Distribution of bivalve larvae at a frontal system in the James River, Virginia

Abstract

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 retentionof downstream-flowing water in the estuary and it's injection into deeper, upstream-flowing water. Therole of the frontal system in retention of bivalve larvae in the James was investigated In a 2-part study: afield exammation of larval distr~bution versus depth along a transect across the front in relation tosalinity and temperature of the converging and diverging water masses, and a laboratory examinationof the ability of oyster Crassostrea virginica larvae to swim in and through salinity gradients comparableto or greater than those encountered near the frontal system. Field studies indicate that larvae arepassively transported through the frontal system and plunge to depth as the more saline water in whichthey are entrained encounters less saline water The deeper, more saline water flows upstream as itleaves the frontal system. Laboratory studies demonstrate that both straight hinge stage (mean length =7 5 ~an)d umbo stage (mean length = 157.5 to 159.7 pm) larvae actively swim through a salinitydiscontinuity of 3 %D when exposed in a column of 22 960 water overlayed by 19 9b. water (extreme valuescharactenstic of bottom and surface water at the frontal system). Further, their mean rates of verticalmovement (0.37 to 1.02 mms-') illustrate the ability of larvae to move through the depth of the watercolumn 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 laboratoryapparatus and did not swim through the salinity interface. Following passive transport through thefrontal system in the lower James straight hinge and umbo stage larvae may employ active depthregulation to redistribute throughout the water