Quantifying estuarine-scale invertebrate larval connectivity: Methodological and ecological insights

The early life history of many marine organisms includes a dispersive planktonic larval phase which allows for the exchange of individuals among subpopulations. Knowledge of the degree of exchange, or connectivity, is critical to understanding the abundance and distribution of marine populations. Here, we applied geochemical tagging to assess estuarine-scale larval connectivity among subpopulations of the commercially and ecologically important eastern oyster, Crassostrea virginica. To generate an “atlas” of geochemical signatures associated with spawning sites and potential dispersal pathways from spawning sites, we outplanted recently spawned oyster larvae to stationary moorings and surface drifters, respectively. Using the atlases generated from both outplant methods, we predicted natal origin, and thus larval connectivity, for newly settled oysters (spat) during three field trials over two summers (June 2013, June 2014, and August 2014), within three regions (∼ 35 km × 15 km quadrants) of Pamlico Sound, North Carolina, U.S.A. Patterns of larval connectivity varied both between months and annually but were predominately directed south to north following wind patterns. Predicted selfrecruitment was variable, as up to 100% of spat in a given region displayed signatures consistent with natal origin within that same region. Predicted connectivity patterns varied significantly based on atlases generated from outplanting on stationary moorings vs. surface drifters. For example, drifter-predicted connectivity followed biophysical larval dispersal models more closely than mooring-predicted connectivity, while mooring-predicted connectivity displayed a higher diversity in larval sources. Both connectivity models highlight the need for resource management strategies such as reserve networks to incorporate designs that account for inherent variability in dispersal pathways.