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This study was carried out to quantify the effects of higher organisms, invertebrate macrofauna, and macrophyte plants on the rates and pathways of microbial respiration coupled to organic matter oxidation in saltmarsh sediments. Sediment geochemistry, rates of microbial metabolism, and the abundance of anaerobic respiratory bacteria were determined at sites differing in the abundance of fiddler crab (Uca pugnax) burrows and vegetation (Spartina alterniflora) coverage. Solid-phase Fe(III) concentrations were 50 to 100 times higher, and solid sulfide concentrations were eight times lower in bioturbated, vegetated sediments (BVL) as compared to nonbioturbated, unvegetated (NUC) sediments. Integrated sulfate reduction rates were 10 times lower in BVL (2 mmol m Ϫ2 d Ϫ2 ) as compared to NUC sediments (20 mmol m Ϫ2 d Ϫ1 ). Directly measured Fe(III) reduction rates were high at the BVL site, whereas no Fe(III) reduction was detected at NUC or in killed sediment treatments. Molybdate, a specific inhibitor of sulfate reduction, inhibited 70% of carbon oxidation when added to NUC sediment but showed no effect on Fe(III) reduction or C oxidation in BVL sediments. Counts of Fe(III)-reducing bacteria (FeRB) were two orders of magnitude higher in BVL sediments (10 7 cells g Ϫ1 ) in comparison to NUC sediments (10 5 cells g Ϫ1 ). Fe(III) respiration comprised up to 100% of carbon oxidation in BVL sediments, whereas sulfate reduction was the dominant respiration process (Ն70% of C oxidation) at NUC. We provide strong evidence to show that macroorganisms stimulate FeRB to outcompete sulfate-reducing bacteria in saltmarsh sediments by supplying an abundance of reactive Fe(III) through reoxidation processes.

The importance of different functional traits of macrobenthos in benthic processes of the Southern Bight of the North Sea was investigated to estimate the effects of density declines and species loss on benthic ecosystem functioning. Two laboratory experiments were performed: before (winter, temperature = 10°C) and after (summer, temperature = 18°C) sedimentation of the spring phytoplankton bloom. Single species treatments of key species (Abra alba, Lanice conchilega and Nephtys sp.) with different functional traits were added to microcosms at 3 density levels (natural, lower, lowest) to account for possible density declines. Sediment -water exchanges of oxygen and nutrients, denitrification and bioturbation were measured. In absence of fauna, benthic mineralisation in the summer experiment was 2.0 times higher than in winter. Fauna stimulated microbial respiration more in summer (up to 100% in L. conchilega treatments) than in winter (negligible fauna effect). As chlorophyll a concentrations were similar in both seasons, the stronger fluxes in summer must be explained by a higher macrobenthic activity owing to the elevated temperature and better condition of the animals. Stimulation of mineralisation by the 3 species in the microcosms was different, and behaviour-related. Owing to its irrigation activity, the tube dweller L. conchilega had more pronounced influences on benthic respiration, nutrient release and denitrification than did the biodiffusers, A. alba and Nephtys sp. A. alba appeared to be a more effective bioturbator than Nephtys sp. Processes such as benthic respiration, nutrient fluxes, denitrification and bioturbation seem to be related to animal densities and therefore decreases in densities can possibly have implications for ecosystem functioning.

The physical mechanism that drives bioirrigation is strongly dependent on the permeability of the sediment. We advance two mechanisms, each described by a corresponding microenvironment model. In muds, burrow water cannot penetrate the sediment, so bioirrigation is intrinsically driven by diffusional transfer across the burrow wall. This ''diffusive'' mode of bioirrigation is accurately described by the classical tube irrigation model. In sands, ventilation flows can penetrate the surrounding sediment via dead end burrows. To quantify this ''advective'' mode of bioirrigation, we propose a novel two-dimensional pocket injection model. This model's principal features are that (1) organisms indent the sediment-water interface with burrow structures, (2) the specific structure of the burrow can be neglected except for the location of a feeding pocket, and (3) burrow water is injected from this feeding pocket into the surrounding sediment. We tested the adequacy of the pocket injection model in a detailed case study of the lugworm Arenicola marina, comparing model simulations and experimental data from core incubations. Simulation of two different sets of inert tracer experiments shows good agreement between model and data, indicating that our model captures the relevant aspects of lugworm bioirrigation in permeable sediments.

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We quantified the fate and transport of watershed-derived ammonium in a tidal freshwater marsh fringing the nutrientrich Scheldt River in a whole-ecosystem 15 N labeling experiment. 15 N-NH was added to the floodwater entering a 3,477 ϩ 4 m 2 tidal marsh area, and marsh ammonium processing and retention were traced in six subsequent tide cycles. We present data for the water phase components of the marsh system, in which changes in concentration and isotopic enrichment of NO , NO , N 2 O, N 2 , NH , and suspended particulate nitrogen (SPN) were measured in concert with a mass balancestudy. Simultaneous addition of a conservative tracer (NaBr) confirmed that tracer was evenly distributed, and the Br Ϫ budget was almost closed (115% recovery). All analyzed dissolved and suspended N pools were labeled, and 31% of added for 30% of 15 N-transformation. In situ whole-ecosystem nitrification rates were four to nine times higher than those in the water column alone, implying a crucial role for the large reactive marsh surface area in N-transformation. Under conditions of low oxygen concentrations and high ammonium availability, nitrifiers produced N 2 O. Our results show that tidal freshwater marshes function not only as nutrient sinks but also as nutrient transformers.

We studied the seasonal exchange of biogenic silica (BSi) and dissolved silica (DSi) between a freshwater and a saltwater tidal marsh and the adjacent coastal waters. Export of DSi was observed from both tidal marshes, whereas BSi was imported in association with suspended solids. The export of DSi was highest (23.4% and 123.8% in the freshwater and saltwater marsh, respectively) in summer when DSi concentrations were low in the nearby coastal waters. Combined data from both marshes suggested a logarithmic decrease in DSi export with increasing DSi concentrations in the inundating waters. BSi import was observed year round in the freshwater marsh, but only in summer in the saltwater marsh. The results show that DSi export from tidal marshes, both freshwater and salt water, contributes significantly to estuarine Si availability in summer and provide new insights regarding potential linkages between tidal marshes and secondary production in nearby coastal waters.Compared with our extensive knowledge concerning N and P processing in the aquatic continuum of watersheds, rivers, lakes, and estuaries, the transport and cycling of silicon has been significantly less well studied (Conley et al.

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