Reports

Marine phytoplankton are responsible for nearly half of global primary productivity (1). They play essential roles in food webs and global cy-cles of carbon, nitrogen, phosphorus, and other elements (2, 3). Empiri-cal studies have shown that recent ocean warming has driven changes in productivity (4), population size (5), phenology (6), and community composition (7). Global ocean circulation models predict further temper-ature-driven reductions in phytoplankton productivity this century, with consequent decreases in marine carbon sequestration (8, 9). The main mechanism these studies have identified is indirect: rising temperatures drive an increase in ocean stratification, which in turn leads to a decrease in nutrient supply to surface waters. However, most models do not con-sider the direct effects of rising temperatures on individual phytoplank-ton species, which experience sharp declines in growth rate above their optimum temperatures for growth. They may, therefore, underestimate the effects of warming on ecosystems