Phytoplankton spring and fall blooms in the North Atlantic in the 1980s and 2000s

Abstract

Phytoplankton chlorophyll‐a (Chl) seasonal cycles of the North Atlantic are described using satellite ocean color observations covering the 1980s and the 2000s. The study region is where warmer SST and higher Chl in the 2000s as compared to the 1980s have been reported. It covers latitudes from 30°N–50°N and longitudes from 60°W–0°W, where two phytoplankton blooms take place: a spring bloom that follows stratification of upper layers, and a fall bloom due to nutrient entrainment through deepening of the mixed layer. In the 1980s, spring and fall blooms were of similar amplitude over the entire study region. In the 2000s, the fall bloom was weaker in the eastern Atlantic (east of 40°W), because of a delayed deepening of the mixed layer at the end of summer (mixed layer depth (MLD) determined from in situ data). Conversely, the spring bloom of the eastern Atlantic was stronger in the 2000s than it was in the 1980s, because of a deeper MLD and stronger winds in winter. In the Northwestern Atlantic (northwest of 38°N–40°W), little differences are observed for spring and fall blooms, and for the wintertime MLD. Our results show that the links between upper layer stratification, SST changes, and biological responses are more complex than the simple paradigm that sequentially relates higher stratification with warmer SST and an enhanced (weakened) growth of the phytoplankton population in the subpolar (subtropical) region.