Effects of the shoot density of seagrass on fish and decapods: are correlation evident over larger spatial scales?

Two conflicting models for the organisation of assemblages of fish and decapods associated with seagrass over large spatial scales, make contradictory predictions about the relationship between density of seagrass shoots and abundance, or diversity, of animals. We tested the predictions of both models by sampling small lish and decapods associated with two species of seagrass (Zostera capricorni Aschers and Posidonia australis Hook) at up to 16 sites within several estuaries in New South Wales, Australia, for 1.5 yr (December 1988 to March 1990). Variation in density of Z. capricorni shoots explained very little of the variation in abundance of animals. However, abundance of one species, the grass shirmp Macrobrachium intermedium, was more closely related to the density of shoots during non-recruitment seasons, suggesting that predation or emigration of individuals after settlement was greater in sparse beds. The effect of variation in density of P. australis shoots was confounded with consistent distribution patterns of most fish and decapod species. As a result, data from P. australis did not provide good tests of the hypotheses. We conclude that density of seagrass shoots explained very little of the large-scale variation in abundance of associated fish and decapods. The data do, however, support the inodel which predicts that the abundance of animals among separate seagrass beds will follow the supply of new individuals to them.     

Comparative acute freshwater hazard assessment and preliminary PNEC development for eight fluorinated acids

Short-term 48, 72 and 96-h aquatic toxicity tests were conducted to evaluate the acute toxicity of eight fluorinated acids to the cladoceran, Daphnia magna, the green alga, Pseudokirchneriella subcapitata, and the rainbow trout, Oncorhynchus mykiss or the fathead minnow, Pimephales promelas. The eight fluorinated acids studied were tridecafluorohexyl ethanoic acid (6:2 FTCA), heptadecafluorooctyl ethanoic acid (8:2 FTCA), 2H-dodecafluoro-2-octenoic acid (6:2 FTUCA), 2H-hexadecafluoro-2-decenoic acid (8:2 FTUCA), 2H,2H,3H,3H-undecafluoro octanoic acid (5:3 acid), 2H,2H,3H,3H-pentadecafluoro decanoic acid (7:3 acid), n-perfluoropentanoic acid (PFPeA) and n-perfluorodecanoic acid (PFDA). The results of the acute toxicity tests conducted during this study suggest that the polyfluorinated acids, 8:2 FTCA, 8:2 FTUCA, 6:2 FTCA, 6:2 FTUCA, 7:3 acid and 5:3 acid, and the perfluorinated acids PFPeA and PFDA, are generally of low to medium concern based on evaluation of their acute freshwater toxicity (EC/LC50s typically between 1 and >100 mg L⁻¹) using the USEPA TSCA aquatic toxicity evaluation paradigm. For the polyfluorinated acids, aquatic toxicity generally decreased as the number of fluorinated carbons decreased and as the overall carbon chain length decreased from 12 to 8. Acute aquatic toxicity of the 5 and 10 carbon perfluorocarboxylic acids (EC/LC50s between 10.6 and >100 mg L⁻¹) was greater or similar to that of the 6-9 carbon perfluorocarboxylic acids (EC/LC50s > 96.5 mg L⁻¹). This study also provides the first report of the acute aquatic toxicity of the 5:3 acid (EC/LC50s of 22.5 to >103 mg L⁻¹) which demonstrated less aquatic toxicity than the 7:3 acid (EC/LC50s of 0.4-32 mg L⁻¹). The cladoceran, D. magna and the green alga, P. subcapitata had generally similar EC50 values for a given substance while fish were typically equally or less sensitive with the exception that PFPeA was most toxic to fish. Predicted no-effect concentrations (PNECs) were estimated using approaches consistent with REACH guidance and when compared with available environmental concentrations, these PNECs suggest that the fluorinated acids tested pose little risk for aquatic organisms.     

Variation in assemblages of fish associated with deep and shallow margins of the seagrass Posidonia australis

Little is known about the fish fauna of deep Posidonia australis seagrass beds in Australia. We investigated associated with the deep and shallow margins of P. australis during two surveys in Jervis Bay, New South Wales. Fish associated with deep and shallow seagrass at each of two areas within a large bed of P. australis were compared over a period of 3 mo (November 1986 to January 1987). This was followed by a larger scale survey in which we compared fish found at the two depths within three large beds of P. australis in Jervis Bay over 2 yr (September 1988 to June 1990). In the smaller scale survey there were marked dissimilarities in relative abhundance and composition of species between samples from deep and shallow P. australis. There were also significant differences in abundance between deep and shallow seagrass on most occasions for the majority of the dominant fish species. As a result of the small-scale survey, we concluded that differences in species composition of fish associated with deep and shallow P. australis may have been due to the presence of species from nearby bare substrate among the sparser leaves of the deep seagrass. The results of the smaller scale survey were not confirmed by the larger survey. The relative abundances of species in samples from deep and shallow P. australis were not dissimilar for all beds. In addition, it was rare for any of the dominant species to have significantly different abundances in deep and shallow seagrass. There was little evidence that the species composition of fish associated with deep P. australis was strongly influenced by the fauna of deep bare substrate. Rather, samples from deep P. australis were far more similar to those from shallow P. australis than to a comparable set of samples collected from deep bare substrata. This study demonstrates how the results and conclusions of small-scale survey work, a feature of many environmental impact asseesments, can be misleading when applied to larger spatial and temporal scales, even within the same bay.