Silt loads: pinpointing their ecological impact

Posted on July 1, 2013

Because high loads of fine sediment in streams can affect freshwater organisms by smothering underwater surfaces, abrading and scouring plants, clogging gills and reducing light levels, it’s important to understand how fine sediment influences stream biodiversity.  Macroinvertebrates are often used as indicators of stream conditions because they occupy an intermediate position in aquatic food chains and are easily sampled.  However, such condition indices usually reflect the combined impacts of all types of human disturbance, rather than the effects of particular stressors such as fine sediment.  Researchers in Idaho and Oregon defined fine sediment as deposited clay, silt and sand particles less than 2 mm in diameter, and related this fraction (as a percentage of all sediment particles) to the occurrence of 206 macroinvertebrate taxa in 1139 streams across 16 ecoregions in the northwestern USA.   Sensitive species were defined as those found only in streams with fine sediment concentrations below 50%.   All macroinvertebrate species tolerated concentrations of up to 10%, but beyond that level there was a steady loss of diversity.  A biotic index was developed, the index scores being based on the occurrence of macroinvertebrate taxa in streams categorized by their sediment concentration.   As a result of this work, the impairment of a particular stream can now be assessed by comparing its index score with the distribution of scores for the ecoregion.  Alternatively, the amount of fine sediment in a stream can be predicted on the basis of its macroinvertebrates.  The main advantages of the new index are its ease of use and the fact that it relies only on the presence of taxa, rather than the identification of every species and the counting of sampled individuals.  It also allows lists of taxa from previous studies to be combined with more recent data to assess changes in stream condition over time.  

Reference:  Relyea, C.D., Minshall, G.W. & Danehy, R.J.  2102.  Development and validation of an aquatic fine sediment biotic index.  Environmental Management, 49, 242–252.