日本海学研究機関等連携事業

日本海洋学会 「沿岸海域における海底湧水と栄養塩のフラックス」


2007年度 日本海洋学会春季大会シンポジウム
日時:2007年3月22日
会場:東京海洋大学品川キャンパス

M. Hatta(University of Hawaii) and ○ J. Zhang (University of Toyama) Keywords: Submarine Ground Water Discharge, Flux, nutrients, Toyama Bay, box model

Introduction

 The estimation of freshwater discharge plays animportant role in understanding interocean fluxes ofbuoyancy in stabilizing the present-day overturningcirculation of heat and salt. Recently, one source offreshwater discharge, Submarine GroundwaterDischarge (SGD), has been recognized as animportant worldwide phenomenon that providesdirect transport pathways for both water and othermaterials between the land groundwater system andthe marine environment. However, few estimates ofSGD flux using oceanographic data have beenreported. Especially in high-latitude marginal seas,important areas for the oceanic circulation, it isdifficult to evaluate the flux because these areas havemultiple interfaces to the open ocean, many sourcesof freshwater and complicated water mass structure.Toyama Bay ? the larges and deepest semi-closed bayin the Sea of Japan - is suitable as a model forestimating SGD flux. This is due to how the bay has asimple water mass structure, the deep water is relatedto the abyssal circulation of Japan  Sea, and theshallow water is influenced by a huge amount offreshwater, especially SGD, which is distributedwidely on the continental shelf. The SGD fluxes offreshwater and nutrients in Toyama Bay wereestimated using a box model in this study.

 

Data and Sampling

 The data sources of temperature and salinity arefrom the expedition of the T/S Kakuyo Maru in 2003(22 stations), the R/V Tansei Maru (KT05-11) in2005 (27 stations), MIRC (Marine InformationResearch Center, Japan Hydrographic Association)and parts by Toyama Prefectural FisheriesExperimental Station from 1987-1998 (12 stations).River water discharge (a total of five large rivers) isfrom the database by the Ministry of Land,Infrastructure and Transport Government of Japan.The data of precipitation, evaporation, and humidityis from a database by Japan Meteorological Agency(http://www.jma.go.jp.html).

 Salinity and dissolved oxygen were measured withCTD sensors, with values calibrated with watersamples. The oxygen isotope ratio was measured inorder to specify the origin of the water mass. Thestable isotope ratio (δ18O) samples of seawater wereanalyzed using an isotope ratio mass spectrometer(Prism, VG) with automatic balancing equipment inlaboratory, University of Toyama. The measurementuncertainty is ±0.1‰. pH samples of seawater wereanalyzed by pH meter.

 

Results and Discussion

 According to the T-S diagram, the water structurein Toyama Bay was classified into three parts: thecoastal surface water, Tsushima Warm Current andthe deep water. It was clarified that water shallowerthan 200m is composed of a mix of high-salinitywater mass and the low-salinity water mass formedby the nearshore freshwater. The SGD's freshwaterflux, which was calculated by a box model(10-200m), was about 30 % of the riverine value. Theamount of SGD in May 2005 was about 2 times thatof July 2003. And the estimation of SGD from 1987to 1998 was 0.93-7.08×108m3 month-1, and thisaverage was 3.8×108m3 month-1. This was12%-101% of the volume of river water flowingtoward the bay (The average of river water was8.1×108 m3 month-1). Further, Ito and Fujii (1993)calculated an estimate of the discharge water volume(33×108 m3 year-1) using a water budget in ToyamaPrefecture. This average volume is 2.8×108m3month-1. This is consistent with our result (3.8×108m3month-1). For the flux of nutrients, P and N are moreimportant than Si in the coastal area of Japan (avolcanic island). The SGD flux was the same for Pand over for N, comparative to the riverine.

 Using model analysis of decadal observation data,it is possible to evaluate the effect of both interannualand seasonal variation for freshwater input in thisstudy. It is helpful to understand the flux and itsimpact of freshwater linked to the various differentnatural phenomena.