The spatial variation of MeHg production, bioaccumulation and biomagnification in marine food webs is poorly characterized but critical to understanding the links between sources and higher trophic levels such as fish that are ultimately vectors of human and wildlife exposure. This paper discusses both large and local scale processes controlling Hg supply, methylation, bioaccumulation and transfer in marine ecosystems. While global estimates of Hg supply suggest important open ocean reservoirs of MeHg, only coastal processes and food webs are known sources of MeHg production, bioaccumulation, and bioadvection. The patterns observed to date suggest that not all sources and biotic receptors are spatially linked and that physical and ecological processes are important in transferring MeHg from source regions to bioaccumulation in marine food webs and from lower to higher trophic levels.

The effects of hypoxia on trophic interactions could vary dramatically depending on whether the benthos is experiencing the onset of a hypoxic event (decreasing dissolved oxygen concentrations from normoxi.a), or its dissipation (increasing dissolved oxygen concentrations from hypoxia). Predator-prey dynamics between the blue crab Callinectes sapidus and an infaunal clam prey Mya arenaria were examined to assess the impact of hypoxia upon predator foraging rates and prey mortality. Laboratory experiments quantified the behavioral response of M arenaria to varying dissolved oxygen levels through the analyses of (1) sediment burial depth, and (2) siphon extension above the sediment surface. Moreover, the funct~onal response (relationship between predator consumption rates and prey density) of single and 2 adult blue crabs to 2 densities of M. arenaria (6 and 24 clams tank-') was examined across 3 dissolved oxygen treatments: (1) normoxia (26.0 mg DO I-'); (2) moderate hypoxia (3.0 to 4.0 mg DO I-'), subsequent to clam acclimation at high oxygen conditions (26.0 mg DO I-'); and (3) moderate hypoxia (3.0 to 4.0 mg DO I-'), subsequent to clam acclimation at low oxygen conditions (51.5 mg DO I-'). M. arenaria sediment burial depth decreased and siphon extension increased during exposure to severe hypoxia. Initiation of moderate hypoxla following normoxia altered blue crab foraging behavior from a destabilizing, type 11 functional response, to a partially stabilizing, type 1 functional response. Conversely, blue crabs exhibited a type I1 functional response under moderate hypoxia subsequent to clam exposure to severe hypoxia. Therefore, low dissolved oxygen concentrations appear to affect the predator-prey ~nteraction between C. sapidusand M. arenaria by either hindering blue crab foraging, or alternatively, increasing clam vulnerability by altering their siphon extension and depth distribution within the sediment column. Moreover, the inclusion of a second blue crab in experimental trials further modified functional responses through both mutual interference and agonist~c behavior or cooperative foraging between predators. The collective results indicate that fluctuations in dissolved oxygen concentrations, as well as both predator and prey density, must be examined jointly to understand their impact upon predator-prey dynamics in marine systems.

Predator-prey dynamics between the sand shrimp Crangon septemspinosa and juvenile winter flounder Pseudopleuronectes americanus were examined in laboratory experiments to assess the joint effects of varying prey density and temperature on shrimp foraging behavior and flounder mortality. The functional response of shrimp to 6 densities of flounder was determined at 2 temperatures (10 and 16°C). The behavioral mechanisms underlying the shrimp's functional response were quantified with visual observations and compared to the foraging parameters predicted by continuous-time functional response models. Shrimp consumption rates increased significantly with increasing flounder density, irrespective of water temperature. At low flounder densities, however, significantly more flounder were consumed at 16°C than at 10°C. Analysis of proportional mortality of flounder across prey density and general functional response models revealed a sigmoidal, Type III functional response at 10°C, and a hyperbolic, Type II functional response at 16°C. Model parameter estimates and visual observations of shrimp foraging behavior suggest that the variable functional responses at different temperatures are the result of cold temperatures decreasing predator activity at low flounder densities, and conversely, shrimp maintaining high attack rates at low flounder densities when exposed to warm temperatures. These findings indicate that shrimp are capable of driving flounder populations to local extinction during warm water conditions. The recent warming trend experienced in northwest Atlantic estuaries, and its impact on trophic dynamics, may therefore explain the failure of the winter flounder stocks to recover in these areas.

Total mercury (Hg) and methylmercury (MeHg) were analyzed in near surface sediments (0–2 cm) and biota (zooplankton, macro-invertebrates, finfish) collected from Narragansett Bay (Rhode Island/Massachusetts, USA) and adjacent embayments and tidal rivers. Spatial patterns in sediment contamination were governed by the high affinity of Hg for total organic carbon (TOC). Sediment MeHg and percent MeHg were also inversely related to summer bottom water dissolved oxygen (DO) concentrations, presumably due to the increased activity of methylating bacteria. For biota, Hg accumulation was influenced by inter-specific habitat preferences and trophic structure, and sediments with high TOC and percent silt-clay composition limited mercury bioavailability. Moreover, hypoxic bottom water limited Hg bioaccumulation, which is possibly mediated by a reduction in biotic foraging, and thus, dietary uptake of mercury. Finally, most biota demonstrated a significant positive relationship between tissue and TOC-normalized sediment Hg, but relationships were much weaker or absent for sediment MeHg. These results have important implications for the utility of estuarine biota as subjects for mercury monitoring programs.

We examined the bioaccumulation and trophic transfer of mercury in two marine finfish species, striped bass (Morone saxatilis) and tautog (Tautoga onitis), collected from the Narragansett Bay (Rhode Island, USA). For each of these target fish, white muscle tissue was analyzed for total mercury (Hg) and results were evaluated relative to fish age, body size, and Hg content of preferred prey. Dietary and stable isotope analysis was also used to elucidate the effect of trophic processes on Hg concentrations in fish. The Hg content of muscle tissue was positively correlated with fish age and length for both species, although striped bass accumulated Hg faster than tautog. Accelerated Hg bioaccumulation in striped bass is consistent with its high trophic level (trophic level = 4.07) and Hg-enriched prey (forage fish and macrocrustaceans; mean Hg content = 0.03 mg Hg kg wet wt(-1)). In contrast, tautog maintain a lower trophic status (trophic level=3.51) and consume prey with lower Hg levels (mussels and crabs; mean Hg content = 0.02 mg Hg kg wet wt(-1)). Despite differences in Hg bioaccumulation between target fish, the mean Hg concentration of tautog exceeded levels in striped bass (0.24 and 0.16 mg Hg kg wet wt(-1), respectively) due to a disparity in age-at-catch between sampled groups (mean age of tautog and bass = 11.3 and 4.3 yr, respectively). Taking into account legal minimum catch lengths further revealed that 75.0% of legal-size striped bass (>70.2 cm TL; n = 4) and 44.8% of tautog (> 40.6 cm TL; n = 29) had Hg levels beyond the US EPA regulatory threshold of 0.3 mg Hg kg wet wt(-1). Moreover, Hg-length relationships suggest that each target fish meets this threshold near their minimum legal catch length. Our findings reiterate the value of species ecology to improve predictions of fish Hg and permit better management of human contamination by this important dietary source.