Microhabitat-scale influences of environmental features on the benthic macroinvertebrate community in a northern Minnesota stream

Background/Question/Methods

There are many environmental factors within streams that affect the macroinvertebrate diversity, which include, but are not limited to, substrate type, coarse particulate organic matter (CPOM), and wood. As water moves over the heterogeneous surface of the substrate it creates microhabitats of fast and slow currents, creating greater niche space for benthic invertebrates.  In addition, CPOM supplies a large amount of nutrients and food source for the macroinvertebrates in streams. Wood supplies habitat for invertebrates, increases the retention and deposition of CPOM, and increases hydraulic complexity as well. To examine the relative importance of these environmental factors to benthic macroinvertebrates, fifty mini-Surber samples were taken in late May from a 40-m reach of a northern Minnesota stream.  The organisms were stored in a seventy percent solution of ethanol and were identified in lab at a later date, usually to the genus level. Submerged and emerged wood were abundant in the study reach, and were measured at each microhabitat location. The substrate within each microhabitat was also characterized by percentages of five categories: boulder, cobble, gravel, fines, and wood. 

Results/Conclusions

The benthic invertebrate communities sampled consisted mainly of the families Chironimidae, Sphaeriidae, Planorbidae, and Culicidae, the class Oligochaeta, and the genera Chimarra, Baetis, and Hydropsyche. Nonmetric multidimensional scaling showed that the abundance of CPOM, fine particulate organic matter, and submerged wood significantly influenced the makeup of these communities. For example, the caddisfly taxa Lype and Cheumatopsyche were found in greater abundance with more submerged wood, possibly due to their behavior, as Lype may tend to settle in wood grooves, and Cheumatopsyche may utilize wood to stabilize their capture nets. However, the surface area of submerged wood had an inverse relationship with overall benthic abundance and species richness. A positive relationship was observed between overall benthic abundance and both CPOM and fine particulate organic matter, suggesting that organic matter contributes to habitat heterogeneity. An example was larvae of the dragonfly genus Stylurus which was more abundant in microhabitat locations with greater amounts of particulate organic matter, into which they can burrow to escape predators. Our results can help inform management decisions that may affect instream wood and organic materials at the microhabitat scale.