台灣海峽表水懸浮顆粒之粒徑與重金屬元素含量變化研究

Abstract

海水中懸浮顆粒大致上分成陸源無機黏土顆粒與海源生物顆粒。本研究藉由2007聯合探測計畫，利用多層次粒徑收集器( Catnet )收集與研究台灣海峽及東海南部表層海水中四種粒徑大小的懸浮顆粒( 0.4~10μm、10~63μm、63~153μm與大於153μm )之分佈狀況，並藉由這些顆粒所含之重金屬元素( Al、Fe、Mn、Cu、Ni、Cd )探討顆粒的組成與來源。 本研究發現台灣海峽與東海南部表水懸浮顆粒在四種粒徑大小皆以沿岸濃度較高，台灣海峽中間與東海南部濃度較低。大部分的懸浮顆粒小於63μm，其中70~80%的顆粒粒徑在0.4~10μm之間，15%的顆粒大小在10~63μm之間。顆粒態鋁、鐵、錳、銅、鎳、鎘濃度受懸浮顆粒濃度分布影響，皆呈現近岸地區濃度高，海峽中間濃度較低的分布型態。台灣近岸懸浮顆粒粒徑大小在0.4~63μm部分的Fe/Al比值為0.52±0.06，主要為陸源礦物顆粒；而大於63μm之顆粒其Cu/Al比值在0.45~4.13之間，平均為1.73，且顆粒所含有機碳濃度與葉綠素甲成正相關( R2=0.75 )，主要應是受陸源物質影響的生物顆粒，大部分由浮游植物組成。 台灣海峽中間63μm大小以下的顆粒，其Cu/Al比值變化較大，在0.21~20.4之間，平均為4.19，這應是包含了海中的浮游生物與來自沿海的陸源顆粒混雜其中。在離岸較遠的海域陸源顆粒大部分已沉降到海底，因此生物顆粒所佔的比例會有所增加。研究發現63~153μm大小之懸浮顆粒主要為生物顆粒，其Cu/Al比平均為6.27，而大於153μm的顆粒主要是浮游動物，其Cu/Al比值平均為9.82。東海南部顆粒則較為特殊，在0.4~10μm粒徑大小部份同時包含陸源顆粒與生物顆粒，而粒徑大於10μm部份則主要由生物顆粒所組成。 由顆粒所含元素比例、顆粒所含有機碳濃度及葉綠素甲濃度，經計算後可大致得知台灣海峽表水陸源顆粒占懸浮顆粒之百分比。台灣沿岸除八掌溪以南之河口海域，陸源顆粒皆占90%以上。台灣海峽西南區域生物顆粒比例較高，陸源顆粒占70~80%。台灣海峽中間及東海南部表水80~90%的懸浮顆粒為陸源顆粒。

Particles, including lithogenic clay mineral and biogenic particles, play an important role in marine element cycling. In this work, size-fractionated particles ( 0.4~10μm, 10~63μm, 63~153μm and >153μm ) in surface water were collected by CATNET at Taiwan Strait and south of East China Sea ( ECS ). The origins and composition of these suspended particles were elucidated by the metal ratio ( Al, Fe, Mn, Cu, Ni, Cd ) in the particles. Particle concentrations in near shore coastal waters are much higher than those in the south of ECS and in the middle of the Taiwan Strait. These particle are mainly composed of smaller than 63μm matters. Due to high suspended particle concentration in coastal water, particulate trace metal concentrations (Al,Fe,Mn,Cu,Ni,Cd ) are also higher in coastal water than in ECS and the middle of Taiwan strait. In Taiwan coastal water, particle size between 0.4~63μm are mainly associated with lithogenic clay mineral from riverine input, with Fe/Al ratio of 0.52±0.06, close to the average Fe/Al ratio of shale. Particles larger than 63μm are mainly biogenic particles with very small fraction of lithogenic material. ( Cu/Al: 0.45 ~ 4.13, averaged 1.73 ) Positive correlation between organic carbon and chlorophyll-a in the particle suggests that most of the biogenic particles in near shore coastal water are phytoplankton. In the middle of the Taiwan Strait, particles between 0.4~63μm contain both planktons and clay minerals. ( Cu/Al: 0.21~ 20.4, averaged 4.19 ), as fraction of biogenic particle increase in off shore region. Particles between 63~153μm are mainly composed of biogenic particles. ( Cu/Al from 0.50 to 28.60, average 6.27 ), which particles larger than 153μm are likely to be zooplankton. Based on the metal concentrations and ratios, chlorophyll-a and particulate organic carbon concentration of these suspended particles, more than 70% are lithogenic particles in Taiwan Strait and south of ECS.