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Global oceanic pH is lowering, which is causing great concern for the natural functioning of marine ecosystems. Current pH predictions are based on open ocean models; however, coastal zones are dynamic systems with seawater pH fluctuating temporally and spatially. To understand how coastal ecosystems will respond in the future, we first need to quantify the extent that local processes influence the pH of coastal zones. With this study, we show that over a single diurnal cycle, the total pH can fluctuate up to 0.2 units in a shallow seagrass‐dominated bay, driven by the photosynthesis and respiration of the vegetation. However, these biologically controlled pH fluctuations vary significantly over small distances. Monitoring conducted at neighboring sites with contrasting hydrodynamic regimes highlights how water motion controls the extent that the local pH is altered by the metabolism of vegetation. The interactive effects of hydrodynamics and vegetation were further investigated with an in situ experiment, where the hydrodynamics were constrained and thus the local water residence time was increased, displaying the counteractive effect of hydrodynamics on the pH change caused by vegetation. With this research, we provide detailed in situ evidence of the spatial variation of pH within marine ecosystems, highlighting the need to include hydrodynamic conditions when assessing the pH‐effects of vegetation, and identifying potential high‐pH refuges in a future low pH ocean.

The Yucatan Channel connects the Caribbean Sea with the Gulf of Mexico and is the main outflow region of the Caribbean Sea. Moorings in the Yucatan Channel show high‐frequent variability in kinetic energy (50–100 days) and transport (20–40 days), but the physical mechanisms controlling this variability are poorly understood. In this study, we show that the short‐term variability in the Yucatan Channel transport has an upstream origin and arises from processes in the North Brazil Current. To establish this connection, we use data from altimetry and model output from several high resolution global models. A significant 40–70 day variability is found in the sea surface height in the North Brazil Current retroflection region with a propagation toward the Lesser Antilles. The frequency of variability is generated by intrinsic processes associated with the shedding of eddies, rather than by atmospheric forcing. This sea surface height variability is able to pass the Lesser Antilles, it propagates westward with the background ocean flow in the Caribbean Sea and finally affects the variability in the Yucatan Channel volume transport. The volume transport in the Yucatan Channel displays a 40–70 day variabilityThis variability is regulated by upstream regions, i.e., the North Brazil CurrentThe 40–70 day variability is caused by intrinsic ocean processes

Joint effects of the dynamic sea‐level rise projected changes in the large‐scale atmosphere/ocean circulation, and wave climate on hurricane‐induced extreme water levels in the Caribbean region are assessed. We use the 2D‐depth integrated ADCIRC + SWAN wave‐ocean model, baroclinically coupled to an ocean‐eddying version of the Community Earth System Model, to compare impacts of the September 2017 hurricanes with projected impacts of similar hypothetical tropical storms occurring in the future. The model predicts only minor changes in the hurricane‐induced extreme water levels for those Caribbean islands which were severely devastated by the 2017 tropical storms (Irma and Maria). That is, provided that the hurricane intensity remains at the present‐day level, the global mean sea‐level rise is the main future coastal flood risk factor. The unique geophysical setting makes the Caribbean region extremely vulnerable to climatic changes experienced over the past few decades. The majority of the population and important infrastructures such as ports and airports are concentrated in the coastal zones which are at risk of permanent flooding as sea level continues to rise. The economy of the small Caribbean islands is heavily dependent on tourism and associated industries which are under pressure due to coral bleaching and coastal erosion. Additionally, the Caribbean region experiences intense and frequent extreme weather events which can potentially become more destructive in the future. As it is uncertain whether the hurricanes themselves will change, we investigate how the present‐day tropical cyclones will impact the Caribbean in the future. Our results indicate that islands in the Northeast Caribbean will not be at higher risk of hurricane‐induced flooding in the future if coastal infrastructure and ecosystems (coral reefs and seagrass meadows) can withstand the sea‐level rise and continue to provide important coastal protection services. In contrast, the projected increase in wave height in the southern Caribbean can lead to more coastal flooding and beach erosion in these areas. Only minor changes in the hurricane‐induced extreme water levels in the Northeast Caribbean are expected under future climate conditionsGlobal sea‐level rise remains the main coastal flood risk factor provided that hurricane intensity is unchangedSouthern Caribbean can become more vulnerable to coastal erosion and flooding by increased wave setup Only minor changes in the hurricane‐induced extreme water levels in the Northeast Caribbean are expected under future climate conditions Global sea‐level rise remains the main coastal flood risk factor provided that hurricane intensity is unchanged Southern Caribbean can become more vulnerable to coastal erosion and flooding by increased wave setup