地下水活動對都市下游河川生態系統的重要性─以台灣景美溪為例

Abstract

在河川環境中，地下水與表面水交換是維持河川水文與物質通量，以及生態系統健康的重要的過程。這種水體垂直交換的特性可提升棲地的複雜性，改變河川表面水的物理化學特性如導電度、水溫、氧化還原電位、溶氧及pH等，進而影響生物的分布及能量的流動，包括提升物種類群豐富度、生物多樣性及生物族群結構的改變等。另外，地下水的活動具有季節的變異，以臺北盆地為例，地下水的豐水期是每年的3-7月間，而枯水期則介於9-10月間，此季節的動態會使河川水體環境隨之變異，增加時間上的複雜性以提升河川棲地的品質。然而，隨著人口快速增加及都市發展，下游河川大多已被渠道化以防止溪水暴漲導致的洪水及增加都市土地的利用，有些河川的河床甚至改造為不透水的水泥底床，這導致都市型河川失去地下水與表面水交換的自然特性。另外，都市化的結果導致外來種魚類成為優勢種族群危害原生種族群，降低河川的棲地品質。因此，本研究針對都市型河川河床的小尺度範圍，分別探討「直接受地下水影響」及「未受地下水影響」的區域，研究地下水活動對河川水質、棲地品質、生物族群結構及食物網結構的影響。 本研究的研究地點位於臺北盆地內景美溪下游的都市河段。臺北盆地含有豐富的地下水層，特別是在景美地區，地下水含量豐富且水位非常接近地表，有利於表面水及地下水交換的過程。然而，臺北盆地是臺灣地區人口最集中及都市化最嚴重的地區之一，盆地中許多河川皆已整治成三面光的不透水河道。景美溪的兩岸有經過渠道化的工程，但其河床未經過整治，仍維持著地下水與表面水交換的特性，地下水活動會改變河床結構因而形成坑洞。然而，景美溪下游的優勢種魚類為外來種口孵尼羅吳郭魚(Cichlidae: Oreochromis niloticus)，其擁有一特殊繁殖行為是公魚會在河床挖一碗狀坑洞做築以吸引母魚進行交配。本研究發現景美溪的河床有許多碗形坑洞，因無法以肉眼辨別坑洞之形成是由於地下水活動或魚類繁殖行為所致，故我們藉由吳郭魚隔絕實驗(fish exclusion experiment)證實地下水活動確實會在河床形成坑狀結構。另外，為了進一步了解景美溪下游地下水活動的機制為上湧(upwelling)還是下沉(downwelling)，我們在坑內及坑外進行水流可視化(flow visualization)的實驗，藉測量顏料受地吸引力及地下水活動往下移動的速率，探討地下水與表面水互動的模式，同時我們亦比較坑內環境及坑外環境以探討景美溪水質受地下水影響的差異。此外，本研究探討坑內及坑外無脊椎生物的族群結構（物種類群豐富度、密度、生物多樣性等）的差異以了解河川生物對地下水活動的反應，以及利用穩定碳氮同位素分析(stable carbon and nitrogen isotope analysis)景美溪下游地下水資源對河川食物網結構之影響。 研究結果顯示在隔絕吳郭魚進入的區域下，河床仍然有坑形成，顯示出底床上的坑洞並非魚類繁殖行為所形成，而景美溪確實保有地下水與表面水交換的特性。水流可視化實驗的結果呈現出坑內顏料下沉的速率顯著較慢(1-way ANOVA, p < 0.05)，這確定景美溪下游地下水和表面水活動以地下水上湧為主。另外，坑內水的導電度、總溶解固體、氧化還原電位、pH及水溫和坑外的河水有顯著的不同(1-way ANOVA, p < 0.05)，表示直接受到地下水活動影響的區域和未受到地下水影響區域的水質有差異。生物的結果方面，雖然坑內的物種類群豐富度及生物多樣性並未高於坑外，但在坑內的底土層中有較高的無脊椎生物密度，顯示出地下水可提供更多的資源供給無脊椎生物生存。此外，景美溪下游魚類的營養基礎來自坑內的資源所佔的比例很高(~ 40 %)，因受地下水影響食物資源的碳氮比(C/N ratio)較低，能較有效的被消費者使用。本研究的結果證實地下水活動會影響都市型下游河川棲地的品質，為將來都市型下游河川復育提供重要的資料。

In river ecosystems, groundwater (GW) and surface water (SW) exchange is an important natural process for maintaining vertical hydrological connectivity, material fluxes and ecosystem health. The vertical GW-SW exchange could increase the habitat complexity and maintain the stability of the water physiochemistry and hydrology in the rivers. Spatial variation of GW-SW exchange pattern could represent a key factor for characterizing aquatic biodiversity and faunal community structure. Also, GW activities exhibit seasonality resulting in the dynamic patterns of river ecosystems. In the northern Taiwan, high GW level in Taipei basin during March to July and low level during September to October are influenced by the seasonal variation of precipitation and evaporation. Such temporal variation could thus be important for enhance the diversity of river habitat and aquatic biota. However, Taiwan as other countries in the world, it has been rapid population increase and urban development since the recent decades, most downstream rivers have been channelized to avoid flood and increase urban land use. River beds of some channelized rivers are commonly covered with impermeable concrete materials. This man-made structure causes the blockage of GW-SW exchange. Also, urbanization usually results in the dominance of invasive species in aquatic ecosystems and this can endanger the native communities. In this study, we first carried out small-scale field manipulative experiment on river bed of Jingmei River and compared the areas of “pit” (depression structure directly caused by GW) and “non-pit” (areas not directly affected by GW activities) to investigate the influences such as water and habitat quality, faunal composition, and the trophic basis associated with GW activities in an urban river ecosystem. Our study site located at the downstream Jingmei River in the Taipei basin where sufficient GW layer lies underground. Taipei basin is one of the densely urbanized areas and most rivers in the basin are channelized with impervious river bed to reduce flood impacts. However, sufficient GW layer widely extends underneath the Taipei basin especially at Jingmei district. The downstream Jingmei River is channelized at both river banks but the river bed remains unmodified so that GW-SW exchange could be maintained. In addition, the dominant fish in downstream Jingmei River is dominated by tilapia (Cichlidae: Oreochromis niloticus) which has a special breeding behavior that male fish make bowl-shaped craters to attract the female for spawning. As we found many bowl-shaped pits on the river bedduring our preliminary survey, we conducted the fish exclusion experiment to confirm whether the pits were formed from GW activity or fish breeding behavior in downstream Jingmei River. Then, flow visualization experiment was undertaken at pits and control locations to determine the direction of the GW activity in Jingmei River. We further compared the water chemistry between pits and non-pits to determine the differences of water quality influenced by GW activities. In addition, we investigated the invertebrate biodiversity and community structure at pits and non-pits to evaluate the biological response to GW activity. Finally, we used stable carbon and nitrogen isotope analysis in conjunction with gut content analysis to study the GW influence on the trophic basis in downstream Jingmei River. As the bowl-shaped pits were formed under both the fish exclusion and control areas, our results confirmed that GW-SW exchange did occur in downstream Jingmei River. Flow visualization experiment results revealed that the sinking rate at pits was significantly lower than at non-pits (1-way ANOVA, p < 0.05). This indicated that the GW activities at pits was via GW upwelling. In addition, conductivity, TDS, ORP, pH, and water temperature were significantly different between pits and non-pits (1-way ANOVA, p < 0.05) suggesting that GW activities strongly affected the water quality in this urban river. Moreover, our study showed that invertebrate density was higher in subsurface sediment at pits although the taxon richness and biodiversity of invertebrates were not particularly higher at pits than non-pits. This indicated that GW influenced pits by providing more suitable habitats to support high faunal density. Our findings also showed that fishes utilized high proportion (~ 40%) of pit resources in Jingmei River. Since food resources of pits were characterized by lower C/N ratio, consumers tended to use the pit resources and the dependence of pit resources was related to ontogenetic development of tilapia. Our study confirmed that GW activities could effectively enhance the habitat quality through increasing the habitat diversity in the hydromorphologically homogeneous urban river. Hence, our results could provide important information for the urban river restoration in the future.