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Aspects of the ecology of a degraded Perth wetland (Lake Monger, Western Australia) and implications for Bio manipulation and other restoration techniques

Lund, Mark Andrew (1992) Aspects of the ecology of a degraded Perth wetland (Lake Monger, Western Australia) and implications for Bio manipulation and other restoration techniques. PhD thesis, Murdoch University.

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Lake Monger (3204'S 115°20'E) was sampled intensively (18 occasions) between October 1988 and October 1989. Ordination and classification of the water chemistry, plankton and macroinvertebrates revealed three seasonal groups, spring/summer, summer/autumn and winter. In terms of the water chemistry these groups corresponded to periods of hypertrophy, eutrophy and mesotrophy respectively. The lake was found to be shallow (1-1.5 m deep) and polymictic. Internal release from the sediments was believed to be responsible for the high levels of P (> 700 jig L1) recorded during summer. In summer, the limiting nutrient for algal growth appeared to be N. Two species alternated in dominance of the zooplankton, the cladoceran Daphnia carinata King in winter and the copepod Mesocyclops sp in summer groups. These species accounted for >80% of the abundance and biomass of the zooplankton when they were dominant. Changes in the edibility of summer algal populations, rather than the effects of zooplanktivorous fish (Gambusia holbrooki (Girard)), invertebrate predators (e.g. hemipterans) or temperature was believed responsible for the summer declines in D. carinata. 70 macroinvertebrate taxa were recorded, substantially higher than found in other studies at the lake. Mean species richness was highest from areas of emergent reeds.

Compilation of available data revealed a decline in water quality in the lake since European settlement, resulting from nutrient enrichment, introductions of exotic biota, removal of native vegetation, physical modification (landfill and dredging) and changes in hydrology (artificial maintenance of water levels). The study year (1988/89) appeared to be similar to other years in the late 1980's. Changes in fertilizer usage around the lake at the start of the 1990's appeared to have been responsible for subsequent significant improvements in the water quality.

Twelve in-lake enclosures (1.5 m3) were used to assess the influence of predation by G. holbrooki, N limitation and gilvin (brown colour), on zooplankton and water chemistry. Increased levels of primary productivity were recorded in untreated control enclosures. Only low levels of gilvin were produced and these resulted in a slight increase in chlorophyll a rather than the anticipated decrease. Gambusia holbrooki was not found to be responsible for any changes in the zooplankton structure. Variability between replicate enclosures and P limitation in the lake meant that N limitation could not be established.

The presence of large numbers of D. carinata was found to significantly improve water quality through grazing and removal of seston. There appeared to be a nutrient threshold of 150 fig T1 of P in the water column, above which algal composition or size was inedible for D. carinata. Biomanipulation, involving fish manipulations, appeared unlikely to be successful in improving water quality as the link between fish predation and summer declines in D. carinata appeared to be tenuous. Reductions in fertilizers used on lawns around the lake appeared to have had a significant effect on improving the water quality of the lake.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): School of Biological Sciences and Biotechnology
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): Davis, Jenny
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