Effects of the vertical structure of the water column on the phytoplankton in a shallow, lagoonal estuary

Phytoplankton community composition is an important determinant of the effects of eutrophication in coastal systems. However, the system specific attributes governing the phytoplankton response to anthropogenic nutrient loading are still poorly resolved. The mixing regime of the Neuse River Estuary is an unusual attribute of the ecosystem. Unlike lakes and open ocean systems, stratification is primarily determined by salinity rather than temperature. Unlike most estuaries where tidal straining is the dominant mixing process, astronomical tides are negligible in the Neuse River Estuary. As a result, riverine discharge and wind stress determine oscillations between well-mixed and poorly mixed conditions on time scales of hours to days rather than the seasonal time scales of lakes and oceans or the daily time scales of most estuaries. The purpose of this study was to determine how this unusual pattern of mixing affects the phytoplankton community. The observed negative correlation between diatom biomass and stratification intensity indicates that settling losses significantly impact diatom biomass. Wind energy for mixing is linked to the maintenance of the diatoms through increases in the eddy diffusivity of the upper layer, deepening of the pycnocline, and resuspension from the sediments. When mixing is weak, growth limiting nutrients often accumulate in the bottom waters. The depth of the euphotic zone approximates the depth of the pycnocline creating a strong tendency for vertically separated light and nutrient resources. Under these conditions flagellates, particularly dinoflagellates and cryptophytes, use vertical migrations to access light and nutrients for growth. Water column stability does not appear to have an effect on flagellate biomass. However, as diatom biomass decreases under stratified conditions, flagellates dominate. The linkage between water column stability and community composition helps explain previously observed spatial and temporal distributions of community composition within the Neuse River Estuary. Additionally, average intensities of wind induced mixing may explain why the Neuse River Estuary is dominated by flagellates while tidally forced estuaries, such as Chesapeake Bay and San Francisco Bay, are dominated by diatoms. Eutrophication models may be improved by separately simulating the ecologically distinct diatom and flagellate groups