The composition and metabolic capability of bacterioplankton communities were examined over seasonal and spatial gradients and related to the source, composition, and quantity of dissolved organic matter (DOM) in the blackwater estuary Winyah Bay, Georgetown County, SC, USA and its tributary rivers. Bacterial community composition (BCC) was measured by terminal restriction fragment length polymorphism, and bacterial metabolic capability (BMC) was measured by defined substrate utilization patterns (Biolog GN2 plates). Spatial patterns were not important, despite the anticipated watershed effects and the well-documented influence of salinity gradients on estuarine bacterioplankton, but DOM, BCC, and BMC all showed varying degrees of temporal patterns; DOM-based groupings differentiated BCC samples better than spatiotemporal categories, but not BMC. BCC was closely related to properties describing DOM composition, particularly those related to DOM source (i.e., cypress swamps vs. in situ phytoplankton production, indicated by chlorophyll a, colored DOM spectral slope, α355/dissolved organic carbon (DOC), and DOC concentration), and to associated physicochemical variables, such as temperature, pH, and salinity. BMC was more strongly related to abiotic factors, such as temperature and dissolved nutrients, as well as to chlorophyll a and percent bioavailable DOC. In contrast with previous studies, BCC and BMC were significantly correlated in this highly heterotrophic estuary, suggesting that DOM source variability may select for specialist phylotypes above a background of generalists. This study, therefore, supports a causative pathway from DOM to BMC to BCC while suggesting that BCC and BMC may be simultaneously influenced by different suites of DOM characteristics and physicochemical parameters.

Groundwater discharge is increasingly recognized as a significant source of nutrient input to coastal waters, relative to surface water inputs. There remains limited information, however, on the extent to which nutrients and organic matter from each of these two flowpaths influence the functional responses of coastal microbial communities. As such, this study determined dissolved organic carbon (DOC) and nutrient concentrations of surface water runoff and groundwater from both an urbanized and a relatively pristine forested drainage basin near Myrtle Beach, South Carolina, and quantified the changes in production rates and biomass of phytoplankton and bacterioplankton in response to these inputs during two microcosm incubation experiments (August and October, 2011). Rainwater in the urbanized basin that would otherwise enter the groundwater appeared to be largely rerouted into the surface flowpath by impervious surfaces, bypassing ecosystem buffers and filtration mechanisms. Surface runoff from the developed basin was most enriched in nutrients and DOC and yielded the highest production rates of the various source waters upon addition to coastal waters. The metabolic responses of phytoplankton and bacterioplankton were generally well predicted as a function of initial chemical composition of the various source waters, though more so with bacterial production. Primary and bacterial productivities often correlated at reciprocal time points (24-h measurement of one with the 72-h measurement of the other). These results suggest human modification of coastal watersheds enhances the magnitude of dissolved constituents delivered to coastal waters as well as alters their distributions between surface and groundwater flowpaths, with significant implications for microbial community structure and function in coastal receiving waters.

Results of intense field sampling for water quality assessment of hypoxia in Long Bay, South Carolina in 2006 (moderate hypoxia), are compared to strategic sampling from 2007 (weak hypoxia) and 2009 (severe hypoxia & anoxia). Results from sampling in 2006 indicated some trends that were persistent in other years including; high chlorophyll (CH) concentrations and lower dissolved oxygen (DO) in bottom waters, constraint of the most compromised water quality to the shallow inner shelf from 0.3–1.0 km offshore, and the centralization of impact in the northern Grand Strand versus shoreline regions in southern Horry county. Analysis of 2006 surface water characteristics from putative terrestrial discharge sites versus control regions did not indicate distinct spatial variability of these regions, but did show clear temporal, and nearshore versus offshore separation. Correlation analysis of the 2006 data revealed that during wet periods salinity was positively related to DO, while in dryer periods it was negatively correlated, suggesting that significant fresh water inputs result in increased heterotrophy and diminished DO in the nearshore. Temporal variability in the extent and magnitude of summertime DO declines in these impacted waters were likely related to spring/summer weather history including temperature and precipitation. Inverse relationships between CH and DO which were found in 11 of 13 category comparisons from 2006 – 2009 suggest that heterotrophic DO consumption overwhelms the capacity for DO production from phytoplankton communities in the shallow inner shelf of Long Bay, South Carolina. Introduction: Hypoxia conditions (Dissolved oxygen (DO) concentrations ≤2.0 > 0.0 mg/l) in marine and estuarine settings have become more severe increasing in; frequency, duration, and extent worldwide over the last 3 decades (Diaz and Rossenberg, 1995; Wu, 2002; Diaz and Rossenberg, 2008; Conley et al., 2009). Hypoxia can significantly impact biological communities in the coastal ocean with few aquatic species or shallow ecosystems adapted to such low oxygen levels (Boesch and Rabalais, 1991; Baird et al., 2004; Grantham et al., 2004). Considerable evidence suggests that impacts may occur above the hypoxia threshold at concentrations closer to 3 mg/L (Howell and Simpson, 1994; Ritter and Montagna, 1999; Gray et al., 2002). The spatial extent of hypoxia and the more severe anoxia conditions within estuarine and coastal marine environments vary widely, ranging from localized (Chesapeake Bay, Hagy et al., 2004; Kemp et al., 2005) to very expansive (Gulf of Mexico, Turner et al., 2005; Rabalais et al., 2009; Baltic Sea, Conely et al., 2002; Conely et al., 2009b). The spatial characteristics of this phenomenon in terms of influenced area or volume, geographic placement, and by depth offer some clues as to the mechanisms which act to cause compromised water quality. The areal extent and volume of hypoxic water within a particular hypoxic zone has been shown to be controlled, in part by nutrient supply. The supply may come from the terrestrial direction in the form of freshwater inputs (Rabalais and Turner, 2001; Kemp et al., 2009; Conley et al., 2009), or from oceanic advection associated with upwelling (Glenn et al., 1996; Glenn et al., 2004). Oxygen concentrations are also modified by water column physical conditions with strong pycnoclines decreasing bottom water DO, by reducing water column mixing and reoxygenation. However microbial DO demand, stimulated by anthropogenic loading of organic and inorganic nutrients, has the capacity to overwhelm even strong columnar mixing to gradually promote hypoxia (Verity et al., 2006). General circulation models predict that climate change alone will deplete oceanic oxygen by increasing thermal stratification. Globally enhanced discharge of freshwater and agricultural nutrients are also predicted, which will acerbate hypoxia development (Diaz and Rosenberg, 2009). Climate change induced shifts in wind patterns may also change the spatial patterns of hypoxia, with some coasts experiencing more frequent upwelling conditions which can affect both thermal stratification and nutrient transport (Rabalais et al., 2009b). Added to these eventualities, continued coastal development will provide additional mechanisms to increase nutrient loading and alter coastal water hydrology in manners that will also enhance hypoxia development (Kleppel et al., 2006). Methodology: High spatial resolution maps of surface water temperature, salinity, pH, dissolved oxygen (DO), colored dissolved organic matter (CDOM), chlorophyll a (CHL), were obtained with flow-through Dataflow TM instrumentation. The results from the chlorophyll sensor were intercalibrated with measurements via acetone extraction (EPA 445.0). All other probes were calibrated pre and post sample collection following manufacturer guidelines. Contour maps were generated according to Boynton and Rohland (2001) using the ESRI ArcGIS 8.1.2 software suite to assist in the interpretation of spatial patterns of different water quality parameters. Interpolation was accomplished using Kriging routines in the Geostatistical Analyst extension within the ArcGIS software. Results and Discussion: Weather conditions in 2006 (Table 1) indicated that precipitation displayed greater than a 2 order of magnitude range across sampling dates. Wind direction displayed a relatively narrow range of directions from 152 – 228 compass bearing, with the exception of 7/10/06 in which wind was oriented from 96 degrees. Wind speed showed less variability ( 2 mg/l of O2) in bottom waters. Figure 2 indicates that the sites had very strong differences in inshore versus offshore depth profiles of chlorophyll, with inshore sites higher in chlorophyll at all depths but especially in the bottom waters. Bottom waters at all 16 sites surveyed on this date displayed higher chlorophyll concentrations versus that observed in the surface waters. a) b)

Anthropogenic alteration of terrestrial shorelines can have pronounced effects on marine environments at the upland-marsh boundary. Possible terrestrial development effects on several physical and biological variables of high-marsh habitats were examined along developed and undeveloped shorelines in an ocean-dominated, southeastern US estuary. Analyses of sediment characteristics suggested development of the upland boundary affected physical conditions within the high-marsh. For example, pore water salinities were greater along undeveloped shorelines during a non-drought period even after rain events. Significant floral and faunal differences also existed between shoreline treatments. Black needle rush stems were significantly taller and marsh periwinkle densities significantly greater, but eastern coffee bean snail densities were significantly reduced along developed shorelines. Benthic infaunal community abundance and composition also were significantly different between shoreline treatments with sand fly larvae, human pest precursors, either only present or present in greater densities along developed shorelines. Sediment respirometry experiments indicated significant differences in heterotrophic and autotrophic processes occurring between shoreline treatments. Greater sediment surface temperatures along developed shorelines provided one possible mechanism driving high-marsh responses to boundary alteration. The history and extent of shoreline development along with a tendency in ocean-dominated southeastern marshes to resist change likely influenced current ecological conditions within our high-marsh study areas. A greater understanding of the driving mechanisms producing localized effects on salt marshes and recognizing regional differences in marsh resistance to change will facilitate predictions of shoreline development consequences and help in proposing effective management strategies for coastal boundaries.

The urchin Diadema antillarum Philippi (Echinodermata: Echinoidea) was extremely rare to absent from reefs throughout Jamaica during much of the 1980s and part of the 1990s following a Caribbean wide mortality event. This survey was conducted in 2007 to assess their distribution on the western forereef of Discovery Bay, Jamaica. Mean urchin densities of 3.93 ± 3.4 m -2 (mean ± SD) are the highest that have been recorded in Discovery Bay since the mortality event. The increases in D. antillarum numbers of the past decade have been accompanied by reductions in macroalgal cover. This study found a strong inverse correlation (Pearson product-moment correlation = -0.597, p <0.000) between macroalgal cover and Diadema density, indicating that the urchins are exerting a strong top-down control on macroalgae. Any future increases in urchin densities will most likely result in further reductions of macroalgal cover.

Coastal ecosystems are easily overexploited and changed by physical and biological factors. In this paper, we discuss current ideas and arguments for coastal ecosystem management with an emphasis on systems that have large bivalve filter feeder components. For centuries the species or population approach has been utilized in fisheries management. With the growing knowledge base on specific environmental effects and relationships, it has become increasingly evident that a broad or holistic approach to fisheries management in these systems is usually more appropriate. An ongoing ecosystem scale experiment in which oysters are completely removed from tidal creeks is described and used as a case study. The experimental design takes estimates of the systems carrying capacity into account. Using the population or species approach to monitor the oysters, the only observable change after the experimental manipulation was a slight increase in summer somatic growth and elevated recruitment of oysters in creeks with oyster reefs removed. These data are interpreted as an indication that the creeks with oysters present are below or near carrying capacity. However, when nekton, plankton and water chemistry data are also examined a much more complicated picture emerges. During the summer growing season, nekton biomass in all creeks is often greater than oyster biomass. Also, our calculations show that oysters do not produce enough ammonium to satisfy phytoplankton productivity, but nekton, water column remineralization and sediments can account for most of the deficit. Finally, microflagellates, which are a preferred food for the oysters, dominate the phytoplankton during the summer growing season and diatoms dominate the colder months. The timing of the change in phase of phytoplankton dominance seems to mirror the seasonal arrival and departure times of nekton in the creeks. We argue that dense bivalve reefs and beds are indicative of intense positive feedback loops that make their ecosystems susceptible to dramatic changes in structure. Such changes have not been reported for natural systems, but are found in systems influenced by over-fishing, nutrient loading and pollution. Thus, the management of sustainable fisheries in coastal ecosystems requires an understanding of the ecosystem science and the realization that systems dominated by bivalves exhibit complex responses that are not easily explained by linear dynamics.

Estuaries integrate atmospheric, watershed, oceanic, and human influences over space and time, Therefore, spatial and temporal patterns in estuarine water-column properties are useful as metrics to evaluate external factors related to internal processes. The National Estuarine Research Reserve monitoring program, including the North Inlet-Winyah Bay complex in South Carolina, provides an ideal setting to track water quality relationships. Our goal was to assess hydrography, chlorophyll a, and particulate and dissolved materials from monitoring data collected at sites from both the salt and estuarine marsh components since 1993-94. Salinity, turbidity, dissolved organic carbon, suspended solids, and chlorophyll a were much greater at the estuarine site, whereas organic nitrogen dominated the total nitrogen pool at both locations. Nitrate was a significant fraction of the total nitrogen pool at the estuarine site but not within the salt marsh. Whereas dissolved organic nitrogen was positively correlated to water temperature, nitrate concentrations were the lowest in the summer, Principal components analysis identified seasonal patterns within the salt marsh for temperature, chlorophyll a, ammonium, suspended solids, and particulate nitrogen. These parameters, grouped together as a primary component, were positively correlated to Spartina alterniflora biomass. In contrast, the estuarine site was more characterized by salinity, pH, and dissolved organic carbon. Although the water-column properties of the salt marsh site reflected a high degree of internal production and remineralization in the summer, patterns at the estuarine site were more likely influenced by seasonal changes in circulation and biogeochemical processing common to coastal plain estuaries.

In the tidal creeks of North Inlet, a high salinity salt marsh estuary near Georgetown, South Carolina, USA, the Eastern oyster Crassostrea virginica is an abundant component of the benthic macrofauna that exerts controls on microbial communities by its grazing and nutrient regenerative activities, The effects of oyster activity on North Inlet microbial food web structure were studied using: (1) water samples collected from tidal creeks with oyster reefs versus tidal creeks without oyster reefs (removed as part of a large-scale field manipulation study); and (2) flow-through flumes. In pair-wise comparisons of creeks with similar hydrography and morphology, the only microbial group found to vary significantly with the presence of oyster reefs was the phototrophic nanoflagellates (pflags), which were 1.25- to 2.25-fold less abundant in creeks with oyster reefs during the summer phytoplankton bloom, Because heterotrophic nanoflagellates (hflags) did not vary in these same comparisons, we hypothesized that preferential feeding for pflags by oysters was responsible for the reduction in pflag abundance. The hypothesis was tested during March and July 1999 using flumes with flowing creek water containing either live oysters or dead oyster shells, Significant reductions in pflags and some types of diatoms were measured in the outflow from live oysters, but oyster effects on other microbial components (hflags, cyanobacteria, and heterotrophic bacterio-plankton) were not evident. The flume study demonstrated preferential feeding by oysters on pflags using naturally occurring microbial assemblages, The differences in pflag abundance in creeks with oyster reefs versus creeks without oyster reefs suggests that this grazing activity can affect the structure of natural microbial communities.

Active and passive mechanisms utilized by many organisms in marsh-estuarine ecosystems couple the water column to the bottom. These linkages are often engineered by dense populations of plants (marshes) or animals (beds and reefs) that use their organismic structure, i.e., bodies or shells, and functional processes, i.e., water pumping, suspension feeding, etc., to enhance the movement of materials between the two habitats. These adaptations to the benthic boundary layer result in organismically mediated fluxes of materials between the water and the bottom that may dramatically alter either or both habitats. Dense stiff blades of grass dominate the salt marsh component of marsh-estuarine ecosystems. This structure ensures low water flow, low shear velocities, high drag and high roughness at the benthic boundary. These physical factors allow molecular diffusion and sedimentation to dominate exchange mechanisms. Marsh mussels magnify benthic-pelagic coupling by their active pumping and filtration of water. The shells of bivalve beds form a rough benthic surface that enhances turbulent mixing and increases the width of the benthic boundary layer. These beds can remove, via sedimentation (passive) and filtration (active) mechanisms, enormous quantities of suspended materials (phytoplankton, etc.) from the water and release, as a result of metabolism, large amounts of dissolved inorganic substances into tidal currents. In some systems, bivalve beds are equivalent to saltmarshes in processing materials. There have been few studies on benthic-pelagic coupling by marsh-estuarine mudflats. In marsh related mudflats, there is low water flow, shear velocities, drag and smooth surfaces. Both passive and active coupling mechanisms are common to mudflats, but there is little direct information available. The high ratio of bottom surface area to tidal water volume over these flats suggests a great potential for material exchanges. At the system level, coupling processes directly involve marshes and animals in the cycling of major nutrients not only within the shallow tidal marsh-estuarine ecosystem, but also with the adjacent coastal ocean. The magnitude of these system level couplings has only been identified in a few locations, but they are almost always related to high productivity sub-systems, i.e., mussel beds and oyster reefs.

We report here the experimental design and observations from the premanipulation year of an ecosystem-level study investigating the hypothesis that oyster reefs control the structure and function of intertidal creeks. A group of eight tidal creeks in North Inlet. South Carolina, USA, dominated by oysters, Crassostrea virginica (Gmelin), were studied using a replicated BACI (Before-After Control-Incident) design in which all creeks are sampled simultaneously. Before the start of the premanipulation year, oyster biomass in the creeks was adjusted to 8 g db/m(3). Detailed geomorphological observations were made on each creek as the study began. Nutrients and chlorophyll a were measured weekly in each creek and exhibited seasonal and interannual influences. Intensive planktonic-microbial loop samplings were conducted seasonally and suggested a diatom-dominated winter community controlled by nutrient availability and a microflagellate-dominated summer community controlled by grazing. Nekton biomass exceeded oyster biomass in most creeks during the summer. As expected, oyster growth decreased from summer to winter, and survival was higher in winter. In the study's second, or manipulation year, the role of oysters will be tested by removing them from four creeks.

In North Inlet, a tidally dominated salt‐marsh estuary near Georgetown, South Carolina, the summer chlorophyll maximum correlates with an annual peak in ambient NH4+ concentration. This relationship suggests that phytoplankton population growth during the summer bloom is limited by factors other than nutrient supply, because NH4+ is the major inorganic nitrogen source available to phytoplankton in North Inlet, and phosphorus should not be limiting (N:P is generally ~7). We tested the hypothesis that phytoplankton population growth during the bloom was controlled by grazing. Natural samples were incubated in treatments designed to differentiate between nutrient and grazing effects, and time‐course changes in total phytoplankton biomass and phototrophic community composition were followed. Marked seasonal differences were observed in the relative contribution of pica‐, nano‐, or microplankton to phytoplankton community biomass, as well as the mechanisms controlling phytoplankton population growth. During the summer bloom, phototrophic picoplankton (mostly Synechococcus spp.) and nanoplankton (mostly flagellates) were relatively abundant, and phytoplankton population growth was unaffected by NH4+ addition, but was greatly stimulated by dilution that reduced microzooplankton grazing pressure. During the winter, when diatoms dominated the phytoplankton, the response to dilution was relatively minor, while NH4+ addition significantly stimulated the growth of various phytoplankton groups and total chlorophyll. The results indicate a seasonal transition in microbial food‐web trophic structure and regulation in North Inlet estuary. During the summer, microzooplankton grazing is an important factor regulating phytoplankton population growth during the nanoflagellate‐ prevalent bloom, whereas in the winter, a diatom‐dominated community is limited by nutrient supply.

The combined effects of nutrient enrichment and grazing by isopods and amphipods on abundances of seagrass epiphytes were tested inZostera marina L. (eelgrass) microcosms. Using epifluorescence microscopy, densities of epiphytic diatoms, cyanobacteria, heterotrophic flagellates, and heterotrophic bacteria were enumerated after 1 mo and 2 mo of treatment. In general, numbers of diatoms decreased, in the presence of grazers and showed little response to nutrient enrichment, whereas numbers of cyanobacteria increased with nutrient enrichment and showed little response to grazing. Thus, macrofaunal grazing maintained a photoautotrophic community domainated by cyanobacteria, particularly under nutrient enriched conditions. Following 2 mo of treatment, dense macroalgal growth under nutrient-enriched conditins with grazers absent appeared to limit populations of both epiphytic autotrophs. Patterns of abundance of heterotrophic bacteria suggested that the original bacteria population was nutrient limited. Bacteria populations may have been limited by organic carbon supplies at the end of the experiment. Abundances of heterotrophic flagellates and bacteria were strongly correlated on both sampling dates. Results suggest that heterotrophic flagellates might serve as a link between heterotrophic bacterial production and higher trophic levels in seagrass epiphyte food webs.

Pore-water dissolved organic carbon (PWDOC) concentrations were examined in vegetated and bare sediments of aHalodule wrightii seagrass bed, and in a mud bottom sediment of a southern Texas estuary. Temporal variability was examined at diel (dawn and noon) and bimonthly time scales. Distribution patterns of PWDOC were compared with physical, chemical, and biological factors thought to exert control on PWDOC. Concentration of PWDOC, bacterial production, and resultant PWDOC turnover times displayed statistically significant spatial and temporal variability. Concentration of PWDOC ranged from 14 mg C 1−1 to 107 mg C 1−1 of pore water, or 9–71 μg C cm−3 wet sediment. PWDOC was more variable and was approximately 5 times higher than DOC concentrations in the water column. Low PWDOC concentrations (mean = 14.6 μg C cm−3) and high bacterial production rates (mean = 1.92 μg C cm−3 h−1) were observed at the mud station, whereas PWDOC concentrations were high (mean = 24.6 μg C cm−3) and bacterial production rates were low (mean = 0.43 μg C cm−3 h−1) at the bare station. PWDOC turnover times (Tt), assuming 50% bacterial growth efficiency (1–840 h) were shortest at the mud station (mean=13 h) and longest at the bare station (mean=180 h). In the overlying water column, Tt values were longer, ranging from 1,000–10,000 h. PWDOC concentrations were 25% higher in vegetated sediments than in neighboring bare sediments. This difference was probably due to inputs of labile photosynthetic excretia, since bacterial production rates in vegetated sediments displayed significant diel variability and were 4 times greater than that of bare sediments. Based upon the entire data set, PWDOC was significantly related to macrofaunal biomass, sediment POC, sediment C:N ratios, and oxygen metabolism, but was significantly correlated only to the latter two variables in stepwise multiple regression. Our findings suggest that organism activities and detrital quality are the major determinants controlling variability in PWDOC.

Using ecosystem development theory and the River Continuum Concept as starting points, we present a new holistic theory to explain the spatial and temporal behaviour of marsh-estuarine ecosystems Along the marine-estuarine-freshwater gradient in response to sea-level rise. In this theory, a geohydrologic continuum represented by tidal channel provides a predictable physical model of how the marsh-estuarine ecosystem adapts until there is a change of state. North Inlet, South Carolina is used as an example of this marsh-estuarine continuum. Mature creeks are at the ocean-estuary interface and are strongly influenced by marine factors. Further into the estuary, less and less mature creeks are encountered which are dominated by smaller scale spatial and temporal controls such as oyster reefs. Immature or ephemeral creeks import both particulate and dissolved materials, while mature creeks export both forms of nutrients. Mid-aged creeks appear to take up particulate materials and release dissolved constituents. Ultimately, the continuum reaches the fresh-saltwater interface where a very young estuarine ecosystem invades a more mature type, under the influence of disturbance. Our new explanation satisfies most criteria for a good theory by being internally consistent to the location specified, generating testable hypothesis, not blindly adapting existing theories, agreeing with known properties of the ecosystem described and by generating new invigorating discussion within the scientific community.

The sensitivity and comparative simplicity of 5N stable isotopic tracer techniques has been used to quantify rates of nitrification in aquatic systems. However, the most commonly used method for recovery of inorganic oxidized nitrogen compounds from aqueous samples, which is based on liquid-liquid partitioning, is time consuming and contamination prone. We describe a solid-phase rapid chromatographic method for recovery of 15NO2− and NO3− produced by nitrification in aqueous samples. Compared to liquid-liquid partitioning, the advantages are significantly reduced processing time and reduced potential for contamination. Typical results are presented for the tidal, freshwater reaches of the James River estuary.

AbstractEcotoxicological effects of creosote contamination on benthic bacterial communities in the Elizabeth River, Virginia were investigated using both structural and functional microbial parameters. Parameters included direct counts, viable counts of heterotrophs and “cresol‐utilizers”, and bacterial production determined using the tritiated thymidine uptake method. Ancillary data included temperature, salinity, Eh profiles, concentrations of polycyclic aromatic hydrocarbons (PAHs), sediment granulometry and total organic carbon. Two reference stations in relatively nonpolluted areas were sampled for comparative data. Results indicated that cell specific and total heterotrophic bacterial production were depressed in a dose‐dependent manner with increasing sediment PAH concentrations. Sediment properties and seasonal changes in temperature appeared to modify the effects of PAHs on bacterial production. Direct bacterial counts and viable counts of total heterotrophs were depressed in the most contaminated sediments. Evidence of creosote adaptation was equivocal, with cresol‐utilizer densities not significantly elevated at contaminated stations. The presence of creosote was associated with shifts of Eh toward more negative values compared to nonpolluted sediments. Toxicants which reduce benthic bacterial production may indirectly impact other trophic groups through aberrant cycling of carbon or nutrients. Of the parameters examined, the tritiated thymidine production assay was found to be the most sensitive for detection of ecotoxicological effects.