Determination of the freshwater origin of Coastal Oyashio Water using humic-like fluorescence in dissolved organic matter

Yu Mizuno,Jun Nishioka,Youhei Yamashita,Yuta Tsuzuki,Yuya Tada,Atsuko Sugimoto,Takahiro Tanaka,Koji Suzuki

doi:10.1007/s10872-018-0477-x

Abstract

Coastal Oyashio Water (COW), defined as a water mass with a temperature lower than 2 °C and a salinity lower than 33.0, is distributed in the North Pacific Ocean off southeastern Hokkaido, Japan, from winter to spring. COW is rich in macronutrients and dissolved iron and is thus considered to affect the spring phytoplankton blooms in the Oyashio region. Although river water and sea-ice melt water have been considered freshwater end-members of COW, the contributions of these freshwater sources to COW have not been well described. In this study, the humic-like components in dissolved organic matter were first applied as a parameter to evaluate the freshwater end-members of COW in March 2015. Linear regressions with negative slopes were determined between the humic-like components and the salinity of COW. The intercepts of the regressions against the humic-like components were within the ranges of those observed for the local rivers of Hokkaido but were very different from those of sea ice. These findings suggest that river water contributed to the COW observed here as a freshwater end-member, although the contribution of sea-ice melt water to COW could not be evaluated. This novel approach also highlighted two different less-saline water masses in COW. The first was characterized by a lower temperature and relatively high levels of humic-like components, while the second was higher in temperature and had higher levels of humic-like components. It is suggested that these different characteristics are due to the contributions of water from different rivers and/or different effects of sea-ice melt water.

Highlights

Riverine humic substances can be considered one of the major components of dissolved organic matter (DOM) in coastal environments because linear negative correlations between the salinity and dissolved organic carbon (DOC) concentration as well as the fluorescence derived from terrestrial humic substances have often been observed (Fellman et al 2010; Yamashita et al 2011; 2015a; Cao et al 2016)
The discharge from the major rivers in eastern Hokkaido from January to March is on the order of 101–102 m3 s−1 for each river. These values suggest that the rivers connected to the Sea of Okhotsk are probably important as freshwater end-members of the Coastal Oyashio Water (COW) observed in this study
This study is the first to apply the relationship between salinity and humic-like fluorescence to evaluate the distribution of two distinct freshwater end-members in a coastal environment

Summary

Introduction

Riverine humic substances can be considered one of the major components of dissolved organic matter (DOM) in coastal environments because linear negative correlations between the salinity and dissolved organic carbon (DOC) concentration as well as the fluorescence derived from terrestrial humic substances have often been observed (Fellman et al 2010; Yamashita et al 2011; 2015a; Cao et al 2016) These negative correlations suggest that the fluorescence intensity of terrestrial humic substances (in other words, the levels of terrestrial humic fluorophores) can be used as a tracer of river water, similar to the oxygen isotopes of water or alkalinity (e.g., Yamamoto-Kawai et al 2009). At present, humic-like fluorophores have scarcely been applied to identify freshwater end-members in coastal environments

Objectives

Methods

Results

Discussion

Conclusion