Long term nutrient loads and chlorophyll dynamics in a large temperate Australian lagoon system affected by recurring blooms of cyanobacteria

While rare globally, blooms of the toxic cyanobacteria Nodularia spumigena are a recurring problem in a few estuaries, such as the Baltic Sea and several southern Australian estuaries. Here, we document recurring Nodularia spumigena Mertens blooms in the Gippsland Lakes, S.E. Australia; a temperate lagoon system with episodic, winter-spring dominated catchment inflows. Physico-chemical conditions exerted a strong influence over bloom development, with blooms consistently occurring at surface water salinities between 9 and 20 (average = 15), inorganic nitrogen concentrations < 0.4 mu M, and inorganic nitrogen to reactive phosphorus ratios < 5. There was a positive correlation between average annual chlorophyll a and total phosphorus (TP) load in years when there was no Nodularia bloom, but this relationship broke down in Nodularia bloom years, even though there was a strong correlation between in-lake TP and chlorophyll a during these years; this highlights the importance of internal sources of phosphorus to bloom development. Large catchment derived nitrate and nitrite (NOx) inputs following wildfires and floods in 2007, led to high concentrations of NOx within the surface waters of the Gippsland Lakes through the second half of 2007 and the start of 2008. We hypothesise that these high NOx concentrations were a key factor leading to an unprecedented Synechococcus sp. bloom that developed in the austral summer of 2007-2008, despite conditions that would otherwise favour a Nodularia bloom.