Disturbance and recruitment: a test of solute and substrate specificity using Mercenaria mercenaria and Capitella sp.1

Disturbances alter sediment surface chemistry, and this alteration promotes rejection of habitats by new benthic recruits. To date, the specific compound(s) affecting the rejection response have not been identified, but model predictions and experimental evidence suggest that oxygen and ammonium concentrations are informative. We evaluated the recruitment response of the opportunistic polychaete Capitella sp. 1 to disturbed, undisturbed and recovering sediments, with parallel measures of oxygen concentration at the sediment surface. Results suggest that Capitella sp. 1 avoid disturbed sediments, but the behavior was not consistent with surface oxygen concentration. We also manipulated porewater ammonium concentration and examined the recruitment responses of Mercenaria mercenaria and Capitella sp. 1 to experimentally altered, as well as naturally disturbed, surfaces. Results suggest that M mercenaria avoid disturbed habitats, regardless of ammonium concentration; however, Capitella sp. 1 responses were consistent with avoidance of high ammonium environments. Finally, we tested a prediction of our disturbance recovery model: that chemical signals associated with disturbance, and responses of recruits, vary as a function of the diagenetic regime. As predicted, chemical concentrations were more dramatically changed in disturbed muds relative to disturbed sands. Capitella sp. 1 rejected disturbed mud surfaces having high ammonium/low oxygen but accepted habitats having low ammonium and high oxygen, including undisturbed mud and sand surfaces, and disturbed sand surfaces. M. mercenaria rejected disturbed sediments regardless of ammonium or oxygen concentration, or sediment type. Our findings confirm that small-scale geochemical processes that are widespread in sedimentary habitats affect recruitment decisions.