Phytoplankton group-specific contributions to the Subantarctic biological pump

Abstract

Summer Subantarctic surface waters are characterized by high concentrations of unconsumed nitrate (NO3-., likely due to combined iron, light and silica limitation of phytoplankton growth. The response of phytoplankton diversity and community structure to these nutrient limitations is not well understood despite the implications of such dynamics for organic carbon (C. export. The degree to which phytoplankton consume NO3- supplied by upward vertical mixing (“new production”. is proportionate to net C removal to the deep ocean, while growth fueled by recycled ammonium (NH4+. yields no net C flux. The N isotopic composition (?15N. of upper ocean biomass can be used as an integrative tracer of NO3- vs. NH4+ uptake. However, surface particles include heterotrophs and detritus in addition to phytoplankton, complicating the use of bulk particle ?15N as a measure of new vs. recycled N uptake. This is overcome by coupling cytometric cell sorting (FACS. – isolating important populations (e.g., cyanobacteria, picoeukaryotes, diatoms. – with group-specific ?15N analysis. During the summer 2016/2017 Antarctic Circumnavigation Expedition cruise across the Indian sector of the Subantarctic Ocean, we collected particles for FACS-?15N analysis and seawater samples for nutrients and NO3- ?15N. Four phytoplankton groups (Synechococcus, and three different types of eukaryotes. appear to be dominant across the basin. Their contribution to new production and C export will be discussed, as will that of Subantarctic island populations occupying waters where iron and possibly silica are no longer limiting due to mesoscale upwelling and island runoff. Indeed, results indicate that surface NH4+ and silica concentrations are higher near and downstream of the islands relative to open Subantarctic waters, indicating terrestrial inputs. Particulate organic N biomass is also higher, as is its ?15N, which can be explained by enhanced NO3- drawdown in near-island waters due to the alleviation of iron limitation of phytoplankton. - Abstract as displayed in the - Abstract booklet. The presentation on the day may differ from the - Abstract.