Spatial and temporal variation in the dispersal of clam populations on intertidal flats

Abstract

In soft-bottom habitats, post-settlement dispersal of infaunal organisms can be important in population connectivity and turnover. However, this phase of dispersal is often overlooked and understanding of how spatial and temporal variability in biophysical processes drives dispersal in the field is lacking. To quantify spatial and temporal variability in post-settlement dispersal of bivalves, we conducted a study at three sites in Whangarei Harbour, New Zealand. At each site on two separate occasions, we stained the sediment and infauna in 2 m diameter plots with fluorescein to measure the post-settlement dispersal of the clam, Austrovenus stutchburyi (Gray). Dispersal was quantified by monitoring changes in the abundance of non-fluorescent individuals within the plots and fluorescent individuals outside the plots. Our experiments show that some A. stutchburyi were able to disperse at least 50 cm into the plots within 1 tidal cycle (12 h). Dispersal was size dependent, with adult A. stutchburyi dispersing less than juveniles. Post-settlement dispersal varied both spatially and temporally, as indicated by both the change in abundance of non-fluorescent individuals in the plots through time, and the turnover rate within the plots. Dispersal was greatest at the most wave exposed site, and during a storm event. By replicating an experiment in both space and time, our results demonstrate the variability in bivalve dispersal in the field setting. Through the use of fluorescein staining in situ, this study is one of the few that provides estimates of population turnover due to post-settlement dispersal in undisturbed communities.