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中物理变异性和跨陆架运输的重要来源，包括美国佛罗里达群岛的珊瑚礁斜坡。内部钻孔到达的特点是近底水温和密度的快速波动，加上强烈的上坡水流的开始。在整个夏季的几个月里，凉爽的次表层水以半日频率强迫上岸，冷水的渗透率和与这种脉冲机制相关的高频物理变率随着距离的增加而降低。地下水到达礁坡与溶解的营养物质、浮游植物和浮游动物的浓度增加有关。内部潮汐强迫是一套物理机制，可以潜在地将近岸底栖生物群落与近海温跃层和相关的次表层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整个佛罗里达群岛的珊瑚礁中都很普遍，新的观测表明，在距珊瑚礁海域至少1.5公里处50米深的海底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，这表明地下冷水侵入了珊瑚礁海域的陆架。这种等温线的上升可能是由佛罗里达洋流的离岸曲折引起的，这表明区域海洋学变异性与高频跨陆架运输之间存在直接联系。本项目将调查内部潮汐强迫对悬浮摄食珊瑚生长速度的影响，以及海洋变异性在调节佛罗里达群岛珊瑚礁区域内潮汐强迫中的作用。已经确定了两个礁石地点，在这些地点，自然地形特征产生了相邻的礁坡微生境，具有相同的深度梯度和相似的上覆环境水团，但暴露于内部潮汐强迫的程度非常不同。在这些配对地点，操纵珊瑚生长速度实验和对本地珊瑚群落骨骼带状模式的分析将与高频近海底物理采样、溶解营养物质通量的测量和浮游动物可获得性的特征相结合。实验工作的结果将在大范围生物模式和可调节内部潮汐强迫的高频海洋过程的区域规模观测的背景下进行审议。由于缺乏实验研究和长期、大范围的观测，对内潮汐强迫的生物学和生态学重要性的理解受到限制。除了影响热和营养动态，与内部潮汐孔有关的跨陆架运输可能是将无脊椎动物幼体和鱼类运送到珊瑚礁的动态管道。这种底栖-中上层耦合机制对珊瑚礁上的生物过程和沿海分布的后生种群之间的连通性具有深远的潜在影响。内潮强迫可以直接影响礁内小尺度、高频率的物理变率，并可能受到区域尺度的水柱层化和沿岸流的海洋过程的调制。因此，这些强迫机制可能起到将小规模的海底生物过程与区域尺度的海洋可变性联系起来的作用。该项目应扩大对珊瑚对内部潮汐强迫的反应和对底栖海洋群落内部潮汐强迫的近岸变异性的重要性的了解。

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.