Nitrogen (N) and phosphorus (P) inputs influence algal community structure and function. The rates and ratios of N and P supply, and different N forms (e.g., NO 3 and NH 4 ), from external loading and internal cycling can be highly seasonal. However, the interaction between seasonality in nutrient supply and algal nutrient limitation remains poorly understood. We examined seasonal variation in nutrient limitation and response to N form in a hypereutrophic reservoir that experiences elevated, but seasonal, nutrient inputs and ratios. External N and P loading is high in spring and declines in summer, when internal loading because more important, reducing loading N:P ratios. Watershed NO 3 dominates spring N supply, but internal NH 4 supply becomes important during summer. We quantified how phytoplankton groups (diatoms, chlorophytes, and cyanobacteria) are limited by N or P, and their N form preference (NH 4 vs. NO 3 ), with weekly experiments (May-October). Phytoplankton were P-limited in spring, transitioned to N limitation or colimitation (primary N) in summer, and returned to P limitation following fall turnover. Under N limitation (or colimitation), chlorophytes and cyanobacteria were more strongly stimulated by NH 4 whereas diatoms were often equally, or more strongly, stimulated by NO 3 addition. Cyanobacteria heterocyte development followed the onset of N-limiting conditions, with a several week lag time, but heterocyte production did not fully alleviate N-limitation. We show that phytoplankton groups vary seasonally in limiting nutrient and N form preference, suggesting that dual nutrient management strategies incorporating both N and P, and N form are needed to manage eutrophication.

Lakes in human‐dominated landscapes often face high loads of nutrients that may alter ecosystem function. High N loads relative to P are especially common in agricultural watersheds, where nitrate (NO3) in particular is elevated due to fertilizer application and runoff. While past research has focused extensively on the impact of nutrient loads on reductions in water quality, we still lack assessment of the impact of high N loading and extreme stoichiometric imbalance on ecosystem process rates, specifically measurements of gross primary production (GPP), ecosystem respiration (ER), and net ecosystem productivity (NEP). We used open‐bottomed experimental mesocosms (limnocorrals) to establish four treatments of increasing N : P by increasing N loads while leaving P loads consistent, and measured chlorophyll concentration and metabolic rates. We observed significant differences in productivity among treatments, with higher biomass and GPP in the highest two N : P treatments and a unimodal pattern between GPP and N concentrations. Declines in GPP at high N may have been due to limitation by P or light. In contrast to other studies, we did not observe any significant differences in ER with fertilization, potentially from a lack of dissolved organic carbon loading often associated with higher nutrient concentrations in those lakes. Maximum GPP and NEP observed here was far above commonly cited thresholds for likely P limitation at a molar N : P of approximately 330. Our results highlight the potential for high productivity despite stoichiometric imbalance that is common in agricultural systems, suggesting N management may be important in reducing primary production in hypereutrophic lakes.

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