Abstract
We investigated the effect of the thalassinid mud shrimp Upogebia pugettensis on organic matter and nutrient cycling on Idaho Flat, an intertidal flat in the Yaquina River estuary, Oregon. Field studies were conducted to measure carbon and nitrogen remineralization rates and benthic fluxes relative to shrimp population density. Organic carbon remineralization rates estimated from incubations increased with burrow density. Similarly, benthic oxygen uptake increased linearly with burrow density. Shrimp bioirrigation enhanced carbon remineralization up to 2.9 times; its effects on nitrogen cycling were even more dramatic, enhancing ammonification rates up to 7 times, nitrification rates between 3 and 9 times, and estimated denitrification up to 4 times relative to portions of the tide flat lacking shrimp. Bioirrigation by shrimp also reduced pore‐water inventories of dissolved inorganic nitrogen (DIN), resulting in DIN fluxes that increased exponentially with burrow density up to 15‐fold relative to No‐shrimp plots. For both C and N fluxes, the strongest density‐dependent relationships occurred with burrow hole density rather than shrimp population density or biomass. Combined with population mapping of mud shrimp populations on Idaho Flat, we estimate that total areal C and DIN effluxes on the tide flat were enhanced 1.9 and 3.7 times, respectively, relative to areal fluxes expected if no shrimp were present. Mud shrimp thus perform as geochemical ecosystem engineers that strongly affect the flux and fate of C and N in Pacific estuaries.