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Comparisons of the benthic and zooplankton communities in the eutrophic Peel-Harvey and nearby Swan estuaries in south-western Australia

Rose, Thomas H. (1994) Comparisons of the benthic and zooplankton communities in the eutrophic Peel-Harvey and nearby Swan estuaries in south-western Australia. PhD thesis, Murdoch University.

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The benthic macroinvertebrates and the zooplankton of the shallow (<1.5 m deep) sandy middle regions of the Peel-Harvey Estuary and of the nearby Swan Estuary were sampled seasonally between the winters of 1986 and 1987. Measurements were also taken of the environmental variables in the benthos and in the water column.

In comparison with the Swan Estuary, the Peel-Harvey Estuary is highly nutrient enriched and is not as well flushed. As a consequence, the water in the basins of the Peel-Harvey experience large seasonal blooms of the blue-green alga Nodularia spumigena and massive growths of benthic and drifting green algae. The first led to marked seasonal declines in redox values, secchi depths and nocturnal dissolved oxygen and, together with the decomposition of the macroalgae, resulted in a marked increase in particulate organic matter in the sediments.

The following comparisons between the benthic invertebrate fauna in the Peel- Harvey with that of the Swan Estuary are consistent with the effects of far greater eutrophication in the former system. (1) The number of individuals was far higher in the Peel-Harvey, whereas the number of species, diversity and biomass was greater in the Swan. (2) The contributions made by small and highly fecund polychaetes (e.g. Capitclla capitata), amphipods (e.g. Corophium minor), tanaids (e.g. Tanais dulongi) and microbivalves (e.g. Arthritica semen) to the numbers and biomass of individuals were much greater in the Peel-Harvey, whereas the contributions by large long-living bivalve molluscs and polychaetes to the numbers and more particularly the biomass of individuals were greater in the Swan. (3) While the densities and biomass of the main components of the benthic community rose to reach a maximum in spring or summer in the Swan Estuary, they peaked in autumn in the Peel-Harvey. The later peak in the latter system is apparently related to deleterious conditions present during spring and early summer, e.g. frequent episodes of hypoxia. Greater variability in the density and biomass of organisms at sites in the Peel-Harvey are assumed to reflect variation in the intensity of deleterious conditions produced by eutrophication within this system.

The following comparisons also indicate that the zooplankton community of the Peel-Harvey Estuary manifests the effects of eutrophication to a greater extent than that of the Swan Estuary. (1) The abundance of individuals in the Peel-Harvey was higher than in the Swan whereas the reverse situation pertained with the number of species. (2) The density of calanoids and harpacticoids was far lower in the Peel- Harvey, presumably reflecting the influence of anoxia on their benthic stages. (3) The numbers of amphipods and nematodes were greater in the Peel-Harvey. (4) As with the benthos, the density and biomass of zooplankton varied more between sites in the Peel- Harvey and the density of the main components of the zooplankton peaked later, ie. after the deleterious conditions produced during spring and summer had amehorated. There were also a much higher number of interactions between season and time of day in the density and biomass of the zooplankton and this was attributed to the combination of eutrophication-induced hypoxia and turbidity, which altered migration patterns in comparison to the Swan.

The above comparisons thus indicate that there have probably been major changes in the faunal composition of the Peel-Harvey Estuary following eutrophication and particularly the advent of Nodularia blooms since the late 1970s. This view is supported by the fact that, in comparison with the situation during the 1970s and early 1980s, the densities of gastropods in the benthos and of calanoids in the plankton are lower, while the densities of amphipods in both the benthos and zooplankton are greater.

Item Type: Thesis (PhD)
Murdoch Affiliation: School of Biological and Environmental Sciences
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: Thank you.
Supervisor(s): Potter, Ian
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