Environmental variability and phytoplankton dynamics in a South Australian inverse estuary

Estuaries are widely viewed as hotspots of primary productivity. The Coorong in South Australia is an inverse estuary divided into two lagoons, extremely important to the associated riverine, lacustrine and marine environments and characterized by a steep, lateral salinity gradient. Here, we analyzed the abundance and distribution of primary producers over two years (August 2011–2013) and investigated the biogeochemical factors driving observed changes. The phytoplankton community was numerically dominated by chlorophytes in the North Lagoon with Chlorohormidium sp. and Oocystis sp. being the most abundant species. In the South Lagoon, diatoms dominated the community, with Cylindrotheca closterium, Cyclotella sp. and Cocconeis sp. being the most prevalent species. Finally, cryptophytes and dinoflagellates were found to be present throughout both lagoons but in comparatively much lower abundances. Salinity was the most important driver of phytoplankton communities and ranged from 0.15 to 72.13PSU between August 2011 and August 2013. Chlorophytes were found to be most prolific in freshwater areas and abundances rapidly declined laterally along the Coorong. Beyond a salinity threshold of 28PSU, extremely limited numbers of Crucigenia sp. and Oocystis sp. were observed, but abundance were seven to ten-fold lower than in less saline waters. The salinity of the North Lagoon was found to be directly controlled by the flow volume of the River Murray, however, no effect of river flow on the South Lagoon was evident. Our findings suggest that management plans for the Coorong need to be put into place which can regulate salinity regimes via river flow, even during periods of drought. This is highly important in order to maintain low enough salinities throughout the North Lagoon, ensuring a continued healthy ecosystem state. •A pronounced spatial salinity gradient along the lateral length of the estuary was observed.•The distribution of phytoplankton was found to be a function of salinity.•Ecological disparity observed between sites with clear ecotonal boundary between lagoons.•Salinity threshold of 28PSU determined for chlorophytes beyond which abundances drastically decrease.•Drought damage-prevention plans are suggested to maintain ecosystem health.