SPATIAL VARIABILITY OFTHE SURFACE PHYTOPLANKTON BIOMASS IN THE ROSS SEA DURING THESUMMER IN 2018/2019

Abstract

Surface biological productivity in the ocean is a key controlling factor to estimate carbon dioxide sink capacity. However, the observational data to understand ecological dynamics in the Ross Sea are still significantly limited. In this study, we present the phytoplankton biomass estimated by continuous underway measurements of O2, pCO2, biological oxygen saturation (ΔO2/Ar), and fluorescence on IBRV Araon cruise across the Ross Sea in December 2018 February 2019. Phytoplankton biomass ranged from <1 mgL-1 to ~10 mgL-1 Chl-a, with the highest values observed in regions of Terra Nova Bay (TNB) and the lowest values observed in Cape Hallett near the continental shelf break. pCO2 and O2 exhibited large spatial gradients (about 150 to 450 μatm and 250 to 500 μM, respectively) and covaried with phytoplankton biomass. In TNB in early-January, significantly under-saturated pCO2 values (~200 μatm) were observed. During the revisit in this area from 21 January, low pCO2 values of 200~250 μatm were still persisted implying bloom period maintained throughout a month of January. On the other hand, in the northern region near Cape Hallett where Chl-a showed the lowest value near zero, pCO2 values near saturation of ~ 450 μatm were observed probably due to low drawdown of CO2. The measurements of ΔO2/Ar using mass spectrometry offer an opportunity to investigate net community production (NCP), the difference between net primary production and heterotrophic respiration. Such NCP derived from ΔO2/Ar can be considered a real-time proxy of primary production. Therefore, comparing the measurement data of pCO2, and O2 with NCP is expected to allow us to determine if the biological controls overwhelm the influences of the deep water entrainment and air-sea gas exchange during the summertime in the Ross Sea.