Drivers of biogeochemical variability in a California kelp forest and implications for local amelioration against ocean acidification

Ocean acidification will negatively affect many marine organisms and ecosystems. Regional-scale restoration, protection, and cultivation of submerged aquatic vegetation, including kelp, has been proposed to mitigate ocean acidification on a local scale. Kelp forests are one of the world’s most productive marine ecosystems; they remove CO₂ from seawater during photosynthesis, potentially reducing the exposure of nearshore marine communities to acidification. We investigated the physical and biological drivers of biogeochemical conditions inside and outside a kelp forest in order to identify the conditions under which local amelioration may be feasible. Here we present a summertime record of vertical variability from two mooring deployments inside and outside a nearshore kelp forest in southern Monterey Bay. Six pH sensors were deployed at approximately 1-meter vertical resolution inside the kelp forest, and one pH sensor was deployed outside of the kelp, approximately 2-5 m below the surface. Dissolved oxygen (DO), temperature, and currents were also recorded at both moorings. Temperature, pH, and DO exhibited large variability across multiple (hourly to multiweek) timescales with a dominant semi-diurnal frequency. Variability in the vertical gradient was primarily driven by the propagation of nearshore internal bores, whose strength depended on the magnitude of upwelling-favorable offshore winds. We observed significant low-frequency biology-driven modification in the surface layer inside the kelp forest. While subsurface (2-5 m below surface) pH and DO were not significantly different between inside and outside the kelp forest, surface pH was on average 0.06 pH units higher inside the kelp compared to outside based on biweekly discrete samples, suggesting the potential for kelp canopy to locally increase surface pH. Kelp modification of seawater properties, and any ameliorating effect against acidification, is likely limited to a narrow band of surface water.