Marine Chemistry, 29 (1990) 77-93 77 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
The Cycl ing of Iodine as Iodate and Iodide in a Tropical Es tuar ine Sys tem
A. DE LUCA REBELLO 1, F.W. HERMS 2 and K. WAGENER 1
1Departamento de Quimica, Pontificia Universidade Catdlica do Rio de Janeiro, 22453, Rio de Janeiro (Brazil) 2Departamento de Oceanografia, Universidade do Estado do Rio de Janeiro, 20550, Rio de Janeiro (Brazil)
(Received September 15, 1988; revision accepted September 27, 1989)
ABSTRACT
De Luca Rebello, A., Herms, F.W. and Wagener, K., 1990. The cycling of iodine as iodate and iodide in a tropical estuarine system. Mar. Chem., 29: 77-93.
The concentration of iodate and iodide were independently determined in seawater samples from Guanabara Bay (Rio de Janeiro, Brazil) taken at depths from 0.15 to 5 m (which was almost the bottom), at various times of day and in three different seasons.
The ratio of the two species varied between 0.3 and 3.9, and their concentrations changed at rates of the order of 10/zM h - 1. This is about two orders of magnitude faster than can be expected from fluxes from sediments. These rates of variation were highly correlated with biological param- eters such as the rates of photosynthesis and respiration, and concentration of phytoplankton. A model is presented which explains the diurnal cycling of iodine by biological activity, in agreement with the observed phenomena.
INTRODUCTION
The concentration of iodine, a minor component of seawater, is affected by processes in the marine environment which are not yet well understood.
Plaff (1825) was the first to detect iodine in seawater, and almost a century later Winkler ( 1916 ) determined 'total iodine' (iodate + iodide ) in waters from the Adriatic Sea and found their concentration to be around 0.3 #M, of which 80% was iodate. Since then, a number of studies have been made to estimate the concentrations and distribution of iodine species in seawater.
According to Sugawara and Terada (1957, 1958) and Johannesson (1958), the main species of iodine in seawater are iodate and iodide, although small quantities of molecular iodine and iodine organo-compounds may exist in sur- face waters.
Under normal conditions for surface waters (oxygen partial pressure 0.21
atm, pH 8.1 and temperature 20°C) the equilibrium concentration ratio of iodate to iodide should be 1019s. Thus, theoretically, iodide should not be de- tectable in seawater; however, measurable concentrations have been reported in the literature, which indicate the existence of a redox disequilibrium.
Organic iodine concentrations up to 0.04 pM can be found in coastal areas and could be responsible for the differences in iodate and iodide concentration encountered by several authors (Truesdale, 1975). This is because the analyt- ical procedures used in the determination of those species generally include either an oxidation or a reduction step.
The spatial variation of iodine species in seawater has been attributed to factors such as adsorption on metal oxides, release from sediments, terrestrial input and biological activity.
Barkley and Thompson (1960) concluded that iodine should have a con- servative behaviour and that variations in I/C1 ratios observed in coastal areas were because of assimilation of iodine by benthic algae. Tsunogai and Henmi (1971) recorded strong enrichment of iodide over iodate in surface tropical and sub-tropical waters, and concluded that iodide could he produced by active reduction in surface waters in association with high primary production and nitrate concentration, although biological activity could result in iodide consumption.
Organisms that reduce nitrate via the enzyme nitrate-reductase can also reduce iodate and contribute to the iodide pool of seawater (Tsunogai and Sase, 1969). However, according to Hichett (in Chapman, 1983), this reduc- tion will take place only when oxygen levels are very low and at high nitrate concentrations. Hirano et al. (1983) observed that some organisms, such as algae and fish, accumulate 10 times more iodide than iodate, but excrete iodate at higher rates. In an earlier study, Kuenzler (1969) suggested that iodide would be the predominant form of iodine excreted by zooplankton under natural conditions.
The nutrient-like behaviour of iodine and its possible association with bio- logical activity were shown by Wong and Brewer (1974), Wong et al. (1976) and Elderfield and Truesdale (1980). Wong and Brewer (1974) pointed out that iodate should have an important role in biological processes, and derived a constant ratio for Apparent Oxygen Utilization of O2" NO3: P04: IO3 = 2440:357:22.7:1 from their measurements in South Atlantic waters. Elder- field and Truesdale (1980) derived an assimilation ratio I/C of 1 × 10 -4 from experimental data and the Redfield relation.
(Note added in revision. After submission of this paper, two publications (Luther and Cole, 1988; Ullman et al., 1988) appeared, on the speciation and behaviour of iodine species in estuarine waters. Ullman et al. (1988) concluded that the major factors controlling iodine speciation and distribution in estu- aries include mineralization of organic iodine from sediments, anthropogenic
IODINE CYCLING IN A TROPICAL ESTUARINE SYSTEM 79
input, resuspension of material from sediments, estuarine flow regime and res- idence time. Luther and Cole presumed that biological processes may play a role in the interconversion of iodine species. However, neither paper produced any data on biological activity, nor any correlation between the conversion rates of iodine and biological activity. )
The present study was designed with the goal of improving the understand- ing of the processes that control iodate and iodide concentrations in estuarine waters. This study was conducted in Guanabara Bay, located in the heart of Rio de Janeiro. This bay is at present a eutrophic system where primary pro- duction can be as high as 2 g C m -2 d a y - ' (Rebello et al., 1988). Turnover rates are very high, as demonstrated by measured respiration rates of > 1.5 g C m -2 d a y - ' (Rebello et al., 1990). The total area of the bay is 380 km 2 and the average depth is 7.7 m. The tidal water exchange is of the order of 1700 m 3 s - ' and the runoff is approximately 140 m 3 s - ' . The average tidal amplitude is 0.69 m.
EXPERIMENTAL
A single sampling site was selected in Guanabara Bay (see Fig. 1), taking into account that previous works (Rebello et al., 1986, 1988) permitted this site to be well defined in terms of its physical-biochemical properties. Sam- pling was carried out on May 7, July 8 and November 13, 1987, to cover the dry and wet seasons. On each sampling date, water was collected from different depths and at several times of day (see Tables 2-4, below). The depth of 5 m corresponds almost to the bottom, and because of the turbidity of the overlying waters it is in the aphotic zone.
A 2-1 van Dorn bottle was used to collect the water samples, which were transferred to polyethylene bottles which had been pre-cleaned with nitric acid and bi-distilled water, for storage or measurement on board. The following parameters were determined: temperature, conductivity, dissolved oxygen, pH, pE (measured on board), seston, pigments (filtration on board), nitrate, ni- trite, soluble phosphate, iodate and iodide.
Nutrient concentrations were determined on the day of sampling, using the methods described by Grasshoff et al. (1983). Dissolved oxygen was ti trated according to the modified Winkler method (Grasshoff et al., 1983 ), and seston and pigments were determined following the method of Strickland and Par- sons (1972). Primary production of surface waters was measured during the second sampling, using the oxygen method and in situ incubation. Samples for determination of iodate and iodide were stored at - 20 ° C in polyethylene bot- tles after filtration through 0.45-/nn Millipore filters. The maximum storage time was 2 months, although experiments had shown that longer periods did not cause detectable concentration changes.
Iodate was determined using differential pulse polarography, basically as proposed by Herring and Liss (1974). To obtain the maximum sensitivity a
80 A. DE LUCA REBELLO ET AL.
NITEROI N
w £
oO . . . . . .
OCEAN 5 km
^NTI C
Fig. 1. Sampling point in Guanabara Bay, Rio de Janeiro.
TABLE 1
Primary production data for the second sampling (depth 0.15 m)
ho hf 02 lib. 02 cons. GP GP (h:min) (h:min) (ml 1-1 ) (mll -l ) (ml I -I ) (gC m -3 h -I )
borate/sodium hydroxide buffer was added to the samples, and pH maintained at 8.2. The analyses were performed in a 384B PAR polarographic analyser coupled to a 303A PAR electrode using the following instrumental parameters: DME, initial potential -0 .700 V, final potential - 1.300 V, drop time 1 s, pulse modulation 75 mV, scan rate 14 mV s-1, purging time 20 min. Iodide was de- termined using a cathodic stripping voltammetry method developed by Herms (1988): to a 5 ml sample were added 10/zl of 0.1 M EDTA solution, 10/A of 5% sulphamic acid and 10/~l of 0.05 M sodium hydroxide solution, to give a final pH of 6.0. The instrumental parameters were: HMDE, initial and deposition potential 0.020 V, final potential -0 .610 V, pulse time 0.5 s, deposition time 180 s, equilibration time 15 s, pulse modulation 100 mV, scan rate 4 mV s-1, purging time 20 rain. The detailed description and discussion of this method will be published elsewhere.
All iodate and iodide analyses were run in triplicate and the concentrations were determined after three standard additions. Peak potentials for iodide were - 0.084 + 0.013 V vs. Ag/AgCl and for iodate - 1.054 + 0.011 V vs. Ag/AgCl. The analytical sensitivity varied from 71.9 to 200.5 nA/~M - 11 min - 1 for iodide and from 14.69 to 44.60 nA/zM- 1 IOn- for iodate. The sensitivity of both de- terminations was closely related to changes in the content of surfactants and other organic substances. The standard deviation for the iodate determination was 3.05 + 1.35%, and that for iodide was 2.96 + 2.03%. The estimated limit of detection for estuarine waters was better than 15 nM.
The results for all species determined, for the three samplings, are summa- rized in Tables 1-4.
DISCUSSION
Results of the different samplings were evaluated separately, as the tidal regimes were different on each occasion. During the first sampling, the tidal amplitude was at a normal level (tidal velocity was -0.08 m h-i); during the second sampling, high tidal amplitudes were observed (tidal velocity was ÷ 0.16 m h- i up to 2:00 p.m. and - 0.13 m h- i from 2:00 to 4:35 p.m.); and during the third sampling, tidal amplitudes were very small (tidal velocity was zero from 10:00 to 12:00 a.m. and - 0.04 m h- i from 12:00 to 4:00 p.m. ). Considering the shallow depth of the selected site, some influence of tidal characteristics on mixing processes and consequently on the measured parameters should be ex- pected. Weather conditions were similar on all occasions: sunny days preceded by a dry week. Temperatures were higher by 2 or 3 ° C in the last sampling.
Although the data obtained in the first sampling are sufficient to allow some observations on the behaviour of iodine species in the system studied, they are not included in the interpretations given below, because the set was incomplete.
IODINE CYCLING IN A TROPICAL ESTUARINE SYSTEM 87
Evaluation of the data from the second sampling
The concentrations of iodide and iodate showed large variations with both time of day and depth; this could be due to salinity changes.
As Table 5 shows, a good linear correlation was found between the average
TABLE 5
Results of linear correlation analyses of the iodate and iodide concentrations with various parameters
Parameter Unit for slope Iodate Iodide
Slope Corr. Slope Corr. coeff, coeff.
July 1987 Salinity /IM (rag 1-1 ) -1 0.020 0.935 -0 .010 0.765 Diss. 02 /zM (ml 021-1) -1 -0 .011 0.870 0.005 0.820 Chl. A /zM (mg 1-1)-1 -0 .503 0.920 0.231 0.790 pH /zM/zipH= 1 -0 .116 0.920 0.056 0.820 Seston /zM (rag l - 1) - 1 - 0.004 0.940 0.002 0.870 Time /~M h - ~ - 8.40 0.920 2.73 0.530 0.15-m depth Time /LM h - 1 2.75 0.630 - 11.96 0.910 5.0-m depth I-/(I- +IO~- ) h -1 - 0.0332 0.901 0.15-m depth l-/(I- +IO~-) h -I - - -0.0328 0.918 5.0-m depth Variation of iodine /Imol/ml- 1 02 0.003 0.108 - 0.030 0.936 species with rated respiration Variation of iodine ] ano l /ml - 1 02 - 0.022 1.000 0.012 0.453 species with photosynthesis rate
h, time between subsequent sampling and incubation time (h).
88 A. DE LUCA REBELLOETAL.
iodate concentrations for each depth and salinity; the correlation for iodide was poorer. As the salinity variations with time were in general small for a given depth, the influence of this parameter on the variations of the iodine species over time was almost negligible.
Following the hypothesis that the distribution of ionic species of iodine is related to biological processes, linear correlations were calculated between io- dide and iodate concentrations and the parameters associated with biological activity. Table 5 shows the results of this exercise and makes clear the impor- tance of biological processes on the species studied.
The measurements of photosynthesis and respiration rates performed at 0.15 m allowed a direct verification of the previous statement. Table 6 gives the data used to calculate the linear correlations shown in Table 5. These re- sults indicate a strong dependence of iodide on respiration, whereas iodate is shown to be related to the photosynthetic process. With this knowledge, the next step was to evaluate and compare surface data (0.15 m ), representing the result of photosynthesis and respiration, and bottom data (5.0 m), which would almost solely result from respiration.
A linear correlation was calculated using the relation I - / ( I - + IO~ ) at 0.15 and 5.0 m and the sampling time. The results shown in Table 5 indicate a relation between liberation of iodine in surface waters and consumption in the bottom waters.
The rates of production and consumption for the individual species are listed in Table 5. These rates characterize net production of iodide and consumption of iodate in reactions occurring in the presence of light and the inverse process in the absence of light.
Evaluation of the data from the third sampling
As in the second set of data, the influence of salinity on the values obtained for iodide and iodate was verified, and the results are shown in Table 5. Here
T A B L E 6
Linear regression parameters for the relat ion of iodide and iodate vs. dissolved oxygen, chlorophyll A, pH and seston
Correlation Slope Correlation coefficient (r)
I- vs. 02 I - vs. Chl. A I - vs. pH I - vs. seston IOn- vs. 02 IOn- vs. Chl. A IO~ vs. pH IOn- vs. seston
0.005 gmol I - m l - 1 02 0.820 0.231/lmol I - mg -1 Chl. A 0.790 0.056 ~mol I - (uni t p H ) - 1 0.820 0.002 #mol I - m g - l seston 0.870
- 0 . 0 1 1 #tool IO~ ml -~ 02 0.870 -0 .503/Lmol IOn- mg -1 Chl. A 0.920 - 0.116/1tool IOn- ( un i t pH ) - 1 0.920 - 0.004 ~mol IO~ rag- ~ seston 0.940
IODINE CYCLING IN A TROPICAL ESTUARINE SYSTEM 89
also, changes in salinity are not predominantly responsible for the variations of iodide and iodate concentrations with time and depth.
Table 5 shows the results of calculated linear correlations between bio-as- sociated parameters and the entire set of iodide and iodate data. In this case, both iodate and iodide are inversely related to the parameters under consid- eration. This differs from the second sampling.
The results for the production and consumption rates, shown in Table 5, also indicate iodide behaviour different from that found in the previous sam- piing. This can be understood by considering the data for oxygen, chlorophyll and pH. The situation described in former works (Rebello et al., 1988) is that encountered in the second sampling, where oxygen is oversaturated in surface waters and depleted in the bottom waters. During the day, pH increases dras- tically, reaching values beyond 9 in the early afternoon.
In the third sampling, surface oxygen concentrations were extremely low in the morning, despite both chlorophyll and seston values being larger than in the second sampling. Similarly, pH was lower, and increased slowly during the day. The concentration of pigments in the surface waters decreased from morning to afternoon; a similar trend was present in the values for bottom- water oxygen.
Such conditions can be found the day after an algae bloom, for example, and are also favoured by reduced horizontal mixing due to the low tidal amplitude prevailing during this sampling. The high pigment concentrations found in the morning were certainly derived from dead and old cells. The presence of this material increased the water turbidity, slowing down the process of photosyn- thesis and favouring both autotrophic and heterotrophic respiration.
CONCLUSIONS
The use of direct and independent methods of determination for iodate and iodide without including the usual oxidation or reduction step was a pre- requisite to obtain the results presented here. The derived rates of consump- tion and production for both species were of the order of 10 nmol h- i and could have been overlooked if any organically bound iodine had been converted into the ionic forms during sample pretreatment.
The changing water masses in the open system may have resulted in lower correlations.
Although correlations have been demonstrated between iodine species and parameters such as 02 and chlorophyll concentration, no co-variations with nutrient constituents were encountered. This is to be expected for a system such as Guanabara Bay, where high turbidity of waters and not nutrients are the limiting factor for primary production. The observed variations in nutrient concentration were due to the input of sewage, remobilization from sediments and suspended material (Rebello et al., 1988), and mixing of waters.
90 A. DE LUCA REBELLO ET AL.
~PHYTOPLANKTON " ~ j ZOOPLANKTON
L IGHT DARK REACTION REACTION
I I ,
J [.
SMALL F ISHES
i OETR'TUS B TER'A ]
~ F
POOL OF INORGANI~ IODINE : IO'3 ~ I--
Fig. 2. Interpretation of the measured field data on iodine species in the water of Guanabara Bay: mineral cycling via the action of biological processes.
The measured pE values were not included in the above discussion, as they could not be related to any of the other parameters of interest. In such a com- plex estuarine system, pE is certainly controlled by a number of reactions not identified here. At the measured pE values, iodide concentrations should be orders of magnitude higher than iodate values. The ratio of the two species, however, varied between 0.3 and 3.9, independently of pE.
The total iodine balance made up from the observed rates of variation of iodide and iodate at the surface and at the bottom gives, for the second sam- piing, a consumption rate of iodine of 14.88 nM h - 1, and, for the third sam- pling, a production rate of iodine of 13.39 nM h - 1. This mass balance indicates that the concentration of iodate and iodide is intimately dependent on the predominance of photosynthesis and autotrophic and heterotrophic respira- tion. Other processes, such as photo-oxidation or photoreduction to I2 and re- lease from sediments, play a marginal role in this estuarine system.
Concerning the release of iodine from sediments, it was possible to estimate from known respiration rates (Rebello et al., 1990) and an I /C ratio of 1 × 10 -4, a flux of 10 Hmol I m - 2 day- 1. For example, using the observed production rate
IODINE CYCLING IN A TROPICAL ESTUARINE SYSTEM
TABLE 7
How well does the model describe the measured data?
91
Conditions observed Consequences according to model
Measured reaction rate of I- and IO~ (nMh -1)
July 1987 Surface (0.15 m)
High primary production: 12-14 ml 021-1 200-243 mg Chl. A m - 3
Bottom 2-3 ml 021 -I (5m) 39-26 mg Chl. A m -~
November 1987 Surface Low primary production: (0.15 m) 2.4-7 ml 021-1
297-71 mg Chl. A m -3
High consumption of IOn- ; I- produced from zooplankton compensates largely I- consumption of algae
I- consumption from algae respiration; IO~ production from bacteria
Low consumption of IO~ ; consumption of I- from algal respiration
- 8 . 4 0 I O ~
+ 2.73 I-
- 1 1 . 9 6 I -
+ 2 . 7 5 IOn-
- 2 .01 IO~-
- 7 . 9 5 I -
Bottom 0.8-1.2 ml 02 l- 1 I- production from zooplankton; + 15.56 I- (5 m) 16-19 mg Chl. A m-3 IO~ production from bacteria + 7.79 IO~
of iodide in the bottom waters (considering a 1-m layer at the bottom) for the third sampling (see Table 7) yields
15.56 nM h-1 × 1000 1 m - 2 × 2 4 h=373 #mol m -2 day - '
This is about 40 times more than can be expected as flux from the sediments. The completeness of measured biological and chemical data allowed the for-
mulation of a biological model for iodate and iodide cycling. The basic facts are summarized in Table 5 as results of linear correlation analyses between the concentrations of iodate and iodide and each of the measured parameters. Of primary importance are those correlations with a steep slope (dependence) and correlation coefficients close to one. Those correlations gave the basis for the model shown in Fig. 2.
This model describes well all observed situations, as summarized in Table 7. The outcome is not surprising, although it has not been discussed before: turn- over rates which are about 40 times higher than diffusion-controlled reactions (release from sediments ) can only be of biological nature.
92 A. DE LUCA REBELLO ET AL.
ACKNOWLEDGEMENTS
T h e a u t h o r s a c k n o w l e d g e t h e v a l u a b l e c o l l a b o r a t i o n o f S t a n v a n d e n B e r g a t t h e b e g i n n i n g o f t h i s work , a n d t h e f i n a n c i a l s u p p o r t o f F I N E P , C N P q a n d C I R M .
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IODINE CYCLING IN A TROPICAL ESTUARINE SYSTEM 93
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