Seasonal patterns in phytoplankton biomass in coastal ecosystems

We test, through a compilation of published data, whether specific trends in phytoplankton biomass seasonality can be generalized in northern temperate enclosed coastal ecosystems (ECE) and northern temperate open coastal ecosystems (OCE). Bimodal cycles (i.e, displaying two annual peaks) are the most frequent trends in both ecosystems, and whereas they mostly peaked at late winter and fall in OCE, they mostly peaked in spring and late summer, and also over a wider range of months, in ECE. The interaction between restricted depth or tidal stirring and light seasonality in ECE may be responsible for these differences. Moreover, bimodal cycles reached higher biomass peaks in ECE, which can result from higher water column nutrient concentrations. Unimodal cycles (one annual peak) also occurred commonly in both ecosystems, and whereas they peaked throughout winter and spring in OCE, they peaked mostly in summer in ECE. We suggest that the high nutrient concentrations typically found in ECE waters can alleviate nutrient limitation of phytoplankton growth and couple phytoplankton growth with light and stratification seasonality. These results reveal broad-scale patterns in phytoplankton seasonality in temperate coastal ecosystems, and help identify physical processes generating these patterns.