Stream Suspended Sediment and Particulate Organic Matter at Harvard Forest 2009-2010

In addition to conveying water and nutrients and providing habitat to a variety of ecosystems, streams transport downstream mineral sediment and other particulate matter washed in from hillslopes and eroded from its channel and banks. At high levels, suspended sediment can be a devastating pollutant for aquatic organisms. The amount of suspended material in a stream varies tremendously with discharge; typically, suspended sediment increases with discharge, as stormwater runoff and overland flow carry particles from the hillslopes into the channel. Suspended sediment can also be a function of land use and vegetation, both of which affect the infiltration capacity of the landscape; more infiltration generally means less surface runoff and thus less sediment. Forested watersheds such as the Bigelow Brook watershed will typically have less suspended sediment than similar watersheds in urban environments. By analyzing how suspended sediment varies with discharge, I will be able to compare the relative effectiveness of overland flow of stormwater in washing materials into the streams. It is also possible that tree loss due to the wooly adelgid, ice storms, or fire in the watershed may increase the amount of sediment to Bigelow Brook, as a loss in tree canopy may result in more soil erosion due to rain splash and more water overall reaching the stream. For this reason, I hope to continue monitoring sediment in Bigelow Brook for an extended period of time to record any significant changes due to changing vegetation. Furthermore, by determining how much of the suspended sediment consists of particulate organic matter (using standard LOI techniques), I will be able to estimate the net carbon export from the two watersheds via that pathway. Preliminary, back-of-the-envelope calculations suggest that as much as 3-5% of the total annual carbon export leaves the Harvard Forest watershed via stream-transported particulate organic matter. To this end, I propose to measure suspended sediment and particulate organic matter in Bigelow Brook over the next several years, and compare these data with those collected in an urban setting; Beaver Brook in Worcester. This work will be conducted at Harvard Forest by me (Sara Mitchell) and undergraduate researchers from the College of the Holy Cross, in Worcester, MA.
Harvard Forest is an ideal site for this analysis. First, the area is protected from development, contains few roads, no storm drains, and very little infrastructure, and thus represents as close to a "natural" and undisturbed setting as can be found in New England. While the area certainly has been disturbed in the past, the land use history is well-documented and mapped. Second, the site has a great combination of being accessible by car yet protected from the general public. Third and most importantly, the site already has an outstanding hydrological data collection system and existing data set, with precipitation and discharge measured every 15 minutes for the past several years. With any project that involves comparing a variable to discharge, one of the most difficult data sets to collect is the discharge data.

除了输送水分与养分、为各类生态系统提供栖息环境之外，溪流还会向下游搬运从山坡冲刷而来、以及从河道与岸坡侵蚀产生的矿物沉积物与其他颗粒物质。当浓度过高时，悬浮沉积物（suspended sediment）会对水生生物造成毁灭性污染。溪流中的悬浮物质含量随流量变化幅度极大；通常而言，悬浮沉积物浓度会随流量增大而升高，因为暴雨径流与坡面流（overland flow）会将颗粒物质从山坡带入河道。悬浮沉积物同时也受土地利用与植被状况影响，二者均会影响景观的入渗能力（infiltration capacity）；入渗量越高，通常地表径流就越少，相应携带的沉积物也越少。像比格洛溪（Bigelow Brook）流域（watershed）这类森林流域，其悬浮沉积物浓度通常低于城市环境中的同类流域。通过分析悬浮沉积物随流量的变化规律，我可以对比暴雨径流坡面流将物质冲入溪流的相对效率。此外，流域内因铁杉球蚜（wooly adelgid）、冰暴或火灾导致的树木损失，可能会增加比格洛溪的沉积物输入量：林冠损失会加剧雨水溅蚀造成的土壤侵蚀，同时也会有更多水分最终汇入溪流。有鉴于此，我计划长期持续监测比格洛溪的沉积物含量，以记录植被变化引发的任何显著变化。进一步而言，通过标准烧失量（loss on ignition, LOI）技术测定悬浮沉积物中颗粒有机质（particulate organic matter）的占比，我可以估算通过该途径从两个流域输出的净碳量。初步的粗略估算显示，哈佛森林（Harvard Forest）流域每年总碳输出中，多达3%-5%是以溪流输送的颗粒有机质形式流失的。为此，我提议在未来数年内监测比格洛溪的悬浮沉积物与颗粒有机质含量，并将这些数据与在城市环境中（马萨诸塞州伍斯特（Worcester）市的比弗溪（Beaver Brook））收集的数据进行对比。本研究将由我（萨拉·米切尔，Sara Mitchell）以及来自马萨诸塞州伍斯特圣十字学院（College of the Holy Cross）的本科生研究人员在哈佛森林开展。
哈佛森林是开展本分析的理想站点。首先，该区域受开发保护，道路稀少，无雨水管网，基础设施极少，因此代表了新英格兰地区所能找到的最接近"自然"且未受干扰的环境。尽管该区域在历史上确实遭受过干扰，但其土地利用历史已有完整记录与测绘。其次，该站点既便于驾车抵达，又能免受普通公众的干扰。第三也是最重要的一点，该站点已拥有一套完善的水文数据采集系统与现有数据集，过去数年来每15分钟即可获取一次降水与流量数据。对于任何需要将某一变量与流量进行对比的研究而言，最难采集的数据集之一便是流量数据。