Seasonal trophic controls drive population variability in a foundational marine copepod.

Understanding the trophic drivers of zooplankton population variability is critical for predicting ecosystem responses to climate change. In the Gulf of Maine, the copepod Calanus finmarchicus is a foundational species linking primary producers to higher trophic levels, yet the biotic drivers shaping its seasonal and interannual abundances remain incompletely understood. Here, we assess how predators impact C. finmarchicus abundances using over four decades of survey data. We find strong evidence for seasonally-structured trophic control, with spring C. finmarchicus abundances driving mid-year predator increases, which subsequently imposes significant top-down pressure on fall C. finmarchicus populations. This interplay is especially pronounced in the deep, retentive inner basins of the Gulf of Maine, where predator-prey dynamics tend to dominate over advective exchange. Our results reveal shifting interactions between bottom-up and top-down controls, highlighting the need to incorporate seasonal trophic mechanisms into ecosystem models to improve projections under further environmental change.