Freshwater Inflow Effects on Larval Fish and Crab Settlement Onto Oyster Reefs

Planktonic larvae of resident, oyster reef-associated decapods and fishes are subject to variable transport and retention whenever estuarine circulation is altered by freshwater inflow. Because freshwater inflow has the potential to advect larvae either toward or away from oyster reef settlement habitats, we compared the monthly distributions and abundances of larvae and postsettlement stages in Estero Bay, FL, under variable inflows during a 2-y period. Positive correlations between inflow and larval abundances of 2 species (Rhithropanopeus harrisii and Gobiesox strumosus) appeared to be caused by advection of upstream larvae downstream into the study area, whereas similar correlations with postsettlement juveniles of other species (Eurypanopeus depressus and Petrolisthes armatus) suggested bottom-up improvements to postsettlement survival. In contrast, the larvae of many species were advected seaward and away from oyster reef habitats during periods of elevated inflow, creating a spatial gap between the larvae and their landward settlement habitat. The size of this gap was larger for reefs that had greater exposure to freshwater inflows. Larvae displaced too far seaward would have a reduced window of opportunity to find oyster substrate for settlement, thus risking increased aberrant drift and predation loss. Because of the stationary nature of the settlement habitat, advection associated with elevated freshwater inflows was beneficial to some species and detrimental to others, producing winners and losers. The study also suggested that live oyster density was a good indicator of the density, biomass, and richness of oyster reef fishes and decapods. Evidence of temporal resource partitioning (successive peaks in larval densities) was present for those fishes that use empty oyster shell as sites for egg laying and nesting, although this pattern was present only during the first year of the study