Marine Science Center

1. Gradients in competition and predation that regulate communities should guide biogenic habitat restoration, while restoration ecology provides opportunities to address fundamental questions regarding food web dynamics via large-scale field manipulations. 2. We restored oyster reefs across an aerial exposure gradient (shallow-subtidal-tomid-intertidal) to explore how vertical gradients in natural settlement, growth and interspecific interactions affected the trajectory of man-made shellfish reefs. 3. We recorded nearly an order-of-magnitude higher oyster settlement on the deepest (subtidal) reefs, but within a year abundance patterns reversed, and oyster densities were ultimately highest on the shallowest (intertidal) reefs by over an order-of-magnitude. 4. This reversal was due to (i) significantly elevated survivorship on intertidal reefs and (ii) larger surviving oysters on intertidal reefs. These patterns are likely to have developed from greater levels of biofouling and predator abundance (e.g. stone crabs, gastropods) on deeper reefs where aerial exposure was <5% of the monthly tidal cycle. 5. Synthesis and applications. The success of restoration initiatives involving habitat-forming species can be enhanced by accounting for the biotic interactions that regulate population fitness. In littoral systems, vertical gradients in predation, competition and disturbance can be exploited to guide restoration of vegetated (e.g. mangrove, seagrass) or biogenic reef habitats. In particular, our results demonstrate that paradigms of vertical zonation learned from the rocky intertidal and saltmarshes also describe the fate of restored shellfish reefs. As with rocky shores, the lower vertical limit of adult oyster distribution in our study system was most likely driven by predatory and competitive (i.e. smothering) interactions, with a threshold depth at c. 5% daily aerial exposure. Below this depth, experimentally restored reefs failed completely. As with Spartina saltmarsh, accumulation of oyster biomass was greatest at an intermediate vertical position relative to mean sea level (i.e. mid-to-low intertidal). Our developing model proscribes a vertical 'hot spot' for restoration efforts to maximize biogenic reef fitness and production.

Ecologists have long been interested in identifying and testing factors that drive top-down or bottom-up regulation of communities. Most studies have focused on factors that directly exert top-down (e.g., grazing) or bottom-up (e.g., nutrient availability) control on primary production. For example, recent studies in salt marshes have demonstrated that fronts of Littoraria irrorata periwinkles can overgraze Spartina alterniflora and convert marsh to mudflat. The importance of indirect, bottom-up effects, particularly facilitation, in enhancing primary production has also recently been explored. Previous field studies separately revealed that fiddler crabs, which burrow to depths of more than 30 cm, can oxygenate marsh sediments and redistribute nutrients, thereby relieving the stress of anoxia and enhancing S. alterniflora growth. However, to our knowledge, no studies to date have explored how nontrophic facilitators can mediate top-down effects (i.e., grazing) on primary-producer biomass. We conducted a field study testing whether fiddler crabs can facilitate S. alterniflora growth sufficiently to mitigate overgrazing by periwinkles and thus sustain S. alterniflora marsh. As inferred from contrasts to experimental plots lacking periwinkles and fiddler crabs, periwinkles alone exerted top-down control of total aboveground biomass and net growth of S. alterniflora. When fiddler crabs were included, they counteracted the effects of periwinkles on net S. alterniflora growth. Sediment oxygen levels were greater and S. alterniflora belowground biomass was lower where fiddler crabs were present, implying that fiddler crab burrowing enhanced S. alterniflora growth. Consequently, in the stressful interior S. alterniflora marsh, where subsurface soil anoxia is widespread, fiddler crab facilitation can mitigate top-down control by periwinkles and can limit and possibly prevent loss of biogenically structured marsh habitat and its ecosystem services.

1. Gradients in competition and predation that regulate communities should guide biogenic habitat restoration, while restoration ecology provides opportunities to address fundamental questions regarding food web dynamics via large-scale field manipulations. 2. We restored oyster reefs across an aerial exposure gradient (shallow-subtidal-tomid-intertidal) to explore how vertical gradients in natural settlement, growth and interspecific interactions affected the trajectory of man-made shellfish reefs. 3. We recorded nearly an order-of-magnitude higher oyster settlement on the deepest (subtidal) reefs, but within a year abundance patterns reversed, and oyster densities were ultimately highest on the shallowest (intertidal) reefs by over an order-of-magnitude. 4. This reversal was due to (i) significantly elevated survivorship on intertidal reefs and (ii) larger surviving oysters on intertidal reefs. These patterns are likely to have developed from greater levels of biofouling and predator abundance (e.g. stone crabs, gastropods) on deeper reefs where aerial exposure was <5% of the monthly tidal cycle. 5. Synthesis and applications. The success of restoration initiatives involving habitat-forming species can be enhanced by accounting for the biotic interactions that regulate population fitness. In littoral systems, vertical gradients in predation, competition and disturbance can be exploited to guide restoration of vegetated (e.g. mangrove, seagrass) or biogenic reef habitats. In particular, our results demonstrate that paradigms of vertical zonation learned from the rocky intertidal and saltmarshes also describe the fate of restored shellfish reefs. As with rocky shores, the lower vertical limit of adult oyster distribution in our study system was most likely driven by predatory and competitive (i.e. smothering) interactions, with a threshold depth at c. 5% daily aerial exposure. Below this depth, experimentally restored reefs failed completely. As with Spartina saltmarsh, accumulation of oyster biomass was greatest at an intermediate vertical position relative to mean sea level (i.e. mid-to-low intertidal). Our developing model proscribes a vertical 'hot spot' for restoration efforts to maximize biogenic reef fitness and production.

In the high-salinity seaward portions of estuaries, oysters seek refuge from predation, competition and disease in intertidal areas 1,2 , but this sanctuary will be lost if vertical reef accretion cannot keep pace with sea-level rise (SLR). Oyster-reef abundance has already declined ⇠85% globally over the past 100 years, mainly from over harvesting 3,4 , making any additional losses due to SLR cause for concern. Before any assessment of reef response to accelerated SLR can be made, direct measures of reef growth are necessary. Here, we present direct measurements of intertidal oyster-reef growth from cores and terrestrial lidar-derived digital elevation models. On the basis of our measurements collected within a mid-Atlantic estuary over a 15-year period, we developed a globally testable empirical model of intertidal oyster-reef accretion. We show that previous estimates of vertical reef growth, based on radiocarbon dates and bathymetric maps 5,6 , may be greater than one order of magnitude too slow. The intertidal reefs we studied should be able to keep up with any future accelerated rate of SLR (ref. 7) and may even benefit from the additional subaqueous space allowing extended vertical accretion.

Acting on the perception that they perform better for longer, most property owners in the United States choose hard engineered structures, such as bulkheads or riprap revetments, to protect estuarine shorelines from erosion. Less intrusive alternatives, specifically marsh plantings with and without sills, have the potential to better sustain marsh habitat and support its ecosystem services, yet their shoreline protection capabilities during storms have not been evaluated. In this study, the performances of alternative shoreline protection approaches during Hurricane Irene (Category 1 storm) were compared by 1) classifying resultant damage to shorelines with different types of shoreline protection in three NC coastal regions after Irene; and 2) quantifying shoreline erosion at marshes with and without sills in one NC region by using repeated measurements of marsh surface elevation and marsh vegetation stem density before and after Irene. In the central Outer Banks, NC, where the strongest sustained winds blew across the longest fetch; Irene damaged 76% of bulkheads surveyed, while no damage to other shoreline protection options was detected. Across marsh sites within 25 km of its landfall, Hurricane Irene had no effect on marsh surface elevations behind sills or along marsh shorelines without sills. Although Irene temporarily reduced marsh vegetation density at sites with and without sills, vegetation recovered to pre-hurricane levels within a year. Storm responses suggest that marshes with and without sills are more durable and may protect shorelines from erosion better than the bulkheads in a Category 1 storm. This study is the first to provide data on the shoreline protection capabilities of marshes with and without sills relative to bulkheads during a substantial storm event, and to articulate a research framework to assist in the development of comprehensive policies for climate change adaptation and sustainable management of estuarine shorelines and resources in U.S. and globally.

Coastal ecosystems provide numerous services, such as nutrient cycling, climate change 16 amelioration, and habitat provision for commercially valuable organisms. Ecosystem functions 17 and processes are modified by human activities locally and globally, with degradation of coastal 18 ecosystems by development and climate change occurring at unprecedented rates. The demand 19 for coastal defense strategies against storms and sea-level rise has increased with human 20 population growth and development along coastlines worldwide, even while that population 21 growth has reduced natural buffering of shorelines. Shoreline hardening, a common coastal 22 35 habitat for nekton) than bulkheads or control marshes. These ecosystem-service enhancements 36 were detected on shores with sills three or more years after construction, but not before. Sills 37 3 provide added structure and may provide better refuges from predation and greater opportunity 38 to use available food resources for nekton than unvegetated bulkheaded shores or control 39 marshes. Our study shows that unlike shoreline hardening, living shorelines can enhance some 40 ecosystem services provided by marshes, such as provision of nursery habitat. 41 42

Rapid population growth and coastal development are primary drivers of marine habitat degradation. Although shoreline hardening or armoring (the addition of concrete structures such as seawalls, jetties, and groins), a byproduct of development, can accelerate erosion and loss of beaches and tidal wetlands, it is a common practice globally. Here, we provide the first estimate of shoreline hardening along US Pacific, Atlantic, and Gulf of Mexico coasts and predict where future armoring may result in tidal wetland loss if coastal management practices remain unchanged. Our analysis indicates that 22 842 km of continental US shoreline -approximately 14% of the total US coastline -has been armored. We also consider how socioeconomic and physical factors relate to the pervasiveness of shoreline armoring and show that housing density, gross domestic product, storms, and wave height are positively correlated with hardening. Over 50% of South Atlantic and Gulf of Mexico coasts are fringed with tidal wetlands that could be threatened by future hardening, based on projected population growth, storm frequency, and an absence of coastal development restrictions.