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The interaction of coral reefs, both chemically and physically, with the surrounding seawater is governed, at the smallest scales, by turbulence. Here, we review recent progress in understanding turbulence in the unique setting of coral reefs?how it influences flow and the exchange of mass and momentum both above and within the complex geometry of coral reef canopies. Flow above reefs diverges from canonical rough boundary layers due to their large and highly heterogeneous roughness and the influence of surface waves. Within coral canopies, turbulence is dominated by large coherent structures that transport momentum both into and away from the canopy, but it is also generated at smaller scales as flow is forced to move around branches or blades, creating wakes. Future work interpreting reef-related observations or numerical models should carefully consider the influence that spatial variation has on momentum and scalar flux. 

Climate change is causing decreases in pH and dissolved oxygen (DO) in coastal ecosystems. Canopy-forming giant kelp can locally increase DO and pH through photosynthesis, with the most pronounced effect expected in surface waters where the bulk of kelp biomass resides. However, limited observations are available from waters in canopies and measurements at depth show limited potential of giant kelp to ameliorate chemical conditions. We quantified spatiotemporal variability of surface biogeochemistry and assessed the role of biological and physical drivers in pH and DO modification at two locations differing in hydrodynamics inside and outside of two kelp forests in Monterey Bay, California in summer 2019. pH, DO, dissolved inorganic carbon (DIC), and temperature were measured at and near the surface, in conjunction with physical parameters (currents and pressure), nutrients, and metrics of phytoplankton and kelp biological processes. DO and pH were highest, with lower DIC, at the surface inside kelp forests. However, differences inside vs. outside of kelp forests were small (DO 6–8%, pH 0.05 higher in kelp). The kelp forest with lower significant wave height and slower currents had greater modification of surface biogeochemistry as indicated by larger diel variation and slightly higher mean DO and pH, despite lower kelp growth rates. Differences between kelp forests and offshore areas were not driven by nutrients or phytoplankton. Although kelp had clear effects on biogeochemistry, which were modulated by hydrodynamics, the small magnitude and spatial extent of the effect limits the potential of kelp forests to mitigate acidification and hypoxia.