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22-2/H56-07/18-3 Contour interval: 2

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Thematic issue

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Benthic flux studies conducted from February 1995 to January 1996 focused on (1) spatial and temporal consistency in benthic fluxes at key sites in Port Phillip Bay (2) benthic chamber calibration (3) bio-irrigation measurements (4) direct measurements of N2 for independent assessments of denitrification rates and efficiencies at key sites. The most important results of the study are: 1) The general pattern in nutrient fluxes observed in 1994, is repeated in the 1995 data. 2) The flux data show no sensitivity toward paddle stir rate indicating that benthic processes are insensitive to diffusive boundary layer thickness over a scale factor of two. 3) The measured N:C flux ratios for the February 1995 study, was always less than the predicted value and the missing N is interpreted to be the result of denitrification. Direct measurements by Gas Chromatography (GC) of N2 fluxes generally supported this conclusion. 4) Added nitrate was advected into the sediments. Only the naturally formed nitrate was denitrified, suggesting that there are microbial microniches in which organic carbon oxidation, ammonification, nitrification and denitrification occurs simultaneously. 5) The mass spectrometric measurements of N2 during January/December 1996 proved beyond doubt that denitrification occurs in the sediments of Port Phillip Bay. 6) There was a significant seasonal effect in the denitrification efficiencies (measured) at site 16 (Yarra Estuary) but seasonal effects on denitrification efficiencies in the central sediments (Site 37) were marginal. 7) Denitrifying efficiencies were consistently high in the central basin sediments.

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