Calcification diversity is key to understanding if the biotic responses of calcareous nannoplankton communities to climate change have biogeochemical consequences

Climate variability impacts key characteristics of marine phytoplankton communities, including community composition, size structure, and biodiversity. As each phytoplankton species has specific ecological and biogeochemical traits, changes in phytoplankton community composition can have consequences for ecosystem processes. The calcareous nannoplankton, a marine phytoplankton group that includes coccolithophores, play a unique role in marine biogeochemical cycles though the production and export of both organic and inorganic carbon (biomass and calcite, respectively). However, it is challenging to investigate if differences in calcareous nannoplankton community composition have consequences for community biogeochemical traits (size, biomass, calcite) because the diversity of cellular calcification traits across calcareous nannoplankton species is poorly quantified. Here, we transform the morphological trait data preserved in the fossil record of Paleogene calcareous nannoplankton assemblages to reconstruct past community cell size structure and its associated biomass and calcite traits through the Oligocene ‘coolhouse’ in the Pacific Ocean. Community composition at the low and mid-latitude sites that we investigated (ODP Site 130-804, ODP Site 143-869, ODP Site 198-1211) was distinctly different to composition at the high latitude site studied (IODP Site 378-U1553). The dominant warm water-affinity taxa at lower latitude sites (Syracosphaera, Discoaster) and temperate-cool water-affinity taxa at the high latitude site (Cyclicargolithus, Reticulofenestra, Chiasmolithus) have distinctly different morphological traits, leading to latitudinal differences in total community biomass and calcite and how biomass and calcite is partitioned across size classes. Within each site, community composition through the Oligocene instead tends to fluctuate between species with substantial degrees of biogeochemical trait overlap, largely moderating the degree of biogeochemical impact resulting from shifts in community composition through time. Our results suggest that the impacts of climate-driven biotic reorganisation on community biogeochemical traits can be minimised when replacement species have similar size and calcification traits (higher levels of trait redundancy). Conversely, the biogeochemical implications of even relatively small changes in community composition can be amplified by strongly dissimilar calcification traits within an assemblage. Physiological and ecological traits that influence production rates of biomass and calcite will also factor into the biogeochemical impact of community composition shifts but are difficult to quantify using existing methods. Our research highlights the importance of considering calcification and size diversity when exploring the wider consequences of calcareous nannoplankton responses to climate change in both past and present-day communities.