本研究之目的在透過分析七家灣溪優勢物種，包括濱岸植物、附生藻、水棲昆蟲及台灣櫻花鉤吻鮭，雨、旱二季共132個標本之碳、氮穩定同位素組成(δ^13C、δ^15N)，探討台灣櫻花鉤吻鮭棲地食物網同位素特徵之意涵，進而了解台灣櫻花鉤吻鮭之營養來源。由δ^13C及δ^15N特徵值顯示，C3型濱岸植物是七家灣溪最重要的基礎營養來源。相對於旱季，雨季有較大程度的土壤淋洗作用，導致雨季時期附生藻類之δ^15N值較旱季之値為輕。因為七家灣溪在雨季之營養條件劣於旱季，因此，雨季溪水中細顆粒有機質(FPOM)受微生物分解作用發生較顯著的同位素分化作用，使得FPOM之δ^15N值呈現雨季重、旱季輕的現象。由IsoSource軟體分析七家灣溪各營養階主要食物來源之結果顯示，在雨季可得到關於台灣櫻花鉤吻鮭的營養關聯；此關聯由基礎生產者至高階消費者依序為：C3型濱岸植物、濾食性水棲昆蟲、掠食性水棲昆蟲、台灣櫻花鉤吻鮭。然而，對於旱季而言，並不能經由IsoSource獲得台灣櫻花鉤吻鮭主要食物來源的訊息；這可能是因為有一些來源食物未被發現且未被列入IsoSource中運算。This study examined the food web of Chichiawan Stream (central Taiwan) and evaluated the trophic sources of Formosan landlocked salmon through analysis of stable carbon and nitrogen isotope compositions of dominant species. The species sampled in both rainy and dry seasons include 132 samples of aquatic insects, periphytons, riparian vegetations, fine particular organic matter, and the Formosan salmon. The characteristic values of δ^13C and δ^15N indicate that C3 riparian vegetation is the primary trophic source of the stream. Because the extent of soil leaching owing to precipitation is higher in rainy season than that in dry season, the δ^15N values of periphytons are more depleted in rainy season relative to dry season. In addition, the feature that δ^15N value of fine particular organic matter is heavier in rainy than in dry season is due to a more significant isotopic fractionation on rainy-season sample. The isotope fractionation resulting from microbiological decomposition is significant in rainy season because the trophic condition in rainy season is inferior to that of dry season. The food-source evaluation for every trophic level performed by IsoSource package indicates that a trophic relation regarding to Formosan salmon in rainy season is established and the trophic level of bio-species from lower to higher are C3 riparian vegetation, filter aquatic insect, predator aquatic insect, and Formosan salmon in order. However, the IsoSource cannot obtain the information of major food source for Formosan salmon in dry season. This unavailable result may be that some food sources had not been found and involved in IsoSource
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