Hydrodynamic trapping in the formation of the chlorophyll a peak in turbid, very low salinity waters of estuaries

In the James River Estuary, a chlorophyll peak occurred in verylow salinity waters (0.5) during periods of low river dischargein the summer and fall. The biomass of phytoplankton, as measuredby chlorophyll concentration, was 5–10 times that formedin adjacent areas further up-and downstream. Comparisons betweenthe peak area and the 2 area for net plankton biomass, biogenicsilica, phaeopigments, POC/PON ratios and microscopic observationindicated the peak biomass was largely composed of high concentrationsof physiologically healthy freshwater diatoms. We applied equationsfor particle sinking rates to diatoms observed in the JamesRiver and compared these rates with calculations of upward verticalwater velocity. During periods of low river discharge (summerand fall) the sinking rate of diatoms in the chlorophyll peakclosely balanced the net upward water velocity, thereby selectivelytrapping diatoms in the very low salinity zone. The turbiditymaximum increased in intensity and moved downriver in winterand spring due to high river discharge. As upward water velocityincreases, phytoplankton are never able to attain a criticalbiomass and are swept downestuary. This seasonally changingplant biomass is significant to food chain and geochemical considerationsof estuaries. ¹Present address: Department of Oceanography, National FisheriesUniversity of Pusan, Korea