Nutrient limitation of phytoplankton in the upper Swan River Estuary, Western Australia

During 1993-94 the phytoplankton community in the upper Swan River estuary had a peak chlorophyll a concentration of 57 mg m(-3) during early summer (December 1993) and a second peak of 35 mg m(-3) during late autumn (May 1994). Mid summer was characterized by low cell densities and low chlorophyll a concentrations. The potential of the phytoplankton community for nutrient limitation was assessed with dilution bioassays given nutrient mixes deficient in one of the following: nitrogen, phosphate, silicate, iron, trace metals, chelators, or vitamins. During the mid-summer period of low phytoplankton abundance, nitrogen was the nutrient with the greatest potential to limit algal biomass. During mid summer, ambient N:P ratios tended to be near unity and bioassays indicated that the available pool of N was up to 20 times more limiting to biomass development than was available P. Also during mid summer, bioassay treatments given no nitrogen and control treatments given no nutrients showed little growth, reaching chlorophyll a concentrations similar to 1/30th of those given a full suite of nutrients. Chlorophyll a concentrations in the bioassay control treatments given no nutrients were correlated (r(2) = 0.74) with measured surface nitrate concentrations; this suggested that nitrate inputs may be a major factor controlling phytoplankton biomass in this ecoystem. The correlation between surface nitrate concentration and rainfall (r(2) = 0.69) further suggests that rainfall may be the most important mechanism supplying nitrate to the surface waters of this estuary.