In shallow lake and pond ecosystems, two alternative regimes can exist over a range of nutrient concentrations: an oligotrophic, clear water state dominated by macrophytes, and a eutrophic, turbid state characterized by high algal biomass. Phytoplankton growth is controlled by the most limiting nutrient (commonly phosphorus (P) or nitrogen (N)). Nutrient thresholds are nutrient concentrations below which phytoplankton growth is limited. The purpose of our project was to determine which nutrient is limiting (N or P or both) and test a method to experimentally determine both N and P nutrient thresholds that is simple, repeatable, and relevant across environmental gradients. Two impounded wetlands near the Great Salt Lake, Utah were used as experimental sites in June 2019. Water was collected from each site (~50 L) and used to fill clear, 1 L Cubitainers. Two studies were done at each site to establish a growth response curve for N and P. Nitrogen starvation was induced in the N study by adding 0.6 mg/L of P to each container. Similarly, P starvation was induced for the P study by the addition of 6.0 mg/L N. The containers in the N study received variable concentrations of N and containers in the P study received concentrations of P. Nutrients were applied on the first day of the incubation, and the containers incubated in place for three days. At the end of the incubation period, phytoplankton biomass was estimated using the concentration of chlorophyll-a and the growth rate (µ) of phytoplankton under various nutrient levels was calculated.
Results/Conclusions
At both locations we found that phytoplankton growth was not limited by P but by N only. In the N studies at each site, phytoplankton growth rate increased with increasing N concentration, however, in the P studies phytoplankton growth rate did not increase with increasing P concentration. At both locations the background concentration of P was so high that even 6 mg/L of applied N did not make P the most limiting nutrient. Additionally, the control treatments within the P study that received no N show a lower growth rate than the treatments receiving N, again suggestion N limitation. These results support management efforts to minimize N as well as P discharge to these sensitive wetlands.