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.

溪流除输送水分与养分、为多样生态系统提供栖息场所外，还会向下游输送从山坡冲刷而来、以及从河道与岸坡侵蚀形成的矿物沉积物与其他颗粒物。当悬浮沉积物浓度过高时，会对水生生物造成毁灭性污染。溪流中的悬浮物质浓度随流量变化幅度极大；通常而言，悬浮沉积物浓度随流量升高而上升，因为暴雨径流与坡面流会将山坡上的颗粒物带入河道。悬浮沉积物浓度同时受土地利用与植被状况调控，二者均会影响景观的入渗能力；入渗量越高，地表径流通常越少，由此携带的沉积物也相应更少。诸如比奇洛溪（Bigelow Brook）流域这类森林流域，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浓度随流量的变化规律，本研究可对比暴雨坡面流将颗粒物冲入溪流的相对效率。流域内因球毡蚜（wooly adelgid）、冰暴或火灾导致的林木损失，也可能使比奇洛溪的沉积物含量上升：林冠丧失会加剧雨滴溅蚀造成的土壤侵蚀，同时会有更多径流直接汇入溪流。基于此，本研究计划长期持续监测比奇洛溪的沉积物含量，以记录植被变化引发的各类显著变化。
此外，通过测定悬浮沉积物中颗粒有机质的占比（采用标准烧失量（LOI, Loss on Ignition）技术），本研究可估算两类流域通过该途径输出的净碳量。初步粗略估算显示，哈佛大学森林（Harvard Forest）流域全年总碳输出中，多达3%至5%是以溪流输送的颗粒有机质形式流失的。为此，本研究计划在未来数年内监测比奇洛溪的悬浮沉积物与颗粒有机质含量，并将所得数据与城市环境下（马萨诸塞州伍斯特市的比弗溪（Beaver Brook））采集的同类数据进行对比。
本研究将由本人萨拉·米切尔（Sara Mitchell）以及来自马萨诸塞州伍斯特市圣十字学院（College of the Holy Cross）的本科生研究团队在哈佛大学森林开展。哈佛大学森林是开展本分析的理想场地：其一，该区域受开发保护，道路稀少，无雨水管网，基础设施极少，近乎代表了新英格兰地区可觅得的最接近“自然”且未受干扰的状态；尽管该区域在历史上曾受干扰，但其土地利用史已有详实的记录与测绘。其二，该场地兼具车辆可达性与对公众的保护性。其三也是最为关键的一点，该场地已配备完善的水文数据采集系统与现存数据集，过去数年间每15分钟即可获取一次降水与流量数据。对于任何涉及变量与流量对比的研究而言，流量数据都是最难采集的数据集之一。