Small diversity effects on ocean primary production under environmental change in a diversity-resolving ocean ecosystem model

Marine ecosystem models used to investigate how global change affects ocean ecosystems and their functioning typically omit pelagic diversity. Diversity, however, can affect functions such as primary production and their sensitivity to environmental changes. Using a global ocean ecosystem model that explicitly resolves phytoplankton diversity within four phytoplankton functional types (PFTs) we investigate the model's ability to capture diversity effects on primary production under environmental change. An idealized scenario with a sudden reduction in vertical mixing causes diversity and primary-production changes that turn out to be largely independent of the number of coexisting phytoplankton types. The model provides a small number of niches with respect to nutrient use in accordance with the PFTs defined in the model, and increasing the number of phytoplankton types increases the resolution within the niches. The variety of traits and trade-offs resolved in the model constrains diversity effects such as niche complementarity, which operate between, but not within PFTs. The number and nature of the niches formulated in the model, for example via trade-offs or different PFTs, thus determines the diversity effects on ecosystem functioning captured in ocean ecosystem models.