By Watershed Conservation Research and Data Specialist Anna Willig

Aquatic macroinvertebrates — animals without a backbone that are visible to the naked eye — are some of the most fascinating creatures found in streams. We tend to lump all macroinvertebrates into one category, but there is tremendous diversity found underwater. 

Some macroinvertebrates live for a few weeks, others for a few years. Some stay burrowed in the streambed their entire lives, others emerge as flying adults and migrate across the country. Some scrape algae off of rocks, others are voracious hunters. To illustrate this diversity, we are highlighting three sample orders from the class Insecta (the insects) that have vastly different life histories. While there are generalities that are shared between all species of each order, this is akin to generalizing the life history of all passerine birds (order Passeriformes, which includes most songbirds). 

Odonata Spotlight | The Dragonflies

First up is the order Odonata, which includes dragonflies and damselflies. Dragonflies and damselflies are similar, but we focus on dragonflies, which are unique among aquatic macroinvertebrates for their large size, long lifespan, and hunting prowess. Dragonflies start their lives as eggs laid near the surface of the water. Eggs hatch within a few weeks, but some dragonflies can delay hatching for a year if they sense stream conditions are suboptimal. Once they hatch, they typically spend less than a year underwater in the larval (juvenile) stage, though some species spend up to six years developing in streams. Larval dragonflies are fearsome hunters that eat anything they can catch, including other insects, small fish, and tadpoles. 

In their final days as larvae, dragonflies stop eating! During their metamorphosis from larvae to adult, their mouthparts change and, for a few days, are not functional. They crawl onto land at the end of their metamorphosis and climb out of their larval skin, emerging as adults. Once their wings dry and harden, adults take to the air, where they are as ferocious as they are underwater. They have remarkable vision and can predict where their prey will go, a feat not seen in other insects. Males establish territories over water, keeping other males out and luring females in. 

Adult dragonflies live for several months to a year, and a few species, including the Common Green Darner (Anax junius), migrate across the country. Much remains unknown about the migration of Common Green Darners, but, like Monarch Butterflies, a migratory cycle likely lasts a few generations. The first generation emerges in the late winter in southern states and flies north to mate and lay eggs. The second generation hatches in late spring and migrates in late summer, making their way south to a place they have never been. They mate and lay eggs in the south, and the final generation emerges in the fall. This generation overwinters in the south, mating and laying eggs for the next migratory generation.

Ephemeroptera Spotlight | The Mayflies

Our next order is Ephemeroptera, the mayflies. Like dragonflies, mayflies have an aquatic larval stage and a terrestrial adult stage, but they share few other similarities. Mayflies are the only insect with an extra phase of metamorphosis called the subimago life stage. They are short-lived as adults and, in ideal conditions, can form massive swarms. Mayfly eggs often hatch within a few weeks, though some species hatch immediately and others delay hatching for a year in suboptimal stream conditions. Mayflies spend up to two years as aquatic larvae, shedding their skin dozens of times before adulthood. While most larval mayflies eat algae, leaves, and other plant matter, a few are predators (though not nearly as efficient as dragonflies).

At the end of their larval stage, mayflies crawl out the stream and shed their larval skin. However, they are not yet adults. This post-emergent, pre-adult form is a life stage unique to mayflies called a subimago. Subimagos have a hairy covering over their wings and must molt one more time to remove this covering, after which point they are adults. Adults only live for a few hours to days. During this ephemeral time, they have one task: reproduce. They lose the ability to eat during their metamorphosis into adults, forcing them to focus all their energy on finding a mate instead of finding food. Males form massive swarms, sometimes large enough to be detectable by weather radar, twirling and dancing to attract females. Females fly into the swarm, find a mate, and speed off to the nearest body of water, where their final act is laying eggs. 

Coleoptera Spotlight | The Water Beetles

Another unique order is Coleoptera, the beetles, which is the largest order in the animal kingdom, containing an estimated 25% of all animal species. Unlike Odonata and Ephemeroptera, which include exclusively insects with an aquatic life stage, only a small fraction of Coleopterans have an aquatic life stage. Common water beetles include water pennies, diving beetles, and riffle beetles. Water beetles are aquatic for their larval and adult stages, only emerging for a few weeks to pupate. They hatch from eggs laid in the water, typically within two weeks, as most cannot delay hatching. Larval water beetles have diverse diets; some scrape algae off of rocks, some shred leaves, and some are predatory. They spend anywhere from a few months to a year in their larval stage and crawl onto the banks to metamorphose.

On the banks, larvae burrow into soft sand and mud to metamorphose, emerging as adults several weeks later. They fly briefly until they find a suitable place to enter the water. Once they re-enter the water, many water beetles never return to the surface. Some even lose their wings. Adults do not have gills and carry an air bubble to breathe, maintaining an equilibrium with the oxygen in the water that constantly replenishes their air. Some adults are short-lived and may not have mouthparts, while others can live for up to three years and have a variety of feeding strategies. Adults mate and lay their eggs underwater, setting the stage for the next generation.

Each of these orders contains multiple families, each family contains multiple genera, and each genus contains multiple species, each with a unique life history. Each stream contains its own community of macroinvertebrates, an unknown diversity hiding just beneath the waters.  And these insects are not exclusive to local streams —  there are thousands of species of dragonflies, mayflies, and water beetles found on all continents except Antarctica. Since 2018, the Watershed Team has collected annual macroinvertebrate samples from the headwaters of Ridley, Crum, and Darby Creeks. We are busy sorting and identifying our samples and are looking forward to sharing what these unique creatures can tell us about local stream health.

References and Resources

Voshell, J. R. (2002). A guide to common freshwater invertebrates of North America. McDonald & Woodward Pub.

Interested in learning more about macroinvertebrates? Check out macroinvertebrates.org for information about and pictures of many of our local macroinvertebrates.  

By Watershed Conservation Research and Data Specialist Anna Willig

One of the most basic, yet most critical, aspects of stream health is temperature. Temperature sits behind the scenes, governing nearly everything that happens in a stream. Make a stream too warm or too cold, and the wildlife will disappear. 

While measuring the temperature of a stream seems straightforward — just stick a thermometer in the water — the reality is much more complicated. In small streams, such as the headwaters of Ridley, Crum, and Darby Creeks, temperature is incredibly variable. At the same spot in a stream, temperature can change by several degrees depending on the time of day, current weather, and time of year. Each day, water temperature rises and falls with the sun. Weather matters — a hot, sunny day will warm a stream more than a cool, cloudy day. Temperature also changes with the seasons, rising throughout the spring and summer and dropping in the fall and winter. 

Location in a stream is important as well. In shady areas, streams will be cooler than in sunny areas. Slow-moving stretches tend to be warmer than fast-moving areas. Muddy water gets warmer than clear water. Shallow areas can warm more than deep sections. The variation of water temperature over time and across space in streams makes it crucial to measure temperature many times at many places in a stream in order to understand temperature trends.

Changes in land use further alter water temperature. When forests are leveled and replaced with strip malls and subdivisions, streams heat up. In undeveloped areas, rainfall soaks into the ground and cools as it flows underground towards streams. In developed areas, rainfall instead runs off of (often hot) parking lots, roads, houses, and sidewalks right into streams. This water cannot cool off and can make streams hotter. Removing streamside trees, building dams, and creating ponds all expose water to more sunlight, ultimately heating the stream.

Add in climate change, which will warm our region drastically in the next few decades, and it is clear that our streams are heating up. But why is this a problem? What does temperature have to do with the animals that live in our streams?

The danger of warm water temperature starts at the molecular level. Temperature governs the rate of chemical reactions and, in water, the amount of oxygen available. As a stream warms, chemical reactions occur faster and the water holds less oxygen. Most animals that live in streams — insects, mussels, fish, frogs, turtles — are cold-blooded, meaning their body temperatures change with the stream temperature. If a stream warms up, so will body temperature, speeding up respiration, metabolic reactions, and all other internal chemical processes that keep the animal alive. However, since warm water holds less oxygen, there will be less oxygen to support these processes, causing stress. 

This thermal stress affects how fast organisms grow, how big they get, and how well they reproduce. Researchers from Stroud Water Research Center tested the impacts of water temperature on aquatic insects, which form the base of the food chain in streams. While there was a threshold above which all insects died, there were concerning effects even before temperature reached lethal levels. As temperatures warmed, insects developed and matured faster, but they did not become as large and consequently struggled to reproduce. If streams become too hot for aquatic insects to survive and reproduce, they will disappear, with catastrophic consequences for the entire ecosystem. 

Fish are also sensitive to changes in stream temperature. Brook trout, Pennsylvania’s only native trout, currently cannot reproduce in local streams because temperatures are too warm. Even though trout are stocked by anglers every spring in local streams, the thermal stress forces these fish to devote so much energy to survival that they have no energy left for reproduction. Even less sensitive types of fish, like bass and sunfish, will show signs of stress when temperatures get too high.

If we want to keep our streams healthy, if we want to protect all the animals that depend upon them, we need to keep them cool. Limiting deforestation, especially along stream banks, will protect shady areas that cool streams. Establishing and expanding forested streamside buffers will create new shade and cool streams. Limiting new development will prevent the expansion of hot pavement. Shifting from stormwater management strategies that hold water, such as retention basins, to strategies that encourage infiltration, such as rain gardens, will force more water into the soil, where it can cool before reaching a stream.

You can make a difference at home by replacing lawns with gardens or meadows full of native plants, which will allow more water to enter the soil. Adding rain barrels will reduce the amount of hot runoff flowing off rooftops into streams. Planting trees, even if you are far from a stream, will also cool water down. Climate change means that rising temperatures will remain a constant threat, but making stream-conscious decisions can help ensure our streams remain healthy and full of life. 

References and Resources

Check out this lecture on temperature from Dr. John Jackson of Stroud Water Research Center (lecture starts at 18:20): https://youtu.be/8Y1ey45053Q?t=1100.

Check out this research article about the impacts of temperature on mayflies from Stroud Water Research Center.

To see a streamside buffer restoration in progress, visit the Watershed Team’s Tree Planting at Ashbridge Preserve! Since 2019, we have put in nearly 1,500 trees with the goal of lowering water temperature in Ridley Creek.