Recruitment dynamics of juvenile blue crabs in North Carolina: Ecological patterns and processes on a landscape scale

Dispersal is a fundamental process influencing how groups of organisms are interconnected and arranged in spatially heterogeneous landscapes. The present study assessed the determinants of multi-stage dispersal and its consequences on spatial population dynamics of a marine invertebrate, the blue crab ( Callinectes sapidus) within the Croatan, Albemarle, Pamlico Estuarine System, NC, USA. Large-scale sampling indicated that primary (postlarval) dispersal led to spatial clumping of juveniles within more seaward habitats, while secondary (juvenile) dispersal soon after settlement altered initial settlement patterns and expanded the estuarine distribution of juveniles. The specific wind characteristics that facilitated primary and secondary dispersal varied according to site. Tropical cyclones had a varied influence on dispersal, depending on the storm's characteristics. It appears that planktonic secondary dispersal is a relatively consistent process, occurring during periods with and without storms. While Inlet region habitats acted as general donor areas to receiver habitats in the Sound, specific dispersal corridors were detected between sites. Relationships between multiple blue crab life stages indicated that density dependence varied according to spatial scale (site, region, estuary), life stage, and location. A density-dependent settler/recruit relationship was strongest at the large estuarine scale; for recruits/juveniles, density dependence was only detected at the small regional scale (Sound only). When the influence of post-settlement immigration was removed, variability in recruit/juvenile relationships was greatly reduced, suggesting that secondary dispersal can mask density dependence and is a strong component in recruitment. A field mark-recapture study determined that emigration and mortality contributing equally to loss of crabs from seagrass beds. Mortality rates were density-dependent and emigration rates were inversely density-dependent for older juveniles. The general mode of juvenile emigration (benthic vs. planktonic) changed with ontogeny. High colonization and loss over short temporal scales demonstrated the dynamic nature of juvenile crab assemblages. This study of recruitment variability is unique in its examination of spatial abundance patterns within the estuarine landscape, incorporating the influence of multi-stage dispersal and habitat connectivity on population dynamics.