to salinity during the entire period the halocline (Figure 3a) may be explained by bacterial (Table 1). Although the correlation coefficients were high decomposition of the accumulated organic matter. Iodate and the uncertainty of the slope low (P<0.0001), the lack and iodide depth profiles at station L1 (Figure 3b), situ- of meaning of the intercept (negative values) and high un- ated in front of the wastewater outlet, reflect high anthro- certainty of the intercept (high standard errors) were pogenic influence on elemental speciation. These mirror- caused by a large scattering of data points. This is partly like profiles of iodate and iodide and a relatively uni- due to temporal and longitudinal variations in iodine dis- form depth distribution of total inorganic iodine, which tributions included in these two composite groups. Bet- exclude the role of organic-iodine species, show how the ter correlation was obtained when each sampling was untreated municipal wastewater discharges may induce analyzed separately, but still the intercept had generally dramatic changes in redox conditions within the water no statistical significance. These results suggest that be- column. side simple dilution, secondary processes affected iodine Concentration maxima at the other estuarine stations distribution and speciation, particularly the inter-conver- were generally less pronounced. In contrast to other sam- sion processes between iodate and iodide. Higher corre- pling periods, the concentration ratios between iodate lation coefficients for total inorganic iodine and similar and iodide below the halocline, at stations E2 and E9 in slopes both for December and the period from February December 2000, were rather low (mainly below 1.0) until April support the proposed inter-conversion mecha- (Figures 4a and 4c). More typical profiles at these sta- nism. These data also indicate that only total inorganic tions are represented in Figures 4b and 4d. The hypoxic iodine behaved conservatively during estuarine mixing, conditions that periodically occur in the Prokljan Lake since the deviations from the hypothetic mixing line be- and near the town of Skradin in autumn and winter 14,36 tween the two end-members during the entire period were may explain the observed low iodate to iodide concen- low only for total inorganic iodine (Figure 5). At higher tration ratio. Hypoxia occurs as a result of several fac- salinities, iodate concentrations mainly drop below the tors; a marine phytoplankton bloom, stabile stratification mixing line, indicating its reduction within the estuarine and a slow rate of water exchange. Hypoxia usually pre- area. At lower salinities, a scattering of data points indi- vails until higher river flux is compensated by a coun- cated higher iodate involvement in the biogeochemical ter-flux of seawater enriched in oxygen.36 Relatively cycle. Contrary to iodate, the estuarine area was enrich- high temperatures below the halocline in December, low ed in iodide and its concentrations were less variable at pH and re-establishment of the prevailing natural condi- low salinities. tions in the deepest layers, which can be also obtained from the iodate and iodide depth distributions (Figure Relationship between Iodate and Iodide and Seasonal 4c), support this explanation. Variations in Iodine Distribution. – In iodine studies, Due to specific iodate to iodide concentration ratios particularly those that deal with natural aquatic systems in December, as mentioned above, two data groups were where salinity changes might affect iodine concentra- compiled for the linear regression analysis between io- tions, it has become common to rationalize iodine con- dine species and salinity. One group includes data from centrations to salinity 35, so that changes in concentra- stations E2 and E9 in December 2000, and the other tion profiles may be linked only to biogeochemical pro- group includes the remaining sampling data. The differ- cesses.37 In this paper, the letter "r" in a subscript stands ence between these two data groups is obvious in Figure 5. for the rationalized value of a substance, e.g., the ratio- The results obtained indicate that both iodate and iodide nalized concentration of a substance X is cr(X) =

Croat. Chem. Acta 79 (1) 143¿153 (2006) IODATE AND IODIDE DISTRIBUTIONS IN THE WATERS OF A STRATIFIED ESTUARY 149

hypothetical relationship between iodate and iodide based on the two end-member mixing model (Figure 6a). The intercepts refer to the lowest and the highest rationalized concentrations of total inorganic iodine at the coastal station C1, while the hypothetical slope is –1 mmol dm–3 / mmol dm–3 (as only inter-conversion processes between iodate and iodide are assumed). At lower salinities (Figure 6b), however, only a fraction of data points are within the hypothetic envelope and the scattering is more pronounced. The deviations during spring, when most of the sampling points fall below the hypothetical relationship, point to the removal of dissolved iodate and/or iodide, while during the late autumn/winter period both species were produced in the brackish surface layer. This seasonal trend in iodate and iodide distributions both at the upper estuarine section between stations E2 and E9 and the lower section between stations E3 and E5 is particularly obvious when rational-

0.5 (a)

0.4

–3

dm 0.3

m 0.2

–

(I ) / mol

0.1

0.0 0.0 0.1 0.2 0.3 0.4 0.5

c – m –3 r3(IO ) / mol dm

0.8 (b)

0.6

–3 ¯ u

Figure 5. Relationship between concentrations of iodate ( , ), dm iodide (,l) and total inorganic iodine (r,p), and salinity. Black symbols refer to December 2000 and white symbols to the period 0.4 from February to April 2001. Solid lines indicate the linear regres- m sion best fit lines. Dotted lines indicate the mixing line between –

(I ) / mol

riverine and marine end-members. c 0.2 c(X)·35·S–1. When rationalized, iodate and iodide concentrations show fewer variations at higher salinities, as 0.0 well as a seasonal trend at lower salinities. 0.0 0.2 0.4 0.6 0.8 c – m –3 When a similar approach to iodate and iodide distri- r3(IO ) / mol dm butions in the Chesapeake Bay38 is applied to the Krka c ³ Figure 6. Relationship between rationalized concentrations ( r)of estuary, at higher salinities (S 10) almost all data points iodate and iodide: a) at higher salinities (S ³ 10), and b) at lower in the relationship between rationalized iodate and io- salinities (S < 10). White symbols refer to the late autumn/winter dide fall within the linear envelope, which represents the period, black symbols refer to the spring period.

Croat. Chem. Acta 79 (1) 143¿153 (2006) 150 V. @IC AND M. BRANICA

E2, E9 E3, E4a, E5 0. 8

–3

dm 0. 6

m 0. 4

–

(IO ) /0. mol 2

0. 0 0 10203040 0 10203040 S S 0. 8

–3 0. 6

0. 4

–

(I ) / mol

c 0. 2

0. 0 0 10203040 0 10203040 S S 1. 2

–3 1. 0

dm 0. 8

m 0. 6

–– 0. 4

(IO +I ) / mol r3 0. 2

c 0. 0 0 10203040 0 10203040 S S

Figure 7. Relationship between rationalized concentrations (cr) of iodate (¯,u), iodide ( ,l) and total inorganic iodine (r,p), and salinity at stations E2/E9 (left side graphs) and E3/E4a/E5 (right side graphs). White symbols refer to the late autumn/winter period, black symbols refer to the spring period. ized concentrations are plotted versus salinity (Figure 7) stations cannot be neglected. During both the late au- and the scattering of data points no more seems to be so tumn/winter and spring periods, iodate showed signifi- randomly distributed at low salinity end. Unlike to cantly higher reactivity than iodide at lower salinities, graphs where concentration is plotted versus salinity whereas variations in the more saline layer were less (Figure 5), the relationship between rationalized concen- pronounced. Iodide concentrations, on the other hand, tration and salinity implies a hyperbolic form when the were tightly coupled along an almost horizontal straight two end-member mixing model is applied, which bends line during spring, suggesting virtual absence of second- up or down according to either the positive or negative ary processes that might affect iodide concentration and intercept at zero salinity, respectively. If the intercept is point to simple dilution. During winter, the concentra- equal to zero, the hyperbola does not bend in any direc- tions were more variable, following the same upward tion, thus representing dilution processes.24 Though the trend as iodate in the upper layer in the lower estuarine intercept can barely represent the concentration at zero section (E3–E5). salinity (like in this case where negative values and large The role of the freshwater phytoplankton for iodate standard errors of the intercept were found), temporal concentration thus seemed more important than that of similarities in iodate and iodide distributions at various the marine phytoplankton, since the obtained variations

Croat. Chem. Acta 79 (1) 143¿153 (2006) IODATE AND IODIDE DISTRIBUTIONS IN THE WATERS OF A STRATIFIED ESTUARY 151

0.6 upper layer 0.6

0.5 0.5

–3

–3 dm 0.4 0.4

m 0.3 0.3

/mol

0.2 r 0.2 c

0.1 0.1

0.0 0.0

0.6 interfacial layer 0.6

0.5 0.5

–3

–3 dm 0.4 0.4

m 0.3 0.3

/mol

0.2 r 0.2 c

0.1 0.1

0.0 0.0

0.6 saline layer 0.6

0.5 0.5

–3

dm 0.4 0.4 dm

m 0.3 0.3

/mol

0.2 0.2 c

0.1 0.1

0.0 0.0 0 5 10 15 20 30 0 5 10 15 20 30 Distance / km Distance / km L1 L1 E0 E2 E9 E3 E4a E5 C1 E0 E2 E9 E3 E4a E5 C1 Station Station Figure 8. Longitudinal profiles of the average iodate (¯), iodide (l) and iodate+iodide (p) concentrations (c – left side graphs) and rationalized concentrations (cr – right side graphs) in the brackish, interfacial and saline layers in the Krka estuary (April 2001). were restricted to low salinities. In the freshwater/brack- depth profiles of rationalized iodate concentrations and ish layer, diatoms represent the most abundant phytoplank- pH. While iodide concentration maxima were mainly re- ton group and the depletion of iodate during spring may corded at the bottom of the interfacial layer, the iodate reflect biological uptake when nitrate is depleted,39 par- maxima were observed either at the surface or directly ticularly as at this concentration level of iodate even re- above the halocline. These iodate maxima agree well with duced biological uptake may cause the observed differ- pH maxima (which may indicate phytoplankton produc- ences. During winter, the phytoplankton biomass accu- tion) and were more pronounced during winter. Z-shaped mulates in the brackish layer, due to higher input of the distributions of rationalized iodate around the halocline Krka River nutritious salts. However, the phytoplankton were also observed during spring but these were far less activity is generally lower compared to that in the spring/ pronounced. The role and importance of freshwater phy- summer period. Indicators of the possible role of living toplankton in the production of inorganic iodine need to freshwater phytoplankton in iodate production were the be further investigated. Besides phytoplankton activity,

Croat. Chem. Acta 79 (1) 143¿153 (2006) 152 V. @IC AND M. BRANICA the processes including formation and degradation of or- iodine from organic matter. Results of the longitudinal ganic iodine, dissimilatory iodate reduction as well as the spring distributions of chlorophyll-a and its breakdown photochemical processes in the surface layer could have products in the Krka estuary33 (indicators of the physio- also contributed to the observed variations in iodate and logical status of the phytoplankton biomass) support iodide distributions in the upper layer.4 these explanations. Distribution of the most abundant degradation product (phaeophorbide-a) showed an increas- Longitudinal Distribution. – Longitudinal profiles of io- ing trend in the downstream direction and the highest con- date and iodide in three well separated water layers in centration in the brackish layer at the estuarine transect the estuary (sections E0–C1) revealed that both specia- from stations E4a to E5, where the chlorophyll-a con- tion and distribution of inorganic iodine were not concentrations were also higher. Allowing for the possibility trolled only by the simple dilution but also by the sec- that these longitudinal distributions in the brackish layer ondary, biochemical processes (Figure 8). Brief attention represent the typical pattern during spring, since iodide will be paid first to the saline layer where longitudinal was virtually uninvolved in these processes, iodate was distributions showed fewer variations and a rather produced either from phytoplankton or from organic smooth pattern, due to less pronounced phytoplankton matter. Most of the recent studies report that dissolved activity. Distributions in this saline layer indicated that organic iodine may be formed either from iodate or iodide beside iodate uptake/reduction and iodide production to- via biotic or abiotic mechanisms, but that the main prod- 3,4,7 ward the head of the estuary, a slight removal of total in- uct of its decomposition would principally be iodide. organic iodine also occurred. The presence of the marine Contrary to these findings, our results show that not only phytoplankton species capable of assimilating iodate and was iodate produced in the upper layers at the outermost releasing dissolved iodine in the form of iodide,2,40,41 estuarine stations, but also that the production of iodate such as the most abundant Skeletonema Costatum and exceeded that of iodide. Though it is less likely that iodate Chaetoceros sp.,31 has been confirmed in the Krka estu- was produced by bacterial or photochemical decomposi- ary. These results are consistent with our findings. Be- tion of the dissolved organic iodine, a contribution of se- side phytoplankton activity, the reduction of iodate at the quential oxidation processes should be considered. Also, boundary between anoxic sediment and oxic overlying the production of iodate at the outermost station due to a water could have also contributed to the observed longi- specific phytoplankton assemblage at higher salinities, de- tudinal variations in the more stagnant marine layer.42 veloped through additional nutrient input, cannot be re- Anoxic conditions in the sediment profiles were deter- jected. 43 mined at stations E2 and E9. Acknowledgements. – I owe great gratitude to my mentor, Differences between the concentration and rational- Professor Marko Branica, for guiding me all these years. I appre- ized concentration scales were higher in the interfacial ciate his efforts to teach me how to watch the sea as an explor- er. I also thank @. Kwokal for scuba diving to collect samples, and brackish layers, but an increasing trend toward the Dr. B. Raspor for instrumental support and Dr. D. Omanovi} estuarine mouth in the concentration of total inorganic for his help with the computer program. This work was sup- iodine can be obtained on both scales (Figure 8). In ad- ported by the Ministry of Science and Technology of the Re- dition to a specific distribution at the stations in the [i- public of Croatia, under Project No. 00981502. benik harbor, an extremely high concentration of iodide (0.09 mmol dm–3) was detected in the water sample at a depth of 2.0 m at station E9 (April 2001). This unex- REFERENCES pectedly high value above the halocline (S = 3.0) might be connected with the marked brownish layer that was 1. J. D. Smith and E. C. V. Butler, Nature 277 (1979) 468– observed by a diver at this water depth. One day earlier, 469. 2. T. A. Moisan, W. M. Dunstan, A. Udomkit, and G. T. F. at station E2 situated upstream, the concentrations of both Wong, J. Phycol. 30 (1994) 580–587. iodate and iodide were slightly enhanced at depths of 1.5 3. P. L. M. Cook, P. D. Carpenter, and C. V. Butler, Mar. Chem. m and 2.0 m, and 2.0 m, respectively. It seems that such 69 (2000) 179–192. an unevenly formed layer of organic and/or inorganic or- 4. G. T. F. Wong and X. -H. Cheng, Mar. Chem. 74 (2001) igin is capable of accumulating a significant quantity of 53–64. inorganic iodine. In the estuarine area situated downstream, 5. A. L. Rebello, F. W. Herms, and K. Wagener, Mar. Chem. which is affected by additional anthropogenic nutrient 29 (1990) 77–93. input (stations E4a, L1 and E5), the variations in iodate 6. W. J. Ullman, G. W. Luther III, R. C. Aller, and J. E. Mackin, Mar. Chem. 25 (1988) 95–106. concentration were also high in the brackish layer. Upon 7. G. W. Luther III, T. Ferdelman, C. H. Culberson, J. Kostka, presumption that the iodate uptake at station E3 was due to and J. Wu, Estaurine Coastal Shelf Sci. 32 (1991) 267–279. high freshwater phytoplankton activity, the production of 8. M. A. R. Abdel-Moati, Mar. Chem. 65 (1999) 211–225. inorganic iodine toward the estuarine mouth may reflect 9. G. W. Luther III and H. Cole, Mar. Chem. 24 (1988) 315– either an opposite process or the production of inorganic 325.

Croat. Chem. Acta 79 (1) 143¿153 (2006) IODATE AND IODIDE DISTRIBUTIONS IN THE WATERS OF A STRATIFIED ESTUARY 153

10. J. D. Smith and E. C. V. Butler, Mar. Chem. 28 (1990) 353– 27. G. T. F. Wong, Rev. Aquat. Sci. 4(1) (1991) 45–73. 364. 28. V. @uti} and T. Legovi}, Nature 328 (1987) 612–614. 11. M. Jura~i}, Ph.D. Thesis, University of Zagreb, Zagreb, 1987, 29. G. Cauwet, Mar. Chem. 32 (1991) 269–283. (in Croatian). 30. R. Sempéré and G. Cauwet, Estaurine Coastal Shelf Sci. 40 12. Z. Grèti}, A. [krivani}, and D. Vili~i}, Rapp. Comm. Int. (1995). 105–114. Mer. Medit. 30 (1986) 36. 31. D. Vili~i}, T. Legovi}, and V. @uti}, Aquat. Sci. 51 (1989) 13. T. Legovi}, Mar. Chem. 32 (1991) 121–135. 31–46. 14. Z. Grèti}, Ph.D. Thesis, University of Zagreb, Zagreb, 1990, (in Croatian). 32. T. Legovi}, V. @uti}, Z. Grèti}, G. Cauwet, R. Precali, and 15. T. Legovi}, Z. Grèti}, and V. @uti}, Mar. Chem. 32 (1991) D. Vili~i}, Mar. Chem. 46 (1994) 203–215. 163–170. 33. M. Ahel, R. G. Barlow, and R. F. C. Mantoura, Mar. Ecol. 16. @. Kwokal, unpublished results. Prog. Ser. 143 (1996) 289–295. 17. G. Kniewald, @. Kwokal, and M. Branica, Mar. Chem. 22 34. D. Fuks, M. Devescovi, R. Precali, N. Krstulovi}, and M. (1987) 343–352. [oli}, Mar. Chem. 32 (1991) 333–346. 18. J. R. Herring and P. S. Liss, Deep-Sea Res. 21 (1974) 777– 35. J. Saliot, Laureillard, S. Peulve, and J. Fillaux, Croat. Chem. 783. Acta 70 (1997) 323–332. 19. G. W. Luther III, C. B. Swartz, and W. J. Ullman, Anal. Chem. 36. T. Legovi}, D. Petricioli, and V. @uti}, Mar. Chem. 32 (1991) 60 (1988) 1721–1724. 347–359. 20. G. T. F. Wong and L. -S. Zhang, Mar. Chem. 38 (1992) 109– 37. V. W. Truesdale, Estaurine Coastal Shelf Sci. 38 (1994) 116. 435–446. 21. G. T. F. Wong and X. -H. Cheng, Mar. Chem. 59 (1998) 38. G. T. F. Wong and L.-S. Zhang, Estaurine Coastal Shelf Sci. 271–281. 56 (2003) 1093–1106. 22. H. Bilinski, @. Kwokal, and M. Branica, Water Res. 26 (1992) 39. Z. Grèti}, R. Precali, D. Degobbis, and A. [krivani}, Mar. 1243–1253. Chem. 32 (1991) 313–331. 23. V. W. Truesdale and R. Upstill-Goddard, Estaurine Coastal Shelf Sci. 56 (2003) 261–270. 40. G. T. F. Wong and X.-H. Cheng, J. Environ. Monit. 3 (2001) 257–263. 24. V. W. Truesdale and K. Jones, Con. Shelf Res. 20 (2000) 1889–1905. 41. G. T. F. Wong, A. U. Piumsomboon, and W. M. Dunstan, 25. M. Petek and M. Branica, Thalassia Jugosl. 5 (1968) 257– Mar. Ecol. Prog. Ser. 237 (2002) 27–39. 261 (and the references within). 42. V. W. Truesdale, D. S. Danielssen, and T. J. Waite, Estaurine 26. V. @ic and M. Branica, Estaurine Coastal Shelf Sci. 66 Coastal Shelf Sci. 57 (2003) 701–713. (2006) 55– 66. 43. V. @ic, unpublished results.

SA@ETAK

Raspodjele jodata i jodida u vodama stratificiranog estuarija

Vesna @ic i Marko Branica

Raspodjele kemijskih vrsta anorganskog joda u visoko stratificiranom estuariju rijeke Krke (isto~na obala Jadranskog mora) prou~avane su u razdoblju od prosinca 2000. do travnja 2001. godine. Kroz promatrano raz- doblje potvr|eno je konzervativno pona{anje ukupnog anorganskog joda, a uo~ene su i zna~ajne vremenske i longitudinalne razlike u raspodjelama. U slanijim slojevima (S ³ 10) koncentracije jodata i jodida su znatno manje varirale u usporedbi s onima u bo~atom sloju i me|usloju, {to ukazuje na slabiju dinamiku i slabije izra- `en utjecaj sekundarnih procesa na raspodjelu anorganskog joda. U gornjim slojevima utvr|en je porast kon- centracija jodata i jodida prema izlazu iz estuarija. Sezonske razli~itosti u raspodjelama jodata i jodida u bo~a- tom sloju pokazatelj su va`ne uloge rije~nog fitoplanktona i organske tvari za kru`enje anorganskog joda u estuariju.

Croat. Chem. Acta 79 (1) 143¿153 (2006)