Climate forcing of the spring bloom in Chesapeake Bay

Abstract

Interannual variability of the spring phytoplankton bloom is strongly expressed in estu- arine ecosystems such as Chesapeake Bay. Quantifying this variability is essential to resolve ecosys- tem responses to eutrophication from variability imposed by climate. We developed a 'synoptic clima- tology' from surface sea-level pressure (SLP) maps to categorize and quantify atmospheric circulation patterns and address climate forcing of phytoplankton dynamics in the Bay. The 10 patterns we iden- tified had unique frequencies-of-occurrence and associated meteorological conditions (i.e. precipita- tion, temperature, wind speed and direction). Four measures of phytoplankton biomass, surface chlorophyll a (B), euphotic layer chlorophyll a (Beu), water column chlorophyll a (Bwc), and total bio- mass (Btot), were obtained from remotely sensed ocean color data spanning 16 yr (1989 to 2004) com- bined with concurrent shipboard data. Years with more frequent warm/wet weather patterns had spring blooms that reached peak biomass farther seaward in the estuary, were greater in magnitude, occurred later in the spring, and covered a larger area than years with a predominance of cool/dry weather patterns. Frequencies of winter weather patterns were used to forecast spring B, Beu, Bwc, and Btot, explaining between 23 and 89% of the variance in the regional time-series. Residuals from these models did not show time-trends attributable to either accelerating eutrophication or manage- ment actions intended to decrease nutrient loadings. These findings extend our understanding of cli- matic influences on phytoplankton dynamics in the Bay by quantifying the effects of synoptic climate variability on spring bloom intensity, thereby supporting forecasts of seasonal phytoplankton biomass based on sub-continental scale weather patterns in this mid-Atlantic estuary.