Temperature, length of growth season and phytoplankton abundance in the Gulf of Maine

Abstract

I show that the relation between annual average phytoplankton concentration (mg Chl-a m–3) and in situ sea surface temperature (SST) is positive (Chl-a=~0.5×SST, r=0.8, P<0.001) at an average temperature of 11°C (range 10–12°C) in the Gulf of Maine. However, within-seasonal observations (for 2005–09) were predominantly negatively associated. For the first relationship that describes annual averages, the extension of the growth season with increasing temperature may be an important factor. I show that an increase of 1°C starts the growing season 8 days earlier and lengthens the season by 13 days (for temperature >10°C). Tentative calculations suggest that the increased length matches the increase in annual phytoplankton concentration. For the second relationship that describes within-seasonal relationships, I suggest that warmer water during late summer increases stratification and limits nutrient supply to the upper productive layer.