Meta-analysis of stage-specific Calanus finmarchicus vertical distribution in relation to hydrography and chlorophyll in the North Atlantic.

Calanus finmarchicus is an important, extensively studied zooplankton species in the North Atlantic. Many studies have explored its abundance and life cycle, but basin-wide relationships between its vertical distribution and environment during the feeding season remain poorly known. We conducted a meta-analysis of stage-specific vertical distribution and its relationships with environmental variables (temperature, salinity, irradiance, chlorophyll-a) in the epipelagic layer (0-200 m) of the North Atlantic during spring and summer (21 March to 21 September). Fitting a GAM model, we analyzed data from 47 years (1971-2018) with the aim to discern common, stage-specific responses to environment across the area. Highest abundances were observed in the upper 50 m in spring (at 5°C) and summer (at 7.5°C). The timing of the phytoplankton bloom emerged as a key driver determining vertical distribution, with all stages found shallower during the seasonal surface Chl.-a maximum. Contrary to reports of mismatch with global warming, the data indicated a region-wide match of spring bloom and Calanus. In the coldest areas of its habitat (< 1°C), the copepods stayed closer to surface, potentially to fulfill development, while in warmest areas (>10°C), early stages stayed deeper likely to avoid warm surface waters.