Nearshore Benthic-Pelagic Coupling: Coral Growth Responses to Internal Tidal Forcing on Florida Keys Coral Reefs

University Of California-San Diego Scripps Inst Of Oceanography, La Jolla CA

Investigators

Abstract

Internal bores generated by internal tides and breaking internal waves represent an important source of physical variability and cross-shelf transport in a range of near-shore marine environments, including the slopes of coral reefs in the Florida Keys, USA. The arrival of internal bores is marked by rapid fluctuations in near-bottom water temperature and density coupled to the onset of strong upslope flows. Cool subsurface water is forced onshore at semi-diurnal frequencies throughout the summer months, and the penetration of cool water and the high frequency physical variability associated with this pulsing mechanism decreases with distance up reef slopes. The arrival of subsurface waters on reef slopes is associated with increases in concentrations of dissolved nutrients, phytoplankton and zooplankton. Internal tidal forcing represents a suite of physical mechanisms that can potentially connect near-shore benthic communities to water and materials associated with offshore thermocline and associated subsurface chlorophyll maximum layers. The impact of internal tidal forcing is widespread throughout reefs in the Florida Keys, and new observations show that very large internal bores can be detected near the bottom at 50 m depth at least 1.5 km seaward of the Keys reef tract. The timing of these large internal bores appears to correspond to upward deviations of offshore isotherms indicative of subsurface intrusions of cool water onto the shelf seaward of the reef tract. This upwelling of isotherms may be caused by offshore meanders of the Florida Current, suggesting a direct link between regional oceanographic variability and high frequency cross-shelf transport. This project will be an investigation of the effects of internal tidal forcing on growth rates of suspension-feeding corals, as well as an examination of the roles of oceanographic variability in modulating internal tidal forcing along the Florida Keys reef tract. Two reef sites have been identified where natural topographic features produce adjacent reef slope microhabitats with equivalent depth gradients and similar overlying ambient water masses but with very different levels of exposure to internal tidal forcing. At these paired sites, manipulative coral growth rate experiments and analysis of skeletal banding patterns in native coral colonies will be combined with high frequency near-bottom physical sampling, measurement of fluxes of dissolved nutrients, and characterization of zooplankton availability. Results from experimental work will be considered within a context of broad-scale biological patterns, and regional-scale observations of high frequency oceanographic processes that can modulate internal tidal forcing. Understanding the biological and ecological importance of internal tidal forcing is limited, in part, by a lack of experimental studies and long-term, broad-scale observations. In addition to influencing thermal and nutrient dynamics, cross shelf transport associated with internal tidal bores may represent a dynamic conduit for delivery of larval invertebrates and fish to coral reefs. Such mechanisms of benthic-pelagic coupling have far-reaching potential consequences for biological processes on coral reefs and for connectivity among meta-populations distributed alongshore. Internal tidal forcing can directly influence small scale, high frequency physical variability within reefs, and may be modulated by regional scale oceanographic processes of water column stratification and alongshore currents. These forcing mechanisms, thus, may function to link small-scale, benthic biological processes to regional-scale oceanographic variability. This project should expand understanding both of coral responses to internal tidal forcing and of the importance of alongshore variability in internal tidal forcing for benthic marine communities.