海岸底棲食碎屑蟹類對有機物質來源的選擇性利用：穩定碳、氮同位素分析應用

Abstract

本研究主要利用穩定碳、氮同位素探討台灣海岸濕地（高雄林園紅樹林復育區）碎屑食物網組成及底棲食碎屑蟹類（雙齒近相手蟹、北方呼喚招潮蟹、網紋招潮蟹、清白招潮蟹、萬歲大眼蟹）對於不同有機物質來源（C3、C4植物與底棲藻類）的選擇與其利用。 紅樹林復育區中各樓地分區的優勢植物碎屑是構成該區內顆粒>62μm表層底泥中最主要的碳源成份，而底棲藻類（δ13C=20.2?）是顆粒＜62?m表層底泥（δ13C＝-23.9?-19.6?）的主要碳源組成。 食碎屑蟹類主要食物並非取決於棲地優勢基礎生產者。以混和模型理論分析螃蟹對於棲地優勢基礎生產者之碳源的吸收利用比例。結果發現吸收比例最多的為單葉鹹草（C4植物）: 45.1?100%，其次為底棲藻類：0?81.0%，最差的為水筆仔與海茄苳（C3植物）：9.3?12.3％。 推測螃蟹最優先吸收利用單葉鹹草(C4植物)碳源的原因為：(1)細菌及微生物分解C4植物的速度比C3植物更快，可以幫助螃蟹更快速的吸收C4植物碳源、(2)掉落的紅樹林葉片(C3植物)含有化學物質會抑制螃蟹的吸收利用。 食碎屑蟹類與表層底泥的δ13C值成直線相關，迴歸線斜率趨近於1，顯示表層底泥內所含的有機質碳源是螃蟹主要的食物來源，而螃蟹對於底泥中所含不同有機質來源(C3、C4植物與底棲藻類)之碳源具有不同的利用。C4植物區內的螃蟹(δ13C=-13.4?)利用了底質顆粒＞62μm表層底泥(δ13C=-14.4?）中的C4植物碎屑(δ13C=-13.9?)；在C3、C4植物混生區中的螃蟹(δ13C=-15.4?-14.5?)利用了表層底泥中所含的C4植物碎屑及部分底藻；C3 植物區內的螃蟹(δ13C=-19.7?)則傾向利用顆粒＜62? m表層底泥（δ13C=-23.9?）中的底棲藻類及部分C3植物碎屑(δ13C= -30.2?-29.4?)。所以，螃蟹食物碳源主要來自C4植物及底棲藻類；C3植物相對並不重要。 食碎屑蟹類（δ15N=10.14?12.08?）的δ15N值與棲地優勢基礎生產者（δ15N = 7.34?13.19?）的值，平均相差約1.77?，所以食碎屑蟹類皆屬於初級消費者。這顯示林園紅樹林復育區內的食碎屑蟹類並不會因為棲地的變化而改變其營養階層。

This study is using the stable carbon and nitrogen isotopes to examine the selective utilization of benthic organic matter sources (C3, C4 plants, and benthic algae) by detrital-feeding crabs (Perisesarma bidens; Macrophthalmvs vanzai; Uca arcuata; Uca lacteal; Uca borealis) and the detrital food web structure in the Kaohsiung Lin-yuan mangrove estuary. The results indicate that the carbon source of sediment particles larger than 62 ? m comes mainly from the dominant plants detritus of each area. The carbon source of the sediment particles smaller than 62 ? m ( δ13C=－23.9?－19.6?) comes mainly from benthic algae (δ13C=－21.2?). The main food sources of detrital-feeding crabs do not lie on the dominant primary producer detritus in the area. Using the two-source mixing model and the dual isotope, three-source mixing model to calculate the utilization percentage of benthic organic matter sources (C3, C4 plants, and benthic algae) by detrital- feeding crabs. Results indicate that according to the utilized percentages from high to low, C4 plant (29.3?92.7%, Cyperus malaccensis), benthic algae (7.3?82.8%), and C3 plants (7.4?10.1%, Kandelia candel and Avicennia marina). We think that the reasons of preferential ingestion of C4 plants carbon by detrital- feeding crabs: (1) Detrital matter made from the C4 plant decomposed more rapidly ands supported a great microbial biomass than did detrital material made from C3 plant. (2) Degraded leaves of C3 plant can secrete chemical matter to restrain the crabs to eat. (3) Assimilation efficiency of detrital-feeding crabs can be increased significantly by microbial microbial colonization of the detritus. The δ13C values of detrital-feeding crabs and surface sediments are positive correlation, and the value of regression line slope is close to 1. The results indicate that the food source of detrital-feeding crabs comes mainly from the organic matter carbon sources of surface sediments. Therefore, Detrital-feeding crabs in different areas have different utilizations of sediments carbon organic sources (C3, C4 plants, and benthic algae). Crabs (δ13C=－13.4?) in the area with C4 plant assimilate C4 plant (δ13C=-13.9?) detritus of the sediment particles larger than 62 ? m (δ13C= -14.4?). Crabs (δ13C= -15.4?－14.5?) in the area with a mixture of C3 and C4 plants assimilate C4 plant detritus and benthic algae of the surface sediments. Crabs (δ13C=－19.7?）in the area with C3 plants assimilate benthic algae and partly C3 plant (δ13C=－13.9?）detritus of the sediment particles smaller than 62 ? m (δ13C=－14.4?). Therefore, Food sources of detrital-feeding crabs come mainly from C4 plant and benthic algae, C3 plants are relatively not important. The δ15N values of the detrital- feeding crabs (δ15N= 10.1?12.1?）differ from the dominant primary producer (δ15N= 7.3?13.2?）by 1.77?. Thus, the detrital-feeding crabs are primary consumers. The result indicates that the detrital-feeding crabs did not change the trophic level in different areas of the Kaohsiung Lin-yuan mangrove estuary.