底泥銅與鉛污染之交互作用與青鱂魚幼魚及胚胎之生物有效性及毒性評估

Abstract

河川底泥具有高累積污染物的能力，因而被認為是污染物的匯聚池。重金屬具不易生物降解的特性，因而廣受關注，且底泥重金屬之生物有效性可能受底泥特性、底泥—水交介面傳輸行為及水生生物生理性質影響。為了解底泥中重金屬與水生生物間的化學及生物作用，此研究使用流水式全底泥暴露系統，將青鱂魚幼魚及胚胎同時暴露於銅及鉛污染底泥達七天，以評估污染底泥對於不同時期魚類之生物有效性及毒性，以及銅及鉛間傳輸與影響。四種環境底泥之基本性質迥異，其中中正橋底泥(ZZ)及外雙溪底泥(WS)屬於粗質地之砂土，淡水河底泥(TS)屬於中質地之壤質砂土，而萬板大橋底泥(WB)屬於細質地的壤土。WB之陽離子交換容量(CEC)最高，而WB及TS之有機碳含量(OC)較ZZ及WS高。此外，pH由高至低分別為TS > WS > ZZ > WB。研究結果顯示，幼魚暴露於四種銅及鉛污染底泥之毒性均高於胚胎。在銅及鉛污染之中，ZZ暴露下幼魚及胚胎均有最高的死亡率，而TS之死亡率最低，顯示粗質地、低pH等底泥性質，會增加底泥之金屬釋放能力。此外，幼魚之體長及體重在所有銅及鉛污染底泥暴露下皆顯著被抑制；相反的，胚胎之孵化率在所有底泥處理組中，未呈現劑量反應關係，證實在污染底泥系統中，青鱂魚幼魚對於毒性之反應較胚胎更為敏感。銅污染或鉛污染底泥處理組[500 mg-Cu/kg (500/0)及500 mg-Pb/kg (0/500)]之幼魚存活率皆較銅及鉛污染底泥處理組[250-Cu/250-Pb mg/kg (250/250)]低。然而，當銅污染底泥及銅及鉛處理組中銅濃度接近時，幼魚之存活率卻有相近的趨勢，因此，幼魚之致死毒性主要貢獻於銅，因底泥較易釋放銅至水相中，而鉛在底泥中的生物有效性較低。本研究以log-logistic模型或線性迴歸模型進行化學指標如底泥金屬總量、水相（表層水、孔隙水）溶解態金屬濃度以及Chelex-100樹脂抽出量與生物毒性及生物有效性之相關分析。其中，Chelex-100樹脂抽出量對於銅及鉛污染底泥之生物體內金屬含量與毒性均具有最好的預測能力，而孔隙水最差。因此，Chelex-100樹脂抽出法能夠作為預測底泥重金屬污染之生物有效性及毒性的化學評估方法。

In the river system, sediment is considered as a long-term source of contaminants for aquatic organisms due to its high accumulating capacity. Among all contaminants, heavy metals are of high concern because of their non-biodegradable nature. The bioavailability of these metals may be affected by sediment properties, transportation behavior between sediment-water system and physiology of aquatic organisms. To understand the chemical and biological interaction of sediment-bound contaminants to the aquatic life, this study used the whole-sediment exposure method with intermittent water renewal system to evaluate the bioavailability and toxicity of two heavy metals [i.e. lead (Pb) and copper (Cu)]. Medaka (Oryzias latipes) embryos and larvae, which were used as model organisms, were exposed to a variety of Pb- and Cu-spiked sediments with different physicochemical properties for 7 days to evaluate the toxicity and bioavailability. Moreover, this study discussed the transportation and effect in the sediment between these two metals. The physiochemical properties of four sediments were totally different. ZZ sediment and WS sediment were sand, TS sediment was loamy sand and WB was loam. In these sediments, WB had the highest CEC, as well as ZZ and WS has lower OC. pH value from high to low were TS > WS > ZZ > WB. Survival rates of both treated embryos and larvae from TS were the highest, and those from ZZ were the lowest. As such, we believed that metals in lower pH, CEC, OC and more sandy sediment were more releasable, and larvae were more sensitive to metal toxicity than embryos. Moreover, the larval morphology i.e. body length and body weight in dual metal spiked sediments groups were significantly inhibited in very low concentrations. The trend of larval mortality spiked with single Cu into ZZ sediment was similar to that with dual metal at in same Cu concentration. Based on the results, the lethal toxicity was mainly affected by Cu, as the bioavailability of Pb was low. In this study, we also used several analytical approaches to assess the bioavailability of these two metals and the correlation (log-logistic model and linear regression) with observed toxic effects. The resuls showed that Chelex-100 resin extraction method can be an effective method for predicting and evaluating bioavailability of sedimental Cu and Pb to medaka no matter in single or dual metal polluted sediment.