A Year in the Life of a Central California Kelp Forest: Physical and Biological Insights into Carbon Cycling

Kerry Jean Nickols1, David Koweek2, Paul Leary3, Steven Yitzchak Litvin3, Timothy Luthin2, Sarah Lummis4, David Allen Mucciarone5, Robert B Dunbar6 and Tom W Bell7, (1)California State University Northridge, Northridge, CA, United States, (2)Stanford University, Stanford, CA, United States, (3)Stanford University, Hopkins Marine Station, Pacific Grove, CA, United States, (4)University of California Santa Cruz, Santa Cruz, CA, United States, (5)Stanford University, Los Altos Hills, CA, United States, (6)Stanford University, School of Earth Sciences, Stanford, CA, United States, (7)University of California Santa Barbara, Santa Barbara, CA, United States
Kelp forests are among the most productive ecosystems, yet little is known about their biogeochemistry. The biogeochemical environment of kelp forests is influenced by regional processes (e.g., upwelling) that deliver offshore waters into the forest as well as local processes (e.g., production, respiration, and local hydrodynamics) that modify local water chemistry. We present a weekly-resolved 14-mo time-series (Jul 2013-Sep 2014) of water column properties (temperature, salinity, total alkalinity, and dissolved inorganic carbon (DIC)) within and around a central California kelp forest at wave-exposed and protected sites in conjunction with water column velocity and satellite-derived estimates of kelp biomass. CTD hydrocasts and water samples revealed strong seasonal cycles in the chemical and physical water column structure. In the winter, the water column was well-mixed, kelp abundance was low, and vertical DIC gradients were typically absent. During the upwelling season kelp growth was high, the water column was stratified, and vertical DIC gradients sometimes exceeded 200 ╬╝mol/kg. Vertical gradients were strongest in the protected side of the forest and weaker on the wave-exposed side. DIC variability is the dominant control on pH, pCO2, and carbonate mineral saturation state variability. pCO2 was generally highest in the wave exposed side of the forest, especially during the upwelling season. This side experienced stronger cross-shore currents and a smaller cross-shore kelp forest extent than the protected side, promoting cross-shore exchange of kelp forest waters with offshore waters. This study emphasizes the importance of long-term, spatially distributed measurements for mechanistic descriptions of kelp forest biogeochemistry. Understanding the contributions of physical and biological processes to CO2 system chemistry is an essential step to predicting responses of temperate nearshore habitats to climate change and ocean acidification.