Climate forcing of phytoplankton dynamics in Chesapeake Bay

Abstract

Climate has long been recognized as an important driver of phytoplankton dynamics. In Chesapeake Bay, climate variability is manifest as differences in timing and magnitude of freshwater flow. Interannual differences of freshwater flow influence phytoplankton through effects on light and nutrient distributions. Understanding how climate forces temporal and spatial patterns of phytoplankton biomass (Chla) and primary productivity (PP) is an important area of research as we attempt to predict effects of climate change and nutrient enrichment on estuarine ecosystems. This Dissertation describes climate forcing of Chla and PP using a synoptic climatology to quantify climate variability and ocean color remote sensing to assess phytoplankton variability. I developed a synoptic climatology using surface sea-level pressure data for the eastern United States to characterize regional climate because large-scale climate indices are not strongly expressed in this region. The long time series (1989-2004) of remotely sensed ocean color measurements provided high spatial and temporal resolution that allowed me to resolve interannual differences of Chla and PP. I show that the frequency-of-occurrence of synoptic-scale weather patterns during winter explained 54% of the variance in spring freshwater flow to Chesapeake Bay through interannual differences in precipitation and water storage in the basin as snow and ice. Winter weather patterns were also linked to interannual variability of several characteristics of the spring phytoplankton bloom (timing, position, magnitude) through their effects on precipitation and freshwater flow. Multiple linear regression models of winter weather pattern frequencies on regional Chla explained between 23-89% of the variance of the time series. Climate variability in winter-spring also influenced summer and annual integral production through nutrient loading associated with the spring freshet, explaining between 43-62% of the variance of integral production. Finally, I quantified the effects of Hurricane Isabel on Chesapeake Bay phytoplankton dynamics and showed that event-scale climate perturbations can have significant impacts on ecosystem dynamics as well as seasonal and regional carbon cycling. Together these analyses highlight the importance of climate forcing of Chla and PP in Chesapeake Bay and support predictive models that explain significant amounts of the variance of these important ecosystem properties.