Phosphorus limits diazotrophic cyanobacteria whereas other phytoplankton are co-limited by nitrogen and phosphorus in the lowland Murray River, Australia

Context Balances in the availability of nitrogen (N) and phosphorus (P) are particularly important in controlling the competitive dynamics between diazotrophic and non-diazotrophic cyanobacteria in riverine environments. In the Murray River, this has not been greatly studied despite frequent cyanobacterial blooms. Aims To assess the influence of N and P concentrations on cyanobacterial and eukaryotic phytoplankton communities. Methods We conducted six in situ nutrient amendment experiments across five riverine sites along the Murray River, Australia. Substantial diazotrophic cyanobacterial biomass was present at four sites on experimental onset. Results Chlorophyll-a measures indicated co-limitation of N and P at Corowa_21 and Kulnine_21, whereas P alone was the key limiting nutrient at Torrumbarry_20, Mildura_20 and Mildura_21. Phytoplankton populations responded differently to nutrient addition, with diazotrophic cyanobacteria being more often limited by P alone whereas algal groups such as chlorophytes were consistently co-limited. Low ambient dissolved inorganic nitrogen concentrations supported a competitive advantage for some diazotrophic species but not for non-diazotrophic species. Conclusions We conclude that within the Murray River, P is the key limiting factor for diazotrophic cyanobacterial growth, and the broader phytoplankton community is co-limited by P and N. Implications Reductions in the levels of P input will potentially be the most effective nutrient-based control method in reducing cyanobacterial biomass in this and analogous river systems.