Phytoplankton Dynamics in the Very Low Salinity Region of the James River Estuary, Virginia, U.S.A.

During summer and autumn discharge from the James River estuary, Virginia, was less than 120 m3sec-1. There was a peak phytoplankton biomass in the very low salinity region (defined as the location where surface salinity measured less than 0.5$ 0/00) and this peak represented five to ten times greater biomass than adjacent waters. The peak biomass occurred independent of the tidal state and the location of nutrient inputs. It disappeared during winter and spring, and nutrient limitation was not responsible for the low phytoplankton biomass, indicating there were physical, not chemical controlling factors. The peak biomass was hypothesized to be caused by hydrodynamic trapping, the same mechanism involved in the formation of the turbidity maximum, and by increased phytoplankton residence time during periods of low river discharge. Close balance of sinking rates of dominant phytoplankton species with the net upward vertical water velocity, relatively large netplankton biomass (retained by 23 um screen), exhaustion of dissolved silicate, relatively high ratio of particulate biogenic silica to particulate organic carbon, and relatively low ratio of particulate organic carbon to chlorophyll a in the very low salinity region indicate that diatoms are selectively trapped within this zone. As river discharge increased during winter and spring, the magnitude of the turbidity maximum increased but the peak biomass disappeared as a result of decreased residence time of phytoplankton, decreased sinking rate of phytoplankton due to increased water viscosity at low temperature, and increased net-circulation which requires larger sinking rates to develope the peak biomass in the turbidity maximum zone. There were some differences between the turbidity maximum zone and the location of the peak biomass. High biomass in the very low salinity region decreased very rapidly before the 1.5 0/00, while the turbidity maximum zone encompassed a much broader area. High ratios of chlorophyll a to phaeopigments, and low ratios of particulate organic carbon to nitrogen in the very low salinity region suggest that phytoplankton within this zone grew under good physiological conditions. Mass mortality due to osmotic stress placed on freshwater phytoplankton appears to be the best explanation for the rapid loss of biomass