Phytoplankton Ecology and Dynamics in the James River Estuary Virginia, U. S. A.

Autotrophic biomass and productivity as well as nutrient distributions and phytoplankton cell populations in the James River estuary, Virginia, from Newport News Shipyard to the 0 o/oo isohaline, were sampled on a monthly basis from August of 1981 to December of 1982. Particular emphasis was placed on the very low salinity region (defined as the location where surface salinity measured between 0.0 and 0.75 o/oo based on conductivity) in order to determine the fate of fresh water phytoplankters upon being advected into the estuary because of their suspected role in the biogeochemical cycling that occurs there. Both chlorophyll a and the production of particulate organic carbon in the very low salinity region peaked during the late summer and early fall months and were significantly correlated with river discharge demonstrating a hydrodynamic control. During certain times of the year (usually coinciding with a decrease in river discharge) greater than 50% of the biomass present at 0 \u27o/oo succumbed within a narrow range of salinity. A continuous culture cyclostat experiment (which incorporates the rates of change of salinity per unit time derived from the Hansen-Rattray model for circulation in a partly mixed estuary) performed on natural populations of phytoplankton from the James River estuary implies that this loss of biomass is most likely due to the osmotic stress encountered by the cells upon advection into the estuary. Confounding factors, which may contribute to the regulation of the abundance of phytoplankters midriver, include the flocculation of colloidal material together with phytoplankton and the reduction of the photic zone due to the turbidity maximum. Furthermore, autotrophic abundance upriver may indirectly regulate the onset of the spring bloom in the more estuarine portions of the river by controlling the amount of nutrients available. This is evidenced by an inverse relationship that exists between the phytoplankton abundance present in very low salinity waters and the abundance of biomass measured in the more estuarine portion of the river. In addition during the time of the year when phytoplankton biomass is increased at the 0 o/oo isohaline there is a concurrent removal of nitrate and phytoplankton chlorohyll a along the low salinity segment (0 -2 o/oo) which demonstrates that the fate of the fresh water phytoplankton does effect biogeochemical cycling