Long-Term Trends of Nutrients and Apparent Oxygen Utilization South of the Polar Front in Southern Ocean Intermediate Water from 1965 to 2008

Takahiro Iida,Mitsuo Fukuchi,Tsuneo Odate

doi:10.1371/journal.pone.0071766

Abstract

The variation of nutrients over decadal timescales south of the polar front in the Southern Ocean is poorly known because of a lack of continuous observational data in this area. We examined data from long-term continuous hydrographic monitoring of 43 years (1965–2008) in the Indian sector of the Southern Ocean, via the resupply of Antarctic stations under the Japanese Antarctic Research Expedition and Australian Antarctic Research Expedition. We found significant increasing trends in phosphate and nitrate, and a decreasing trend in apparent oxygen utilization (AOU) in intermediate water (neutral density = 27.8–28.1 kgm−3) south of the polar front. The rates of phosphate and nitrate increase are 0.004 µmol yr−1 and 0.02 µmol yr−1, respectively. The rate of decline of AOU was 0.32 µmol yr−1. One reason for this phosphate and nitrate increase and AOU decline is reduced horizontal advection of North Atlantic Deep Water, which is characterized by low nutrients and high AOU. The relationship between climate change and nutrient variability remains obscure, emphasizing the importance of long-term monitoring.

Highlights

The Southern Ocean is very important in global ocean circulation and climate change
Intermediate Water was defined as Upper Circumpolar Deep Water (UCDW) from 200–800 m depths, which was characterized with neutral density [32]
Lower circumpolar deep water (LCDW) is distributed under UCDW, which was characterized by a salinity maximum and nutrient minimum and whose source is North Atlantic Deep Water (NADW) [33,34]

Summary

Introduction

The Southern Ocean is very important in global ocean circulation and climate change. A long-term observational studies reported decreasing salinity in the intermediate water on the decadal scales [3], and warming of intermediate water has been reported [4]. Polynyas are important in the Southern Ocean for deep and intermediate water [6,7]. Tamura et al [8] reported that ice production where decreased by 30% from the 1990s to the 2000s, this represents one candidate for causing the recent freshening of Antarctic Bottom Water (AABW) [9,10], which affects deep ocean nutrients and carbon cycles in the Southern Ocean. Warming trends have been found in the Weddell Sea in long-term monitoring datasets because of these deep water mass changes [11]

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Conclusion