Abstract

The relative contributions of the spring-neap tides, river discharge and vertical behavior to the dispersal of eastern oyster (Crassostrea virginica) larvae in Delaware Bay is investigated with a coupled circulation-individual-based larvae model. The coupled model simulates larval growth and behavior along particle trajectories using temperature and salinity conditions obtained from an implementation of the Regional Ocean Model System (ROMS) for Delaware Bay. Particles, representing oyster larvae, are released at five-day intervals during the spawning seasons (mid-June to mid-September) of 1984, 1985, 1986, 2000 and 2001 from areas that correspond to natural oyster reefs in Delaware Bay. Successful larvae are those that reach a settlement size of 330 μm within the planktonic larval duration of 30 days, as calculated from the growth model. The fortnightly variability of the along-estuary flow associated with the spring-neap tides is the primary factor affecting larval dispersion. Larvae released during neap tides are transported mostly up-estuary into areas with reduced salinity, which decreases growth rates and reduces the chance of reaching settlement size within 30 days. Fewer than 20% of the larvae released from upper bay reefs reach settlement size. However, connectivity to mid-bay oyster reefs by the up-estuary transport provides a potential source of larvae to maintain the upper bay oyster reefs. Larvae released during spring tides are often transported down-estuary into regions of higher salinity, which provide the majority of the potential recruits for down-estuary oyster reefs. Low salinity caused by periods of high river discharge overwhelms the up-down estuary transport associated with the spring-neap tides and reduces the ability of the oyster larvae to reach settlement size. Simulated larval trajectories show that vertical behavior makes a small contribution to larval dispersion and that turbulent mixing can overwhelm this process. Thus, for average river discharge conditions, the intra-seasonal dispersion of eastern oyster larvae in Delaware Bay is mostly controlled by the spring-neap tides and the timing of spawning relative to the occurrence of these tides may determine patterns of transport and movement eastern oyster larvae among oyster reefs in Delaware Bay.

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