Nutrient availability can influence the growth and composition of phytoplankton communities. Phytoplankton can be limited by either nitrogen (N) or phosphorus (P), and studying which element is limiting is important for managing eutrophication and potential future algal blooms. In addition, phytoplankton taxa may respond differentially to N forms, i.e., ammonium (NH4) vs. nitrate (NO3). The supply of N and P, and different N forms, to phytoplankton often varies seasonally. We quantified seasonal dynamics of limitation by N, P and the response to different N forms, in eutrophic Acton Lake. We conducted 23 weekly growth bioassay experiments (May-October 2019) to quantify nutrient limitation of phytoplankton groups, including chlorophytes, cyanobacteria and diatoms. We hypothesized that 1) a transition would happen from P to N limitation for all phytoplankton groups during the study period; 2) cyanobacteria would dominate the phytoplankton communities during the N limitation period; and 3) cyanobacteria and chlorophytes would show a stronger preference for NH4, while diatoms would show an equal or stronger preference for NO3. Each experiment lasted 48 hours and response of phytoplankton groups was assessed using a spectrofluoroprobe.
Results/Conclusions
Phytoplankton were P-limited May-July, but N-limited from mid-August to October. Chlorophytes dominated the assemblage through July, but cyanobacteria dominated during the period of N-limitation. In general, as hypothesized cyanobacteria and chlorophytes showed a preference for NH4 over NO3, whereas diatoms showed a stronger preference for NO3. The dynamics of nutrient limitation contrasted somewhat with those of the previous year (2018), likely due to differences in precipitation and runoff which influences the concentrations and ratios of limiting nutrients. The transition to N-limitation occurred 6 weeks later in 2019 than 2018, but late summer N-limitation was more severe in 2019 than 2018. Precipitation and runoff were lower overall, but more temporally constant, in 2018 compared to 2019. Thus, spring 2019 was very wet (high streamflow), but summer was very dry and stream inflow ceased by August. In contrast, spring runoff was lower in 2018, but flow persisted at low levels throughout summer. Contrasting temporal dynamics of N and P limitation probably reflect interannual differences in watershed N inputs (especially NO3), and differences in the relative importance of stream inputs (high N:P) vs. internal nutrient supply (low N:P). Understanding whether N or P is limiting, and which N forms influence phytoplankton communities, can help manage and prevent harmful algal blooms in freshwaters.