Environmental controls on the nitrogen-fixing cyanobacterium Nodularia spumigena in a temperate lagoon system in SE Australia

Estuarine blooms of N-2-fixing cyanobacteria are rare, and the factors controlling them remain poorly understood outside the Baltic Sea. We measured in situ physico-chemical conditions and undertook nutrient-addition bioassays and dilution-based grazing experiments fortnightly over the 2010 to 2011 Austral spring-summer-autumn, to evaluate the role of physical environmental drivers, grazing and macronutrients on the growth of Nodularia spumigena Mertens (hereafter Nodularia) in a temperate lagoon system (Gippsland Lakes, Australia). Nodularia appeared in late summer, following a period of warm weather (water temperature > 22 degrees C), high solar exposure, calm conditions, relatively low salinity (<22) and low dissolved inorganic nitrogen (similar to 0.4 mu mol l(-1)). A major bloom did not eventuate. Bioassays indicated N limitation prior to Nodularia appearing, and P limitation after, while in situ nutrient concentrations indicated N limitation throughout. Nodularia grew rapidly (similar to 0.4 d(-1)) in control bioassays (no nutrients added), whereas other phytoplankton taxa showed zero or negative growth. N addition caused significantly lower Nodularia growth, while other taxa had higher growth. P addition did not significantly enhance Nodularia growth. This suggests Nodularia was able to obtain P from the water column and from phytoplankton breakdown. A reduction in grazing pressure via dilution led to lower growth rates of Nodularia and higher growth rates of other phytoplankton taxa. This suggests that the grazers selectively consumed taxa other than Nodularia, and that grazing positively influences Nodularia growth in this system. These data indicate that Nodularia growth was limited in situ by physical conditions (primarily solar radiation) and that nutrients and grazing were of second order importance.