Vertical patterns of phytoplankton size distribution in the Cantabric and Balearic Seas

In this paper we analyze plankton size distribution (2–100 μm of equivalent spherical diameter) under different hydrographic structures, such as the seasonal thermocline of the Cantabric and Balearic seas and the oceanographic front present in the Balearic Sea. Nannoplanktonic fraction (2–20 μm) tends to present biomass maxima at shallower depths than the chlorophyll maximum in stratified waters, as well as in the more productive waters near a front. The slope of the normalized size-biomass spectrum was more negative in these zones of phytoplankton biomass maxima indicating a higher proportion of smaller cells. The main hypotheses to explain chlorophyll and biomass maxima as well as phytoplankton accumulation in productive areas are reviewed. According to the observed results, the most suitable mechanism to explain subsurface biomass maxima is the active net growth of the smaller phytoplanktonic cells. The importance of the methodology employed for analyzing the size distribution of plankton communities and its relationship with hydrographic variability are also discussed.