1402 / 2024-09-26 20:09:26

Early phytoplankton community development in the Southern Ocean’s marginal ice zone.

Atlantic Southern Ocean,Southern Ocean Marginal Ice Zone,Biogeochemistry,phytoplankton community structure,community development,succession,sea ice,net primary productivity (NPP),nitrogen uptake

Session 30 - Planktonic and Microbial Contributions to Marine Ecosystems and Biogeochemistry: Insights from Observations, Experiments, and Modeling

The dynamic plankton ecosystem of the Antarctic marginal ice zone (MIZ) is thought to play a key role in Southern Ocean productivity, yet the spatial heterogeneity of the MIZ complicates our understanding of how its phytoplankton community develops and functions. We investigated early phytoplankton community development in the Atlantic MIZ (~52-60˚S) from winter (July) and spring (October-November) measurements of hydrography, (macro)nutrient concentrations, phytoplankton community composition, and rates of primary production and inorganic nitrogen uptake from which we additionally derived estimates of carbon export potential. Using a multivariate statistical framework, we elucidate a succession scheme for the early productive season that reveals a plankton ecosystem evolving rapidly in response to changing environmental conditions. The statistical aggregation of our stations is unrelated to station location and time of sampling, underscoring the influence of the highly variable sea-ice dynamics on community structure and function, and by extension, carbon production and export. Four successional stages, driven largely by differences in light-, silicate-, and ammonium availability, are identified: the winter “reset” period of low, nanoflagellate-dominated biomass is followed by an early growth stage where biomass remains low and is largely attributable to picoeukaryotes. This community is succeeded by fast-growing, chain-forming diatoms (e.g., Chaetoceros spp.) that increase the biomass before being replaced by larger, slower-growing species (e.g., centric diatoms) that are known to persist even as resource limitation sets in. Our estimates of carbon export potential indicate a roughly 12-fold increase (from 49-589 µM.C.day^-1) over the stages of succession. We conclude that the rapid evolution and ephemeral nature of the early springtime MIZ phytoplankton community reflects the regional hydrographic and biogeochemical heterogeneity. Moreover, it highlights the need for an improved understanding of the MIZ at appropriate time and space scales if we are to better predict Southern Ocean phytoplankton response to climate change.