Estuarine benthic habitats serve as essential feeding grounds for demersal finfishes and shellfishes and provide a link between the water column and demersal fisheries in coastal foodwebs. We hypothesize that the cascading linkages of water-column conditions to benthic invertebrates and from benthic invertebrates (as prey) to demersal fishes are a primary mechanism by which water-quality degradation affects sustainable production of demersal fisheries in coastal ecosystems. To evaluate these linkages, we related changes in water quality to changes in bottom-habitat quality for fishes (defined by the availability of prey resources) and assessed how changes in habitat quality affect fish diet. We examined the first link (water-column conditions to benthos) by intensively sampling the benthic community in the Neuse River Estuary, North Carolina, during 2 summers (1997 and 1998) in which depletion of bottom-water oxygen occurred. Dramatic decreases in the abundance of benthic macroinvertebrates were evident after hypoxia/anoxia in both years. Abundances of the clam Macoma balthica, a key prey item for fishes and crabs and the biomass dominant in the benthos, decreased by over 90% in deep and mid-depth areas (> 2 m) throughout the estuary after hypoxia/anoxia in 1997. Although summer decreases in benthic macroinvertebrates were also exhibited in 1998, a year of less frequent and less severe hypoxia, the magnitude of decline was less than that of 1997. To evaluate how these changes in prey abundance may affect demersal fishes, we analyzed the diet of Atlantic croaker Micropagonias undulatus, the most abundant demersal fish in the system, prior to and after summer hypoxia of 1998. This analysis demonstrated a shift in croaker diet away from clams, an abundant item before hypoxia, to less nutritional items such as plant and detrital material afterwards. This dietary shift reflects the decreased abundance of clams as a result of hypoxia/anoxia. We conclude from our study of the Neuse River Estuary that hypoxia/anoxia events degrade essential fish habitat and that this degradation has the potential to reduce the capacity of the system to support production of demersal fisheries.

Several interrelated factors affect, water quality in the Albemarle-Pamlico Estuarine System (APES) including land use change in the upland and coastal watersheds, legislatively mandated basin-wide nutrient management plans, intense storms, and global and local changes in sea level. Despite its importance as an essential fish habitat, the APES has not been monitored as intensively or extensively for habitat impacts associated with decreased water quality as other estuaries have been, such as with the North Carolina tributary estuaries or Chesapeake Bay. To support the sustainable use of these estuaries, we are developing an automated water quality monitoring system aboard ferries that traverse the APES. This program, FerryMon, provides a unique, long-term, and cost-effective monitoring system to evaluate status and trends in APES water quality. Intensive temporal and spatial data obtained from all ferry routes provide an environmental baseline and are used to assess the patterns and variability in surface water hydrography, dissolved constituents, and particulate matter. The data are useful to calibrate estimates of ocean color and sea surface temperature from aircraft and satellite sensors. We are creating a searchable geographic database that is intended for scientists, managers, and the general public. Using ferries as sampling platforms to monitor estuarine water quality is a tractable approach and FerryMon represents a model for use in other large bodies of water traversed by ferries.

Bottom-water hypoxia (less than or equal to2 mg l(-1) dissolved oxygen [DO]) greatly modifies the benthic habitat of estuaries, depending upon spatial extent, duration, and frequency. Bottom-water hypoxia often develops under conditions of density stratification, which inhibits vertical mixing, and warm temperatures, which enhance biological oxygen demand. Long-term, mid-channel data from the Neuse River Estuary in North Carolina permitted evaluation of how stratification and temperature combined to affect DO concentrations at the bottom. Salinity stratification (AS) and water temperature (T) explained respectively 30 and 23 % of the variance in bottom-water DO concentrations. The amount of salinity stratification required to induce bottom-water hypoxia declined with increasing water temperature. About 80 % of observed hydrographic profiles exhibited bottom hypoxia when AS exceeded 5 psu and T exceeded 20degreesC. Using cross-channel hydrographic surveys as verification, we derived a general set of methods to estimate the lateral extent of low-DO bottom water from mid-channel hydrographic profiles, The method involves cross-estuary and along-estuary extrapolation based on assumption of a flat oxycline. Occasional violation of this assumption resulted in modest overestimation in cross-channel extent of low DO. Application of this method produced estimates ranging from 0 to 116 km(2) of bottom area (0 to 42 % of the estuarine study area) exposed to hypoxia over all sample dates in summer 1997. The maximal bottom area exposed to hypoxia corresponded closely with an independent estimate of the area (100 km(2)) that experienced almost complete mortality of Macoma spp. clams, the key benthic resource for demersal fishes and crabs. Consequently, mid-channel hydrographic profiles taken along the mid-channel of the estuary can be employed to assess the spatial scale of bottom habitat degradation due to hypoxia.

Three sequential hurricanes, Dennis, Floyd, and Irene, affected coastal North Carolina in September and October 1999. These hurricanes inundated the region with up to 1 m of rainfall, causing 50- to 500-year flooding in the watershed of the Pamlico Sound, the largest lagoonal estuary in the United States and a key West Atlantic fisheries nursery. We investigated the ecosystem-level impacts on and responses of the Sound to the floodwater discharge. Floodwaters displaced three-fourths of the volume of the Sound, depressed salinity by a similar amount, and delivered at least half of the typical annual nitrogen load to this nitrogen-sensitive ecosystem. Organic carbon concentrations in floodwaters entering Pamlico Sound via a major tributary (the Neuse River Estuary) were at least 2-fold higher than concentrations under prefloodwater conditions. A cascading set of physical, chemical, and ecological impacts followed, including strong vertical stratification, bottom water hypoxia, a sustained increase in algal biomass, displacement of many marine organisms, and a rise in fish disease. Because of the Sound's long residence time (≈1 year), we hypothesize that the effects of the short-term nutrient enrichment could prove to be multiannual. A predicted increase in the frequency of hurricane activity over the next few decades may cause longer-term biogeochemical and trophic changes in this and other estuarine and coastal habitats.

Mathematical modelling is a useful tool to investigate potential ecological responses to variations in hydrodynamic and nutrient inputs in managed aquatic landscapes. The objective of this study was to develop a periphyton growth model and use it to identify critical factors in ecosystem phosphorus dynamics in pristine freshwater wetlands of Everglades National Park (ENP). We simulated changes in periphyton biomass under increased total phosphorus (TP) input for comparison to an ongoing manipulative field experiment. The field experiment analyzes the responses of periphyton, macrophytes, soils, and fauna to increased TP concentrations in replicated flow-through flume channels built within ENP. Both field and model experiments introduce TP enrichments of 5, 15, and 30 μg l−1 above ambient concentrations (typically 5–10 μg l−1). The model domain was an individual flume channel (3 m wide×85 m long) with incident light, water temperature, and volume flux at the head as forcing functions. Base case model periphyton biomass increased with incoming TP during the wet season (June–November) and reached a maximum of 70 g C m−2 in late August. There was strong evidence of P-limited primary production as water column TP accounted for 95.5% of the variability in average daily carbon biomass. In addition to positive responses in biomass, water column TP and net production also increased under increased TP input. TP enrichment led to lower recycling within and increased export from the model flume channel. Although our simplified model was not structured to simulate shifts in periphyton composition frequently observed as a consequence of P enrichment, the processes and ramifications of community structure are complicated and are not well understood. We suggest a two or more component formulation that can account for a continuum of oligotrophic to eutrophic conditions in order to model periphyton biogeochemical relationships in the Everglades. Refined versions of this model will be used to better simulate observed changes during the course of the field experiment in order to generate new hypotheses for further investigations.

The littoral zone of Chesapeake Bay contains a mosaic of shallow vegetated and nonvegetated habitats with biotic components that are sensitive to changes in biological and physical driving factors. Static and dynamic modelling frameworks provide an integrative way to study complex hydrodynamic and biogeochemical processes in linked estuarine habitats. In this study we describe a spatial simulation model developed and calibrated relative to a specific littoral zone, estuarine ecosystem. The model consisted of four distinct habitats that contained phytoplankton, sediment microalgae, Zostera marina (eelgrass), and Spartina alterniflora. There was tidal exchange of phytoplankton, particulate and dissolved organic carbon and dissolved inorganic nitrogen between the littoral zone ecosystem and the offshore channel. Physical exchange and biogeochemical transformations within the habitats determined water column concentrations in each habitat. Predicted subtidal water column concentrations and Z. marina and S. alterniflora biomass were within the variability of validation data and the predicted annual rates of net primary production were similar to measured rates. Phytoplankton accounted for 17%, sediment microalgae 46%, the Z. marina community 24% and S. alterniflora 13% of the annual littoral zone primary production. The linked habitat model provided insights into producer, habitat and ecosystem carbon and nitrogen properties that might not have been evident with stand-alone models. Although it was an intra-ecosystem sink for particulate carbon, the seagrass habitat was a DOC source and responsible for over 30% of the littoral zone carbon and nitrogen primary production. The model predicted that the Goodwin Islands littoral zone was a sink of channel derived POC, but a source of DOC to the surrounding estuary. The framework created in this study of estuarine ecosystem dynamics is applicable to many different aquatic systems over a range of spatial and temporal scales.