Reply on RC1

Liang Zhao

doi:10.5194/bg-2021-296-ac1

Abstract

The Amundsen Sea is one of the regions with the highest primary productivity in the Antarctic. To better understand the role of the Southern Ocean in the global carbon cycle and in climate regulation, a better understanding of the variations in and environmental controls of primary productivity is needed. Using cluster analysis, the Amundsen Sea was divided into nine bioregions. The biophysical differences among bioregions enhanced confidence to identify priorities and regions to study the temporal and spatial variations in primary productivity. Four nearshore bioregions with high net primary productivity or rapidly increasing rates were selected to analyze temporal and spatial variations in primary productivity in the Amundsen Sea. Due to changes in net solar radiation and sea ice, primary production had significant seasonal variation in these four bioregions. The phenology had changed at two bioregions (3 and 5), which has the third and fourth highest primary production, due to changes in the dissolved iron, nitrate, phosphate, and silicate concentrations. Annual primary production showed increasing trends in these four bioregions. The variation in primary production in the bioregion (9), which has the highest primary production, was mainly affected by variations in sea surface temperatures. In the bioregion, which has the second-highest primary production (8), the primary production was significantly positively correlated with sea surface temperature and significantly negatively correlated with sea ice thickness. The long-term changes of primary productivity in bioregions 3 and 5 were thought to be related to changes in the dissolved iron, nitrate, phosphate, and silicate concentrations, and dissolved iron was the limiting factor in these two bioregions. Bioregionalization not only disentangle multiple factors that control the spatial differences, but also disentangle limiting factors that affect the phenology, decadal and long-term changes in primary productivity.

Highlights

The Southern Ocean, known as the Antarctic Ocean, encompasses 10% of the global ocean and contains parts of the South Pacific Ocean, the South Atlantic Ocean, the South Indian Ocean, and the marginal seas around Antarctica, such as the Ross Sea, Weddell Sea, and the Amundsen Sea
Previous studies have indicated that the phenology, decadal and longterm changes in primary productivity in the Southern Ocean have been and will continue to be affected by the current and predicted changes in ocean circulation and hydrology associated with climate variability (Lannuzel et al, 2007; Herraiz-Borreguero et al, 2016; Kim and Kim, 2021)
Significant spatial differences exist in the changes in primary productivity in the Southern Ocean, both over large latitudinal scales and at regional scales (Arrigo et al, 2008; Ardyna et al, 2017)

Summary

Introduction

The Southern Ocean, known as the Antarctic Ocean, encompasses 10% of the global ocean and contains parts of the South Pacific Ocean, the South Atlantic Ocean, the South Indian Ocean, and the marginal seas around Antarctica, such as the Ross Sea, Weddell Sea, and the Amundsen Sea. Significant spatial differences exist in the changes in primary productivity in the Southern Ocean, both over large latitudinal scales and at regional scales (Arrigo et al, 2008; Ardyna et al, 2017). These spatial differences are related to nutrient availability (mainly iron and possibly nitrate and silicic acid), temperature, light availability, and mortality factors (Boyd, 2002; Behernfeld and Boss, 2014; Arrigo et al, 2015). These factors are controlled by vertical mixing, advection, sea ice cover, and seasonal variations in solar irradiance (Ardyna et al, 2017). Primary productivity shows significant spatial differences in the Amundsen Sea, the Amundsen Sea

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