Initial dominance in coccolithophore communities affects community structure but does not translate into altered community functioning

Climate change has the potential to profoundly influence the community structure and function of marine ecosystems. Prior to testing the consequences of altered environmental conditions on ecosystem functioning, it is first necessary to better understand how the functioning of an ecosystem is affected by its structure. Using phytoplankton communities with 4 naturally co-occurring coccolithophores including species of Emiliania, Gephyrocapsa, and Calcidiscus collected off the Azores, we experimentally tested whether varying initial dominance leads to different competitive outcomes and consequently affects community functioning, such as biomass and carbon accumulation. We manipulated initial community structure by creating 5 different dominance scenarios: (1) all species contributing evenly to total initial biomass, and (2–5) one of each species contributing 4× that of the remaining 3 species to total initial biomass. All 4 species were simultaneously grown in monocultures starting with the same total initial biomass as the communities. Monocultures differed significantly in total final biomass, particulate inorganic carbon, and particulate organic carbon content. Priority effects in the communities caused the initially dominant species to remain dominant during the stationary phase in 3 out of 4 cases. However, despite varying dominant species and different outcomes in the monocultures, community functioning was unaffected. We suggest that selective and facilitative effects are responsible for the equalization of community functioning. We conclude that monoculture experiments are not sufficient to predict whole-community responses, since species interactions can significantly alter the expected functional outcome.