Stable Isotopic Composition of Nitrates and POM in BES Streams

In the Baltimore urban long-term ecological research (LTER) project, (Baltimore Ecosystem Study, BES) we use the watershed approach to evaluate integrated ecosystem function. The LTER research is centered on the Gwynns Falls watershed, a 17,150 ha catchment that traverses a gradient from the urban core of Baltimore, through older urban residential (1900 - 1950) and suburban (1950- 1980) zones, rapidly suburbanizing areas and a rural/suburban fringe. Stable isotopic analyses were carried out on stream samples collected bi-weekly from June 2005 through December 2005 as part of the routine Baltimore LTER sampling. Sites included POBR (forest), MCDN (agricultural), BARN (low-residential), GFGL (suburban), DRKR (urban), GFCP (urban), and RGHT (storm drain). Samples were also taken from a small tributary to the Gwynns Falls (GFGR), approximately 300 m above GFCP, that was highly contaminated with sewage. A major sewer leak to this stream was identified and repaired in April 2004. Stable isotopic analyses of soil water underneath fertilized lawns and atmospheric deposition was measured in long-term lawn study plots on the campus of the University of Maryland Baltimore County. Storm samples were also collected from 6 locations (DR1, DR3.1, DR3.2, DR4, DR5, DRKR gauge) within the Dead Run watershed over July 2005. At one site (DR3), two sets of samples were collected, one just above (DR3.1) and just below (DR3.2) an overflowing sewer. All stormflow samples were collected on the receding limb of the storm hydrograph, as the flashy nature of these urban streams makes sampling the rising limb difficult in terms of both timing and personal safety. Nitrate concentrations are a separate set of chemical analyses than the routine weekly analyses.

在巴尔的摩城市长期生态研究（Long-Term Ecological Research, LTER）项目（又称巴尔的摩生态系统研究（Baltimore Ecosystem Study, BES））中，研究团队采用流域方法评估综合生态系统功能。该LTER研究以格温斯福尔斯流域（Gwynns Falls watershed）为核心研究区域，该流域面积达17150公顷，涵盖从巴尔的摩城市核心区，经1900-1950年建成的老旧城市居住区、1950-1980年建成的郊区地带、快速郊区化区域，直至乡村/郊区边缘的完整土地利用梯度。作为巴尔的摩LTER常规采样工作的一部分，研究人员于2005年6月至12月间每两周采集一次溪流样本，并开展稳定同位素分析。本次采样覆盖的点位包括：POBR（森林）、MCDN（农业）、BARN（低居住区）、GFGL（郊区）、DRKR（城市）、GFCP（城市）以及RGHT（雨水管道）。研究团队还在格温斯福尔斯的一条小型支流GFGR（位于GFCP上游约300米处）采集了样本，该支流曾遭受严重的生活污水污染。2004年4月，针对该支流的一处主要污水管道泄漏问题已完成排查与修复工作。在马里兰大学巴尔的摩分校校区的长期草坪研究样地内，研究人员对施肥草坪下方的土壤水以及大气沉降物开展了稳定同位素分析。2005年7月，研究团队在死流流域（Dead Run watershed）内的6个点位（DR1、DR3.1、DR3.2、DR4、DR5、DRKR监测站）采集了暴雨径流样本。其中在DR3点位设置了两组采样：分别位于一处溢流污水管的上游（DR3.1）与下游（DR3.2）。鉴于城市溪流具有暴涨暴落的特性，涨水期采样在时间把控与人员安全层面均存在较大难度，因此所有暴雨径流样本均采集于洪水退去阶段（即暴雨水文过程线退水段）。硝酸盐浓度检测属于独立于常规周度化学分析的另一套检测项目。