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 densitydependent 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.

Study of the flux and fate of reactive organic material (OM) within Debidue Flat, an intertidal sandflat in the North Inlet estuary, South Carolina, demonstrated that this coarse-grained deposit is a dynamic, open system that experiences rapid OM decomposition and exchange of solutes in the top 30 cm of the sediment column. The fluxes of reactive OM through Debidue Flat were high during all seasons (27-170 mmol C m 2 d Ϫ1 ) and were comparable to fluxes in muddy portions of the North Inlet estuary. Porewater decomposition products were N-and P-rich, the modeled reactivity of organic carbon undergoing decomposition was high (first-order rate constant, k ϭ 0.02 d Ϫ1 ), and abundant extractable chlorophyll a was measured year-round; all properties were consistent with marine algalderived substrates. Porewater solute profiles were controlled by advective flow that rapidly exchanged porewater with overlying waters to ϳ25 cm depth on timescales of hours. Thus, these sandflats act like an unsteady ''trickling bed filter,'' capturing or generating reactive organic particles, rapidly remineralizing OM, and recycling nutrients. Macrobiological structures within the flat altered the amounts and reaction rates of OM on various spatial and temporal scales. Relatively elevated OM decay rates were associated with the burrows of Callichirus major, a deepburrowing thalassinid shrimp. Large stingray feeding pits accumulated fine grained OM, locally clogging the ''trickling bed filter,'' and inhibiting porewater advection. As illustrated by Debidue Flat, intertidal sands can be sites of high OM flux and turnover and play an important role in biogeochemical cycling in estuarine systems.Much of the work on benthic biogeochemical cycling has centered on organic-rich muds. Organic matter remineralization in low carbon sands, however, can have rates comparable to those in organic rich muds (Rowe et al. 1988, Cammen 1991 Grant et al. 1991). Porewater advection results in the rapid exchange of pore and overlying water and has been implicated as the primary process responsible for enhancing remineralization rates and carbon cycling in sands.Physical and biological structures, tidal currents, and waves create pressure gradients that can drive advective porewater flow deep into permeable sandy sediments (Shum and Sundby 1996;Huettel and Webster 2001). These flows move along two-or three-dimensional flowpaths and can increase the effective diffusion coefficients of permeable sediments 5-10 times (Vanderborght et al. 1977;Huettel and Gust 1992). Intertidal systems can also have gravitational 1 Corresponding author (dandrea.tony@epa.gov). Present address: US EPA, Pacific Coastal Ecology Branch, 2111 SE Marine Science Drive, Newport, Oregon 97365. AcknowledgmentsWe thank Joe D'Andrea, Bonnie Willis, Karen Hudson, and Ryan Pigg for their help during field work. We would also like to acknowledge the Belle W. Baruch Marine Laboratory for providing laboratory and office space and Bill Johnson for the use of the chemical laboratory. We appre...

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