Physical and microbial processing of dissolved organic nitrogen (DON) (Salinity Experiment) along an oligotrophic marsh/mangrove/estuary ecotone (Taylor Slough and Florida Bay) for August 2003 in Everglades National Park (FCE), South Florida, USA

A better understanding of the biogeochemical cycling of nutrients entering Florida Bay is a key issue regarding the restoration of the Everglades. In addition to precipitation, the other major source of freshwater to Florida Bay is from Taylor Slough and the C-111 Basin in the northeast section of the Bay. While it is known that these areas deliver significant amounts of N to the Bay, a significant portion of this is in the form of dissolved organic N (DON). The sources, environmental fate and bioavailability to microorganisms of this DON are however, not known. Should this DON be readily available, any increased load as a function of restoration changes might have an impact on internal phytoplankton bloom dynamics. No significant flocculation or precipitation of DOM occurred with increase in salinity, meaning that terrestrial DOM does not get trapped in the sediments but stays in the water column where it subjected to photolysis and advective transport. Sunlight has a significant effect on the chemical characteristics of DOM. While the DOC levels did not change significantly during photo-exposure, the optical characteristics of the DOM were modified. The environmental implications of this are conflicting: photo-induced polymerization may stabilize the DOM by reducing its bioavailability while photolysis may make the DOM more labile. Overall, DON bioavailability was relatively low in this region. Even though the amount of DON loaded to the bay may be significant, the fraction of DON available for microbial cycling is much smaller. The amount of N supplied by recycling may be a significant portion of the total DIN pool. All this must be considered in context with the proposed CERP modifications to flows. As of the latest initial Comprehensive Everglades Restoration Project (CERP) update, the flows to Taylor Slough and C-111/Panhandle Basis are not predicted to change very much from base conditions. Therefore we do not expect any great increases in TN loading in this region. In contrast, the proposed flow increases to the Shark River Slough are large and may have significant effects on transport of DOM to the Southwest Florida Shelf. We believe that future efforts in DON characterization and bioavailability should be concentrated in this area.

深入探究输入佛罗里达湾的营养盐生物地球化学循环，是大沼泽地（Everglades）修复工作的核心议题之一。除大气降水外，佛罗里达湾的另一主要淡水来源为海湾东北部的泰勒水道（Taylor Slough）与C-111流域（C-111 Basin）。现有研究表明，上述区域会向海湾输入大量氮素，其中相当一部分以溶解态有机氮（dissolved organic nitrogen, DON）的形式存在。然而，这类溶解态有机氮的来源、环境归趋及其对微生物的生物可利用性尚未明确。若这类溶解态有机氮易于被生物利用，则修复措施带来的输入负荷增加，可能会对湾内浮游植物藻华的动态变化产生影响。

研究发现，随着盐度升高并未出现显著的溶解态有机物（dissolved organic matter, DOM）絮凝或沉淀现象，这意味着陆地来源的溶解态有机物不会被截留于沉积物中，而是留存于水柱内，并将经受光解作用与平流输送过程。太阳光对溶解态有机物的化学特征具有显著调控作用：尽管光辐照过程中溶解态有机碳（dissolved organic carbon, DOC）的浓度未发生明显变化，但溶解态有机物的光学特性却发生了改变。由此带来的环境效应存在矛盾性：光诱导聚合作用可通过降低其生物可利用性来稳定溶解态有机物，而光解作用则可能使溶解态有机物更具生物易降解性。

整体而言，本研究区域内溶解态有机氮的生物可利用性相对较低。尽管输入海湾的溶解态有机氮总量可观，但可参与微生物循环的组分占比却极低。通过循环再生途径供给的氮素，可能占溶解态无机氮（dissolved inorganic nitrogen, DIN）总库的重要组成部分。所有这些结论，都需要结合拟推行的《综合大沼泽地修复计划》（Comprehensive Everglades Restoration Project, CERP）对水流量的调整方案进行综合考量。

截至最新的《综合大沼泽地修复计划》初始更新版本，泰勒水道与C-111/潘汉德尔区域（Panhandle Basis）的水流量预计将与基准条件相差无几。因此，我们预计该区域的总氮（total nitrogen, TN）输入量不会出现大幅增长。与之形成对比的是，拟推行的鲨鱼河水道（Shark River Slough）水流量增加方案规模较大，可能会对溶解态有机物向佛罗里达西南陆架（Southwest Florida Shelf）的输送过程产生显著影响。我们认为，未来针对溶解态有机氮的表征与生物可利用性研究，应聚焦于该区域。