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

由内潮和破碎的内波产生的内孔是一系列近岸海洋环境（包括美国佛罗里达群岛珊瑚礁的斜坡）中物理变异性和跨大陆架运输的重要来源。内钻孔到达时的特点是近底水温和密度的迅速波动，并伴有强烈的上坡水流的开始。在整个夏季，凉爽的地下水以半日频率被强迫向岸上流动，随着离礁坡越远，冷水的渗透和与这种脉冲机制相关的高频物理变异性就会减少。地下水到达礁石斜坡与溶解的营养物、浮游植物和浮游动物的浓度增加有关。内部潮汐强迫代表了一套物理机制，可以潜在地将近岸底栖生物群落与近海温跃层和相关的地下叶绿素最大层相关的水和物质联系起来。内部潮汐强迫的影响在佛罗里达群岛的珊瑚礁中广泛存在，新的观测表明，在基斯珊瑚礁地带向海至少1.5公里处50米深的底部附近可以检测到非常大的内部钻孔。这些大的内部钻孔的时间似乎与近海等温线的向上偏差相对应，表明冷水在地下侵入到礁带向海的陆架上。等温线的上升流可能是由佛罗里达洋流的近海弯曲引起的，这表明区域海洋变化与高频率的跨大陆架运输之间存在直接联系。该项目将调查内部潮汐强迫对悬浮食性珊瑚生长速度的影响，以及研究海洋变率在调节佛罗里达群岛礁带内部潮汐强迫方面的作用。已经确定了两个珊瑚礁遗址，在那里，自然地形特征产生了相邻的珊瑚礁斜坡微生境，它们具有相同的深度梯度和相似的上覆环境水团，但受到内部潮汐强迫的程度却大不相同。在这些配对的地点，人工珊瑚生长速率实验和本地珊瑚群落的骨骼带状模式分析将与高频近底物理采样、溶解营养物质通量测量和浮游动物可用性表征相结合。实验工作的结果将在大尺度生物模式的背景下加以考虑，并在区域尺度上对可以调节内部潮汐强迫的高频海洋过程进行观测。对内部潮汐强迫的生物学和生态学重要性的理解有限，部分原因是缺乏实验研究和长期、大范围的观测。除了影响热动力学和营养动力学外，与内部潮孔有关的跨大陆架运输可能是将无脊椎动物和鱼类幼虫运送到珊瑚礁的动态通道。这种底-上层耦合机制对珊瑚礁的生物过程和分布在沿岸的元种群之间的连通性具有深远的潜在影响。内部潮汐强迫可以直接影响珊瑚礁内小尺度、高频率的物理变化，并可能受到水柱分层和沿岸流等区域尺度海洋学过程的调节。因此，这些强迫机制的作用可能是将小尺度的底栖生物过程与区域尺度的海洋学变化联系起来。这个项目应该扩大对珊瑚对内部潮汐强迫的反应的理解，以及对底栖海洋群落内部潮汐强迫的沿岸变化的重要性的理解。

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.