Kelp forests are known as key habitats for species diversity and macroalgal productivity; however, we know little about how these biogenic habitats interact with seawater chemistry and phototroph productivity in the water column. We examined kelp forest functions at three locales along the Olympic Peninsula of Washington state by quantifying carbonate chemistry, nutrient concentrations, phytoplankton productivity, and seawater microbial communities inside and outside of kelp beds dominated by the canopy kelp species Nereocystis luetkeana and Macrocystis pyrifera. Kelp beds locally increased the pH, oxygen, and aragonite saturation state of the seawater, but lowered seawater inorganic carbon content and total alkalinity. Although kelp beds depleted nitrate and phosphorus concentrations, ammonium and dissolved organic carbon (DOC) concentrations were enhanced. Kelp beds also decreased chlorophyll concentrations and carbon fixed by phytoplankton, although kelp carbon fixation more than compensated for any difference in phytoplankton production. Kelp beds entrained distinct microbial communities, with higher taxonomic and phylogenetic diversity compared to seawater outside of the kelp bed. Kelp forests thus had significant effects on seawater chemistry, productivity and the microbial assemblages in their proximity. Thereby, the diversity of pathways for carbon and nitrogen cycling was also enhanced. Overall, these observations suggest that the contribution of kelp forests to nearshore carbon and nitrogen cycling is greater than previously documented.