Developing a Diagnostic Tool for Assessing Excessive Sediment Harm to Stream Communities

Abstract

Excess sediment is a top cause of impairment in U.S. rivers and streams. A number of streams on the north shore of Lake Superior’s western arm are on the Minnesota Pollution Control Agency’s impaired waters list due to turbidity problems. The underlying geology of the north shore, in addition to the steep slopes of the Lake Superior escarpment, forms a stream base vulnerable to erosion and excessive sediment deposition in streams. This vulnerability is created, at least in part, by an area of clay loam soil that many north shore stream channels intersect as they come down the escarpment to the shore of Lake Superior. The steep slopes cause high stream velocities which, combined with the high erodability of this soil layer, create high erosion potentials, particularly on outside channel bends. The increased fine sediments traveling through and accumulating in stream substrates potentially presents several problems for aquatic biota. Excess sediment deposits reduce habitat space for aquatic macroinvertebrates, which are vital components of the food web. In addition to potentially decreasing food sources for fish, the excess sediment deposits can bury fish spawning habitats. Even if the fish can clean off nesting areas, they will expend extra energy doing so. There are many stream condition indicators using stream fish or macroinvertebrates, but none address excess sediment specifically. In many areas of the country there are any number of human‐caused stressors affecting stream condition, including agricultural runoff, high stormwater discharges, loss of stream shoreline habitat, deforestation, development, and industrial discharges. When there are many stressors impacting streams, it is hard to differentiate among them to determine which stressors are creating which problems for stream biota. While some north shore streams have non‐turbidity impairments, there are considerably fewer than in other parts of the country. The dominance of erosion‐based impairments provided the opportunity to develop an indicator diagnostic of excessive sediment deposition in stream substrate as the cause of biotic impairment in north shore streams. We selected stream macroinvertebrates for indicator development for several reasons. They are less mobile than fish, meaning that they have limited ability to escape from disturbance, and even more limited ability to return after a disturbance ceases (at least until the next generation begins). Macroinvertebrates are easy to collect, are present in relatively high abundances, and have high morphological diversity. For all of these reasons, macroinvertebrates are commonly used in stream condition assessments, and their use is ubiquitous across the US and across agencies. Because most agencies collect stream macroinvertebrate information already, their use to create a diagnostic indicator could allow agencies and managers to get more information out of data they already have, without the need for additional sampling. The goal of this project was to develop a suite of stream macroinvertebrate metrics diagnostic of invertebrate community impairment caused by excessive fine sediment deposition in stream substrate; in other words, burial or partial burial of streambed rocks by sand, silt, and clay. Such a diagnostic tool would aid managers in their stream assessment work. While similar projects have been previously attempted (and failed) in other parts of the country, most have been in areas suffering from a number of stressors, making development of an indicator diagnostic of just sediment impairment more difficult. Our hope in attempting such work using north shore streams was that the relative lack of other stressors in northeastern Minnesota would make the development of such an indicator more possible. Having such an indicator should help agencies make a stronger connection between the Total Maximum Daily Load (TMDL) turbidity measurements and sediment deposition presumed to be causing harm to stream biota.