Award: OCE-1737176

Kelp forest ecosystems are of ecological and economic importance globally and provide habitat for a diversity of fish, invertebrates, and other algal species. In addition, they may also modify the chemistry of surrounding waters. Uptake of carbon dioxide (CO2) by giant kelp, Macrocystis pyrifera, and production of oxygen may play a role in ameliorating the effects of increasing ocean acidity and hypoxia on nearshore marine communities. Predicting the capacity for kelp forests to alter seawater chemistry requires understanding of the oceanographic and biological mechanisms that drive variability in seawater chemistry. This project linked kelp forest community attributes and hydrodynamic properties to kelp forest biogeochemistry (including the carbon system and dissolved oxygen) to understand mechanistically how giant kelp modifies surrounding waters and affects water chemistry. We studied four kelp forest sites along the Monterey Peninsula in Central California, characterized by different oceanographic settings. During 8-week field seasons, continuous measurements of water column velocity, temperature, dissolved oxygen, pH, and photosynthetically active radiation were augmented by twice-weekly measurements of dissolved inorganic carbon, total alkalinity, and nutrients as well as periods of high frequency sampling of all carbonate system parameters. We combined these data with additional biological sampling of kelp, benthic communities, and phytoplankton to determine 1) how seawater residence time influence natural ranges in carbon system chemistry and dissolved oxygen in kelp forests, 2) to what extent kelp biomass, understory algal biomass, and phytoplankton impact carbon system chemistry within kelp forests, and 3) what physical and biological characteristics of kelp forests promote or decrease susceptibility to acidification and hypoxia. Intellectual Merit: We found that seawater residence time drove differences in carbon chemistry and oxygen among sites, largely determined by wave energy and currents. Between kelp forest sites, seawater chemistry was influenced more by physical conditions than kelp canopy growth or phytoplankton concentrations. We found limited contributions to chemical signals from understory algal communities but that epibionts in the kelp canopy (organisms growing on kelp blades) can impair production, thus decreasing oxygen concentrations. We found differences in seawater chemistry inside and outside kelp forests which were related to the presence of kelp and not related to phytoplankton concentrations. However, the chemical signal from kelp photosynthesis was limited to a narrow band of surface water within kelp forests and was small in magnitude (2-8% difference for dissolved oxygen concentrations and 0.01-0.05 pH unit difference for pH). Overall, kelp forests showed limited potential to ameliorate ocean acidification and hypoxia. Although kelp forests may not protect species from acidification and hypoxia stress, their important role as a foundation species and contributions to other ecosystem services still warrant protection and restoration. Broader Impacts: This project provided opportunities for 1 postdoctoral researcher, 2 PhD students, 2 master of science students, and 8 undergraduate researchers in interdisciplinary marine science data collection, project development, data analysis, and dissemination of results. This project was integrated into courses and presentations for undergraduate students at California State University Northridge, exposing students to techniques and results of interdisciplinary marine science research and how data can be collected to address science needs of policymakers and stakeholders. Results were also shared in policy briefings for the State of California and integrated into advisory reports for the State of California’s Ocean Protection Council. Project data are available at https://www.bco-dmo.org/project/748778, and all publications are open access. Last Modified: 12/26/2021 Submitted by: Kerry Nickols