The organizing effects of elevated CO₂ on competition among estuarine primary producers

Fossil fuel combustion, eutrophication, and upwelling introduce excess CO₂ into coastal zones. The extent to which marine autotrophs may benefit from elevated CO₂ will be a function of their carbon limitation and, among other factors, competition with other primary producers. Here, we report on experiments performed with North Atlantic species of Ulva and Gracilaria grown in situ or exposed to ambient (~400 µatm) and elevated pCO₂ (~2500 µatm) and/or subjected to competition with each other and/or with natural plankton assemblages. Elevated pCO₂ significantly increased the growth rates of Gracilaria and Ulva and yielded significant declines in tissue δ¹³C, suggesting that increased growth was associated with increased CO₂ use relative to HCO₃^-. Gracilaria growth was unaffected by competition with plankton or Ulva, while Ulva experienced significantly reduced growth when competing with Gracilaria or plankton. Dinoflagellates experienced significantly increased growth when exposed to elevated pCO₂ but significantly slower growth when competing with Gracilaria. Elevated carbon-to-nitrogen ratios among macroalgae suggested that competition for nitrogen also shaped interactions among autotrophs, particularly Ulva. While some estuarine autotrophs benefit from elevated pCO₂, the benefit can change when direct competition with other primary producers is considered with Gracilaria outcompeting Ulva and dinoflagellates outcompeting diatoms under elevated pCO₂.