Macrofaunal modification of porewater advection: role of species function, species interaction, and kinetics

Abstract

Sedimentary habitats are complex associations of biotic, chemical, and physical pro- cesses comprising 'ecosystem function'. The relative importance of these processes to biogeochemi- cal cycling in highly reactive, permeable sediments remains poorly understood. We report results from several field experiments in a muddy-sand intertidal flat dominated by 2 functionally different types of bioturbating macrofauna in False Bay, Washington, USA: (1) the arenicolid polychaete Abarenicola pacifica and (2) 2 species of thalassinid shrimp (Upogebia pugettensis and Neotrypaea californiensis). Experimental plots composed primarily of one of the study taxa or mixed communities of both were evaluated for their effects on porewater advection, solute concentrations, and sediment characteristics. Fluorescein-impregnated acrylamide gels were used to infer rates of transport, and acrylamide gel peepers were used to record porewater concentrations of diagenetically important constituents among experimental plots. Laboratory studies evaluated rates of diffusive transport in non-bioturbated sediments for comparative analysis. We found that (1) functionally different macro- fauna affect rates of porewater advection in permeable sediments, (2) organism effects are not attrib- utable to changes in average measures of sediment granulometry, (3) species interactions may further complicate the advective environment and the resulting diagenetic processes, and (4) species effects vary according to reaction rate kinetics. We hypothesize that species-related effects on trans- port are due to inhibition of arenicolid feeding by thalassinid tubes that serve to block sediment fluidization and advective flow. Thus, specific behaviors and interactions among organisms appear to affect transport rates and sediment function in advectively permeable habitats. The results indicate the importance of integrating behavior, kinetics, and transport into future studies of sedimentary biodiversity and ecosystem function.