Virginia Institute of Marine Science
Marine Ecology Progress Series
James River is the southernmost of the major subestuaries of the Chesapeake Bay, USA. A frontal system develops on the early flood tide in the Hampton Roads region of the lower James. This system, together with a cyclonic gyre in Hampton Roads, is in part responsible for partial retention of downstream-flowing water in the estuary and it's injection into deeper, upstream-flowing water. The role of the frontal system in retention of bivalve larvae in the James was investigated In a 2-part study: a field exammation of larval distr~bution versus depth along a transect across the front in relation to salinity and temperature of the converging and diverging water masses, and a laboratory examination of the ability of oyster Crassostrea virginica larvae to swim in and through salinity gradients comparable to or greater than those encountered near the frontal system. Field studies indicate that larvae are passively transported through the frontal system and plunge to depth as the more saline water in which they are entrained encounters less saline water The deeper, more saline water flows upstream as it leaves the frontal system. Laboratory studies demonstrate that both straight hinge stage (mean length = 75~) and umbo stage (mean length = 157.5 to 159.7 pm) larvae actively swim through a salinity discontinuity of 3 %D when exposed in a column of 22 960 water overlayed by 19 9b. water (extreme values charactenstic of bottom and surface water at the frontal system). Further, their mean rates of vertical movement (0.37 to 1.02 mms-') illustrate the ability of larvae to move through the depth of the water column in the James in less than one tidal cycle. Pediveliger stage oyster larvae (mean length 317.2 pm), by contrast, restricted swimming to small but frequent excursions above the bottom in the laboratory apparatus and did not swim through the salinity interface. Following passive transport through the frontal system in the lower James straight hinge and umbo stage larvae may employ active depth regulation to redistribute throughout the water column; however, pediveliger stage larvae probably remain near the sediment-water interface.
Mann, Roger L., Distribution of bivalve larvae at a frontal system in the James River, Virginia (1988). Marine Ecology Progress Series, 50, 29-44.