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.Oyster-reef communities (Crassostrea virginica) provide numerous ecosystem services, including production of oysters, water filtration 8,9 , provision of habitat for fishes and crustaceans 10 , shoreline stabilization 11,12 , and maintenance of estuarine-water alkalinity 13 . In the face of natural and anthropogenic stressors, such as harvesting, degrading water quality, increasing rates of SLR, warming, disease, ocean acidification, and parasitism, reef habitats and associated services are becoming unsustainable. Loss of these reefs in estuaries that otherwise lack alternative hard substrates is a global problem 4 . SLR, in particular, threatens oyster reefs in the high-salinity seaward portions of estuaries because there, oysters seek refuge from biofouling (space competition), predation and disease in intertidal areas 1,2 . The importance of the intertidal area to individual oyster growth in lower estuaries is apparent from experimental work that has shown intertidal oysters grow 34% faster and exhibit an order of magnitude less fouling (percentage cover) than subtidal oysters 14 . Constructed subtidal reefs in no-harvest sanctuaries (North Carolina, USA) were also found to have few, if any, live oysters (mean density of 0-92 live oysters m −2 ) after six years in euhaline waters, whereas intertidal reefs faired significantly better (200-225 live oysters m −2 ; ref. 15). Restoration is a common mitigation option for historic oyster-reef loss, but project success with accelerating SLR will depend on a reef 's ability to maintain an intertidal position. Model simulations presume that reef-accretion rates cannot exceed the rate of SLR, but parameterizations are not verified with direct measures of reef-scale growth 16,17 . Without direct measures of reef-scale growth, our ability to assess restoration and conservation ...

Summary1. 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.

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Within intertidal communities, aerial exposure (emergence during the tidal cycle) generates strong vertical zonation patterns with distinct growth boundaries regulated by physiological and external stressors. Forecasted accelerations in sea-level rise (SLR) will shift the position of these critical boundaries in ways we cannot yet fully predict, but landward migration will be impaired by coastal development, amplifying the importance of foundation species’ ability to maintain their position relative to rising sea levels via vertical growth. Here we show the effects of emergence on vertical oyster-reef growth by determining the conditions at which intertidal reefs thrive and the sharp boundaries where reefs fail, which shift with changes in sea level. We found that oyster reef growth is unimodal relative to emergence, with greatest growth rates occurring between 20–40% exposure, and zero-growth boundaries at 10% and 55% exposures. Notably, along the lower growth boundary (10%), increased rates of SLR would outpace reef accretion, thereby reducing the depth range of substrate suitable for reef maintenance and formation, and exacerbating habitat loss along developed shorelines. Our results identify where, within intertidal areas, constructed or natural oyster reefs will persist and function best as green infrastructure to enhance coastal resiliency under conditions of accelerating SLR.

Sea level anomalies are intra-seasonal increases in water level forced by meteorological and oceanographic processes unrelated to storms. The effects of sea level anomalies on beach morphology are unknown but important to constrain because these events have been recognized over large stretches of continental margins. Here, we present beach erosion measurements along Onslow Beach, a barrier island on the U.S. East Coast, in response to a year with frequent sea level anomalies and no major storms. The anomalies enabled extensive erosion, which was similar and in most places greater than the erosion that occurred during a year with a hurricane. These results highlight the importance of sea level anomalies in facilitating coastal erosion and advocate for their inclusion in beach-erosion models and management plans. Sea level anomalies amplify the erosive effects of accelerated sea level rise and changes in storminess associated with global climate change.