Despite its widespread use, the ecological effects of shoreline armoring are poorly synthesized and difficult to generalize across soft sediment environments and structure types. We developed a conceptual model that scales predicted ecological effects of shore-parallel armoring based on two axes: engineering purpose of structure (reduce/slow velocities or prevent/ stop flow of waves and currents) and hydrodynamic energy (e.g., tides, currents, waves) of soft sediment environments. We predicted greater ecological impacts for structures intended to stop as opposed to slow water flow and with increasing hydrodynamic energy of the environment. We evaluated our predictions with a literature review of effects of shoreline armoring for six possible ecological responses (habitat distribution, species assemblages, trophic structure, nutrient cycling, productivity, and connectivity). The majority of studies were in low-energy environments (51 of 88), and a preponderance addressed changes in two ecological responses associated with armoring: habitat distribution and species assemblages. Across the 207 armoring effects studied, 71% were significantly negative, 22% were significantly positive, and 7% reported no significant difference. Ecological responses varied with engineering purpose of structures, with a higher frequency of negative responses for structures designed to stop water flow within a given hydrodynamic energy level. Comparisons across the hydrodynamic energy axis were less clear-cut, but negative responses prevailed (>78%) in high-energy environments. These results suggest that generalizations of ecological responses to armoring across a range of environmental contexts are possible and that the proposed conceptual model is useful for generating predictions of the direction and relative ecological impacts of shoreline armoring in soft sediment ecosystems.

Proliferation of macroalgal blooms is regulated by grazing pressure and nutrient availability, which may be mediated directly by benthic macroinvertebrates or indirectly through feedback mechanisms. Using invertebrates common to a shallow estuary in Cape Cod, Massachusetts (USA), we determined effects of faunal diversity on benthic microalgae, net ecosystem metabolism, sediment nutrient fluxes, and macroalgal biomass and productivity. Laboratory microcosms contained sediments with single-and mixed-species invertebrate assemblages, in the presence of (1) no macroalgae, (2) a macroalgal monoculture, and (3) a realistic macroalgal polyculture. The depositfeeding gastropod Ilyanassa obsoleta suppressed benthic microalgae, enhanced nitrate efflux from sediments, and maintained macroalgal standing stocks. Conversely, the burrowing, omnivorous polychaete Alitta (formerly Nereis) virens stimulated benthic microalgal growth, inhibited efflux of ammonium, and drastically reduced macroalgal biomass via grazing and translocation of thalli below the sediment surface. In the polyculture experiment, A. virens sequentially removed Gracilaria sp. (Rhodophyta), Ulva sp. (Chlorophyta), and finally Fucus vesiculosus (Phaeophyta). The bivalve Mya arenaria exhibited limited effects on benthic dynamics. In mixed-fauna assemblages, biomass and productivity of benthic microalgae and macroalgae were consistently lower than predicted, revealing non-additive effects of biodiversity. Communities dominated by I. obsoleta or other surficial grazers could indirectly promote macroalgal blooms via sustained release of sediment-derived nutrients and reduction of benthic microalgae. In contrast, omnivorous burrowers such as A. virens may buffer symptoms of eutrophication through inhibition of ammonium supply and direct grazing of bloomforming macroalgae. Overall, our results highlight species-specific effects on key ecosystem functions, and demonstrate important feedbacks between top-down and bottom-up controls in shallow estuaries.

Small-scale armoring placed near the marsh-upland interface to protect single-family homes is widespread but understudied. Using a nested, spatially blocked sampling design on the coast of Georgia, USA, we compared the biota and environmental characteristics of 60 marshes adjacent to either a bulkhead, a residential backyard with no armoring, or an intact forest. We found that marshes adjacent to bulkheads were at lower tidal elevations and had features typical of lower elevation marsh habitats: high coverage of the marsh grass Spartina alterniflora, high density of crab burrows, and muddy sediments. Marshes adjacent to unarmored residential sites had higher soil water content and lower porewater salinities than the armored or forested sites, suggesting that there may be increased freshwater input to the marsh at these sites. Deposition of Spartina wrack on the marsh-upland ecotone was negatively related to elevation at armored sites and positively related at unarmored residential and forested sites. Armored and unarmored residential sites had reduced densities of the high marsh crab Armases cinereum, a species that moves readily across the ecotone at forested sites, using both upland and high marsh habitats. Distance from the upland to the nearest creek was longest at forested sites. The effects observed here were subtle, perhaps because of the small-scale, scattered nature of development. Continued installation of bulkheads in the southeast could lead to greater impacts such as those reported in more densely armored areas like the northeastern USA. Moreover, bulkheads provide a barrier to inland marsh migration in the face of sea level rise. Retaining some forest vegetation at the marsh-upland interface and discouraging armoring except in cases of demonstrated need could minimize these impacts.

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In this study, we present data to support the hypothesis that removal of epiphytes by grazers is an important control of nitrogen fixation in temperate seagrass meadows during the summer. Previous work in West Falmouth Harbor, Massachusetts, USA, found highest rates of epiphytic nitrogen fixation in the part of the harbor (Snug Harbor) with the greatest nitrogen load and the lowest phosphate concentrations, a somewhat paradoxical result suggesting that biogeochemical controls are not the major factor regulating this nitrogen fixation. Here we report that the density of invertebrate grazers on epiphytic algae (predominantly Bittiolum alternatum) was least in Snug Harbor, where nitrogen fixation rates were greatest. Reciprocal transplant experiments showed that seagrass shoots transplanted into Snug Harbor from the part of the harbor (Outer Harbor) where external nitrogen loading was lower but grazer densities were 4-fold higher, had a more than 5-fold increase in epiphytic nitrogen fixation after a 12 d incubation period. Shoots transplanted from Snug Harbor to Outer Harbor showed a large, rapid reduction in epiphytic nitrogen fixation rates after only 6 d, likely due to consumption of epiphytes. Our results suggest that trophic control is a potentially important determinant of epiphytic nitrogen fixation rates in temperate seagrass meadows.