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Abstract 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., NO₃and NH₄), 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 hyper‐eutrophic 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 NO₃dominates spring N supply, but internal NH₄supply becomes important during summer. We quantified how phytoplankton groups (diatoms, chlorophytes, and cyanobacteria) are limited by N or P, and their N form preference (NH₄vs. NO₃), 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 NH₄whereas diatoms were often equally, or more strongly, stimulated by NO₃addition. 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.