Kelp forests are among the world’s most productive marine ecosystems, yet little is known about their biogeochemistry. This study presents a 14-month time series (July 2013–August 2014) of surface and benthic dissolved inorganic carbon and total alkalinity measurements, along with accompanying hydrographic measurements, from six locations within a central California kelp forest. We present ranges and patterns of variability in carbonate chemistry, including pH (7.70–8.33), pCO₂ (172–952 µatm), and the aragonite saturation state, Ω_Ar (0.94–3.91). Surface-to-bottom gradients in CO₂ system chemistry were as large as the spatial gradients throughout the bottom of the kelp forest. Dissolved inorganic carbon variability was the main driver of the observed CO₂ system variability. The majority of spatial variability in the kelp forest can be explained by advection of cold, dense high-CO₂ waters into the bottom of the kelp forest, with deeper sites experiencing high-CO₂ conditions more frequently. Despite the strong imprint of advection on the biogeochemical variability of the kelp forest, surface waters were undersaturated with CO₂ in the spring through fall, indicative of the strong role of photosynthesis on biogeochemical variability. We emphasize the importance of spatially distributed measurements for developing a process-based understanding of kelp forest ecosystem function in a changing climate.