Highest rates of gross primary productivity maintained despite CO2 depletion in a temperate river network

Abstract Aquatic primary productivity produces oxygen (O2) and consumes carbon dioxide (CO2) in a ratio of ~1.2. However, in aquatic ecosystems, dissolved CO2 concentrations can be low, potentially limiting primary productivity. Here, results show that a large drainage basin maintains its highest levels of gross primary productivity (GPP) when dissolved CO2 is diminished or undetectable due to photosynthetic uptake. Data show that, after CO2 is depleted, bicarbonate, an ionized form of inorganic carbon, supports these high levels of productivity. In fact, outputs from a process‐based model suggest that bicarbonate can support up to ~58% of GPP under the most productive conditions. This is the first evidence that high levels of aquatic GPP are sustained in a riverine drainage network despite CO2 depletion, which has implications for freshwater ecology, biogeochemistry, and isotopic analysis.