應用濾食性貝類台灣蜆生物累積效應為都市型河川的金屬污染評估

Abstract

近年來，由於頻繁的人為活動，導致全球都市化的興起，使得河川生態系統時常經由人為排放而遭受到過度的金屬污染影響。由於河川中的水生生物具有累積金屬的特性，因此透過生物監測的方式，能夠有效的幫助我們了解水生生態系統隨時間的變化下，受到的金屬污染影響。在全世界，濾食性貝類台灣蜆 (Corbicula fluminea) 已成為一種被廣泛用於環境污染監測上的指標生物，然而，在的亞洲國家的研究中 (如:台灣)，卻鮮少將此物種作為監測金屬污染的指標生物。因此，本實驗將會利用台灣蜆作為監測都市型河川金屬污染物的指標生物，並且探討 (1) 環境中的金屬污染物與台灣蜆的生物累積之間的相互關係; (2) 確認不同的金屬種類在暴露實驗過後，會在不同的軟體組織中 (鰓、內臟團和肌肉) 反應出不同的累積程度; (3) 透過不同的軟體組織比較台灣蜆的生物累積的模式，並揭示不同金屬在台灣蜆體內的累積途徑; (4) 確認濾食性貝類台灣蜆適合作為監測台灣都市型河川的指標生物。為了達成本實驗的研究目的，於實驗期間我們將台灣蜆定置於籃內，並且挑選兩條位於北台灣的都市型河川 (淡水河及大屯河)，進行現地暴露的實驗。 我們的實驗結果顯示，無論將台灣蜆暴露於輕度或是中度污染的都市型河川當中，都能夠在短時間之內 (21-28天) 觀察到金屬污染物 (如鋅、銅、鎳、鉻、鉛、鎘) 有效的累積在蜆的軟體組織內，並且針對不同種類的金屬，反應出不同的累積程度。另外，在本實驗進行河水金屬濃度測量時發現，河川中所有的顆粒型金屬污染濃度皆高於溶解型的金屬污染濃度，但經由實驗結果卻發現，最能夠影響台灣蜆的生物累積傳輸型式中，還是以溶解型的金屬污染物為主。而透過本實驗的生物累積因子 (Bioaccumulation Factors; BAF) 比較後發現，所有的溶解型的金屬中，鉛、銅及鎳所計算出的BAF值顯示出較高的累積量，而顆粒型的金屬累積量中則是由鎘、銅及鎳計算出較高的BAF比值。雖然，從總BAF的數據顯示，台灣蜆體內的金屬累積量在兩地之間存在著差異，但將三種不同的軟體組織之間的BAF數值比較後則可發現，無論是暴露在淡水河或大屯河中的台灣蜆，都可觀察到不同類型的金屬，皆依循著相似的模式累積在軟體組織內，其中又以內臟團為主要累積金屬的部位。因此，本研究結果建議，台灣蜆對於金屬累積的主要來源，是經由攝食行為取得環境中的金屬污染物質。 本實驗結果指出，台灣蜆是一種非常適合做為河川生態環境監測的指標生物，並且能夠有效的反應出環境中的多種金屬污染物 (如: 鋅、銅、鎳、鉻、鉛及鎘) 所造成的生態毒理影響。另外，我們的實驗結果也發現，台灣蜆對於鎘金屬污染物具有較敏感的累積反應，因此建議可將此物種所傳遞的累積訊息，作為警惕環境中潛在鎘污染風險的初期訊號。最後，本實驗建議，為完善台灣都市型河川的金屬污染監測，應考量生態毒理的評估方式，並以台灣蜆作為長期監測環境污染的指標生物。

Global urbanization in recent decades has caused severe pollution impacts to river ecosystems due to excess metal contaminations by various human activities. The filter-feeding bivalve Corbicula fluminea, which is capable of bioaccumulating metallic contaminants, have been increasingly used as biomonitor for the time-integrated toxic impacts of metallic pollution of freshwater ecosystems worldwide. However, limited studies have been done for long-term biomonitoring programs for riverine metallic pollution using C. fluminea as bioindicator in rapid urbanized Asian countries including Taiwan. In this study, We carried out in-situ exposure experiment using caged C. fluminea in two urban rivers in Taiwan including Danshuei River and Datun River from the highly urbanized region of northern Taiwan aiming to (1) evaluate the relationship between metallic concentrations in water and body tissues of C. fluminea after exposure; (2) determine the bioaccumulation extent of different metals in the body tissues after exposure; (3) reveal the potential accumulative pathways of different metals in C. fluminea by comparing their bioaccumulation patterns among different body tissues; (4) confirm if C. fluminea could serve as an appropriate bioindicator for bioaccumulation monitoring the urban rivers in Taiwan. Our results indicated that most metallic contaminants, including Zn, Cu, Ni, Cr, Pb and Cd, could effectively accumulated in the soft tissues of caged C. fluminea during the in-situ exposure in the urban rivers with low to moderately metallic pollution for relatively short period of time (i.e. 21–28 days). Corbicula fluminea had different bioaccumulation tendency for different metallic contaminants. Despite the higher particulate concentrations comparing to the dissolved concentrations were measured for all metals in rivers, C. fluminea demonstrated effective bioaccumulation of dissolved metals than particulate metals. Although there were inter-site differences in the total metallic bioaccumulation capacities of the caged C. fluminea, similar patterns of BAF variation among different body tissues were apparent. The visceral mass was most important type of body tissues showing metallic bioaccumulation suggesting that the contamination pathways was dominated by food uptake. Thus, our findings suggested that C. fluminea served as good biomonitor for the ecotoxicological impacts of most metallic contaminants (e.g. Zn, Cu, Ni, Cr, Pb, and Cd). In particular, C. fluminea could be used as sentinel organisms for Cd contamination in rivers as an early signal for potential ecological risk due to its high sensitivity to Cd contamination. Therefore, we suggested that C. fluminea should be considered as the bioindicator for long-term monitoring of the ecotoxicological impacts of metallic pollution monitoring of urban river environments in Taiwan.