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We investigated the seasonal dynamics of flow cytometrically-defined populations of viruses, heterotrophic bacteria, and the picoeukaryotic and prokaryotic phytoplankton at three sites in the temperate oligotrophic inverse estuary of Spencer Gulf (South Australia). We consistently identified two sub-populations of viruses, three sub-populations of heterotrophic bacteria, one population of picoeukaryotic phytoplankton and two populations of prokaryotic phytoplankton (cyanobacteria Prochlorococcus and Synechococcus). Both the cytometric community composition and the abundance of viruses, heterotrophic bacteria and both prokaryotic (Synechococcus and Prochlorococcus) and eukaryotic picophytoplankton were consistent with previous observations conducted in South Australian continental shelf waters. Noticeably LDNA bacteria (i.e. inactive or dormant cells) were consistently significantly the most abundant group of heterotrophic bacteria (totaling from 29% to 68% of total bacterial abundance) and were up to 10-fold more abundant than that previously reported in South Australian continental shelf waters, including the nearby Saint Vincent Gulf. These results suggest an overall low activity of the microbial community, and are consistent with previous evidence that LDNA cells may play a greater role in heterotrophic processes than HDNA cells in oligotrophic waters. In an attempt to further assess the qualitative and quantitative nature of the mortality of these organisms, we used a specific dilution assay to assess the relative contribution of viruses and microzooplankton grazers to the mortality rates of heterotrophic bacteria, and picoeukaryotic and prokaryotic phytoplankton. We consistently reported site-specific, population specific and sea-son-specific viral lysis and grazing rates of heterotrophic bacteria and the picoeukaryotic and prokaryotic (cyanobacteria Prochlorococcus and Synechococcus) phytoplankton across sites and seasons. Specifically, both viral lysis and micro-zooplankton grazing rates of heterotrophic bacteria were consistently relatively low across sites and seasons, even though their seasonality suggested an overall dominance of grazing over viral lysis in both summer and winter. In contrast, no seasonality is found in either lysis or grazing rates of prokaryotic and eukaryotic picophytoplankton, which are comparable to previous observations conducted in oligotrophic waters, suggesting the mortality dynamics of these populations is similar to those encountered in other oligotrophic waters. The observed patterns of mortality rates of heterotrophic bacteria and both prokaryotic and eukaryotic picophytoplankton are consistent with the low chlorophyll concentration and production previously observed in the waters of the Spencer Gulf.

Virally-Mediated Versus Grazer-Induced Mortality Rates in a Warm-Temperate Inverse Estuary (Spencer Gulf, South Australia)