Seasonal metal remobilization in the sediments of Great Bay, New Hampshire

Marine Chemistry, 15 (1984) 173--187 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

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SEASONAL METAL REMOBILIZATION IN THE SEDIMENTS OF GREAT BAY, NEW HAMPSHIRE MARK E. HINES

Jackson Estuarine Laboratory, University of New Hampshire, Durham, NH 03824 (U.S.A.) WM. BERRY LYONS, PETER B. ARMSTRONG, WILLIAM H. OREM*, MARY JO SPENCER and HENRI E. GAUDETTE

Department of Earth Sciences and Ocean Process Analysis Laboratory, University of New Hampshire, Durham, NH 03824 (U.S.A.) GALEN E. JONES

Department of Microbiology, University of New Hampshire, Durham NH 03824 (U.S.A.) (Received July 21, 1983; revision accepted April 24, 1984)

ABSTRACT Hines, M.E., Lyons, W.B., Armstrong, P.B., Orem, W.H., Spencer, M.J., Gaudette, H.E. and Jones, G.E., 1984. Seasonal metal remobilization in the sediments of Great Bay, New Hampshire. Mar. Chem., 15: 173--187. Concentrations of dissolved iron, manganese, molybdenum, copper, and organic carbon (DOC) were measured in the pore waters from surficial sediments of a temperate estuary to delineate seasonal metal remobilization from 1978 through 1980. Iron and DOC data were collected for 31 months and covaried inversely and exponentially. Iron dissolution occurred during the spring and during periods of active bioturbation with concentrations as high as 18 mg 1-1. Iron values were low during winter due to oxidation to ferric oxides. The lack of active bioturbation during the summer of 1978 allowed for the nearly complete removal of iron as a monosulfide precipitate. However, bioturbation resumed during the summer of 1979 and 1980 and dissolved iron concentrations as high as 10 mg 1-1 were observed at those times. The iron and DOC data were a qualitative measure of bioturbation activity. Dissolved manganese, molybdenum, and copper data were collected for 18 months during 1978 and 1979. All three metals displayed spring maxima covariate with iron, suggesting that they behaved chemically like iron and/or were associated with iron- or manganese-rich phases during this time of the year. In general, manganese and m o l y b d e n u m varied temporally with iron while copper concentrations mimicked iron variations only during the spring.

INTRODUCTION

Organic-rich nearshore marine sediments support large and active populations of microorganisms (Fenchel and Blackburn, 1979). Because of the rapid deposition of these sediments, decomposition of organic matter takes *Present address: United States Geological Survey, 923 National Center, Reston, VA 22092, U.S.A.

0304-4203/84/$03.00

© 1984 Elsevier Science Publishers B.V.

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place under anaerobic conditions, mainly via sulfate-reduction processes {J~irgensen, 1982). During sulfate reduction there is a net production of inorganic nutrients (Richards, 1965; Berner, 1977; Martens et al., 1978)and dissolved organic matter (Nissenbaum et al., 1972; Krom and Westrich, 1980). Therefore, the concentrations of these pore water components generally increase with depth in anoxic marine sediments and often demonstrate an inverse and stoichiometric relationship with sulfate concentrations (Goldhaber et al., 1977; Martens et al., 1978; Rosenfeld, 1979). The production of reduced sulfur compounds during sulfate reduction has a controlling influence on the chemistry of trace metals in anoxic sediments (Berner, 1970; Goldhaber and Kaplan, 1974; Elderfield et al., 1981). In clastic marine sediments, the high iron concentrations result in the precipitation of large quantities of iron-sulfide minerals (Berner, 1969, 1970; J~irgensen, 1977). In temperate environments, where temperature and microbial activities vary seasonally, a proportion of precipitated iron-sulfide minerals (i.e., FeS, Fe2S3) are reoxidized during colder months due to oxygen penetration in the absence of rapid microbial activity (J¢rgensen, 1977; Aller, 1977). As temperature and microbial activity increase in the spring, the production of reduced end products reduces Fe 3÷ to Fe 2÷, causing a dissolution and remobilization of iron (Hines et al., 1982). This iron reduction may occur by direct reduction by bacteria (SCrensen, 1982; Jones et al., 1983). Iron, and possibly other trace metals, undergoes seasonal dissolution and precipitation reactions as a result of changing redox regimes. Seasonal cycles of pore water and metal chemistry in sediments are complicated by the activities of infaunal organisms which influence redox conditions and microbial activities through reworking and irrigation (Aller, 1977; Goldhaber et al., 1977; Yingst and Rhoads, 1980). This results in subsurface recycling of iron and sulfur which remobilizes iron for precipitation of reduced sulfur compounds (Hines et al., 1982; Hines and Jones, 1984). Even when sulfide production is rapid, bioturbation can result in a net increase in the concentration of dissolved iron. Conversely, bioturbation is responsible for the removal of several dissolved chemical species in pore waters due to facilitated bioadvection of material into the overlying water (Aller, 1977; Aller and Yingst, 1978). In this communication we present data demonstrating the large seasonal variations in estuarine pore water chemistry during a 2.5 year period. Emphasis is directed toward the effects of varying microbial metabolic activities and bioturbation on the sedimentary cycling of trace metals. METHODS

Sampling Sediment samples were collected from a shallow water location (about 0.5 m deep at mean low tide) near the Jackson Estuarine Laboratory (JEL)

175 in Great Bay Estuary, New Hampshire, U.S.A. (Fig. 1). The sediments consisted of clay and silt-sized particles interspersed with fine sand. These sediments are bioturbated throughout the upper 10 cm from June into December (Hines et al., 1982; Hines and Jones, 1984). The dominant macroorganisms present are the capitellid polychaete Heteromastus filiformis and the tellinid bivalve Macoma balthica (Black, 1980). Sediment samples were taken by hand at low tide using a Plexiglas box corer. Cores were extruded in a N2-filled glove bag and sliced into 2-cm thick horizontal sections which were placed into pre-cleaned 250-ml linear polyethylene centrifuge bottles. After centrifugation at 5000 g for 1 h at ambient temperature, pore water was filtered under N 2 using acid-cleaned 0.4-#m Nuclepore filters and polycarbonate filtration units. Samples for metal analysis were acidified with Ultrex nitric acid to a final concentration of 1% and stored in precleaned conventional polyethylene or linear polyethylene bottles. Extreme precautions were taken to minimize metal contamination during sample handling and analysis. All plastic ware used in trace metal sample storage and analysis was cleaned in a fashion similar to that outlined by Patterson and Settle (1976). Samples for dissolved organic carbon (DOC) analysis were stored frozen (--80°C) in precombusted (450°C)test tubes.

Fig. 1. Map depicting the location of the JEL Sampling site in Great Bay, New Hampshire.

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An inert atmosphere was used throughout sediment and pore water processing to minimize oxidation of reduced species.

Analysis Pore waters were analyzed colorimetrically for dissolved iron using the ferrozine method (Stookey, 1970) as modified b y Murray and Gill (1978), with a precision of + 1.0% at 2.0 mg 1-1. Dissolved manganese was analyzed colorimetrically (Goto et al., 1962) with a precision of +3% at a concentration of 500 pg 1-~ for smaller volume pore water samples (Armstrong et al., 1979). Dissolved m o l y b d e n u m was analyzed by electron paramagnetic resonance spectrometry (Contreras et al., 1978) with a precision of -+12% at a concentration of 20ttg 1-] . Dissolved copper was analyzed by flameless atomic absorption s p e c t r o p h o t o m e t r y using a Perkin-Elmer instrument with standard addition techniques and background correction, with a precision of ~ 1 0 % at all concentrations encountered. The majority of the DOC was analyzed on a Sybron-Barnstead PHOTOchem organic carbon analyzer with potassium hydrogen phthalate as a standard (Orem, 1982), with a precision of ~ 3 . 0 % at 1 2 m g l -~ DOC. Samples collected early in the study (1978) were analyzed for DOC using the hot combustion technique of Van Hall et al. (1963). RESULTS AND DISCUSSION

Temporal variations in dissolved iron and DOC Figure 2 depicts variations in the concentrations of dissolved iron and DOC in the upper 6 cm o f J E L sediments over the 2.5 year study period. Average values of three 2-cm horizontal sections are presented to demonstrate the breadth of the variations noted. Although these solutes displayed

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vertical variability (Fig. 3), the temporal trends noted for averaged data (Fig. 2) were similar to trends using data from individual depths. Iron and DOC covaried inversely and exponentially throughout the year (p < 0.05). The concentrations of these solutes were extremely low in the overlying waters and considered insignificant compared to the concentrations encountered in the sediments. Since the dissolved concentrations of iron and organic carbon are affected strongly by microbial activity and bioturbation, they were instructive in interpreting the interactions among biogeochemical processes and metal remobilization in these sediments. The chemistries of these parameters are complicated due to the occurrence of competing dissolution and precipitation processes which are often mediated by oxidation and reduction reactions. For example, iron remobilization can occur from the mineralization of organic compounds (Volkov and Formina, 1974; Fenchel and Blackburn, 1979; Matisoff et al., 1981), the reduction and dissolution of metal oxides (Berner, 1969; Murray and Gill, 1978), and the oxidation of metal sulfides (Aller, 1978; Elderfield et al., 1981). Iron removal is a result of the precipitation of iron sulfides during sulfate reduction or the precipitation of iron oxides and oxyhydroxides when exposed to oxygenated conditions (Goldhaber and Kaplan, 1974; J~rgensen, 1977). Causes of DOC production and consumption are complicated because of the complex nature of sedimentary organic matter and the preferential uptake and dissolution of specific organic compounds. However, it appears clear that net DOC production occurs during periods of active sulfate reduction while decreases in DOC concentrations occur once sulfate reduction diminishes and sediments become more oxidizing in character (Krom and Westrich, DISSOLVED IRON (rag/I) 0

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1980). Furthermore, bioturbation tends to enhance the removal of solutes like DOC (Aller, 1977) while causing increases in the concentration of dissolved iron relative to non-bioturbated sediments (Hines et al., 1982). The processes responsible for iron and DOC consumption and production continue throughout the year. However, the magnitude and depth distribution of reactions vary greatly seasonally. Hence, the depth profiles of iron and DOC in Fig. 3 are due to vertical and temporal changes in the ratio of production and removal rates for these solutes as well as to temperatureregulated changes in the vertical extent of oxygen penetration into the sediments. Average temporal changes depicted in Fig. 2 reflected the result of the dominant processes which occurred between sampling periods. Even though competing reactions occurred year round, the data in Fig. 2 revealed the seasonal changes in the importance of certain biogeochemical processes and allowed us to generate the following scenario of the temporal regulation of dissolved iron concentrations in these sediments. Dissolved iron concentrations were low during winter, particularly in 1979 (Dec. and Feb.) and 1980 (Feb. and March; Fig. 2). Conversely, DOC was generally abundant during winter. The low dissolved iron concentrations were probably a result of increased sediment oxidation and precipitation of ferric oxides or oxyhydroxides (JOrgensen, 1977). These sediments were orange-brown in color throughout the upper few cm during winter. The occurrence of FeS oxidation during winter was also apparent from differences in the acid-volatile sulfide content of these sediments between winter and summer (Hines and Jones, 1984). The cause of the high winter DOC concentrations was not clear. However, as will be discussed later, the large winter DOC concentration may have been due to seasonal variations in bioturbation. Dissolved iron concentrations increased rapidly during early spring. This was demonstrated in May 1978 and May/April 1980 (Fig. 2). Concomitant with the increased iron levels was a sharp decrease in DOC concentrations. DOC was not measured during the winter of 1979. The close time interval sampling of 1980 clearly demonstrated the DOC decrease in early spring. In addition, DOC decreased from 46 to 7 mg 1-1 from March to early May, 1978. Sulfate reduction activity in these sediments does not usually increase until mid April (Hines et al., 1982). Therefore, the increasing temperature in March and April may have stimulated heterotrophic activity other than sulfate reduction, thereby consuming DOC, producing reduced end products, lowering Eh, and remobilizing iron by the reduction and dissolution of iron oxyhydroxides to Fe2÷(aq). Lyons et al. (1979) reported that a greater percentage of the dissolved iron present during the 1978 spring maximum was associated with DOC than iron sampled during other times of the year. We are not sure what role direct bacterial reduction of iron plays in creating the spring iron maxima. SOrensen (1982) and Jones et al. (1983) suggested that microbial iron reduction is a significant process in sedimentary environments.

179 During late spring (May--June), iron was partially removed from solution and DOC concentrations increased (Fig. 2). This was seen during 1978 and 1980. Increased sulfate reduction was u n d o u b t e d l y responsible for these events as iron was removed in a sulfide phase and net DOC production occurred. Measurements of sulfate reduction rates using 3sS (Hines et al., 1982) and the accumulation of acid-volatile sulfides in these sediments (Hines, 1981) demonstrated the increased rate of sulfate reduction during May and June. The lag in the onset of sulfate reduction and, therefore, sulfide precipitation during the spring allowed the net remobilization of iron to occur. However, this event was short lived. The influence of sulfate reduction on dissolved iron removal and DOC production was seen clearly during the summer o f 1978 when dissolved iron concentrations were below our detection limit and DOC exceeded 50 mg 1-1 (Fig. 2). Sulfate reduction in J E L sediments is approximately 10-times more rapid during summer than winter (Hines et al., 1982). A similar temporal increase in sulfate reduction rate was reported b y J¢rgensen (1977) for Danish estuarine sediments. Iron and DOC values for the summers of 1979 and 1980 revealed trends opposite to those found in 1978. Iron concentrations were as high as 8-10 mg 1-1 during July of 1979 and 1980 while DOC was less than 5 mg 1-1 (Fig. 2). Differences in bioturbation intensity appeared to be the best explanation o f this drastic reversal in DOC and iron concentrations. X-radiographs of box cores taken at this site in 1978 showed that even though bioturbation was observed during early summer, maximum activity was n o t apparent until the fall (Hines and Jones, 1984). However, bioturbation commenced rapidly in June during 1979 and 1980 and remained active throughout both summers (Hines et al., 1982; Hines and Jones, 1984). The absence of significant bioturbation during June and July 1978 was probably due to the severity o f the preceding winter when several storms and t w o major blizzards occurred. Yeo and Risk (1979) described the long-term impact of violent storms on shallow water and intertidal infauna, noting that infaunal populations removed by storm events did n o t recolonize to former levels until as much as a year later. The following two winters were mild compared to 1978 and summer bioturbation was active. As stated above, the tendency o f bioturbation to facilitate the removal of dissolved components from pore waters is well d o c u m e n t e d (Aller, 1977; Callender, 1982). Hence, net removal of DOC occurred during the summers in which bioturbation was active. Conversely, infaunal reworking and irrigation activities tend to maintain high dissolved iron concentrations in the presence of rapid sulfide production by enhancing iron and sulfur cycling (Aller, 1977; Hines and Jones, 1984). Bioturbation in Great Bay sediments during 1979 and 1980 overwhelmed the processes regulating iron and DOC concentrations during 1978. Therefore, our chemical data may have demonstrated relative annual changes in infaunal abundance and, hence, bioturbation in these Great Bay sediments. Although the removal of surface sediments during storms must

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have had a profound influence on the subsequent types and rates of biogeochemical processes at this site, regardless of whether or not infauna were removed, the relative changes in the concentrations of the solutes studied were explained using the present seasonal framework. The interactions between sulfate reduction, bioturbation and the temporal variations in iron and DOC concentrations were readily apparent from the close time interval sampling during the spring and summer of 1980. Details of these relationships and further microbiological data were published previously (Hines et al., 1982) and demonstrated, among other things, that the large dissolved iron maximum in June 1980 was a result of the very rapid onset of bioturbation which occurred that year. During the fall, bioturbation and microbial acitivity decreased with temperature. This time of year marked the transition from biologically active conditions to a situation dominated more by diffusion. The anomalous increase in DOC during fall and winter may have been due simply to relative changes in the rate of removal of DOC by bioturbation and production of DOC by sulfate reduction. This net increase in DOC concentration during fall and winter completed the trend where DOC and iron covaried inversely. It was not surprising to observe an inverse relationship between these parameters during periods of the year when sulfate reduction and/or bioturbation displayed a dominant influence on the sediments. However, the inverse relationship also occurred during winter and early spring despite the absence of significant bioturbation and sulfate reduction. To summarize the seasonal remobilization of iron in Great Bay, Fig. 4 presents an idealized annual trend of dissolved iron in bioturbated and nonbioturbated estuarine sediments presenting the reactions which dominate changes in dissolved iron concentrations throughout the year. The nonbioturbated model is not only from Fig. 2 but also from data collected from a non-bioturbated site in Great Bay. This latter site is discussed in detail in Hines and Jones (1984). In general, net iron mobilization occurred twice per year; first during the spring and second during the entire summer if bioturbation occurred. As demonstrated in Fig. 2 the idealized trend in Fig. 4 is subject to large annual FeOOH

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variations and sensitive to weather events in many situations. We have depicted low winter dissolved iron levels in this model because of the low concentrations encountered during the winters of 1979 and 1980 (Fig. 2). However, as pointed out by others (Aller, 1978; Elderfield et al., 1981), metals may be remobilized during winter by the oxidation and dissolution of metal sulfides. The seasonal scenario depicted for iron in Fig. 4 will form the basis for the following discussion concerning the mobilization of other metals in these sediments.

Copper, manganese, and molybdenum Figure. 5 depicts the temporal variations in dissolved copper (Cu), manganese (Mn), and molybdenum (Mo) in JEL sediments. These values are for the upper 2 cm of sediment only since the majority of the temporal changes occurred in this uppermost section. The concentrations of these metals were not determined in samples collected after April 1979. The copper and manganese values are within ranges reported for subtidal, surficial sediments in other temperate estuaries (Holdren et al., 1975; Aller, 1977; Murray et al., 1978; Elderfield et al., 1981). The molybdenum values are similar to those reported previously for a 40-cm core from Great Bay (Contreras et al., 1978) and those observed in pore waters from the top few cms of anoxic sediments from the Gulf of California (Brumsack and Gieskes, 1983). The highest concentrations of dissolved copper, manganese, and molybdenum occurred simultaneously with the spring 1978 iron maximum shown in Fig. 2. Manganese concentrations varied seasonally in a manner nearly identical to iron (Fig. 5). This was not surprising considering the similarity between redox cycling of these metals (Berner, 1980). However, manganese is "~3,0

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reduced more easily and oxidized less readily than iron and probably exists mostly in a solid phase separate from iron in oxidizing sediments (Stumm and Morgan, 1970). It is possible, therefore, that the mobilization of manganese in the spring precedes iron remobilization. However, this was not observed in our Great Bay samples probably because they were collected at intervals of several weeks. Dissolved manganese removal during the summer of 1978 occurred simultaneously with iron, except that manganese concentrations remained at 1.0 mg1-1 in June and July while iron values were much lower. This discrepancy is due to the fact that manganese concentrations in anoxic surficial marine sediments are apparently controlled by the solubility of Mn--Ca-carbonate as opposed to iron concentrations which are controlled primarily by the solubility of sulfide minerals (Berner, 1969; Goldhaber and Kaplan, 1974; Elderfield, 1976; Elderfield et al., 1981). The similarity of the seasonal trends for manganese and iron during the remainder of the study period suggested that manganese is remobilized during bioturbation {Sept. and Nov. 1978, Fig. 5). Dissolved molybdenum concentrations varied similarly to manganese and iron throughout most of the study period (Fig. 5). Factors controlling the deposition and dissolution of molybdenum in sedimentary environments are less clear than those controlling manganese and iron. Several mechanisms have been proposed which explain molybdenum chemistry in anoxic environments including: (a) coprecipitation with manganese oxide in oxidized sediments (Berrang and Grill, 1974); (b) association with high molecular weight organic matter (Jones, 1974; Nissenbaum and Swaine, 1976; Brumsack and Gieskes, 1983); (c) coprecipitation with ferrous sulfide (Bertine, 1972; Volkov and Formina, 1974); (d) remobilization from organic matter during organic decomposition (Contreras et al., 1978); (e)remobilization from ferrous sulfide during crystallization of pyrite due to the large difference in ionic radii between Mo 6 ÷ and Fe 2+ (Volkov and Formina, 1974); (f) adsorption and precipitation with almost any solid phase (Krauskopf, 1956). In general, molybdenum geochemistry appears to be related to the geochemistries of iron and manganese (Berrang and Grill, 1974; Pilipchuk and Volkov, 1974; Contreras et al., 1978). The molybdenum maximum during May 1978 (Fig. 5) may have been due to the dissolution of iron hydroxide and/or manganese oxide and solubilization of both metals. However, this mechanism cannot totally explain this remobilization since molybdenum concentrations did not increase with iron and manganese during April 1979. The rapid decrease in molybdenum in June is probably due to coprecipitation with ferrous sulfide. The fact that molybdenum increased in concentration during September suggested that this metal is remobilized during bioturbation as are iron and manganese. Copper also was remobilized during May 1978 when a value of 20/agV ~ was noted {Fig. 5). This metal may have been released from iron or manganese oxides when the latter were solubilized. The scavenging of trace metals

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by metal oxides is well documented (Murray, 1975a, b; Turekian, 1977). Copper was removed rapidly from solution during June. This removal was probably due to the precipitation of copper sulfide which is extremely insoluble (Ksp = 10 -36 ; Elderfield et al., 1981). The similarity between the seasonal trend for copper and the other metals studied was restricted to the spring 1978 data. While iron, manganese, and molybdenum concentrations increased during the fall, copper remained at levels less than 1.0pg1-1 during that period. Elderfield et al. (1981) suggested that oxygen input during bioturbation may remobilize copper during the oxidation of metal-rich sulfides. Copper was the only metal examined which displayed an increase in concentration during winter (Fig. 5). It is possible that the oxidation of these sediments during winter was sufficient to remobilize copper as suggested by Elderfield et al. (1981). However, bioturbation-mediated oxidation apparently was insufficient to increase dissolved copper concentrations. Dissolved metals may form strong complexes with organic compounds (Nissenbaum et al., 1972; Nissenbaum and Swaine, 1976; Krom and Sholkovitz, 1978). In addition, copper may form strong associations with dissolved metastable, inorganic and organic sulfur compounds such as polysulfides and organic polysulfides (Boulegue et al., 1982; Jacobs and Emerson, 1982). Since the Great Bay sediments studied were undergoing redox transitions during spring and winter, it is possible that these metastable sulfur compounds were important in determining the solubility of copper at those times. Many investigators have suggested that estuarine sediments may be an important source of dissolved manganese and copper to estuarine waters (Evans et al., 1977; Morris et al., 1978; McCaffrey et al., 1980; Windom et al., 1983; Hunt, 1983; Hunt and Smith, 1983; Buckley and Winters, 1983). The remobilization of metals during 1978 in Great Bay was most pronounced during spring. Assuming the diffusion coefficient of Li and Gregory (1974), correcting for porosity (75%), and assuming input from only the subtidal area of the Great Bay system, we calculated a diffusional benthic copper flux into the overlying water during this spring event of about 103 g copper per day. This is approximately 20% of the daily riverine input into Great Bay Estuary at this time (Lyons et al., 1982) and, therefore, represents a minor but significant source of dissolved copper to the system. Most importantly, this spring event may contribute a significant flux during a short period of time. It is likely that the flux of metals from the surficial sediment of the present study was less than predicted by diffusional calculations alone due to reoxidation and precipitation of metals in surface sediments. In Great Bay Estuary, metals may be redistributed in the sediments during spring remobilization instead of being removed from them. The quantity of metal diffusing into the overlying water from the sediments depends on the rate at which a particular metal oxidizes upon contact with oxidizing sediments or

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water. For example, Aller (1977) found that manganese diffused more readily than iron into overlying waters. However, manganese oxidation is accelerated by the presence of oxidized iron (Sung and Morgan, 1981). Giblin (1982) and Luther et al. (1982) reported that iron remobilized in salt marsh soils remained within the marsh system due to oxidation and precipitation. It was not possible to develop a seasonal model of metal remobilization for copper, manganese, and m o l y b d e n u m as was presented for iron in Fig. 4 since data for the former metals were collected for only 15 months. The compilation of several seasons of data with varying weather conditions, complementary data, and close interval sampling during 1980 were necessary for development of the iron model. However, our seasonal data for the former metals demonstrated that wide variations in dissolved metal concentrations can occur at one location. These results emphasize the need for studies which (1) employ close time interval sampling; (2) examine the flux of metals into overlying waters during short or long term mobilization events; and (3)investigate the physiochemical state of metals during these mobilization events.

ACKNOWLEDGEMENTS This is Contribution No. 171 from the Jackson Estuarine Laboratory. This work was supported by National Science Foundation Grants D E S 75-04790, O C E 77-20484, and O C E 80-18460.

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