Transport Velocity of Flounder Larvae (Platichthys flesus L.) in the Dollard (Ems Estuary)

Abstract

Larval concentrations of flounder (Platichthys flesus L.), longitudinal current velocity and physical variables were measured simultaneously in the Dollard to calculate the mass transport of the larvae, salinity and suspended matter. Transport velocities of the three substances (salinity, suspended matter and larvae) and of water were calculated and compared. The concept of larval velocity was extended to incorporate the effect of varying vertical larval distribution. The contributions of variations in vertical and temporal concentration to the transport velocity were calculated separately. The differences in velocity between the substances in relation to the vertical and temporal variation provide more insight in estuarine transport processes in the nursery.Temporal salinity variation dominated over vertical variation during flood, and indicated that longitudinal advection was important. Overall, during flood the transport velocity of salinity was 3% lower than that of water. During ebb, there was hardly any difference between the transport velocity of salinity and water. The contributions of temporal and vertical variation in suspended matter were each in the order of 10%, but of opposite signs. The net result is a transport velocity of suspended matter which is 3% (flood) or 4% (ebb) higher than that of water. Erosion-deposition seemed the dominant process. During flood, transport velocity of larvae is 9% higher than that of water, which is mainly caused by vertical concentration variation (6%), while during ebb, larval transport velocity is 9% lower, which is almost exclusively due to the temporal concentration variation (8%). Swimming influences transport during flood, while during ebb it is not clear which process dominates. The results indicate that flounder larvae make use of selective tidal stream transport inside the nursery.