Gary R. Gaston

A benthic index of environmental condition of Gulf of Mexico estuaries

An index was developed for estuarine macrobenthos in the Gulf of Mexico that discriminated between areas with degraded environmental conditions and areas with undegraded or reference conditions. Test sites were identified as degraded or reference based on criteria for dissolved oxygen levels, sediment toxicity tests, and sediment contamination. Discriminant analysis was used to identify a suite of measures of benthic community composition and diversity that would most successfully distinguish degraded from undegraded sites. The resultant benthic index was composed of a linear combination of three factors: the Shannon-Wiener diversity index, the proportion of total benthic abundance as tubificid oligochaetes, and the proportion of total benthic abundance as bivalve molluscs. This index was used to evaluate the spatial patterns of degraded benthic resources in the Gulf of Mexico.

MACROBENTHIC RESPONSES TO NATURAL AND CONTAMINANT‐RELATED GRADIENTS IN NORTHERN GULF OF MEXICO ESTUARIES

Effects of pollution on biotic integrity are difficult to identify when correlations occur between environmental gradients and contaminant effects, as they do in estuaries. In this broad-scale study, we used canonical correspondence analysis (CCA) to distinguish influences of natural and contaminant-related gradients on macrobenthic community structure among 319 sites from estuaries throughout the northern Gulf of Mexico. Natural gradients in salinity, depth, and sediment composition obscured the detection of macrobenthic responses to sediment contamination. After adjusting for natural environmental variability, however, partial CCA revealed important macrobenthic variation in relation to sediment contamination. A rotated principal component analysis (PCA) distinguished five composite environmental factors, each largely reflecting contaminant or natural variation. Two complex gradients in sediment contamination identified by the PCA diverged in partial CCA space and correlated with different macrobenthic indicator taxa. Contaminant gradients represented variation in two different classes of sediment contaminants: trace metals and organic chemicals. Dispersion patterns of CCA site coordinates enabled cross validation of implied contamination-related variation in community function and the utility of several interpretive or management metrics. Trophic diversity decreased with sediment contamination, linking shifts in macrobenthic community function and community structure along contaminant gradients. The CCA model complemented an earlier benthic index developed from these data to examine biotic integrity, but the benthic index could not discern macrobenthic responses to the different contaminant gradients. Neither was the benthic index useful for showing transitions in macrobenthic community structure commensurate with different levels of contamination. Ampelisca amphipod sediment bioassays were inadequate for identifying contaminant effects on biotic integrity, whereas Mysidopsis mysid sediment bioassays conservatively reflected sediment contamination and associated macrobenthic indicators.

Effects of Sediment Contaminants and Environmental Gradients on Macrobenthic Community Trophic Structure in Gulf of Mexico Estuaries

Macrobenthic communities from estuaries throughout the northern Gulf of Mexico were studied to assess the influence of sediment contaminants and natural environmental factors on macrobenthic community trophic structure. Community trophic data were also used to evaluate whether results from laboratory sediment toxicity tests were effective indicators of site-specific differences in benthic trophic structure. A multiple regression model consisting of five composite factors (principal components) was used to distinguish the effects of sediment contaminants and environmental variables on benthic community trophic structure. This model explained 33.5% of the variation in macrobenthic trophic diversity (p<0.001), a variable derived from the distribution of taxas among nine original trophic categories. A significant negative relatinship was found between principal components reflecting concentrations of sediment contaminants and macrobenthic trophic diversity.Detritivores including surface deposit-feeders (SDF), subsurface deposit-feeders (SSDF), and filter feeders (FF) were numerically dominant at 201 random sites, each group accounting for 25–30% of total macrobenthic abundance. The relative abundance of SDFs was considerably lower (12.1±2.9% to 17.1±4.4%) at sites where sediment contaminant concentrations exceeded minimum biological effects thresholds (ER-L values from Long and Morgan 1990 than at sites sampled at random (29.3±5.7%). SSDFs were proportionally more abundant at contaminated sites (42.0±7.7% to 63.6±10.3%) versus random sites (27.5±5.7%), and the relative abundance of SSDFs was positively correlated with concentrations of particular contaminants. Benthic trophic structure was also found to be a function of salinity, where the proportion of SSDFs was negatively correlated with salinity (p=0.035, r=−0.223, n=326). Silt-clay content loaded fairly strongly on the first principal component, but trophic structure parameters were not significantly correlated with sediment grain size or dissolved oxygen (perhaps due, in part, to covariation). Results from laboratory sediment toxicity tests with mysids were predictive of differences in macrobenthic trophic structure in situ (i.e., mysid survival was negatively correlated with %SSDF; p<0.001, r=−0.292, n=326). Results from laboratory sediment toxicity tests with ampeliscid amphipods were not indicative of site-specific differences in benthic trophic structure.Results from this study demonstrated that sediment contaminants can be quite important in structuring macrobenthic communities in soft-bottom estuarine habitats. The fact that macrobenthic trophic diversity decreased significantly with increasing sediment contamination indicates that important general differences in benthic community function may exist between contaminated and random sites. These data suggest that benthic trophic structure analysis may be an effective tool for assessing integrated community responses to chronic sublethal exposure and may be useful for assessing toxicological responses at ecologically relevant levels of organization.

Species-abundance-biomass responses by estuarine macrobenthos to sediment chemical contamination

Macrobenthic community responses can be measured through concerted changes in univariate metrics,including species richness, total abundance, and totalbiomass. The classic model of pollution effects onmarine macrobenthic communities recognizes thatspecies/abundance/biomass (SAB) curves varydistinctively in a nonlinear manner with the magnitudeof organic enrichment. For example, at moderatelevels of organic enrichment, small-bodiedopportunistic species boost the abundance curve, whilespecies richness falls. Ratios among the metrics formuseful indicators of how the community changes withorganic enrichment. However, the classic SAB model isbased on organic enrichment effects over small spatialand temporal scales, and the applicability of the SABmodel to sediment chemical contamination and acrossbroad natural estuarine gradients is largely unknown. Here, SAB responses were examined with respect toprimary gradients in metals and organic chemicalsbased on an extensive dataset comprising 319 estuarinesites from throughout the northern Gulf of Mexico. Each SAB metric was first adjusted with respect to thethree primary natural estuarine gradients, salinity,depth, and sediment silt/clay content. Adjusted SABrelationships varied in their details with respect todifferent classes of sediment contamination, but alltypes of SAB stress responses appear to exhibitsimilar basic characteristics. As in the SAB model,all three SAB metrics were notably low at the highestconcentrations of both metal and organic-chemicalcontaminants. Moreover, rapid decreases in the B/Aratio with increasing contamination supported theconcept that relatively long-lived, large-bodied,equilibrium taxa decline markedly at highconcentrations of toxicants.

Biomass Variations of Estuarine Macrobenthos Preserved in Ethanol and Formalin

Three preservative treatments were compared for their effects on biomass of macrobenthic species collected at Biloxi Bay, Mississippi. We tested the hypothesis that biomass did not differ among treatments once specimens were fixed in 10% formalin. Two commonly used analyses, wet-weight biomass and dry-weight biomass, were tested. Wetweight variations among treatments were compared over time. Dry-weight calculations were not possible on individuals over time because specimens could be dried only once, so ratios of dry weight to wet weight were calculated on specimens at the conclusion of each treatment. Specimens were fixed in 10% formalin for 2 wk, then transferred to either 1% formalin or 70% ethanol, or left in 10% formalin. Wet-weight biomass of the three treatments was determined weekly four times, and then specimens were dried for 72 h at 60°C for dry-weight determinations. Biomass of most taxa fixed in formalin and preserved in ethanol or formalin did not vary significantly in wet weight over time. Minor variations among the treatments occurred in dry-weight biomass. Whereas previous investigators found that ethanol effected biomass when specimens were not formalin-fixed, our results supported the premise that three commonly used preservative treatments did not differ in their effects on biomass of estuarine macrobenthos. Therefore, we propose that estuarine macrobenthos be fixed in formalin, then transferred to ethanol for biomass procedures in order to avoid exposure of laboratory personnel to carcinogenic formalin.

Macrobenthic community colonization and community development in dredged material disposal habitats off coastal Louisiana

We examined marine benthic macroinvertebrate colonization and community structure at multiple spatial scales (study areas, reference and disposal sites, and depth zones within sites) within a 3-day period at three relatively widely separated (ca 60 km) dredged material disposal areas (Mermentau and Atchafalaya Rivers and Freshwater Bayou) in coastal Louisiana. Study areas had different histories of dredged material disposal, but all three are subject to frequent natural habitat disturbances (e.g. freshets). Nine phyla and 51 taxa were represented among the three study areas at reference (R) and disposal (D) sites (Freshwater Bayou: 21(R), 18 (D); Mermentau River: 14 (R), 17 (D) and Atchafalaya River: 38 (R), 40 (D)). Only 15 taxa were common to all three study areas. At the Freshwater Bayou, average taxa richness and abundance responded to water depth, not sites. These response variables averaged higher mean values at the Mermentau River disposal than at the reference site. No consistent pattern in the average of these response variables was detected between sites at the Atchafalaya River. Multidimensional scaling ordination and non-parametric multivariate inferential analysis provided a distinctly different picture of community structure within study areas compared to parametric analyses. A relatively moderate to strong separation in community structure between sites was detected depending on study area. Non-parametric multivariate inferential analysis detected significant differences in internal community structure at the scale of stations and sites within study areas. The weight of evidence suggests that frequent natural disturbances explain differences in macrobenthic animal community structure more than effects of dredged material disposal.