Animals can have strong effects on ecosystems by cycling nutrients, but the importance of this process is context-dependent and temporally variable. I present >25 years of research on nutrient cycling by fish (gizzard shad, Dorosoma cepedianum) in a eutrophic lake, within the context of temporally variable watershed-derived nutrient subsidies. In Acton Lake, watershed subsidies of nitrogen (N), phosphorus (P), and sediments are changing due to changes in agriculture. Furthermore, nutrient cycling by gizzard shad has varied temporally. Over roughly the first decade of our study, concentrations of P and sediments in Acton inflow streams declined, and lake phytoplankton biomass increased greatly. Phytoplankton increased because decreased sediment concentrations alleviated light limitation, and because nutrient excretion by fish increased, due to increased biomass. However, in the past 10-15 years, stream soluble P concentrations increased, nitrate concentrations declined, and lake phytoplankton biomass stabilized. Furthermore, phytoplankton have become increasingly limited by N relative to P, coinciding with decreasing nitrate and increasing P in streams, and with increased relative importance of nutrient excretion by fish, which excrete nutrients at low N:P. Our research shows that nutrient cycling by fish is an important but temporally variable process, which interacts with watershed subsidies to regulate primary producers.