Smith et al. (2020) Stormwater runoff and tidal flooding transform dissolved organic matter composition and increase bioavailability in urban coastal ecosystems

AbstractCoastal drainages contain multiple sources of dissolved organic matter (DOM) that influence OM transformation and fate along inland-to-marine gradients. Anthropogenic activities have altered DOM composition in urban drainages, thereby influencing in-stream breakdown rates, primary productivity, and downstream export. Yet, it is uncertain how hydrologic conditions (i.e., rainfall, tides, shallow groundwater inputs) interact with different sources of DOM to regulate the transformation and export of DOM through urban coastal drainages. We characterized how seasonal changes in hydrologic conditions influence DOM composition and bioavailability in tidally-influenced drainages in Miami, FL, USA. We estimated the quality and bioavailability of DOM using compositional proxies based on fluorescence spectroscopy, including parallel factor analysis, and measured dissolved organic carbon (DOC) degradation during laboratory incubations containing a local bacterial community. Interactions between stormwater runoff and tidal amplitude increased bioavailability of DOM and was positively correlated with predominantly humic-like components in the wet season and protein-like components in the dry season. Further, increases in tryptophan fluorescence intensity corresponded with elevated concentrations of <i>Escherichia coli</i> and enterococci – likely from waste-impacted groundwater – and contributed substantially to overall DOM bioavailability. Our results provide new evidence of an urban priming effect in which labile autochthonous DOM from anthropogenic sources facilitates microbial degradation of DOM that is driven by seasonal differences in stormwater runoff and tides. As hydrologic conditions in near-shore aquatic ecosystems shift with watershed urbanization and climate-driven changes in sea level rise, increases in autochthonous sources of bioavailable DOM may impact ecosystem metabolism and affect the quality of DOM exported downstream.

摘要
滨海排水系统蕴含多种溶解态有机质（dissolved organic matter, DOM）来源，此类来源会沿内陆至海洋的梯度调控有机质的转化与归趋。人类活动已改变城市排水系统中的DOM组成，进而影响河道内有机质分解速率、初级生产力以及下游输出通量。然而，目前尚不清楚水文条件（即降雨、潮汐、浅层地下水补给）与不同DOM来源如何相互作用，共同调控城市滨海排水系统中DOM的转化与输出过程。
本研究以美国佛罗里达州（FL）迈阿密受潮汐影响的排水系统为研究对象，解析了水文条件的季节变化对DOM组成与生物可利用性的影响。研究基于荧光光谱法构建组成替代指标（包括平行因子分析（parallel factor analysis）），以此评估DOM的质量与生物可利用性；同时通过添加本地细菌群落的实验室培养实验，测定了溶解有机碳（dissolved organic carbon, DOC）的降解情况。
暴雨径流与潮汐振幅的相互作用提升了DOM的生物可利用性，且该提升效应在湿季与优势类腐殖质组分呈正相关，在旱季则与优势类蛋白质组分呈正相关。此外，色氨酸荧光强度的升高与大肠杆菌（Escherichia coli）和肠球菌浓度的增加显著相关——此类微生物大概率源自受废弃物污染的地下水——且该现象对整体DOM的生物可利用性贡献显著。
本研究结果为城市激发效应提供了新的实证：源自人类活动的不稳定内源DOM，可促进由暴雨径流与潮汐的季节差异驱动的DOM微生物降解过程。随着近岸水生生态系统的水文条件随流域城市化以及气候驱动的海平面上升发生改变，生物可利用性DOM的内源来源占比提升，可能会影响生态系统代谢过程，并改变下游输出的DOM质量。