有機物質蓄積對河口底棲化學性質、微生物群聚，以及底棲動物的衝擊

Abstract

全世界水域環境污染的情況日益嚴重。以台灣為例，約有三分之二的河川呈現中度 (moderately) 或嚴重 (seriously) 污染，而北部最大河川淡水河由於承受大量生活污水的排放，使得像文蛤Meretrix lusoria這類生存在河口的生物受到嚴重影響。相對在沿岸環境的文蛤人工養殖區同樣也面臨到底質惡化，造成漁獲量降低，以及養殖池壽命縮短的問題。其實，不論河口污染或者是養殖池底泥的惡化，主要都是有機物質蓄積之後，所引發一連串底泥化學、微生物群聚，乃至於底棲動物的變動。 有機物質蓄積是底棲環境的關鍵性議題。水域中有機物質的堆積與否與流體運動及生物地球化學 (biogeochemistry) 代謝兩者息息相關。當流況減弱時，水中懸浮有機物質會沉澱積聚在底泥上，一般水中溶氧會與這些有機物質進行分解作用。當氧氣缺乏時，則以其它替代的化學物質，像是硝酸鹽 (nitrate)、硫酸鹽 (sulfate) 之類與有機物質進行厭氧性的降解 (anaerobic degradation)。這些好氧 (aerobic) 及厭氧代謝的結果都直接反映在底泥化學性質上。為了解有機物質蓄積下對底泥化學性質的影響，本論文測量淡水河河口至中游河段底泥中化學性質的縱向水平空間分布：於2001年採集下游與中游河段之底泥樣品進行硫化物質 (硫化氫, 酸可萃取硫 [acid-volatile sulfide; AVS]，黃鐵礦 [pyrite]) 含量及硫酸還原速率 (sulfate reduction rate; SRR) 的分析；於2002年採集近河口區、下游與中游河段之底泥樣品進行營養鹽含量 (硝酸鹽 [nitrate]、亞硝酸鹽 [nitrate]、磷酸鹽 [phosphate] 及矽酸鹽 [silicate]) 的分析。 綜合底泥化學性質的結果顯示，由間隙水中硝酸鹽、磷酸鹽與硫化氫特性的縱向空間組成分布，可將淡水河系底泥有機物降解的過程界定為三個類型：硝酸鹽富集型 (nitrate-abundance type) 出現在近河口區，間隙水硝酸鹽及磷酸鹽濃度分別為114.24及1.01

To understand the integrated effects of organic matter loading on the benthic chemical properties, microbial communities, and benthos in an estuarine system, two field studies and a set of laboratory experiments were conducted. In the Tanshui River, the horizontal characteristics of the estuarine sediments were revealed in three spatial settings from the river mouth to a mid-reach section: a nitrate-abundance type, a phosphate-release type, and a sulfide-generation type; the vertical characteristics in four layers of Guandu sediment were also elucidated. With the exception of the mixed layer, a variety of metabolic processes by denitrifier strains occur in different vertical layers including nitrification-denitrification coupling processes in the nitrate-concentrated layer, denitrification or dissimilatory nitrate reduction to ammonium in the denitrifier-aggregation layer, and fermentation or spore formation in the ammonium-enriched layer. In addition, two laboratory experiments indicated that environmental condition gradually became worse, which leads to decreasing of dissolved oxygen, increasing of ammonium, nitrate and sulfide concentrate. However, high levels of treatment with the photosynthetic bacterium (PSB, Ectothiorhodospira sp.) decreased sulfide and nitrogenous nutrients in both aerated and hypoxic conditions, and then significantly improved the survival rate of the hard clam (Meretrix lusoria). In summary, the degree of organic matter degradation could be defined by the chemical properties in both the horizontal and vertical sediment profiles. A variety of metabolic processes by denitrifiers, which occur in the comparative sediment profile, indirectly correspond to these different stages. The use of the PSB, Ectothiorhodospira sp., actively removed some chemicals and mediated the sediment properties (from polluted to a transitory stage). Benthic hard clams can serve as a good indicator of the shift in benthic chemical processes toxic to the benthos, which are incapable of being detected by environmental measurements.