東海底棲性魚類群聚與環境因子之關聯：論優養化之影響

Abstract

長江每年輸入大量無機營養鹽及有機物質進入東海海域，並造成其產生嚴重的優養化與缺氧情形。而近年來因氣候變遷而引發的洪水事件會進一步加強陸源物質對於臨近海域生態系統的影響。然而，關於優養化及其他陸源物質對於東海底棲生態系統之影響的相關研究卻很缺乏，且對洪水所帶來的潛在貢獻與風險之評估也仍鮮為探討。故本實驗首先欲探討東海底棲性魚類群聚結構與東海環境特性之相關性為何。此外，我們也透過分析魚類群聚組成與穩定性同位素組成，進一步討論提高的初級生產力是否能貢獻予底棲消費者，亦或會沉降造成嚴重缺氧而危害底棲生物。本研究發現，東海近岸的禁漁範圍，最容易產生底部低氧甚至是缺氧情形。夏季時，多數生物指標，如：生物多樣性、物種豐富度等，皆與表層營養鹽及葉綠素濃度呈現顯著負相關，而與底部溶氧濃度呈顯著正相關，顯示近岸區域的魚類群聚很可能受到優養化及缺氧的負面影響。且近岸測站多以小型的 六線長鯊蝦虎魚(Amblychaeturichtyhs hexanema)為最主要的優勢物種，反映了多數物種無法存活於該低氧環境。所以，原劃定為養護漁業資源的近岸禁魚區之效益，可能相當有限。穩定性碳、氮同位素分析則是顯示絕大部分的東海底棲消費者，皆仰賴海源性的初級生產力；相反地，鮮少利用陸源性有機物質。而近岸測站的魚類與甲殼類標本體內有較高的穩定性碳同位素值(δ13C)，反映其攝取了表層的藻華生物。 發生於2010年的長江洪水事件，會於東海近岸造成更加嚴重的缺氧情況，以致於魚類多樣性低迷。然而部分水深較深，且不易受到缺氧影響的區域，其魚類物種豐富度與長江輸水量呈現顯著正相關，此結果可能反映了長江洪水發生時，適度提高的初級生產力能間接地提高魚類的食物可利用性，因而貢獻到魚類的物種豐富度。故整體而言，長江淡水輸入所刺激的高初級生產力，僅能夠在無缺氧發生的環境下，對東海的底棲生態系統有所貢獻。

The East China Sea (ECS) received large quantities of freshwater, nutrients, and organic matters from the adjacent Changjiang and experienced severe eutrophication and hypoxia in recent decades. Frequent flood events related to climate change may further magnify the impacts of land-based materials on the ECS. Nevertheless, the basic knowledge about response of ECS benthic organisms to these disturbances remain poorly understood. Risk and benefit assessments of flood-induced high primary production and riverborne POM are also scarce. The present study aimed to recognize the links between demersal fish assemblage structures and environmental characteristics across the ECS. Also, we assess the balance between food source contribution and hypoxia risk caused by the increased primary production by analyzing fish community structures and stable isotope compositions for zooplankton, crustaceans and demersal fish. The inshore area of ECS, corresponding to the trawling-prohibited zone, was prone to low DO condition and hypoxia. Biological indices, including species richness and diversity exhibited significantly negative correlations with nutrient concentrations and positive correlations with bottom-water dissolved oxygen, indicating the negative impacts of eutrophication and accompanied hypoxia on the inshore fish assemblages. This area was also heavily dominated by the opportunistic fish Gobiids— Amblychaeturichthys hexanema, reflecting low survival rate of most fishes over there. Thus, the efficacy of inshore trawling-prohibition practice, which intended to restore the ECS fish stocks, was probably very limited. The stable isotope analyses indicated great reliance of benthic organisms on the marine productions regardless of distance from shore. Relatively higher δ13C of demersal fish and crustaceans in the inner shelf implied their uptake of marine blooming materials. Conversely, terrestrial POM plays minor role in nourishing the benthic consumers in the ECS. The Changjiang flood event in 2010 further enhanced the hypoxia formation and resulted in the lowest fish diversity in the inner shelf. Yet fish assemblages inhabited the normoxic mid to outer shelf demonstrated positive response to the Changjiang flood event. Therefore, the Changjiang River discharge only benefits the ECS benthic ecosystem on condition that the bottom waters maintain higher levels of DO.