Phytoplankton Biomass and Growth Limitation During Fifty Years of Reduced Nutrient Loading in Shallow Lake Arresø

We examine phytoplankton development and limitation in the 3‐m deep, well‐mixed water column of hypereutrophic, alkaline Lake Arresø during 50 years of improved sewage treatment that has reduced phosphorus (P) and nitrogen (N) availability and phytoplankton biomass. In the 1970s and 1980s, the high input of sewage provided inorganic nutrients year‐round, but phytoplankton biomass (chlorophyll, Chl, as a proxy) was likely light‐limited by coloured dissolved organic matter and particles other than phytoplankton. From the biomass maximum in the early 1990s, Chl, P and N concentrations decreased in concert over time, and very low inorganic P and N became frequent. The organic P:Chl mass ratio stabilized at close to 1.0, while organic N:Chl varied extensively, indicating that phytoplankton biomass was more closely related to P than N. Although very low inorganic P and N during summer suggest co‐limitation of phytoplankton biomass, P was more frequently below limiting thresholds and likely constrained the biomass more effectively due to flexible N circulation experiencing substantial denitrification and potential N‐fixation. Mean irradiance and light saturation index of photosynthesis were consistently low in the turbid water, but increased as nutrients and biomass declined. Intense photosynthesis elevated pH > 10 and carbonate precipitation with variations over the years corresponding to variations in Chl and, specifically, to the biovolume of chlorophytes. CO 2 declined to 0.01–0.1 μM and photosynthesis was sustained by HCO 3 − . When biomass decreased, photosynthesis limitations by carbon and light decreased as well. Green algae ( Chlorella , Scenedesmus ) dominated at peak biomass and the lowest light and CO 2 availabilities, while gelatinous colonies of cyanobacteria ( Microcystis , Woronichinia ) took over as light and CO 2 conditions improved, suggesting alternating competitive exclusion among algae groups. Nutrients were the primary constraint on phytoplankton biomass, while high biomass was accompanied by reduced light and inorganic carbon limiting photosynthesis. Phytoplankton community structure changed from dominance of chlorophytes depressing cyanophytes and diatoms to cyanophyte dominance as nutrient input, biomass and maximum pH decreased over the years.