Stream Connections: Benthic Macroinvertebrates, Native Fish and PA Geology

There are many ways to enjoy Pennsylvania's wonderful waterways through boating, fishing, swimming, hiking on shore, or relaxing streamside in a lawn chair. These activities allow us to find solace in nature, enjoy family time, catch fish, and mix up our daily routine. What we might not realize here on land is that there is an incredible story below the water's surface. There are "benthic connections" happening all the time and the creatures below have their own way of interacting with the more than 86,000 stream miles in Pennsylvania. Here we’ll explore the connections between stream biology, native fish, and one of the major factors that dictates water quality and in-stream habitat, geology.

Benthic Macroinvertebrates

Benthic, meaning "bottom-dwelling," macro, meaning we can see them with the naked eye (no microscope needed), and invertebrate, meaning they lack a backbone. This overarching term describes a group of small aquatic organisms that include insects (mayflies, caddis flies, stoneflies, midges, beetles), crustaceans (crayfish, shrimp), worms, snails, and clams (Pennsylvania Department of Environmental Protection, PADEP). They serve as a vital food source for fish and other aquatic organisms and are essentially the glue that connects plants, algae, and fish in an aquatic ecosystem. Macroinvertebrates spend all or most of their time in the water, consume bacteria, algae, and plant matter or other animals and therefore are excellent bio-indicators of water quality. Some macro species require exceptionally clean water, while others are more tolerant of excess nutrients and less than ideal water chemistry. Having information on the benthic community plays a key role in telling the water-quality story.

Native Fish of Pennsylvania

There are over 160 species of fish in Pennsylvania comprised of about 24 different families (PA Fish and Boat Commission, 2000). Some of these species are limited to lakes and ponds, some migrate to the ocean, and some are primarily found in the streams and rivers in the state, which is where we’ll focus.

Native fish in Pennsylvania streams need basic items we can all list off: food, shelter, clean water with good oxygen levels, appropriate water temps, and areas to spawn/lay eggs. In-stream habitat requirements vary for each fish and one of the broadest factors dictating where species live includes water temperature—most fish are ectothermic. For example, brook trout need cold water, but largemouth bass inhabit warmer water. Regardless, a sustainable food source is important across the board and macroinvertebrates fill that role for at least part the lifecycle of most stream fish in the state.

For more details on all the fish species in Pennsylvania waters, check out the comprehensive resource from PA Fish and Boat Commission: Pennsylvania Fishes (PDF).

Geology

To understand stream health and macroinvertebrates, we have to first understand the geology—it all starts with the rocks!

The most common type of geology found across the state is sedimentary rock, which has been formed from gravel, sand, silt, or clay and become naturally cemented and layered together over time. If you would like to explore further, check out the PaGEODE interactive map offered through the Department of Conservation and Natural Resources (DCNR). Here you can pinpoint your location to learn more about the geology in your area.

Rock Types Across Pennsylvania in each Physiographic Province are unique. The limestone components within many rock types are formed by calcium and magnesium carbonates from marine organisms. Limestone rock often leaches these important minerals back into the water creating a unique stream chemistry. Take note of the swaths, or patterns of rock types across the state and compare them to the stream categories on the map on the next page to glean more information about the aquatic community that could be found within each section.

Stream Types

So once we're streamside, and we understand the general geology of the area, we ask, "Is this a limestone or a freestone (riffle-run) stream?"

Limestone streams have limestone geology, are fed by limestone springs, have high alkalinity (hardness), and consistent water temperature throughout the year (the stream does not freeze in the winter). Alkalinity refers to the ability of the water to resist change in pH: the high alkalinity of limestone streams means they are more stable in holding a consistent water pH. The water in a limestone stream originates from the underground caverns and pockets formed by the limestone rock and is purified as it percolates through the rock. Limestone streams have a highly productive, abundant macroinvertebrate community, with few different species (low diversity) (PADEP). The majority of macroinvertebrates associated with limestone streams are from five taxa: Lirceus, Gammarus, Ephemerella, Optioservus, and Chironomidae (PADEP, 2009).

Out of the approximately 86,000 stream miles in Pennsylvania, less than 800 miles constitute limestone streams (PADEP, 2009). There are few, but they offer great economic and cultural importance due to their world-renowned trout populations, sustained by the abundant macroinvertebrates and consistently cold water.

Freestone streams are the most common type found across Pennsylvania and are generally associated with a variety of geologic types such as sandstone, igneous, schist, clay, sandstone, gneiss, slate, and quartz, among others. Freestone streams receive a variety of source water from runoff, rainfall, and/or groundwater. They typically have cobble and gravel bottom substrate, are more likely to warm in response to air temps, and have lower flows in the summer months.

Macroinvertebrates are typically found in greater density in the riffle areas of freestone streams where moving water (and thus higher dissolved oxygen) provide suitable habitat.

Stream types in Pennsylvania - The majority of streams in the state are characterized as freestone. Limestone streams overlap with limestone geology. PA Department of Environmental Protection (PADEP). 

Hellgrammites, seen below, are the larval stage of eastern dobsonflies and can be found in higher-quality freestone streams, inhabiting the fastest-moving sections. The larvae are predators of macroinvertebrates, small fish, and other organisms and are also eaten by fish. The adults live for a few days to reproduce and do not eat.

The last term used to categorize stream type is called 'multi-habitat,' which simply describes the more low gradient (flat water) streams that have slow-moving water and silty or muddy bottom substrate (PADEP). Multi-habitat streams lack the more biologically productive riffle habitat.

In terms of macroinvertebrate and fish communities, notable differences are documented in relation to the underlying geology, water chemistry, sediment, nutrient inputs, and dissolved oxygen. The surrounding land use along a waterway impacts the quality of runoff into a stream. All waterways function as the arteries of our landscape. On a watershed scale, they receive, transport, and filter nutrients, sediment, and contaminants from the land and in return, provide us the opportunity to have clean drinking water, healthy food, and recreation area. Just like the arteries in our own bodies, the output is impacted by the input, and in countless instances, waterways become impaired by the excess.

The story doesn't end there, though. Through studying the aquatic ecosystem, particularly the macroinvertebrate community, we are able to gather valuable information on reasons for impairment and ways to improve water quality. More information on that subject can be found at PADEP Looking Below the Surface.