Large-scale season-dependent effects of temperature and zooplankton on phytoplankton in the North Atlantic

We analyzed 13 yr of monthly time series (1998 to 2010) of satellite-derived chlorophyll, sea-surface temperature and zooplankton abundance (copepod taxa from continuous plankton recorder surveys) in order to better understand the ecological processes that regulate the seasonal phytoplankton dynamics in different regions of the North Atlantic Ocean. Results showed large-scale, seasonally varying effects of temperature and zooplankton abundance on chlorophyll concentration. Specifically, we found positive temperature effects in most of the regions, which tended to be most pronounced in spring and early summer, especially in the warmer water regions. The situation changed in late summer (mostly July to September), when we detected a negative correlation between chlorophyll concentration and the abundance of copepod nauplii, suggesting top-down control by grazing when phytoplankton generally suffer nutrient limitation and low growth rate. Our results demonstrate that both physical (bottom-up) effects and zooplankton (top-down) effects alternate to shape the seasonal dynamics of phytoplankton in the North Atlantic.