What is a benthic macroinvertebrate?
Macroinvertebrates are animals with no backbones that are of ”macro” size. That implies they are visible to the naked eye and thus not require a microscope to be seen. A rough reference measure is normally that they should be retained by mesh sizes equal to 250 micrometers (0.25 mm). Although macroinvertebrates do exist on land, they are generally regarded as being aquatic – implying that at least one of their life-stages is entirely spent in a body of water. Since most pelagic and free-swimming small animals would get swept away by the current in running waters, the majority of them are constrained to be bottom dwelling. To emphasize this, the prefix ”benthic” is often added to the term.
Why live at the stream bottom?
Apart from being an excellent structure to cling on to when current drags, what else could make the stream bottom a good place to spend your time? There are two main additional reasons for this. First of all, bottom structures of various shapes and types also provide a refugia from larger predators (such as fish), and secondly bottom substrates are often good places to acquire food. Stony and woody structures in water are often not only coated with yummy and energy rich biofilm, but also, in between them small pockets form where carbon rich organic matter get accumulated. Organic matter of different sizes and level of degradation gets utilized as food by detritivorous macroinvertebrates.
Who are they then?
A quick answer would be that the majority of them are immature stages of various insects, often land living as adults. In fact, apart from a few non-arthropods (molluscs, leaches, roundworms, ringworms, gastropods) the arbitrary ”benthic macroinvertebrate” grouping mainly includes various arthropods, of which 95% are aquatic insects and the rest being crustaceans and chelicerates (aquatic spiders and mites) (figure 1.). Since it is not possible to group them by monophyletic entities, they are often divided into guilds depending on feeding traits. Some might scrape algae from substrates, some are predators, whereas others filter bypassing particles or chew parts of broken down organic material.
What characterises them?
General characteristics for many stream dwelling macroinvertebrates, especially insect larvae, are specialized features to acquire food in fast currents, such as filtering apparatuses aimed for gathering drifting particles. Some larvae even build trap nets that do the collecting work for them (figure 2.). Also, streamlined body shapes and hook-like appendages to ease adherence to slippery stones are common morphological features. Those latter characteristics help them withstanding the dragging forces (i.e. shear stress) caused by rapid currents, but also to actively control downstream transportation within the watercourse. Many benthic macroinvertebrates use the stream current to transport themselves downstream when looking for new habitats to colonize. This, and other types of dispersal forms, is described in deeper depth here.
What are macroinvertebrates good for?
Apart from being key components in the aquatic food web, the abundance and diversity of macroinvertebrates have traditionally been used as indicators for ecosystem health and local biodiversity. Many species are sensitive to various stressors (e.g. pollutants, low oxygen, homogenous flow), and if not present theory predicts you most likely face a pertubated system.
Invertebrate: Organism without a spinal column (backbone). For example, all insects are invertebrates.
Arthropod: Invertebrates with exoskeleton (external skeleton), segmented bodies, and jointed appendages. The vast majority of all macroinvertebrates are arthropods.
Aquatic macroinvertebrate: A macroinvertebrate having at least one of it’s life stages inside water.
Benthic: Stems from the term ”benthic zone” which is the ecological region at the lowest level of a water body, that also includes the sediment surface and some sub-surface layers.
Bottom substrate / substratum: The material that rests at the bottom of a stream (e.g. inorganic such as mud, sand, gravel, pebbles, cobbles, boulders, bedrock, or organic such as wood or accumulation of organic matter).
Pelagic: The zone in a water body that is neither close to the bottom nor near the shore.
Shear stress: The force per unit area that acts on the bed of the channel and thus affects benthic macroinvertebrate habitat.
Periphyton: A green slimy and complex mixture of algae, cyanobacteria, heterotrophic microbes and detritus that is attached to submerged surfaces in most aquatic ecosystems. Periphyton serves as an important food source for invertebrates, tadpoles, and some fish. It also has the ability to absorb contaminants, and limit their movement through the environment. Periphyton biofilm can therefore be an important testifier for aquatic ecosystem health.