Nutrient limitation of phytoplankton in a seasonally open bar-built estuary: Wilson Inlet, Western Australia

The potential for nutrient limitation to affect the phytoplankton community was assessed from 1997 to 1998 in Wilson Inlet, Western Australia. Samples of the endemic phytoplankton community were assayed by giving them nutrient mixes deficient in one of the following: nitrogen, phosphate, silicate, iron, trace metals, or vitamins. When ranked from those treatments with the most potential to limit phytoplankton biomass to those with less potential, the bioassay results suggested that N>P>>Si>Fe, and trace metals and vitamins were never potentially limiting. In summer and autumn, the bioassay data suggested that nitrogen and phosphorus were the nutrients with greatest potential to constrain phytoplankton biomass. Though the concentrations of soluble nutrients were high in winter, bioassays indicated that the phytoplankton community was potentially nutrient limited. Physical conditions such as high flow rate, greater turbidity, lower temperature, and greater light attenuation contributed to lower phytoplankton growth rates during winter. The bioassay data indicated that the phytoplankton biomass was least likely to be constrained by dissolved inorganic nutrients in spring than any other time of the year. In spring the endemic phytoplankton community responded to salinity-stratification induced sediment-nutrient release with a marked increase of biomass. Nitrogen was the nutrient with greatest potential to limit the phytoplankton biomass during spring. The bioassay data conflicted with the dissolved inorganic nitrogen to phosphorus molar ratios, which suggested that P was more likely to limit the phytoplankton biomass during spring. The discrepancy between the two data sets was probably caused by inaccuracies in measuring the concentration of dissolved inorganic phosphorus in spring. Therefore, the results from the current study suggest that the Redfield paradigm, based on the ratio of dissolved inorganic nutrient concentration, provided an inadequate description of phytoplankton nutrient limitation in Wilson Inlet.