Cranes roosting in shallow channels during migration, eagles diving for food and returning to nests on the river’s bank, turtles sunning themselves on logs protruding from the water, and frogs calling from adjacent wetlands. We’re familiar with the more apparent wonders of wildlife, but what about the life that exists under the water- in the sediment, leaves, and substrate? Dozens of small, often overlooked, aquatic insects call the river home and keep our fresh water systems functioning.
In fact, I shouldn’t be calling them insects but rather macroinvertebrates. The group macro (big enough to be seen without a microscope) invertebrates (no backbone) includes more than just insects. It also comprises crustaceans (crayfish, scuds), annelids (leeches, aquatic worms) and mollusks (clams, mussels, snails). Despite their small size, their role in fresh water systems like the Platte River Basin is big.
As part of my research with PBT, we have water-quality sensors installed in the Central Platte River. These instruments are prime real estate for small organisms; stable, submersed in the water column, and not covered in sand– a rarity in the ever-changing Platte River. I used to get squeamish removing all of the creepy crawlies from the sensors to download data, but after discovering how fascinating and integral each species is, I’m now only a little squeamish.
Macroinvertebrates are important as a food source and helping to break down organic matter and nutrients, but what I find most fascinating is their role as bioindicators. These tiny organisms can tell us how healthy a river is just by their presence. They are easy to collect, relatively easy to identify, and have life cycles in the water that can last over a year, exposing them to many conditions. Certain species are highly sensitive to pollutants and changes in physical conditions while others are more tolerant.
For example, a healthy stream will be abundant in aquatic insects such as the stonefly nymph, mayfly nymph, and caddisfly which are sensitive to changes in dissolved oxygen and pollutants. (Field note: When a stonefly nymph needs more oxygen they do pushups to move water with fresh oxygen across their body).
In contrast, a section of the river that has higher levels of pollutants may have leeches or midge fly larvae as the dominant speces as they are moderately tolerant of pollution and low levels of dissolved oxygen.
Macroinvertebrates are not only essential to the functioning of freshwater systems, but play a part in terrestrial systems as well. Larvae that once inhabited the sensors underwater emerge as winged insects, becoming food for terrestrial wildlife such as bats, birds, and spiders. (Field Note: The closer you are to a river, the more food/aquatic insects are available and the more spiders you will encounter.)
Shredders, collectors, grazers, & predators: they sound like character names but they are really functional feeding groups for macroinvertebrates, defined by the way they obtain food.
- Shredders feed on large, coarse pieces of plants or wood that has fallen into the stream. This group, which includes stonefly, mayfly, and some caddisfly larvae, shred large pieces into finer particles that travel further downstream, helping out the organisms who prefer smaller bite sizes.
- Collectors such as midges and blackfy larvae, eat the fine particles of decomposing organic matter in two ways- filtering and gathering. Filterers use special adaptations such as stiff hairs or nets to collect food that is suspended in the water column. Gatherers collect food that has accumulated on surfaces.
- Grazers are the underwater cows of the river: organisms like water pennies, certain mayflies, beetles and case-building caddisflies that scrape algae from rocks or hard surfaces.
- Predators like leeches, dobsonfly larva(hellgrammites), and dragonfly nymphs, capture and feed on prey by piercing or engulfing them.
Each species have adaptations and features that are unique to helping them survive in a river environment. Some have flattened bodies to allow them to live in flowing water, some have mouthparts for scraping algae off of rocks, and others have fan-like structures on their heads to collect food that is drifting down the river. Mayfly larvae molt multiple times in their aquatic life, but after the last molt it uses its shed skin as a boat to float on top of the water while it dries its wings in the sun. Some species of caddisfly larvae (named after 15th century word ‘caddis,’ meaning ribbon) build homes, or cases, out of substrate held together with a silk they make to keep them from drifting downstream.
So next time you’re at the river, flip over a rock, pick up some leaves, or siphon through the sand and you might find an entire community of macroinvertebrates. And maybe you’ll get that creepy crawly feeling, or maybe, just maybe, you’ll find them cute (or at least ecologically important in a cute kind of way).