Macroinvertebrate Impacts

One of the impacts of a dam on a river is the creation of a lake system for macroinvertebrates. Craig Layne of the Dartmouth College Biology Department notes that a dam would disrupt river continuity, the continuous flow of water. As a result, macroinvertebrates would become caught behind the dam, and this would lead to a decrease in them below the dam. Additionally, the dam would trap sediment, leading to a disruption of nutrients and fine particular organic matter; this would have an especially large negative impact on filter-feeding macroinvertebrates. Craig Layne also noted that the dam would produce thermal pollution in the river, with water behind the dam warming up. This change in temperature would be especially hard for sensitive species like mayflies, caddisflies, and stoneflies that prefer colder water; some species of crayfish though would likely benefit. The oscillation in water flow and temperature resulting from a pneumatic crest gate would produce conditions that would be hard on macroinvertebrates, which would therefore impact other animals that depend on macroinvertebrates for food, such as fish. Unfortunately, depending on the nature of the river itself, “thermal alteration and biological disruption can persist for tens of kilometers downstream” (1).

Macroinvertebrates are important indicators of water quality in streams and rivers, as certain taxa only occur in water that is clean and free of major impacts by people. Finding certain types of macroinvertebrates can reveal a lot of information about the health of that body of water, and therefore these insects are frequently used when analyzing the ecology of rivers and streams. Below is a diagram, similar to those that are used when sampling stream or river water quality, of the variety of macroinvertebrate taxa that can be found in a river. For a larger image of the diagram, please see: Macroinvertebrates.

One of the macroinvertebrates that are an indicator of a healthy river are odonates, dragonflies and damselflies, and specifically their nymphs or pre-mature young that live in the water. Some odonate species require particular habitat requirements in streams and rivers, whether that be riffles and rapids, calm water, slow-flowing water, or fast-flowing water. A dam which may alter water flow could negatively impact habitat-specific odonates and lead to their disappearance from stretches of the river. Below are some images of odonates that may be found along the Mascoma River and therefore could be impacted by the introduction of a dam.

  • Springtime Darner (Basiaeschna janata)- male
    Springtime Darner (Basiaeschna janata)- male

© Kyle Kittelberger