Mapping Hypoxia Response to Estuarine Nitrogen Loading Using Molybdenum in Sediments.

Quantitative relationships between nitrogen loading and ecological effects such as hypoxia are critical to developing nitrogen (N) standards for coastal waters, but spatial and temporal variability within estuaries can make the determination of such relationships difficult. Accumulation of molybdenum (Mo) in surface sediments has been proposed as a quantitative indicator of the duration of hypoxia (defined as dissolved oxygen concentrations below 2.8 mg/L) in overlying waters, providing a metric to evaluate the relationship between varying N loads and the occurrence and duration of hypoxic conditions. Nitrogen loads were estimated for seven Rhode Island embayments based on watershed land use and normalized for embayment volume and local residence times (LRT) derived from hydrodynamic modeling. Mo was measured in surface sediments from sampling sites selected within and across the embayments to span the range of N loads. The spatial distribution of sediment Mo within the embayments closely followed that of normalized N loads, and Mo concentrations approximated a second-order relationship with normalized N loads. Sediment Mo concentrations were converted to mean annual duration of hypoxia using a previously derived linear relationship between Mo in surface sediments and annual duration of hypoxia in overlying water, and a quantitative relationship derived between normalized N loads and annual duration of hypoxia. Evaluation of that relationship provides an approach to develop standards for N loading in coastal waters.

Molybdenum accumulation in sediments: a quantitative indicator of hypoxic water conditions in Narragansett Bay, RI.

Authigenic molybdenum (Mo) accumulation in marine sediments has often been used as a qualitative indicator of hypoxic bottom water. To investigate its use as a quantitative indicator of hypoxic exposure, sediment cores were collected from water quality monitoring sites in Narragansett Bay (RI, USA) that experience varying periods of hypoxia. Total Mo concentrations in surficial (0-1 cm) sediments were determined by total digestion and ICP-MS analysis. Lithogenic contributions to total Mo concentrations were estimated by multiplying measured concentrations of aluminum (Al) by the mean crustal Mo:Al ratio and subtracting them from the total concentrations to yield the authigenic fraction. 210Pb dating was used to determine sediment accumulation rates at each site. Mean annual periods of hypoxia in bottom waters were determined from continuous monitoring data for the years coinciding with the top 1 cm of sediment. Results indicated a linear relationship between authigenic Mo concentrations and frequency of hypoxia, although the relationships differed between different sampling periods. These results demonstrate the potential of sedimentary Mo as a tool for assessing the spatial and temporal extent of hypoxia in coastal waters.

Population genetic structure of a nonmigratory estuarine fish (Fundulus heteroclitus) across a strong gradient of polychlorinated biphenyl contamination.

Populations of the estuarine fish Fundulus heteroclitus indigenous to contaminated sites exhibit heritable resistance to some of the toxic effects of early life-stage exposure to polychlorinated biphenyls (PCBs). This evolved tolerance provides evidence of strong selection by PCBs, and it suggests other potential genetic effects of these stressors on resident populations. Environmental contaminants have the potential to affect the genetic structure of populations and to reduce genetic diversity, but species life-history traits, particularly patterns of migration and dispersal, also influence the distribution of genetic variation among populations. Therefore, the present was conducted to determine whether genetic diversity or genetic structure is altered in populations of F. heteroclitus indigenous to 18 sites in Massachusetts (USA) and Rhode Island (USA), representing a steep gradient of sediment PCB contamination and culminating in a Superfund site at New Bedford Harbor (NBH; MA, USA). Allele frequencies at enzymatic loci were used to assess genetic structure and diversity. Selection experiments using a highly toxic PCB congener (3,3',4,4',5-pentachlorobiphenyl) were conducted to determine if genetic patterns at field sites could be associated with contaminant exposures. Although allele frequencies clearly reflected a pattern of isolation by distance, the results indicated neither significant loss of genetic diversity nor alteration of allele frequencies for populations of F. heteroclitus in NBH.

Population genetic structure and tolerance to dioxin-like compounds of a migratory marine fish (Menidia menidia) at polychlorinated biphenyl-contaminated and reference sites.

The present study was conducted to evaluate evidence of genetic adaptation to local contaminants in populations of the migratory marine fish Menidia menidia residing seasonally in reference sites or an industrial harbor contaminated with dioxin-like compounds (DLCs). For this purpose, we compared DLC sensitivity and genetic patterns of populations sampled from sites both inside and outside New Bedford Harbor (NBH; MA, USA), a U.S. Environmental Protection Agency Superfund site with extreme polychlorinated biphenyl (PCB) contamination. Offspring of M. menidia collected from NBH were significantly less sensitive regarding embryonic exposure to the dioxin-like PCB congener 3,3',4,4',5-pentachlorobiphenyl (PCB 126) than offspring of M. menidia from a reference site. Analysis of 10 polymorphic enzymatic loci indicated little genetic differentiation among populations in the study area. However, genotype frequencies of juveniles from both NBH and an adjacent site in Massachusetts exhibited significant deviations from Hardy-Weinberg equilibrium expectations at one locus, phosphoglucomutase (PGM*). Genetic analysis of survivors of embryonic laboratory exposure to PCB 126 indicated that genotypes at PGM* were related to survivorship. Although a relationship was identified between DLC tolerance and PGM* genotype, regional mixing of M. menidia populations during migration and absence of multigeneration exposure at contaminated sites may limit localized adaptation.

Predicting the occurrence of genetic adaptation to dioxinlike compounds in populations of the estuarine fish Fundulus heteroclitus.

A population of the nonmigratory estuarine fish species Fundulus heteroclitus (mummichog) indigenous to a polychlorinated biphenyl (PCB)-contaminated Superfund site (New Bedford Harbor, MA, USA) demonstrated an inherited tolerance to local, dioxinlike contaminants (DLCs). These findings suggest that DLCs have acted as selective agents, allowing the survival of only the most tolerant individuals, forming DLC-adapted populations. We hypothesized that DLC-tolerant mummichog populations would reside where local conditions are toxic to sensitive individuals, and that toxic environmental conditions could be predicted based on responses of sensitive early life stages to laboratory exposures of DLCs. As a measure of DLC tolerance, progeny of field-collected fish were tested in the laboratory with a dioxinlike PCB congener, 3,3',4,4',5-pentachlorobiphenyl (PCB 126). Mummichog populations were collected from sites with sediment PCB concentrations predicted to range from nontoxic to toxic. Consistent with predictions, tolerant populations were indigenous to sites with elevated sediment PCB concentrations. Also, as predicted, DLC-tolerant populations were resident to sites far less contaminated than the Superfund site. These results suggest that exposures to persistent, bioaccumulative, and toxic contaminants may produce evolutionary effects on a geographic scale larger than previously envisioned. This study presents an approach and describes a model system that may improve understanding of the scale of occurrence for these potentially irreversible ecological effects.