Ecological and Biogeochemical Impacts of Internal Waves on Mesophotic Coral Ecosystems: Testing Eddy Covariance and Isotope Approaches, Iriomote, Japan

Mesophotic coral ecosystems (MCE) occur in the ‘twilight zone’ of decreasing light between 30 – 150 m water depth where they may be protected or damped from disturbances impacting shallower reefs. However insufficient information is available on the environmental conditions that support MCE to allow us to understand and conserve these ‘deep water refugia’. For instance, nutrient inputs and recycling have rarely been quantified over MCE, but deeper reefs may differ fundamentally to that of shallow counterparts due to the reduction in light and increasing use of oceanic nutrients at the base of the food web, leading to increased reliance on heterotrophy over autotrophy at species and ecosystem levels and stronger links to oceanic processes. For instance, due to their depth relative to typical water column density stratification, MCE are particularly likely to experience internal wave forcing, the significance of which should vary spatially depending on aspect and exposure. In this study we are focusing on MCE occurring along a continuum of oceanic-exposure along Funauki Bay on the west coast of Iriomote, Japan. Here our preliminary observations indicate that ocean-exposed MCE are subject to semi-diurnal temperature oscillations of up to 4 C during summer (range 23 – 29 deg C), while inner bay MCE occur at shallower depths in more turbid but stable environments. This continuum of oceanic exposure is ideal for testing a range of approaches for quantifying the relative ecological and biogeochemical influence of internal waves. Stable isotope analyses (SIA) are a particularly useful tool for understanding functional links between oceanic processes, local-scale nutrient cycling, and trophic ecology, with results from shallow reefs showing they likely function along a continuum of reliance on external inputs versus internal recycling depending on the degree of oceanic exposure. Although challenging to implement in deep water habitats, the combination of SIA with compound-specific isotope analyses of amino acids (CSIA-AA), depth-specific radioisotope markers such as radiocarbon and iodine ratios (¹²⁹I/¹²⁷I), and eddy covariance experiments offers a promising path towards elucidating the functional importance of internal waves in the development and persistence of MCE at local to regional scales.