Winter Macroinvertebrates: What do bugs do in the cold?
Like every season, winter poses a unique set of challenges and obstacles when it comes to monitoring a stream’s health. Some of our key biological indicators of water chemistry are benthic macroinvertebrates, or macros for short. A Macroinvertebrate by definition is an organism that is large enough to see with the naked eye (macro), but lacks a spine (invertebrate). Benthic is a term meaning “bottom-dwelling”. Benthic macros specifically are those deemed as “bottom-dwellers” who reside in or originate from the bottom of a body of water. Macros are indeed what we commonly think of as bugs, bugs that buzz our ears and crash our picnics, bugs that live in or near water getting slurped up by a fish or frog, and everything in between. All of these critters serve a purpose, whether we think of them as pests or as a sign of stream health.
Now, we may not fancy a streamside picnic in the winter, or ever think to notice, but we don’t imagine bugs crashing our party during winter in a four-season temperate climate like West Virginia. Like any species of living organism there are some specialists that fill a specific niche in the macroinvertebrate community as well, meaning they thrive in specific conditions that others may not. Some of these macros are indeed specialized to thrive in winter such as the Blue Wing Olive mayfly (Baetis tricaudatus) that hatches in the low 40 degree range whereas many other benthic macros will not hatch in low winter temperatures. We can visualize this by comparing benthic macros to terrestrial macros. A terrestrial macroinvertebrate is a land-dwelling macroinvertebrate such as ants, bees, spiders, flies, etc. During winter we don’t see as many of these species whereas some terrestrial macros like to sneak into our homes in search of warmth and food. Though we typically think we won’t find bugs outside in winter, there are many species that flourish during this time. This can make sampling a stream for benthic macros in winter helpful, just like the rest of the year. Perhaps winter sampling isn’t as fruitful as the peak spring and summer seasons. The normal sampling period runs from March to September depending on location, as this is the abundance period that will give us the most representative sample from a given waterway. Species profiles consist of a fixed-count sample, so we generally want to see the highest volume of macros possible. Sampling in winter won’t give us this full profile, but could it tell us more of a stream’s story by looking for macros in winter?
The macro size of these organisms allows us to visually survey an environment for the presence of these insects easily. Most of these live part or most of their life cycle attached to submerged rocks, logs, and vegetation. A common stream macroinvertebrate survey method is the practice of “kick netting”, which uses a screening-style net and disturbance of a streambed to capture insects that were removed from their habitat by agitation. Stream water generally carries these insects downstream into the kick net where the captured specimens can be lifted from the water and transferred to a tray for inspection & release, or bottled for later viewing. Sometimes it is beneficial to quickly survey live organisms and release them back to the stream, whereas other times specimens are ethically euthanized in ethanol or isopropanol to preserve them for long term studies. A kick net or similar device can be utilized to perform two types of studies, quantitative collections or qualitative collections. A quantitative study intends to capture specimens from a specific area to form a defined insect per unit area value (or quantity). This usually entails a kick net or device that has an opening with known dimensions where you can calculate a ratio of insects captured per the given area. This helps us determine changes or differences in the abundance of insects present. A qualitative study doesn’t rely on area & abundance, rather the diversity & taxonomical richness of specimens captured (or quality). This may pertain to types of, sizes of, or the age class / life cycle of insects present. The qualitative survey is generally easier to control the introduction of potential surveying errors. To learn more about stream surveying, or about the macroinvertebrates themselves, check out the following link to West Virginia Division of Environmental Protection’s resources from “WV Save our Streams” below:
Benthic macroinvertebrates exhibit some unique life cycle characteristics. Some aquatic macroinvertebrates spend their entire lives living in water like snails, clams, and crustaceans, although many just live in the water when they are immature. As they reach maturity, larvae metamorphose and leave the water, spending their adult life on land. In many cases, the insects are adults for a very short time. For example, many mayflies live in streams for months to years but last on land for just a few days. During this time, they mate and lay their eggs in or near water so the cycle can continue. The larval and adult forms do not look alike. For those familiar with fly fishing, fishermen try to “match the hatch” by identifying macroinvertebrate species present along a waterway and use a fly that replicates the insects sighted in the adult stages, larval stages, and the “emerger stage” between larvae and adult. Sometimes the most explosive fishing comes from matching the adult stage of a fly landing on the water during the process of laying eggs. Fish instinctively know this is a limited short-term resource, so they often key to feeding on these specific bugs. If you know fly fishing, most of these “hatches” occur during the spring and summer in West Virginia when the temperatures are increasing. Why doesn’t this occur as often in winter?
Insects, as we know, are cold-blooded organisms and they share this trait with many other species, reptiles being one example. This could explain why fishermen don’t experience many hatching macros over the winter months. We typically do not see snakes or turtles in the wild over winter in West Virginia, so why should we still be sampling for insects or using flies to catch fish? Most aquatic macroinvertebrates live in streams throughout the winter. Water is an incredible insulator and resists changes in temperature. Although streams can freeze over, the ice is typically only a few inches thick. This layer of ice also provides extra insulation. Moving water in streams oftentimes resists freezing more than stagnant pools or ponds. Many of these macroinvertebrates also will gain warmth from the ground temperature below rocks and sediment in the stream bed. In some smaller slow-moving streams, “anchor ice” (ice that extends from the water surface clear to the stream bottom) can form, trapping fish, macroinvertebrates, and other aquatic animals. It is extremely rare for anchor ice to affect an entire stream, so many macros will re-inhabit areas affected by anchor ice. One difference in macroinvertebrate populations from season to season is the life-cycle stage that the organisms exhibit at that point in time. In winter, we usually don’t see the adult stages, rather the larval forms of species gathering nutrients and preparing for the upcoming spring or summer.
From a stream health perspective, it can be helpful to sample benthic macros during winter to see how the macroinvertebrate populations are changing seasonally. Water conditions also change rapidly through the seasons and the presence or lack of macros can give us a hint as to how environmental factors are influencing our stream health. Benthic macros are a key stream health indicator because of their sensitivity to physical or chemical factors that may affect a stream. By monitoring the benthic macro populations, we can quickly note changes that could alert us to a problem. Oftentimes, in periods of low water in winter, chemicals or toxins will move more slowly through the stream system & exist in higher concentrations. This may cause chemical or physical characteristics to be worse during the winter months. For example, one of the Save The Tygart Watershed Association (STTWA)’s project streams, Beaver Creek of the Tygart Valley River, exhibits lower pH readings during winter months with the same consistent chemical treatment. There are other variables that can affect this pH drop, however; it is an example of how stream conditions can change seasonally. It is also important to sample the water’s individual chemical and physical parameters during winter.
If you would like to learn more about the life cycle and classification of benthic macroinvertebrates, check out this resource from Macroinvertebrates.org:
If you would like to get involved or learn more about stream sampling techniques or how this article ties in to the Save The Tygart Watershed Association’s mission, please contact us for more information.