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

Abstract

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.

Assessment of Sessile Benthic Communities in Jeju Island, Republic of Korea, Using Autonomous Reef Monitoring Structures (ARMS)

“…In this study, the plate faces showed the strongest variation in sessile benthic composition, in agreement with previous studies that demonstrated the importance of the plate surface in the sessile benthic community composition [7]. For example, Steyaert et al [28] reported that the plate faces (top or bottom) had a significant impact on the percentage covers of various organisms, including bryozoans, gastropods, soft and calcified tube worms, CCA, and fleshy red, brown, and green encrusting macroalgae on ARMS. Our results show that the high occurrences of CCA and Rhodophyta on the top faces, coupled with the high occurrences of Bryozoa, Annelida, and Chordata on the bottom faces, distinctly illustrate a pattern that is likely influenced by light exposure.…”

Environmental DNA captures diurnal fluctuations of surface eukaryotes on a tropical coral reef

“…At each sampling site, water temperature was recorded and was between 29.3°C and 30°C during the whole sampling period. A previous study at a nearby site in the southwest of the lagoon described this area to have a low and stable flow velocity of ~0.2 m/s and wave height of ~0.25 H s (m) (Steyaert et al., 2022). Conditions during this study were likely to be similar, and there were no significant weather events during the sampling period.…”