Biological Contributions to Biogeochemical Variability of Kelp Forest Habitats

Sarah Traiger1, Heidi Hirsh2, Brian Cohn3, Demetra Panos1, Yuichiro Takeshita4, David Mucciarone2, Robert B Dunbar2 and Kerry Jean Nickols1, (1)California State University Northridge, Northridge, CA, United States, (2)Stanford University, Stanford, CA, United States, (3)Occidental College, Vantuna Research Group, Los Angeles, United States, (4)Monterey Bay Aquarium Research Institute, Moss Landing, CA, United States
Abstract:
Concurrent with rising sea surface temperature due to climate change, biogeochemical conditions are shifting, with expected decreases in pH and dissolved oxygen (DO) concentrations. Canopy-forming macroalgae, like giant kelp (Macrocystis pyrifera), may locally increase DO and pH due to their large, productive canopy, which also alters hydrodynamic conditions. Work by our group suggests that biogeochemical differences in the upper water column observed between kelp forests and areas outside of the kelp forest are more pronounced in surface waters. In this study, we investigated the contributions of the kelp canopy and phytoplankton to biogeochemical variability in surface waters. We deployed moorings with pH, DO, and temperature sensors inside and outside of two kelp forest sites during the summers of 2018 and 2019. Moorings were paired with twice-weekly discrete samples of dissolved inorganic carbon and total alkalinity. We measured particulate organic carbon (POC), chlorophyll, and phytoplankton community composition twice-weekly, and kelp blade and frond growth weekly. Inside the kelp forest surface DO rose above subsurface levels from mid-day to early evening. Outside, DO rose above subsurface levels on an event-scale, corresponding with phytoplankton blooms. While phytoplankton community composition was similar inside and outside of kelp forests, POC was generally lower inside the kelp forest, indicating that kelp rather than phytoplankton drives diel patterns in surface DO in the kelp forest. The upper few meters of the water column were usually more thermally stratified within the kelp forest than outside. This is likely due to the presence of the canopy increasing water residence time and reducing turbulent mixing, which allows more time for warming as well as for kelp to modify surface biogeochemistry. The combination of the kelp canopy productivity and its physical structure may play an important role in surface biogeochemical variability.
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