Coastal systems are increasingly impacted by over-enrichment of nutrients, which has cascading effects for ecosystem functioning. Oyster restoration and aquaculture are both hypothesized to mitigate excessive nitrogen (N) loads via benthic denitrification. The degree to which these management activities perform similar functions for removing N, however, has not been extensively examined in New England, a place where nutrient runoff is high and increasing oyster (Crassostrea virginica) restoration and aquaculture activity is taking place. Here, we use a novel in situ methodology to directly measure net N 2 and O 2 fluxes across the sediment-water interface in a shallow (∼1 m) coastal pond in southern Rhode Island. We collected data seasonally during 2013 and 2014 at restored oyster reefs, oyster aquaculture, oyster cultch (shell), and bare sediment. Restored oyster reefs and aquaculture had the highest mean (±SE) denitrification rates, 581.9 (±164.2) and 346 (±168.6) µmol N 2 −N m −2 h −1 , respectively, and are among the highest recorded for oyster-dominated environments. Denitrification rates at sites with oyster cultch were 60.9 (±44.3) µmol N 2 −N m −2 h −1 , which is substantially less than the sites with active oysters but still more than 50% higher than denitrification rates measured in bare sediment (24.4 ± 10.1 µmol N 2-N m −2 h −1). The increase in denitrification rates at treatments, however, varied by season and the greatest rates for restored reefs were in the fall. Overall, the greatest aggregate denitrification rates occurred in the fall. Sediment oxygen demand (SOD) followed similar patterns but with greater overall rates in the summer, and displayed a strong linear relationship with denitrification (R 2 = 0.93). Our results demonstrate that habitats associated with live oysters have higher net denitrification rates and that oyster reef restoration and oyster aquaculture may provide similar benefits to the ecosystem in terms of N removal. However, gas fluxes may also be affected where three-dimensional structure is introduced via oyster shell cultch and this appears to be seasonally-dependent. These data will be important for managers as they incorporate oysters into nutrient reduction strategies and consider system-level trade-offs in services provided by oyster reef restoration and aquaculture activities.

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ABSTRACT1. Anthropogenic-derived nutrient inputs to coastal environments have increased dramatically worldwide in the latter half of the 20th century and are altering coastal ecosystems. We evaluated the effects of nitrogen loading on changes in macrophyte community structure and the associated fauna of a north temperate estuary. We found that a shift in primary producers from eelgrass to macroalgae in response to increased nutrient loading alters habitat physical and chemical structure and food webs. As nitrogen load increased we found increased macroalgal biomass, decreased eelgrass shoot density and biomass, decreased fish and decapod abundance and biomass, and decreased fish diversity.2. The central importance of macroalgae in altering eelgrass ecosystem support of higher trophic levels is evident in the response of the ecosystem when this component was manipulated. Removal of macroalgae increased eelgrass abundance and water column and benthic boundary layer O 2 concentrations. These changes in the physical and chemical structure of the ecosystem with lower macroalgal biomass resulted in higher fish and decapod abundance and biomass.3. Both a 15 N tracer experiment and the growth of fishes indicated that little of the macroalgal production was immediately transferred to secondary consumers. d15 N values indicated that the most abundant fishes were not using a grazing food web based on macroalgae. Fish tended to grow better and have a greater survivorship in eelgrass compared to macroalgal habitats.4. Watershed-derived nutrient loading has caused increased macroalgal biomass and degradation and loss of eelgrass habitat, thus reducing the capacity of estuaries to support nekton.

Detached macrophytes (seagrass and macroalgae) are transported from more offshore areas and accumulate in substantial volumes in surf zones, where they are commonly called wrack. Fishes were sampled using seine nets in 4 volume categories of detached macrophytes (bare sand, low, medium and high volumes) in the surf zone at 2 sandy beaches in southwestern Australia to determine how increasing volumes of surf-zone wrack influences fish community and size composition. Species composition and densities of fish, which were dominated by juveniles, differed between areas where wrack was present or absent, and also among volumes of wrack in the surf zone. Total fish abundance and biomass increased as the volume of wrack increased. Cnidoglanis macrocephalus and Pelsartia humeralis were the dominant species and were most abundant in medium and high wrack volumes. Fish gut contents were analysed for C. macrocephalus and P. humeralis, and verified that Allorchestes compressa is a major prey item for juveniles of these species. A series of habitat preference trials conducted in outdoor aquaria tested whether juvenile C. macrocephalus and P. humeralis showed a preference for different types of detached macrophytes as a habitat, i.e. seagrass, brown algae, or a mixture of both macrophyte types. Non-parametric goodness-of-fit binomial tests for differences in the number of fish between each habitat type showed no clear pattern in habitat preference for either species of fish. Field and laboratory results suggest that the amount, rather than type, of detached macrophytes is more important in providing a habitat for juvenile C. macrocephalus and P. humeralis.