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The spatio-temporal variability of marine copepods, like other aquatic and terrestrial organisms, is controlled by both bottom-up (through changes in food resource) and top-down (through changes in predation) forcing. Canonically, climate-related changes in hydrography, nutrient chemistry and circulation can modulate phytoplankton production, thus imposing a bottom-up control on marine copepods, whereas human activities such as fishing may affect the predation mortality of copepods through food-web re-organization such as trophic cascading. Evaluating the sensitivity of copepod populations to bottom-up and topdown forcing is an essential step toward the prediction of future marine planktonic ecosystem changes. In this study, we used a coupled hydrodynamics/food-web/ population-dynamics model to identify the key processes controlling the observed seasonality and distributional patterns of two copepod groups in the Gulf of Maine (GoM) region, including Pseudocalanus spp. and Centropages typicus. Numerical experiments were conducted to assess the sensitivity of the modeled species to changes in phytoplankton biomass and bloom timing, as well as the changes in mortality regime. The results show that both copepod groups are more sensitive to changes in mortality rates than to food availability and peak timing. Bottom-up processes alone cannot explain the observed variability in Pseudocalanus and Centropages population sizes, while top-down controls play a critical role in copepod population dynamics in the GoM region.

Sensitivity of copepod populations to bottom-up and top-down forcing: a modeling study in the Gulf of Maine region