Water column nutrient, chlorophyll and dissolved organic carbon concentrations and dissolved organic matter characteristics in Roskilde Fjord, Denmark@en

Anthropogenic nutrient enrichment is changing the structure and the function of coastal ecosystems. These coastal zones are transitions between freshwater and marine systems where multiple biogeochemical processes remove, produce and transform organic matter. The extent to which the coastal zone is merely a conduit for terrestrial (allochthonous) organic matter, versus a distinct source of autochthonous organic matter fueled by eutrophication, is unclear. To address this issue, we characterized the freshwater and marine dissolved organic matter (DOM) pools in an eutrophic estuary with a long water residence time (Roskilde Fjord, Denmark) over an annual cycle. We combined elemental, optical (absorbance and fluorescence) and isotopic analyses to obtain insight about the bulk properties of the DOM pool during this period. We also used sediment traps to analyze the changes related to the exchange of organic matter between the particulate (POM) and dissolved (DOM) fractions. The results showed that labile autochthonous DOM from in situ primary production was rapidly transformed to more recalcitrant DOM that accumulated in the estuary despite continuous exchange with the open sea. Also, parts of the POM pool were degraded rapidly (within 24h) and transformed into the DOM pool. Accumulated DOM was characterized by relatively low molecular size and stable carbon isotopic value, and by high protein-like fluorescence. These results indicate that autotrophic material can be a major source of specific recalcitrant DOM in eutrophic coastal waters, contributing significantly to the flux of organic carbon to the ocean.人为营养盐富集正在改变海岸生态系统的结构与功能。海岸带作为淡水与海洋系统的过渡区域，其中多种生物地球化学过程会对有机质进行移除、生成与转化。目前学界尚未明确：海岸带究竟仅为陆地（外源）有机质的输运通道，还是会因富营养化作用成为内源有机质的独立生成源。为解答这一科学问题，本研究针对具备长水停留时间的富营养化河口湾——丹麦罗斯基勒峡湾（Roskilde Fjord）开展了为期一年的监测，对其淡水与海洋溶解态有机质（dissolved organic matter, DOM）库进行了系统表征。研究联合运用元素、光学（吸光度与荧光）及同位素分析技术，解析了该监测周期内DOM库的整体理化性质；同时借助沉积物捕集器，分析了颗粒态有机质（particulate organic matter, POM）与溶解态有机质（DOM）组分间的交换相关变化。结果表明：原位初级生产产生的易降解内源DOM会快速转化为更难降解的DOM，尽管该河口湾与外海存在持续的水体交换，此类难降解DOM仍会在湾内持续累积。此外，部分颗粒态有机质库可在24小时内快速降解，并转化为溶解态有机质库。最终累积的DOM具有相对较小的分子尺寸、稳定的碳同位素比值，以及较高的类蛋白荧光信号特征。上述研究结果证实，自养生物源物质可成为富营养化海岸水域中特定难降解DOM的主要来源，对海洋有机碳通量具有显著贡献。