探討金屬污染物對亞熱帶地區都市化感潮河段造成之生態風險：以北臺灣淡水河為例

Abstract

隨著經濟發展，許多都市型河川都面臨著金屬污染的危機。然而，金屬污染物在感潮河段對生物體金屬濃度及生態的影響及其在環境因子影響下的變化仍有許多未知，也鮮少被探討。因此，本研究在臺灣北部一典型都市型河川淡水河本流設立樣點WZW，WG和JZC，並試圖1) 探討生物體金屬濃度 (鉻、鎳、銅、鋅、鎘、鉛) 隨著不同季節及鹽度梯度下的改變趨勢，以瞭解控制生物體金屬濃度的環境因子；2) 探討不同季節及鹽度梯度下的環境金屬濃度趨勢然後以生物累積指數(Bioaccumulation factor; BAF)，生物-底泥累積指數(Biota-sediment accumulation factor; BSAF)和生物放大指數(Trophic magnification factor; TMF)探討金屬污染物從環境至生物的傳遞路徑；3)使用生物體金屬濃度和生物累積、生物放大探討傳統生態風險評估(Ecological risk assessment; ERA)指數，其中包括potential ecological risk index (RI)、effects range median (ERM)和potential effects level (PEL) 是否適合評估都市型感潮河川生態風險。 實驗結果顯示，生物體金屬濃度普遍有夏高於冬的趨勢, 尤其鉻、鎳、銅更呈顯著差異。季節方面，生物體夏天的銅濃度，和冬天的銅、鋅、鎘濃度有沿河口方向呈顯著增加趨勢。此外，棲地或攝食習慣與底泥息息相關的攝食功能群：濾食者、沉積物攝食者、碎食者的生物體金屬濃度在靠近河口的樣點會明顯比獵食者和雜食者高。然而，由於生物-底泥累積在淡水河不是金屬傳遞的主要路徑，所以底泥對生物體金屬濃度有可能有其他非直接的影響。本實驗顯示淡水河中大的主要金屬傳遞路徑為生物累積、生物放大和攝食互動。 實驗結果也説明，流量可以透過擾動和調整酸堿度改變金屬的生物可利用性和影響動物生理狀況而影響金屬的累積，是主要影響生物體金屬濃度的季節因子。樣點間，夏天的生物體銅、鋅、鎘濃度有通往河口增加的趨勢，應該為河口水中金屬濃度較高的影響。而冬天生物的生物體銅、鋅、鎘濃度往河口有增加的趨勢，有可能只是金屬排除率較低，所以呈現和夏天類似的趨勢。 經比較傳統ERA指數和生物體金屬濃度的差異顯示，ERA指數會低估都市化感潮河段的生態風險，極有可能是因爲傳統ERA指數並沒考量底泥金屬以外路徑造成的金屬毒性，且每考慮生物體的實際金屬濃度。探討金屬傳遞路徑可以協助探討環境金屬濃度和生物體金屬濃度之間的關係，以利評估生態風險。

Most tidal rivers are seriously threatened by metallic pollution associated with urbanization and economic development. In face of this increasing risk to urbanized tidal river ecosystem and the biotic species therein, it would be important to understand the biotic metal burdens and the environmental factors influencing these metal burdens. In this study, we investigated the seasonal and inter-site variation in ambient concentrations and biotic metal burdens of six dominant metallic pollutants (Cr, Ni, Cu, Zn, Cd and Pb) in response to different environmental factors in three sites, i.e. WZW, WG and JZC, along the tidal main reach of Danshuei River from the Northern Taiwan. Functional feeding groups (FFGs) of the aquatic fauna were used to categorize various taxa to delineate the influences on metal burden from different trophic uptake sources. Thus, the major transfer pathways between ambient metallic concentrations and metal burdens in different FFGs were determined with respect to bioaccumulation factors (BAFs), biota-sediment accumulation factors (BSAFs) and trophic magnification factors (TMFs). Moreover, our findings were compared with the traditional ecological risk assessment (ERA) indices including the potential ecological risk index (RI), effects range median (ERM) and potential effects level (PEL) to evaluated the representativeness and accuracy on the assessment of metal burden, bioaccumulation and trophic magnification in urbanized tidal rivers.. The metal burdens for Cr, Ni and Cu for all FFGs was clearly higher in summer than winter. The inter-site trend of increasing metal burdens along the seaward direction was significant for Cu in summer, as well as Cu, Zn and Cd in winter. FFGs strongly associated with demersal habitats, i.e. filter feeders, deposit feeders and detritus feeders, had the highest uptake of all metals near the river mouth. However, biota-sediment accumulation was not a major uptake pathway for all study metals in Danshuei River. Therefore, bioaccumulation, trophic magnification and trophic interactions represented the dominant uptake pathway. The concentrations of all particulate metals increased along the seaward direction in summer, but decreased along the same direction in winter. This suggested that pollutants were washed towards WZW under high river discharge in summer, but were trapped nearer to upstream pollution sources in winter. In both seasons, dissolved Cr and Cd decreased along the seaward direction, but dissolved Ni, Cu and Zn increased along the upstream direction, possibly reflecting the anthropogenic sources for these three metals. There were no significant seasonal differences for sediment metallic content, but concentrations were always highest in WG, which was the confluence of all major tributaries in Taipei and a hydrological sink of sediments. Results showed that ERA indices underestimated the ecological risk in urbanized tidal rivers since they neglect toxicity pathways from water and trophic interactions, and do not take into consideration actual metal burdens. The assessment of uptake pathways is essential for understanding the influences of ambient concentrations on metal burdens to help assess the ecological risk to urbanized tidal rivers.