Globally, coral reefs are threatened by ocean warming and acidification. The degree to which acidification will impact reefs is dependent on the local hydrodynamics, benthic community composition, and biogeochemical processes, all of which vary on different temporal and spatial scales. Characterizing the natural spatiotemporal variability of seawater carbonate chemistry across different reefs is critical for elucidating future impacts on coral reefs. To date, most studies have focused on select habitats, whereas fewer studies have focused on reef scale variability. Here, we investigate the temporal and spatial seawater physicochemical variability across the entire Heron Island coral reef platform, Great Barrier Reef, Australia, for a limited duration of six days. Autonomous sensor measurements at three sites across the platform were complemented by reef-wide boat surveys and discrete sampling of seawater carbonate chemistry during the morning and evening. Variability in both temporal and spatial physicochemical properties were predominantly driven by solar irradiance (and its effect on biological activity) and the semidiurnal tidal cycles but were influenced by the local geomorphology resulting in isolation of the platform during low tide and rapid flooding during rising tides. As a result, seawater from previous tidal cycles was sometimes trapped in different parts of the reef leading to unexpected biogeochemical trends in space and time. This study illustrates the differences and limitations of data obtained from high-frequency measurements in a few locations compared to low-frequency measurements at high spatial resolution and coverage, showing the need for a combined approach to develop predictive capability of seawater physicochemical properties on coral reefs.

在全球范围内，珊瑚礁受到海洋变暖和酸化的威胁。酸化对珊瑚礁的影响程度取决于当地的水动力、底栖群落组成和生物地球化学过程，所有这些都在不同的时间和空间尺度上有所不同。表征不同珊瑚礁海水碳酸盐化学的自然时空变化对于阐明未来对珊瑚礁的影响至关重要。迄今为止，大多数研究都集中在选定的栖息地，而很少有研究关注珊瑚礁规模的变化。在这里，我们在有限的六天内调查了澳大利亚大堡礁整个赫伦岛珊瑚礁平台的时空海水物理化学变化。平台上三个地点的自主传感器测量辅以全珊瑚礁船只调查和早上和晚上对海水碳酸盐化学的离散采样。时间和空间物理化学性质的变化主要由太阳辐照度（及其对生物活动的影响）和半日潮汐周期驱动，但也受到当地地貌的影响，导致低潮期间平台被隔离，涨潮期间快速洪水泛滥。因此，先前潮汐周期的海水有时会被困在珊瑚礁的不同部分，导致空间和时间上出现意想不到的生物地球化学趋势。这项研究说明了在高空间分辨率和覆盖范围下从一些地点的高频测量获得的数据与低频测量相比的差异和局限性，表明需要采用组合方法来开发珊瑚礁海水物理化学特性的预测能力。