Phosphorus limitation of the freshwater benthic diatom Didymosphenia geminata determined by the frequency of dividing cells

Abstract

SUMMARY 1. Unlike other nuisance algal species, the freshwater benthic diatom Didymosphenia geminata typically forms blooms in low-nutrient rivers. The negative association between D. geminata blooming behaviour and nutrient levels appears at both catchment and smaller scales. We conducted a series of trials in streamside experimental channels colonised with D. geminata using water from the D. geminata-affected, oligotrophic Waitaki River, South Island, New Zealand to determine how elevated nitrate and phosphate concentrations affected D. geminata cell division. Because D. geminata blooms are typically most pronounced in unshaded waters, we also investigated the growth response to shading. In all experiments, we used the frequency of dividing cells (FDC) as a metric of cell division. 2. Concentrations of nitrate and dissolved reactive phosphorus (DRP) in the Waitaki River were very low (4 mg m )3 NO 3 -N and <1 mg m )3 DRP). In pilot trials, substrata colonised by D. geminata were subjected to enrichment by either switching the water source to N- and P-rich spring water or by adding a stock NO 3 PO 3 solution. Both trials resulted