淡水竹圍紅樹林濕地有機物質在底棲碎屑食者中的傳遞：穩定同位素分析之應用

Abstract

本研究主要利用穩定碳、硫兩種同位素分析比較的方法，探討淡水竹圍紅樹林濕地不同棲地環境中，有機物質來源，包括維管束植物、底棲性微藻、漲潮時水層中顆粒性有機物質、與退潮時水層中顆粒性有機物質，在底泥有機物質及底棲碎屑食者，包括多毛類與螃蟹，的傳遞情形。 不同的有機物質來源具有不同的穩定同位素組成，其值分別為底棲性微細藻類δ13C＝-19.4?、水筆仔δ13C＝-26.1?、δ34S＝5.6?、蘆葦δ13C＝-14.5?、漲潮時顆粒性有機物質δ13C＝-23.5?、退潮時顆粒性有機物質δ13C＝-24.5?。底泥有機物質的δ13C與δ34S值分別落在-26.8?-24.5?與5.1?9`.1之間，推測其有機物質主要來自於水筆仔與水層中的顆粒性有機物質；在不同粒徑間，其δ13C值與δ34S值皆有差異，表示不同粒徑的有機物質，其來源或組成的差異較大。 多毛類中，腺帶沙蠶（δ13C＝-21.4?-20.8?、δ34 S＝8.6?13.8?）與白線纓鰓蟲（δ55.4F＝-22.2?-21.9?、δ34S＝10.2?11.7?）的穩定同位素組成不同，顯示出腺帶沙蠶的營養階層較白線纓鰓蟲高，而其主要利用水筆仔與退潮時的有機物質，而漲潮時的有機物質也是來源之一，底棲性微細藻類則較少。同時，其穩定同位素δ值與其所棲息環境中底泥有機物質的δ值不同，表示此兩種多毛類會從底泥或水層中選擇性地利用有機物質。招潮蟹的穩定同位素組成，會隨其棲地的優勢植物種類而有差異，在潮溪水筆仔區δ13C＝-19.4?-19.3?、δ34S＝10.9?12.6?；河岸高地蘆葦區δ13C＝-15.2?-14.9?、δ34S＝12.5?13.7? 。顯示在潮溪地區招潮蟹主要利用底棲性微細藻類與水層中的有機物質；而在河岸高地地區則利用蘆葦有機物質，漲潮時的有機物質也是食物來源之一。潮溪台灣厚蟹(δ13C＝-21.5?、δ34S＝9.3?）的營春階層較高，且所利用的基礎生產者來源與多毛類相似。

In the mangrove estuary of the Tanshui River in northern Taiwan, the components of the detrital food chain, including the particulate organic matter (POM) of sea water, river water and sediment, primary producers, deposit feeders, and a suspension feeder, were examined at three sites using stable carbon and sulfur isotopes to reveal the possible flow of organic matter. The fresh leaves of the vascular plant Kandelia candel had aδ13 C as -26.2? indicating that it is a C3 plant. K. candel had a δ34 S value of +5.6?. The δ13 C level of particulate organic matter (POM) of sea water (collected at flood tides) averaged-23.5?, and that of river water (collected at ebb tides) was -24.6?, whereas benthic microalgae had a δ13 C level of -19.4?. Theδ13 C andδ34 S levels of sediments ranged from -26.1 to -24.6 ?o and +5.0 to + 9.1?, respectively, and were close to the values of the mangal leaves, suggesting that the organic source of sediments comes mainly from the vascular plant K. candel. The isotopic values of the sediment did not significantly differ according to the site (3 sites) or the depth (2 depths), but differ according to particle size. These results indicate that particle size is a key factor in harboring different sources of organic matter. In polychaetes, the deposit feeder (the nereid Neathes glandicincta), and the suspension feeder (the sabellid Laonome albicingillum) had δ13 C values ranging from -21.4 to -20.8?, and from -22.2 to -21.9 ?, respectively; whereas δ34 S values ranged from +8.6 to +13.8 ?, and +10.2 to +11.7 ?, respectively. The two polychaetes significantly differed in their stable isotopic compositions and N. glandicincta was at slightly higher trophic rank than L. albicingillum. In addition, the nereids exhibited a greater variation in the sources of the detrital organic matter than the sabellids. Based on the δ13 C and δ34 S values, the crabs were well separated into three groups, also according to the sites. The river living crabs containing Uca arcuata and U. lactea had δ13 C values averaged -15.5 ? andδ34 S values averaged +13.1 ?o, the creek living crabs containing U. arcuata again and U. borealis had -19.4 ? and +11.7 ?, and the creek living Helice. formosensis as -21.5 ? and +9.9 ?, respectively. Such a discrepancy found in the detritivores may be attributed to their different feeding sites and modes. The present study suggests that the detritus consumed by the two studied polychaete species and H. formosensis are probably from K. candel and POM of water. As regards the food of the crabs, both benthic microalgae and Phragmites commuuis are possible sources and the relative importance of the two sources depends on where the crabs live. In conclusion, detrtus in the study mangrove swamp are distinguishable from one another and detritivores differentiate their food sources from a common pool.