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

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‐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 toN‐ and P‐rich spring water or by adding a stock solution. Both trials resulted in periods of rapid cell division lasting at least 8 days. 3. Experimental addition of alone triggered an initial cell division which was not sustained. However, addition of alone or together with resulted in prolonged elevation in cell division indicating that the cell division rate was P‐limited. 4. Reduced light levels resulted in decreased FDC in D. geminata in both ambient and N, N + P and P‐enriched river water. 5. Stimulation of D. geminata division rate by addition of above ambient levels confirms that, while blooming behaviour is often associated with oligotrophic rivers, the cells divide faster with greater levels of phosphorus enrichment.