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

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Abstract
In the James River Estuary, a chlorophyll peak occurred in very low salinity waters (0.5‰) during periods of low river discharge in the summer and fall. The biomass of phytoplankton, as measured by chlorophyll concentration, was 5–10 times that formed in adjacent areas further up-and downstream. Comparisons between the peak area and the 2‰ area for net plankton biomass, biogenic silica, phaeopigments, POC/PON ratios and microscopic observation indicated the peak biomass was largely composed of high concentrations of physiologically healthy freshwater diatoms. We applied equations for particle sinking rates to diatoms observed in the James River and compared these rates with calculations of upward vertical water velocity. During periods of low river discharge (summer and fall) the sinking rate of diatoms in the chlorophyll peak closely balanced the net upward water velocity, thereby selectively trapping diatoms in the very low salinity zone. The turbidity maximum increased in intensity and moved downriver in winter and spring due to high river discharge. As upward water velocity increases, phytoplankton are never able to attain a critical biomass and are swept downestuary. This seasonally changing plant biomass is significant to food chain and geochemical considerations of estuaries.

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