Environmental forcing on the interactions of plankton communities across a continental shelf in the Eastern Atlantic upwelling system

To investigate whether phytoplankton is the main factor determining mesozooplankton distribution in a continental shelf affected by upwelling, oceanographic surveys were conducted off NW Portugal in 2002 and 2003. During four different seasons, we investigated how the mesoscale relationship between these two communities was forced by environmental conditions across the shelf. The horizontal and vertical distribution of phytoplankton was shaped by wind stress over the water. Diatoms dominated mixed and upwelled waters, whereas dinoflagellates prevailed with thermal stratification. Mesozooplankton was less influenced by wind forcing and concentrated mostly at mid-shelf, on the outer margin of main phytoplankton patches. We found that this pattern, under strong thermal stratification conditions, can be associated with localized grazing “hot spots”. Copepods were dominant, contributing to the mesozooplankton community variability between cruises whereas phytoplankton presented a clearer seasonal pattern. Nevertheless, the distribution and abundance of mesozooplankton were directly correlated with all phytoplankton groups at the inner-mid-shelf, while no correlation was observed offshore the outer-shelf. The relative composition of mesozooplankton did not vary between depth strata, whereas a cross-shelf separation occurred between nearshore and widespread clusters of species. This reflected a dependence on phytoplankton and reflected the high variability of oceanographic conditions of the study area.