Remote reef cryptobenthic diversity: Integrating autonomous reef monitoring structures and in situ environmental parameters

Coral reef sessile organisms inhabiting cryptic spaces and cavities of the reef matrix perform vital and varied functional roles but are often understudied in comparison to those on exposed surfaces. Here, we assess the composition of cryptobenthic taxa from three remote tropical reef sites (Central Indian Ocean) alongside a suite of in situ environmental parameters to determine if, or how, significant patterns of diversity are shaped by local abiotic factors. To achieve this, we carried out a point-count analysis of autonomous reef monitoring structure (ARMS) plate images and employed in situ instrumentation to recover long-term (12 months) profiles of flow velocity, wave heights, temperature, dissolved oxygen, and salinity, and short-term (3 weeks) profiles of light and pH. We recovered distinct environmental profiles between sampling sites and observed that ocean-facing reefs experienced frequent but short-lived cooling internal wave events and that these were key in shaping in situ temperature variability. By comparing temperature and wave height profiles recovered using in situ loggers with ex situ models, we discovered that global satellite products either failed to recover site-specific profiles or both over- and underestimated actual in situ conditions. We found that site choice and recruitment plate face (top or bottom) significantly impacted the percentage cover of bryozoans, gastropods, soft and calcified tube worms, as well as crustose coralline algae (CCA) and fleshy red, brown, and green encrusting macroalgae on ARMS. We observed significant correlations between the abundance of bryozoans, CCA, and colonial tunicates with lower mean temperature and higher mean dissolved oxygen profiles observed across sites. Red and brown encrusting macroalgae abundance correlated significantly with medium-to-high flow velocities and wave height profiles, as well as higher pH and dissolved oxygen. This study provides the first insight into cryptobenthic communities in the Chagos Archipelago marine-protected area and adds to our limited understanding of tropical reef sessile communities and their associations with environmental parameters in this region. With climate change accelerating the decline of reef ecosystems, integrating analyses of cryptobenthic organisms and in situ physicochemical factors are needed to understand how reef communities, if any, may withstand the impacts of climate change

Loss of reef roughness increases residence time on an idealized coral reef

AbstractThrough idealized, numerical models this paper investigates flows on a reef geometry which has received significant attention in the literature; a shallow, fringing reef with deeper, shore-ward pools or lagoons. Given identical model geometries and varying only reef flat drag coefficients between model runs ($$C_D = [0.001,0.005,0.01,0.05,0.1]$$ C D = [ 0.001 , 0.005 , 0.01 , 0.05 , 0.1 ] ), two distinct circulation patterns emerge. One is related to low reef water levels and high roughness, and efficiently flushes the entire reef system resulting in low residence times (an ‘open reef’). The other is related to high reef water levels and low roughness, and in spite of the development of an offshore undertow, this dynamic is inefficient at flushing the reef-pool system and facilitating exchange flow with offshore waters (a ‘closed reef’). This paper shows that even given indistinguishable geometry and offshore conditions, this information is insufficient to predict reef dynamics, and suggests that reef roughness (and thus reef health) plays a comparable role in determining circulation patterns and residence times. Furthermore, a transition from open to closed or vice versa caused by e.g., a loss of reef roughness or increase in mean sea level could have implications for transport and mixing of nutrients and water masses, as well as larval dispersal.</jats:p

Data for: Alongshore Setup Gradients Drive Reef Flat Currents on a Coral Atoll Island

This study presents extensive in situ observations from a coral atoll island in the Maldives to investigate the alongshore variability of wave setup and reef flat currents driven by heterogeneity in morphology and wave forcing. The island’s diverse reef morphology provided an ideal natural laboratory for assessing its impact on spatial hydrodynamic variability. Results indicate that shoreline wave setup increased with incident wave height and low reef flat water levels, regardless of reef geometry. Despite the significant variations in topography, bathymetry, and wave exposure, theoretical predictions of wave setup using existing parameterizations demonstrated strong agreement with observations across three distinct areas of the island, with the best performance at the most wave-exposed locations. Additionally, normally-incident waves produced higher local wave setup, thus inducing alongshore gradients in wave setup. These alongshore setup gradients were found to be the dominant driver of reef flat currents, although wave direction had minimal impact on the current speeds and directions. These findings highlight the importance of resolving alongshore hydrodynamic variability, which is often overlooked in reef hydrodynamic studies due to their great morphologic complexity and resource limitations. Improved understanding of these processes is essential for forecasting sediment dynamics and atoll island evolution. Future work should integrate spatially extensive field observations with high-resolution, process-based modeling to better model reef hydrodynamics and morphodynamics to assess coral island resilience under changing storms and sea levels

Data for: Thresholds of wave forcing: Implications for atoll reef dynamics under sea level rise

We investigate the response of shallow reef flow to tidal variability and wave exposure during a four-month field campaign in southern Huvadhu Atoll, Maldives. Incident waves breaking on steep fore reefs and reef crests generate a setup proportional to offshore wave height that drive a cross-reef flow. We identify a critical threshold—where the depth on the reef flat equals the depth at wave breaking—that separates two distinct reef flow regimes: one dominated by strong, unidirectional flow from the ocean into the lagoon, and another where wave breaking ceases, flow rates decrease, and occasionally reverse direction. Recognising the relative importance of water depth and wave energy, we develop a framework for interpreting shallow reef hydrodynamics in a combined tide-wave parameter space. This framework allows us to project how rising sea levels may alter reef flows - potentially leading to prolonged and more frequent periods of limited wave breaking and a decline in wave-driven transport

Wave, light, and dissolved oxygen exposures drive novel coastal eelgrass (Zostera pacifica) transplant performance

The high ecological and economic value of seagrass has been long recognized, with these foundational habitats providing myriad ecosystem services. Yet through cumulative anthropogenic impacts, seagrasses are exhibiting extensive declines globally. A litany of studies and active restoration trials have demonstrated practical methodologies to restore seagrass habitats and effectively return critical habitat functions to degraded coastal zone systems worldwide. Seagrass loss along the U.S. West Coast has precipitated decades of seagrass protection, conservation, and restoration efforts. Yet, mitigation transplanting efforts have prioritized Zostera marina (narrow-leaved eelgrass) in shallow, protected environments, while a dearth of information is available on species inhabiting offshore islands and exposed mainland coasts. In this study, we conducted a novel transplant of Zostera pacifica, a wide-leaved species found in depths of 7 – 20 m along the offshore islands and mainland coast of California. Transplants were conducted at three geographically distinct sites in Santa Monica Bay, coupled with continuous monitoring of biophysical parameters providing insight into physical drivers at transplant and donor sites. Utilizing in situ data, and environmental thresholds adapted from the literature for Z. marina, we performed exposure analyses to evaluate factors influencing Z. pacifica transplant performance. Exceedances of threshold values for environmental parameters, specifically, wave exposure and near-bed flow speeds (Hrms &gt; 0.59 m and Urms &gt; 0.1 m s-1), photosynthetically active radiation (&lt; 3 and &gt; 5 mol m-2 day-1) and dissolved oxygen (&lt; 3 mg O2 L-1) exposure impacted transplant survivorship. These results suggest Z. pacifica persist in biophysically dynamic conditions and are sensitive to exceedances of thresholds, underlining the importance of pre-transplant site-selection processes to this species. These data represent the first holistic study of Z. pacifica transplanting on an exposed mainland coast, which provides a view into the baseline environmental envelopes within existing Z. pacifica habitat, and further, may serve as a model for investigating scalable open coast seagrass restoration for temperate regions

DataSheet_1_Remote reef cryptobenthic diversity: Integrating autonomous reef monitoring structures and in situ environmental parameters.zip

Coral reef sessile organisms inhabiting cryptic spaces and cavities of the reef matrix perform vital and varied functional roles but are often understudied in comparison to those on exposed surfaces. Here, we assess the composition of cryptobenthic taxa from three remote tropical reef sites (Central Indian Ocean) alongside a suite of in situ environmental parameters to determine if, or how, significant patterns of diversity are shaped by local abiotic factors. To achieve this, we carried out a point-count analysis of autonomous reef monitoring structure (ARMS) plate images and employed in situ instrumentation to recover long-term (12 months) profiles of flow velocity, wave heights, temperature, dissolved oxygen, and salinity, and short-term (3 weeks) profiles of light and pH. We recovered distinct environmental profiles between sampling sites and observed that ocean-facing reefs experienced frequent but short-lived cooling internal wave events and that these were key in shaping in situ temperature variability. By comparing temperature and wave height profiles recovered using in situ loggers with ex situ models, we discovered that global satellite products either failed to recover site-specific profiles or both over- and underestimated actual in situ conditions. We found that site choice and recruitment plate face (top or bottom) significantly impacted the percentage cover of bryozoans, gastropods, soft and calcified tube worms, as well as crustose coralline algae (CCA) and fleshy red, brown, and green encrusting macroalgae on ARMS. We observed significant correlations between the abundance of bryozoans, CCA, and colonial tunicates with lower mean temperature and higher mean dissolved oxygen profiles observed across sites. Red and brown encrusting macroalgae abundance correlated significantly with medium-to-high flow velocities and wave height profiles, as well as higher pH and dissolved oxygen. This study provides the first insight into cryptobenthic communities in the Chagos Archipelago marine-protected area and adds to our limited understanding of tropical reef sessile communities and their associations with environmental parameters in this region. With climate change accelerating the decline of reef ecosystems, integrating analyses of cryptobenthic organisms and in situ physicochemical factors are needed to understand how reef communities, if any, may withstand the impacts of climate change.</p