Macrobenthic Responses to Natural and Contaminant-Related Gradients in Northern Gulf of Mexico Estuaries

Abstract

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.

Spatial and temporal variation of the benthic macrofauna in a grossly polluted estuary from southwestern Spain

“…The natural gradients in salinity, granulometry and organic content have been described as the most important factors to explain the distribution and abundance of macrobenthic community in numerous estuarine ecosystems (Wolff 1983;Warwick et al 1991;Attrill et al 1996;Rakocinski et al 1997;Ysebaert et al 2002;Mucha et al 2003;Sousa et al 2006). In general, The Tinto-Odiel system has shown typical water and sediment characteristics of estuarine areas at the spatial scale.…”

“…All estuaries are characterized by a soft-bottom macrobenthic communities impoverished in relation to those from marine sediments (Wolff 1983;Warwick et al 1991;Rakocinski et al 1997;Peeters et al 2000) and this can difficult the interpretation of the effects of pollution on the structure of the animal communities. However, it has been demonstrated that the human impact increases this impoverishment in species diversity, for example, Marques et al (1993) observed that the subtidal macrofauna in the Mondego Estuary appeared to be clearly impoverished compared to other Portuguese estuary much less exposed to human impacts.…”

“…This often makes difficult the interpretation of the effects of disturbance on the animal communities and confounds the impacts of anthropogenic activity on estuarine biotic integrity (Rakocinski et al 1997;Dauvin et al 2006;Dauvin 2008).…”

Avoidance of contaminated sediments by an amphipod (Melita plumulosa), A harpacticoid copepod (Nitocra spinipes), and a snail (Phallomedusa solida)

“…Sediments can vary greatly in physical and chemical properties, and these variations can influence the nature of the resident ecosystem structure [10,11]. Before the avoidance of M. plumulosa, N. spinipes, and P. solida was determined for contaminated sediments, we determined whether varying sediment grain size and organic carbon content influenced the behavior of organisms when exposed to uncontaminated sediments (Table 1).…”

“…The concentration of toxicants in surface sediments will often vary spatially, and a number of organisms have demonstrated the ability to detect and actively avoid areas of contamination [10,11]. In the natural environment, individual organisms, and eventually populations, will migrate away from a contaminated site before the exposure results in the uptake of a harmful dose (sublethal or lethal).…”