Integrative ecosystem analyses of phytoplankton dynamics in the York River estuary (USA)

The relative importance of biotic (top-down) vs. abiotic (bottom-up) controls on phytoplankton dynamics was investigated in the York River estuary, Virginia (USA) by a combination of extensive analyses of long-term data sets collected by the U.S. Environmental Protection Agency (EPA) monitoring program over 17 years (1984–2001), field studies (1996–1997) and ecosystem modeling analyses. Results from the analysis of long-term data records collected at three stations along the salinity gradient suggested that phytoplankton are more likely controlled by abiotic mechanisms such as resource limitation than biotic mechanisms such as grazing since annual cycles of primary production and phytoplankton biomass were similar and no grazing effects were evident based on the observed relationship between phytoplankton and zooplankton biomass (R2 < 0.2, p > 0.1). This scenario was supported by short-term field observations made over an annual cycle at three stations in the mid-channel of the estuary where both chlorophyll a and primary production demonstrated similar patterns of seasonal variation. Ratios of fluorescence before and after acidification at all study sites were relatively high suggesting low grazing pressure in the estuary. A tidally-averaged, size-structured plankton ecosystem model was previously developed and verified for the lower York River estuary. The validated ecosystem model was also used to examine this issue and simulation results supported the importance of bottom-up control in the York River estuary.