Picoplankton community structure and virioplankton abundance in offshore waters of the central Great Australian Bight

The Great Australian Bight (GAB) was once thought to be an area with low biological activity due to a perceived lack of nutrient enrichment processes. Coastal upwelling processes in the eastern GAB are now known to promote... [ view full abstract ]

The Great Australian Bight (GAB) was once thought to be an area with low biological activity due to a perceived lack of nutrient enrichment processes. Coastal upwelling processes in the eastern GAB are now known to promote elevated chlorophyll concentrations and support pelagic ecosystem productivity. However, limited information is available on the lower food web dynamics underpinning these ecosystem processes. The objective of this study was to document picoplankton and virioplankton community structure in the offshore eastern and central GAB. Picoplankton and virioplankton were investigated across five shelf to offshore transects in April 2013. Prochlorococcus, Synechococcus and picoeukaryotes were all resolved using flow cytometry. Prochlorococcus numerically dominated picophytoplankton (up to 1.8 × 10^5) at all stations, exceeding Synechococcus and picoeukaryotes by up to 2 and 3 orders of magnitude respectively. Synechococcus and picoeukaryotes showed a clear shelf to offshore trend with highest abundances occurring close to the shelf. Bacterioplankton (4.8 × 10^5 – 10.9 × 10^5) and virioplankton (6.6 x 10^6 – 21.9 × 10^6) exhibited a patchier distribution over the study region. Regionally, highest picophytoplankton biomass (up to 19.2 g C l-1) occurred close to the shelf. Here, picoeukaryotes accounted for up to 65% of picophytoplankton, with approximately equal C biomass derived from Synechococcus and Prochlorococcus. Bacterioplankton C biomass accounted for a further 9.4 g C l-1. The percentage contribution of bacterioplankton to overall picoplankton biomass changed significantly from shelf (averaging 40%) to offshore (averaging 70%). Viral derived C was negligible compared to picoplankton derived C, however significant positive relationships between virioplankton, Prochlorococcus and bacterioplankton suggest virioplankton are important agents of microbial mortality and subsequent nutrient recycling in the GAB pelagic ecosystem. Multivariate analyses showed microbial community structure to be driven by nutrient status of the water column. The relatively higher picophytoplankton biomass occurring near the shelf, with values comparable to global estimates of picoplankton derived C, indicate the important contribution of picophytoplankton to productivity and ecosystem function in the GAB. This study was in part funded by the Great Australian Bight Research Program, a collaboration between BP, CSIRO, SARDI, the University of Adelaide and Flinders University.