Toward an Improved Understanding of the Subantarctic Biological Pump: Phytoplankton Group-specific Contributions to and Potential Drivers of Carbon Export in the Indian Sector of the Subantarctic Ocean

Heather Forrer1, Angela N Knapp2, Thomas G Bornman3, Rachel Thomas4, Samantha C Waterworth5, Rosemary Dorrington5 and Sarah Fawcett6, (1)Florida State University, Earth, Ocean and Atmospheric Sciences, Tallahassee, United States, (2)Florida State University, Earth, Ocean, and Atmospheric Sciences, Tallahassee, FL, United States, (3)South African Environmental Observation Network (SAEON), Elwandle Coastal Node, Gqeberha, South Africa, (4)Florida State University, Tallahassee, FL, United States, (5)Rhodes University, Department of Biochemistry and Microbiology, South Africa, (6)University of Cape Town, Oceanography, Cape Town, South Africa
Iron (and silicate) (co-)limitation of phytoplankton is considered a primary cause of the Southern Ocean’s inefficient biological pump, although the role of phytoplankton community structure and response is less well understood. In a mass balance sense, phytoplankton consumption of (new) nitrate is proportional to net carbon (C) export, while growth fueled by recycled ammonium yields no net C flux. The nitrogen (N) isotope ratio (d15N) of surface biomass can be used as an integrative tracer of nitrate vs. ammonium uptake. This method is rendered more accurate by coupling either cell sorting (FACS; nano- and picophytoplankton; 0.4-20 um) or microscopy (microphytoplankton; >20 um) with group-specific d15N analysis. Nutrients and nitrate-, FACS-, and microscopy-d15N samples were collected on a mid-summer transect of the Subantarctic Indian basin. The data show that phytoplankton near the Subantarctic islands rely primarily on new N, with 72% of micro- and ~100% of nano- and picophytoplankton growth apparently supported by nitrate, likely due to alleviation of iron and silicate stress by bathymetric upwelling and island runoff. C export potential is lower in the open ocean region where iron stress is higher; here, nitrate supports 64% of micro- and pico- and 42% of nanophytoplankton growth. In terms of relative abundance (RA), the open Subantarctic is dominated by picoeukaryotes (64%), while nanoeukaryotes thrive (50%) near the islands and Polar Front. In contrast to subtropical observations, Synechococcus (Syn) relies heavily on nitrate across the basin, increasing in RA near the islands and fronts; however, the Syn contribution to biomass is, on average, <1%. By contrast, microphytoplankton (largely chain-forming pennate diatoms) constitute >50% of the biomass despite their RA of ~1%. This, coupled with our estimate that nitrate fuels >60% of microphytoplankton growth across the basin, supports traditional models of C export trends in the Subantarctic Southern Ocean.