Simulated effects of seagrass loss and restoration on settlement and recruitment of blue crab postlarvae and juveniles in the York River, Chesapeake Bay

Seagrass meadows provide important settlement habitat, food and refuge for postlarvae and young juveniles of the blue crab, Callinectes sapidus. In the York River, Chesapeake Bay, areal cover and distribution of seagrass beds has declined historically. Beds which existed 12–25 km upriver from the mouth disappeared and have not recovered. A model for planktonic postlarval behavior, coupled with a 3-dimensional hydrodynamic transport model for the York River, was used to investigate potential effects of the decline in seagrass abundance, and hypothetical restoration, on blue crab settlement and recruitment to the benthos, both in seagrass and to unvegetated bottom. Effects of habitat loss were investigated in model simulations using two historic patterns of seagrass cover (ca. 1965 and ca. 1996), five patterns with intermediate loss of cover, and three settlement rates. Declines of crab settlement in seagrass (<40%) and total settlement (<25%) were not as great as the reduction in seagrass cover (70%). Although settlement was higher when seagrass cover was greater, a "settlement shadow" created by seagrass near the river mouth reduced settlement in historic upstream seagrass beds and ameliorated effects associated with the loss of those beds. Increases in recruitment associated with restored seagrass beds differed significantly with restoration location, such that seagrass restoration in some locations enhanced recruitment substantially more than equivalent restoration in other locations, due to spatial variation in transport processes. Thus, landscape-level spatial patterns of existing and lost seagrass habitat across the landscape interact with transport processes and postlarval behavior to determine settlement and recruitment, and should be considered when evaluating population impacts of habitat loss or restoration.