Benthic fluxes in San Francisco Bay

Measurements of benthic fluxes have been made on four occasions between February 1980 and February 1981 at a channel station and a shoal station in South San Francisco Bay, using in situ flux chambers. On each occasion replicate measurements of easily measured substances such as radon, oxygen, ammonia, and silica showed a variability (±1) of 30% or more over distances of a few meters to tens of meters, presumably due to spatial heterogeneity in the benthic community. Fluxes of radon were greater at the shoal station than at the channel station because of greater macrofaunal irrigation at the former, but showed little seasonal variability at either station. At both stations fluxes of oxygen, carbon dioxide, ammonia, and silica were largest following the spring bloom. Fluxes measured during different seasons ranged over factors of 2–3, 3, 4–5, and 3–10 (respectively), due to variations in phytoplankton productivity and temperature. Fluxes of oxygen and carbon dioxide were greater at the shoal station than at the channel station because the net phytoplankton productivity is greater there and the organic matter produced must be rapidly incorporated in the sediment column. Fluxes of silica were greater at the shoal station, probably because of the greater irrigation rates there. N + N (nitrate + nitrite) fluxes were variable in magnitude and in sign. Phosphate fluxes were too small to measure accurately. Alkalinity fluxes were similar at the two stations and are attributed primarily to carbonate dissolution at the shoal station and to sulfate reduction at the channel station. The estimated average fluxes into South Bay, based on results from these two stations over the course of a year, are (in mmol m^–2 d^–1): O₂ = –27 ± 6; TCO₂ = 23 ± 6; Alkalinity = 9 ± 2; N + N = –0.3 ± 0.5; NH₃ = 1.4 ± 0.2; PO₄ = 0.1 ± 0.4; Si = 5.6 ± 1.1. These fluxes are comparable in magnitude to those in other temperate estuaries with similar productivity, although the seasonal variability is smaller, probably because the annual temperature range in San Francisco Bay is smaller.Budgets constructed for South San Francisco Bay show that large fractions of the net annual productivity of carbon (about 90%) and silica (about 65%) are recycled by the benthos. Substantial rates of simultaneous nitrification and denitrification must occur in shoal areas, apparently resulting in conversion to N₂ of 55% of the particulate nitrogen reaching the sediments. In shoal areas, benthic fluxes can replace the water column standing stocks of ammonia in 2–6 days and silica in 17–34 days, indicating the importance of benthic fluxes in the maintenance of productivity.Pore water profiles of nutrients and Rn-222 show that macrofaunal irrigation is extremely important in transport of silica, ammonia, and alkalinity. Calculations of benthic fluxes from these profiles are less accurate, but yield results consistent with chamber measurements and indicate that most of the NH₃, SiO₂, and alkalinity fluxes are sustained by reactions occurring throughout the upper 20–40 cm of the sediment column. In contrast, O₂, CO₂, and N + N fluxes must be dominated by reactions occurring within the upper one cm of the sediment-water interface. While most data support the statements made above, a few flux measurements are contradictory and demonstrate the complexity of benthic exchange.     

Denitrification measured by a direct N2 flux method in sediments of Waquoit Bay,MA

Denitrification was directly estimated in estuarine sediments of Waquoit Bay, Cape Cod, MA by detection of N₂ increases above ambient in the water overlying sediment cores. Denitrification rates (–9 to 712 mol N₂ m^–2 h^–1 ) were high compared to previous studies, but compared well with estimates of N loss from mass balance studies. The precision of the estimate depended on the N₂/0₂ flux ratio. The N₂/0₂ flux ratio was lower in Waquoit Bay than previously studied estuaries, and estuaries had lower N₂/0₂ flux ratios than shelf sites. The contribution of temperature-driven solubility changes to estuarine fluxes was estimated by modeling sediment temperature variations and found to be potentially important (43 mol N₂ m^–2 h^–1); however, control incubations indicate the temperature model overestimates solubility driven fluxes. The relatively low fluxes under anaerobic conditions and the low rate of N0₃ ^–/N0₂ ^– removal from the overlying water indicates coupled nitrification/denitrification produced the observed N₂ fluxes.     

Acoustic Telemetry Reveals Large-Scale Migration Patterns of Walleye in Lake Huron

Fish migration in large freshwater lacustrine systems such as the Laurentian Great Lakes is not well understood. The walleye (Sander vitreus) is an economically and ecologically important native fish species throughout the Great Lakes. In Lake Huron walleye has recently undergone a population expansion as a result of recovery of the primary stock, stemming from changing food web dynamics. During 2011 and 2012, we used acoustic telemetry to document the timing and spatial scale of walleye migration in Lake Huron and Saginaw Bay. Spawning walleye (n = 199) collected from a tributary of Saginaw Bay were implanted with acoustic tags and their migrations were documented using acoustic receivers (n = 140) deployed throughout U.S. nearshore waters of Lake Huron. Three migration pathways were described using multistate mark-recapture models. Models were evaluated using the Akaike Information Criterion. Fish sex did not influence migratory behavior but did affect migration rate and walleye were detected on all acoustic receiver lines. Most (95%) tagged fish migrated downstream from the riverine tagging and release location to Saginaw Bay, and 37% of these fish emigrated from Saginaw Bay into Lake Huron. Remarkably, 8% of walleye that emigrated from Saginaw Bay were detected at the acoustic receiver line located farthest from the release location more than 350 km away. Most (64%) walleye returned to the Saginaw River in 2012, presumably for spawning. Our findings reveal that fish from this stock use virtually the entirety of U.S. nearshore waters of Lake Huron.     

Zooplankton filtration rates in Lake Huron,Georgian Bay and North channel

On three research cruises in 1981, zooplankton community filtration rates were measured at 4 stations: Saginaw Bay, mid-Lake Huron, Georgian Bay and North Channel. For all four stations, the highest rates were observed during the late-September cruise. The maximum observed rate was 137 000 ml d^–1 m^–3, while the lowest rate was 7200 ml d^–1 m^–3. The grazing experiments were performed on three size classes of radioactively labelled algal food (0.45–5 µm, 5–20 µm and 20–64 µm). In 11 of 12 experiments, the smallest size class of food yielded the highest filtration rate. For the late-May cruise we used published data on phytoplankton biomass for the Georgian Bay and North Channel stations to calculate community feeding rates of 0.09 and 0.015 mg C mg C m^–3 d^–1, respectively, and percent cropping rates of 0.74 and 0.35 per day, respectively. A comparison of our feeding rates to literature values for zooplankton biomass suggests that algal food alone may not be sufficient to sustain zooplankton growth at those stations.     

Fluxes of bacterioplankton between a tidal estuary and the sea: returning to the “Outwelling Hypothesis”

The tidal dynamics of bacterioplankton communities at the outer part ofa shallow estuary (Ria de Aveiro, Portugal) were studied during 6 tidal cyclesat a fixed sampling site. Bacterial numbers (0.2–8.1 ×10⁹ cells l^–1), aminopeptidase activity(189–1662 nmol l^–1 h^–1),-glucosidase activity (1.7–67.0 nmoll^–1 h^–1) and potential glucoseincorporation (0.48–3.99 nmoll^–1 h^–1) followed a consistent patternof increase during ebb and decrease during flood.Fluxes of bacterioplankton populations and associated heterotrophic activitiesbetween the estuary and the coastal area during a tidal cycle were estimatedfrom the water flux as derived from a two-dimensional vertically-integratednumerical model. The net fluxes estimated for a tidal cycle ranged from–26.0 to –2.5 to × 10¹⁶ bacterial cells. The nettidal fluxes of potential heterotrophic activities ranged from –10 to–80 mol h^–1 for aminopeptidase, –0.33to –1.10 mol h^–1 for -glucosidase and–0.18 to +0.03 mol h^–1 for glucoseincorporation. Net fluxes were generally negative in sign indicating thetransfer of phyto- and bacterioplankton, as well as potential capacities for thedegradation and recycling of organic matter, from the outer estuarinecompartment to the sea.     

The spring-time abundance and feeding ofEurytemora affinis (Poppe) in Volkerak-Zoommeer,a newly-created freshwater lake system in the Rhine delta (The Netherlands)

Eurytemora affinis, a calanoid copepod, has been encountered in Volkerak-Zoommeer (Rhine delta region, S.W. Netherlands) both before this lake system was isolated in 1987 from the estuarine influence, and after. It was the main particle-feeding crustacean at all the 3 sampling stations in March–April 1990 when it reached densities of up to 215 ind.l^–1. Its decline from mid April onwards, and low densities through summer, coincided with increase in cladocerans, especiallyDaphnia spp. (D. pulex andD. galeata), a decrease in seston (<33m) and chlorophyll concentrations and in primary production rates. The clearance rates (CR) ofEurytemora measured in the spring period varied enormously (0.6–24 ml.ind^–1.d^–1) depending mainly on size (0.44–1.06 mm), food concentration (0.8–2.2 mg C.l^–1), and the water temperature which varied only narrowly (8.0–9.0°C). Mean ingestion rates of the animals measuring 0.68±0.02 mm during the study was 6.7±3.2 gC.ind^–1.d^–1; and assimilation efficiency varied between 27 and 53% (mean: 41±9%). The weight specific CR (SCR) varied between 0.96 and 6.4 litre.mg^–1 body C.d^–1. Pooled regression of SCR on the animal's body weight at the 3 study stations revealed a significant inverse relationship. Also daily ration and specific assimilation ofE. affinis varied greatly and inversely with the body weight. This calanoid contributed from about 50 to 100% to the zooplankton community grazing rates and assimilation rates, the former often exceeding the phytoplankton primary production.     

Influence of inorganic microelements on the production of camptothecin with suspension cultures of <Emphasis Type="Italic">Camptotheca acuminata</Emphasis>

The effects of eight microelements (I^–, BO₃ ^3–, MoO₄ ^2–, Co²⁺, Cu²⁺, Mn²⁺, Fe²⁺, Zn²⁺) on the biosynthesis of camptothecin and the growth of suspension cultures of Camptotheca acuminata were studied. The increase of I^– to 25 M l^–1, Cu²⁺ to 1 M l^–1, Co²⁺ to 2 M l^–1 and MoO₄ ^2– to 10 M l^–1 in Murashige and Skoog (MS) medium resulted in 1.66, 2.84, 2.53 and 2.04 times higher of camptothecin yield than that in standard MS medium respectively. Combined treatment of I^– (25 M l^–1), Cu²⁺ (1 M l^–1), Co²⁺ (2 M l^–1) and MoO₄ ^2– (10 M l^–1) lead to improve cell dry weight, camptothecin content, and camptothecin yield to 30.56 g l^–1, 0.0299%, and 9.15 mg l^–1, respectively, which were 20.2, 208.9 and 273.8% increment respectively when compared with those of control.     

Nutrient dynamics in the floodplain ponds of the Paraná River (Argentina) dominated by the water hyacinth Eichhornia crassipes.

Some aspects of nutrient status and dynamics prevailing during low and high water conditions in the fringing floodplain ponds of the Paraná River dominated by the floating macrophyte Eichhornia crassipes are described. During summertime low water conditions, low DIN:DRP ratios (0.16–1.0) and low DIN (0.5–4.8 mol.liter^–1) in the root-zone of the floating meadows suggest that macrophyte growth is limited by nitrogen. DRP concentrations appear to be controlled more by abiotic sorption-dissolution than by biological reactions. Preflood nutrient fluxes from the sediments, as estimated from porewater profiles, show that a minimum of 1.19 and 0.38 mmol.m^–2.d^–1 of DIN and DRP were regenerated from the sediments, respectively. Heterotrophic N₂ fixation is primarily associated with decaying litter (0.4 to 3.2 molN₂.g^–1.d^–1). Nutrient recycling from sediments and meadow-litter, and heterotrophic N₂ fixation (1.4 mmolN.m^–2.d^–1) appear sufficient to sustain high floating macrophyte productivity for long periods of time, without invoking large inputs from the river. The high water and early isolation periods are characterized by a very dynamic behavior of DIN, reflecting marked imbalances between N supply and demand by the biota. After hydrologic isolation of the ponds, DIN rapidly decreases to undetectable levels and stays low for the following 3 weeks, presumably as a result of high demand by phytoplankton and sediment bacteria. DIN increases again to high values 3–8 weeks after the flood, following the re-establishment of NH₄ ⁺ fluxes from the sediments. Compared to DIN, DRP concentrations remain relatively high and change little during and after the flood. Because of their small amplitude and short duration, floods do not appear to stimulate floating macrophyte production in the Paraná.     

Sediment and interstitial water chemistry of the Orbeteflo lagoon (Grosseto,Italy); nutrient diffusion across the water-sediment interface

The Orbetello lagoon is now highly eutrophic and has experienced increasing incidence of anoxia causing serious economic damage. A multidisciplinary study was commenced in March 1987 to investigate the part played by the lagoon sediments and interstitial water in recycling nutrients and contributing to the observed anoxia. Eleven undisturbed cores were collected and sub-sampled at 2 cm intervals. Interstitial water was obtained by centrifugation and analyzed for pH, Eh, nutrients, major and some minor elements. Differential fluxes from the sediments of 0.2–10 g cm^–2 day^–1 of ammonia and 0.02–0.7 g cm^–2 day^–1 of orthophosphate were obtained depending on the season, temperature of the sediments and the sampling location. A highly significant linear correlation (r ² = 0.86) was found between bicarbonate and ammonia concentrations in the interstitial waters, due to the release of these compounds during the degradation organic matter. A diagenetic model was developed to predict the alkalinity of interstitial water from the theoretical reactions involved in the decomposition of organic matter. The predicted values for bicarbonate and ammonia agreed well with the experimental results.     

Seasonal changes in the respiratory electron transport system (ETS) and respiration of the zebra mussel,Dreissena polymorpha in Saginaw Bay,Lake Huron

Electron transport system activity (ETS) and respiration rates (R) of the zebra mussel, Dreissena polymorpha, were determined monthly from April to November over 2 years at two sites in Saginaw Bay, Lake Huron. The sites were located in the inner and outer bay and contrasted in food quantity and quality. ETS ranged from 2 to 40 g O₂ mg DW^–1 h^–1 over the study period. Both ETS and respiration were strongly related to temperature, and maximum values were found between June and August. ETS also peaked in June/July when assays were conducted at a constant temperature (25 °C), indicating other factors besides temperature affected metabolic activity. R:ETS ratios decreased with increased temperature at the inner bay site, but trends were minimal at the outer bay site. In late summer, blooms of the cyanophyte Microcystis occurred in the inner bay, likely depressing filtration rates, and leading to lower respiration rates relative to ETS. ETS activity was consistently higher in the outer bay and was likely a result of higher food quality. Despite these spatial differences, annual mean R:ETS ratios varied only from 0.04 to 0.09 at the two sites over the 2-year period. Based on these values, ETS may be useful as an indicator of long-term metabolic activity in annual energy budgets of D. polymorpha. However, food conditions differentially affect respiration relative to ETS, and variability in this ratio must be considered when interested in shorter time scales.     

Carbon and nitrogen cycling in intertidal sediments near Doel,Scheldt Estuary

Carbon and nitrogen cycling in intertidal mud flat sediments in the Scheldt Estuary was studied using measurements of carbon dioxide, methane and nitrous oxide emission rates and pore-water profiles of CO₂, ammonium and nitrate. A comparison between chamber measured carbon dioxide fluxes and those based on CO₂ pore-water gradients using Fick's First law indicates that apparent diffusion coefficients are 2 to 28 times higher than bulk sediment diffusion coefficients based on molecular diffusion. Seasonal changes in gaseous carbon fluxes or CO₂ pore water concentrations cannot be used directly, or in a simple way, to determine seasonal rates of mineralization, because of marked seasonal changes in pore-water storage and exchange parameters.The annual amount of carbon delivered to the sediment is 42 mol m^–2, of which about 42% becomes buried, the remaining being emitted as methane (7%) or carbon dioxide (50%). Each year about 2.6 mol N m^–2 of particulate nitrogen reaches the sediment; 1.1 mol m^–2 is buried and 1.6 mol m^–2 is mineralized to ammonium. Only 0.42 mol m^–2 yr^–1 of the ammonium produced escapes from the sediments, the remaining being first nitrified (1.2 mol m^–2 yr^–1) and then denitrified (1.7 mol m^–2 yr^–1). Simple calculations indicate that intertidal sediments may account for about 14% and 30% of the total estuarine retention of nitrogen and carbon, respectively.     

Plant regeneration from thin cell layers in Spinacia oleracea

Caulogenesis and somatic embryogenesis were induced from transverse thin cell layers (tTCLs) of two European (Spinacia oleracea L.) spinach genotypes. Regeneration occurred mostly when tTCLs had been excised from seedlings grown on a preconditioning medium consisting of White's macroelements, Nitsch's microelements, Murashige and Skoog's (MS) vitamins, 6 g l^–1 agar and 20 g l^–1 glucose. The explants were cultured on MS medium supplemented with sucrose (10, 30, 50 or 80 g l^–1) or fructose (5, 10 or 30 g l^–1) and several combinations of indole-3-acetic acid (IAA), -naphtalene acetic acid (NAA), 6-benzylaminopurine (BAP) and gibberellic acid (GA₃). Most of the regeneration events were obtained from root explants of the cultivar Carpo. The best result was observed on MS medium supplemented with 50 g l^–1 sucrose, 100 M NAA, 1 M BAP and 10 M GA₃. After an 8-week culture, the calluses were transferred onto MS medium where shoots and somatic embryos appeared 1 week later. The best root development was obtained on MS medium supplemented with 4.9 M indole-3-butyric acid (IBA) and 8 g l^–1 Phytagel. The plantlets were, then, transferred to soil and developed into well-conformed, fertile plants.     

A review of assessment and remediation strategies for hat spot sediments

Sediments at the extreme end of the scale of contamination, designated as hot spot sediments, are considered. The characterization of such hot spots, and an approach for the quantitative assessments of the behaviour and fate of pollutants in such sediments are covered. Experiments with sediments containing extreme levels of heavy metals showed release rates of 56 mg m^–2 d^–1 of dissolved zinc and 0.004 mg m^–2 d^–1 of dissolved mercury. When these sediments were resuspended, the dissolved fluxes were increased by a factor of 2.2 and 128 for zinc and mercury, respectively. The biological implications of hot spot sediments are dealt with, since sediments are an important habitat for many organisms. Various alternatives for clean-up operations including dredging and capping are discussed.     

Evidence for salt diffusion from sediments contributing to increasing salinity in the Salton Sea, California

Geochemical investigations of interstitial waters from the Salton Sea, CA reveal evidence of concentrated brines in the sediments underlying the lakes two basins. The brines are likely caused by the gradual dissolution of evaporite deposits. The chemical composition of the brine in the northern basin is dominated by magnesium and sulfate and differs from the southern basin where the dominant components are sodium and chloride. Sediment depth distributions of major ions and porosity indicate diffusion of salts from the sediments into the overlying waters in both basins. Benthic fluxes have been calculated for the four most abundant ions: magnesium, sodium, sulfate and chloride. For the northern basin we calculate diffusive fluxes of 3.7 × 10^–2, 8.2 × 10^–2, 44 × 10^–2, and 5.4 × 10^–2 g cm^–2 yr^–1 for magnesium, sodium, sulfate and chloride, respectively. For the southern basin we calculate diffusive fluxes of 0.9 × 10^–2, 9.7 × 10^–2, 6.9 × 10^–2, and 25 × 10^–2 g cm^–2 yr^–1 for these same ions. By scaling up our results we estimate the salinity flux from the sediment to the water column to be between 3.6 × 10⁴ and 3.6 × 10⁵metric tons per year, equivalent to 1–10% of the riverine input. These results are important for developing strategies to combat rising salinity in the Salton Sea, CA.     

Cycling of inorganic and organic sulfur in peat from Big Run Bog,West Virginia

Total S concentration in the top 35 cm of Big Run Bog peat averaged 9.7 mol·g — wet mass^–1 (123 mol·g dry mass^–1). Of that total, an average of 80.8% was carbon bonded S, 10.4% was ester sulfate S, 4.5% was FeS₂­S, 2.7% was FeS­S, 1.2% was elemental S, and 0.4% was SO₄ ^2–­S. In peat collected in March 1986, injected with³⁵S­SO₄ ^2– and incubated at 4 °C, mean rates of dissimilatory sulfate reduction (formation of H₂S + S⁰ + FeS + FeS₂), carbon bonded S formation, and ester sulfate S formation averaged 3.22, 0.53, and 0.36 nmol·g wet mass^–1·h^–1, respectively. Measured rates of sulfide oxidation were comparable to rates of sulfate reduction. Although dissolved SO₄ ^2– concentrations in Big Run Bog interstitial water (< 200 µM) are low enough to theoretically limit sulfate reducing bacteria, rates of sulfate reduction integrated throughout the top 30–35 cm of peat of 9 and 34 mmol·m^–2·d^–1 (at 4 °C are greater than or comparable to rates in coastal marine sediments. We suggest that sulfate reduction was supported by a rapid turnover of the dissolved SO₄ ^2– pool (average turnover time of 1.1 days). Although over 90% of the total S in Big Run Bog peat was organic S, cycling of S was dominated by fluxes through the inorganic S pools.     

Quantification of the contribution of surface outgrowth to biocatalysis in sol-gels: oxytetracycline production by <Emphasis Type="Italic">Streptomyces rimosus</Emphasis>

A technique was developed for differentiating the activity of microbes solely within sol gels by using the contribution of biomass outgrowth. Streptomyces rimosus was immobilised in colloidal silica gels and biomass growth, oxytetracycline synthesis, pH and carbohydrate consumption were compared for UV surface-sterilised gels, untreated gels, and liquid cultures. Absolute and biomass specific oxytetracycline yields were higher for non-sterile gels than for liquid culture. Biomass solely within colloidal silica gels (1.7 mg ml^–1), and gels obtained from colloidal silica modified by addition of larger silica particles (1.2 mg ml^–1) yielded 27 and 21 g ml^–1 oxytetracycline compared with 97 and 104 g ml^–1 for unsterilised gels (3.6 and 5.2 mg ml^–1 biomass) displaying outgrowth. It was therefore apparent that biomass and antibiotic production within the gels was limited and that optimisation requires gel modification.     

Gonadal neoplasms in wild carp-goldfish hybrids from the Welland River near Niagara Falls,Canada

An experimental approach was taken to examine the processes of detritus decomposition in river sediments.Addition of macrophyte detritus (Alternanthera philoxeroides (Mart.) Griseb) to river sediment resultedin an increase in carbon mineralization from a sbasal rate of 200 up to 500 mg C m^–2 d^–1.Carbon mineralization after addition of oak detritus was only slightly higher than mineralization in sediments thatreceived no addition ( 200 mg C m^–2 d^–1). Bacterial biomass and production in sediments \s+ alligator-weedwere higher than in sediments + oak or zero-addition. In these experiments the major fate of addedalligatorweed was mineralization. For alligatorweed detritus, microbial metabolism depletes organic carbonrather than leading to increases in food quality. Therefore, a pulse input of alligatorweed detritus would notbe available as a long-term source of organic carbon. Oak detritus was not rapidly decomposed, and sopersists in these sediments for a longer period.     

Low salinities adversely affect photosynthetic performance of the mangrove,Avicennia marina

Photosynthetic performance of the highly salt tolerant mangrove, Avicennia marina, was compared at two sites differing insubstrate soil salinities. Carbon dioxide exchange and chlorophyll fluorescence weremonitored at a high salinity site in Durban Bay (35) and at a low salinitysite in Beachwood (< 12). Mean CO₂ exchange, conductanceand transpiration were consistently higher at the high salinity site. Carbondioxide response curves indicated that carboxylation efficiency was higherand stomatal limitation lower at the Durban Bay site. PSII quantum yield,electron transport rates (ETR) and intrinsic PSII efficiency(F_v/F_m) were significantly higher at the high salinity site.Quenching analysis indicated a higher degree ofphotoinhibition/photoprotection in leaves at the low salinity site. Predawnand midday leaf water potentials were –1.6 and –3.1 MPa at Beachwood,compared to –2.6 and –3.8 MPa, respectively, at Durban Bay. Leafconcentrations of Na⁺, K⁺, Ca²⁺, Mg²⁺,Cl^- and N were significantly higher at Durban Bay. Photosyntheticperformance is apparently impaired at the low salinity site in Beachwood asa result of K⁺ and N deficiencies in the leaves.     

The molecular basis of HbH disease in Taiwan

Summary We have determined the molecular characteristics of -thalassemia in 12 HbH subjects from Taiwan by restriction endonuclease mapping with -and -specific probes. We have found four types of defects in the -thalassemia-2 genetic determinant: ^3.7 type I; ^4.2; ^CS; and ^T. All HbH subjects carried the ——^SEA genotype in the -thalassemia-1 determinant. At least two different subtypes of ——^SEA genotype were observed in this study.     

Isotope exchange reactions with radiolabeled sulfur compounds in anoxic seawater

The isotope exchange between³⁵S-labeled sulfur compounds of sulfate (SO₄ ^2–), elemental sulfur (S⁰), polysulfide (S_n ^2–), hydrogen sulfide (HS^–: H₂S + HS^– + S^2–), iron sulfide (FeS), and pyrite (FeS₂) was studied at pH 7.6 and 20 °C in anoxic, sterile seawater. Isotope exchange was observed between S⁰, S₂ ^2– HS^–, and FeS, but not between³⁵S labeled SO₄ ^2– or FeS₂ and the other sulfur compounds. Polysulfide mediated the isotope exchange between S⁰ and bisulfide (HS^–). The isotope exchange between S⁰ and S_n ^2–) reached 50% of equilibrium within < 2 min while exchange between S₂ ^2– and HS^– approached equilibrium within 0.5-1 h. In all the experiments HS^–, revealed a fraction exchange from 0.79 to 1.00. Isotope exchange between S^2– and FeS took place only via S₂ ^2– and/or HS^–. The isotope exchange between iron sulfide and the other sulfur compounds was not complete within 24 h as shown by a fraction exchange of 0.07–0.83. This lack of equilibrium (fraction exchange < 1) was due to the isotope exchange between dissolved compounds and surfaces of sulfur particles. The isotopic exchange reactions limit the usefulness of radiotracers in process studies of the inorganic sulfur species. Exchange reactions will also affect the stable isotope distribution among the sulfur species. The kinetics of the isotopic exchange reactions, however, depend on both pH and temperature.