In an attempt to control for effects of eutrophication, management has focused on the importance of minimizing nitrogen (N), as well as phosphorous (P), inputs into aquatic ecosystems. Nutrient limitation is commonly determined using inorganic N, typically assumed to be the most accessible to lake phytoplankton. However, due to the variety of N chemical species that exist in most aquatic systems, it is unclear which chemical species and sources of N should be targeted for reduction efforts. In order to determine the effect of organic and inorganic N chemical species on phytoplankton community biomass, we deployed an in-situ microcosm experiment across 27 lakes in northeastern North America in summer 2018. Following a one-week incubation, we analyzed all treatments with a factorial design of added N species (organic, inorganic, and combined) and P for chlorophyll a, used as a proxy for phytoplankton biomass. We predicted that phytoplankton would be able to use organic N in addition to inorganic N. We determined whether limitation type remains the same when calculated with respect to inorganic, organic, or combined inorganic and organic N additions.
Results/Conclusions
Inorganic and organic N chemical species produced identical chlorophyll a concentrations in 56% of lakes tested.Overall, organic N elicited the greatest chlorophyll a response in 37% of lakes tested. Inorganic N provoked the greatest chlorophyll response in 7% of lakes. These findings indicate that phytoplankton communities can use organic N as readily as inorganic N. Across all 27 lakes, we observed all types of limitation: P limitation, colimitation, serial N limitation, serial P limitation, and no limitation; when nutrient limitation was determined with inorganic N, colimitation was the dominant type (72%). After determining nutrient limitation with respect to inorganic N, organic N, and combined inorganic and organic N, 40% of lakes changed nutrient limitation type at least once. Most of the lakes (74%) had similar magnitudes of phytoplankton community response across all N chemical species. The ability of phytoplankton to use different species of N complicates our understanding of nutrient limitation. Our conception of nutrient limitation may have to be amended to include organic N in order to capture the full potential of phytoplankton to use different species of N. Managers must fully consider their aquatic system when deciding what species of N to target for reduction.