Abstract. Salt marsh ecosystems have been considered not susceptible to nitrogen overloading because early studies suggested that salt marshes adsorbed excess nutrients in plant growth. However, the possible effect of nutrient loading on species composition, and the combined effects of nutrients and altered species composition on structure and function, was largely ignored. Failure to understand interactions between nutrient loading and species composition may lead to severe underestimates of the impacts of stresses. We altered whole salt marsh ecosystems (;60 000 m 2 /treatment) by addition of nutrients in flooding waters and by reduction of a key predatory fish, the mummichog. We added nutrients (N and P; 15-fold increase over ambient conditions) directly to the flooding tide to mimic the way anthropogenic nutrients are delivered to marsh ecosystems. Despite the high concentrations (70 mmol N/L) achieved in the water column, our annual N loadings (15-60 g NÁm À2 Áyr À1 ) were an order of magnitude less than most plot-level fertilization experiments, yet we detected responses at several trophic levels. Preliminary calculations suggest that 30-40% of the added N was removed by the marsh during each tidal cycle. Creek bank Spartina alterniflora and high marsh S. patens production increased, but not stunted high marsh S. alterniflora. Microbial production increased in the fertilized creek bank S. alterniflora habitat where benthic microalgae also increased. We found top-down control of benthic microalgae by killifish, but only under nutrient addition and in the opposite direction (increase) than that predicted by a fish-invertebrate-microalgae trophic cascade. Surprisingly, infauna declined in abundance during the first season of fertilization and with fish removal. Our results demonstrate ecological effects of both nutrient addition and mummichog reduction at the whole-system level, including evidence for synergistic interactions.

Despite the paradigm that Spartina spp. detritus is the basis for estuarine food webs, other primary producers may contribute to the diets of saltmarsh consumers. To determine the dietary contribution of primary producers to benthic infauna in the Plum Island Estuary, Massachusetts, USA, we examined natural abundance stable isotopes in 4 intertidal saltmarsh habitats and conducted an 15 N enrichment experiment in 2 habitats. Natural abundance isotope data suggested that Spartina spp. detritus was of limited dietary importance to infauna in all habitats (including Spartina spp. understory) and instead benthic algae and phytoplankton were the dominant food sources.15 N enrichment was used to improve dietary resolution of benthic algae and phytoplankton sources that had similar natural abundance values. To label only benthic algae, 15 N-enriched Na 15 NO 3 was applied daily for 14 d to sediment in mudflat and creek-wall habitats. Food-web incorporation of 15 N-labeled benthic algae was found in most species. However, label uptake in the polychaetes Manayunkia aestuarina, Fabricia sabella and Streblospio benedicti indicated that phytoplankton was the most important food source for these consumers. Label uptake in the polychaete Nereis diversicolor differed between habitats, suggesting a large dietary contribution of microphytobenthos (MPB) in mudflat and phytoplankton in creek wall. The oligochaete Paranais litoralis consumed both MPB and phytoplankton regardless of habitat. The harpacticoid copepod Heterolaophonte sp. consumed primarily epiphytic diatoms. Overall, infauna in this system relied on phytoplankton and benthic algae as dominant food resources, and dietary contributions from primary producers varied among species and habitats. KEY WORDS: Food web · Stable isotopes · Saltmarsh · Isotope addition · Infauna · Microphytobenthos · PhytoplanktonResale or republication not permitted without written consent of the publisher Mar Ecol Prog Ser 359: 37-49, 2008 (Cammen 1980, Lopez & Levinton 1987. For example, van Oevelen et al. (2006a) found that bacteria contributed minimally to the diet of intertidal benthic infauna. Recent attention has been given to the dietary role of the less conspicuous MPB, macroalgae, filamentous algae and epiphytic algae (here collectively called benthic algae) that inhabit marsh mudflats and surrounding areas (Haines & Montague 1979, Kwak & Zedler 1997, Quiñones-Rivera & Fleeger 2005. Specifically, isotope studies have revealed the importance of MPB and other algae to the diet of saltmarsh infauna (Herman et al. 2000, Levin et al. 2006). Because they are relatively nutritious and easy to digest, algae may be a preferred food source for deposit-and suspension-feeding infauna even though they may live in a sediment matrix rich in Spartina spp. detritus (Lopez & Levinton 1987, Tenore 1988, Kreeger & Newell 2000, Sullivan & Currin 2000.Infaunal invertebrates play an important role in the structure and function of saltmarsh ecosystems, especially because they are abundant acros...

[1] Inlets provide a critical ecological link between restricted bays and estuaries to the coastal ocean. The net fluxes of water and suspended sediment are presented in this study. These fluxes are obtained based on data from a multidisciplinary, full tidal cycle survey across Barataria Pass in southern Louisiana on 31 July to 1 August 2008. The velocity profiles were obtained with an acoustic Doppler current profiler mounted on a small boat continuously crossing the inlet, which contains swift and turbulent tidal currents. Water samples were collected six times in a 24 h period at three discrete depths and three locations across the inlet. The observations delineated a clear eddy on the western side of the inlet which causes a low R 2 value of the tidal harmonic analysis on the edges of the inlet. The net flux of total suspended sediment out of the bay was determined to be 8800 t of which 20% was organic matter, demonstrating a significant source of organic matter to the base of the coastal ocean detrital food chain. The time evolution and net fluxes of water, and suspended sediments showed that the net flow resembles conventional estuarine circulation patterns with net outward flow on the surface and shallow ends of the inlet and with net inward flow in the center and at the bottom of the center of the inlet. The west side has a much larger outward flow than the east side while the east side is fresher. These differences suggest that the Louisiana Coastal Current from around the Bird's Foot Delta derived from the mixing of shelf water with the Mississippi River freshwater may have entered the bay. This must have been mostly from the east side during the survey, which resulted in a smaller outward flow on the eastern side. A numerical experiment further confirmed this assumption and the model was verified by field observations on 5 May 2010.

in the fertilized system but could not be accurately calculated in the reference system due to rapid (,4 h) NO 3 À turnover. Over the fiveday paired tracer addition, sediments sequestered a small fraction of incoming NO 3 À , although the efficiency of sequestration was 3.8% in the reference system and 0.7% in the fertilized system. Gross sediment N sequestration rates were similar at 13.5 and 12.6 molÁha À1 Ád À1 , respectively. Macrophyte NO 3 À uptake efficiency, based on tracer incorporation in aboveground tissues, was considerably higher in the reference system (16.8%) than the fertilized system (2.6%), although bulk uptake of NO 3 À by plants was lower in the reference system (1.75 mol NO 3) than the fertilized system (;10 mol NO 3 À Áha À1 Ád À1 ). Nitrogen processing efficiency decreased with NO 3 À load in all pools, suggesting that the nutrient processing capacity of the marsh ecosystem was exceeded in the fertilized marsh.

Responses of infaunal saltmarsh benthic invertebrates to whole-ecosystem fertilization and predator removal were quantified in Plum Island Estuary, Massachusetts, USA. Throughout a growing season, we enriched an experimental creek on each flooding tide to 70 µM NO 3 -and 4µM PO 4 -3 (a 10 x increase in loading above background), and we reduced Fundulus heteroclitus density by 60% in a branch of the fertilized and a reference creek. Macroinfauna and meiofauna were sampled in creek (mudflat and creek wall), marsh edge (tall form Spartina alterniflora) and marsh platform (Spartina patens and stunted S. alterniflora) habitats before and after treatmentswere begun; responses were tested with BACI-design statistics. Treatment effects were most common in the mid-range of the inundation gradient. Most fertilization effects were on creek wall where ostracod abundance increased, indices of copepod reproduction increased and copepod and annelid communities were altered. These taxa may use epiphytes (that respond rapidly to fertilization) of filamentous algae as a food source. Killifish reduction effects on meiobenthic copepod abundance were detected at the marsh edge and suggest predator limitation. Fish reduction effects on annelids did not suggest top-down regulation in any habitat; however, fish reduction may have stimulated an increased predation rate on annelids by grass shrimp. Interactions between fertilization and fish reduction occurred under S. patens canopy where indirect predator reduction effects on annelids were indicated. No effects were observed in mudflat or stunted S. alterniflora habitats. Although the responses of infauna to fertilization and predator removal were largely independent and of similar mild intensity, our data suggests that the effects of ecological stressors vary across the marsh landscape.