This is part 2 of a two-part post on aquatic ecosystems. If you haven’t already, check out part 1!
In my last post, I shared my experience working as a science technician studying aquatic ecosystems like streams, ponds, and wetlands. These water features are critically important to many ecosystems and the creatures that live there. They also provide us with a lens through which we can glimpse the overall health of an ecosystem. In this post, I’ll teach you some signs to look for to recognize healthy aquatic ecosystems in your area. How do your local ponds, streams, and wetlands measure up?
Free of Trash
This one seems obvious, but it is worth including first because of how important it is, and because it is something we can all help prevent and fix. Litter and trash can be harmful to wildlife and plants. Lots of trash floating on the surface of water can impact the amount of light and oxygen in a pond or stream. Below the water’s surface, aquatic plants need sunlight and oxygen for photosynthesis, the process through which plants make their own food. Furthermore, our trash, especially plastics, contains harmful chemicals that may harm aquatic animals. Animals like fish, birds, and turtles confuse trash for food and eat it, which can be very toxic to them. Healthy aquatic ecosystems should be free of human trash.
Diverse Plant Communities
Any time you see an area dominated by just one plant, that might be a clue that something is wrong. Unhealthy ecosystems are more susceptible to invasive plant species. When an invasive plant species is introduced to an ecosystem, it can outcompete native plant species. Where there were once many diverse plant species, a single invasive plant may take over. This will send ripple effects through the whole local food chain. Examples of invasive plants species in North America include cheat grass, kudzu, garlic mustard, and buckthorn trees.
Some Algae, but not Too Much
It is natural for stagnant bodies of water (non moving bodies of water like ponds and wetlands), and slower moving streams to have a good bit of algae. Fish, aquatic insects, and other invertebrates need algae as a source of food. But how much is too much? When ponds are near farm fields, the runoff into them may contain too much of the nutrients phosphorous and nitrogen, which is found in many fertilizers. After rain, water flowing off of farm fields and lawns will wash fertilizers containing phosphorous, nitrogen, and other chemicals into low-lying ponds and wetlands. This can cause massive, unnatural plumes of algae to form, which block out sunlight, deplete oxygen, and make it difficult for any other life forms to survive in the pond. When one species takes over, such as a massive plume of algae, it is often a sign that something is out of balance in an ecosystem.
Abundant and Diverse Aquatic Macroinvertebrates
Have you ever flipped over a rock in a pond or stream? Chances are you saw tons of wriggling little creatures underneath! These creatures are aquatic insect larvae, such as mayflies, dragonflies, and caddisflies (“case-makers”) and other aquatic macroinvertebrates! “Aquatic macroinvertebrates” is a general word for tiny creatures with no backbones that live in water. They include insect larvae, worms, snails, crayfish, and more. Many aquatic macroinvertebrates are herbivores of algae and provide an important food source for many fish species. A healthy ecosystem should have a variety of different aquatic macroinvertebrates!
Many macroinvertebrate species are sensitive to disturbances and contaminants in an ecosystem, so their populations will reflect an ecosystem’s health. The aquatic macroinvertebrates in a pond or stream are actually such an excellent indicator of ecosystem health that scientists routinely use studies of the aquatic macroinvertebrates in a stream or pond as a holistic measure of aquatic ecosystem health! A healthy aquatic community will have abundant and diverse macroinvertebrates.
Multiple Trophic Levels
A healthy ecosystem should also contain a variety of different living things including plants, herbivores, predators, and decomposers. These life forms represent different trophic levels, or positions in the food chain, and all play different roles in the ecosystem to maintain balance. A classic example is the relationship between wolves, elk, and riparian plants. When an ecosystem is healthy, all three thrive. However, if wolves (or other top predators) are driven out of an area, the elk population will increase since no one is eating them any more. That will cause the plant populations to suffer, because more elk eat more plants. Additionally, without fear of predation, herbivores like elk will be emboldened to graze out in the open near wetlands and streams, rather than hiding in the woods. This leads to greater stress on the plants around aquatic ecosystems.
Scientists study this effect by building fences around small areas of an ecosystem, to keep out elk, and to see what an area would look like without exaggerated grazing. This same sort of imbalance between plants, herbivores, and predators can happen on a smaller scale too. In some pond ecosystems, the top predators (predators that are not prey to any other animals) might be herons, hawks, or otters, rather than wolves. See if you can identify a top predator in a local pond or stream ecosystem near you. In a healthy ecosystem, look for a variety of insects, fish, birds, amphibians, and other animals.
Free of Chemicals
Chemicals can get washed into water from surrounding human settlements and agriculture. Some chemicals might be noticeable, and appear as a white or yellow froth on top of water, an oily sheen floating on the surface, or an unnatural metallic or rusty color. Some chemical pollutants are invisible, but their impact on wildlife is not! For example, road salts, when dissolved in water might be invisible, but it can impact the number of female frogs that are born in a pond – which can impact a frog species’ ability to reproduce.
Another example of a harmful but nearly invisible chemical is a mosquito pesticide called DDT. When DDT gets into pond and stream water, it accumulates in the tissues of plants. These contaminated plants are eaten by macroinvertebrates and small fish, which are in turn eaten by larger fish, which are eaten by raptors like bald eagles. On each step up the food chain, the amount of the DDT chemical would build up in the organisms, in an effect called biomagnification. The bioaccumulated DDT in the eagles caused the shells of the eggs they laid to be too weak, which caused an enormous threat to this species. Luckily a biologist named Rachel Carson figured out the cause of the problem and DDT was later banned in the United States. DDT is still legal in some countries.
We love visiting streams and wetlands, but too many human or livestock visitors traveling over fragile surfaces can have a big impact. Human visitors and livestock that are not natural to an area can cause soil compaction by repeatedly walking over the same area, this can make it very difficult for plants to grow. Plants are an important part of a pond or stream ecosystem because their roots help stabilize the banks or shores, preventing erosion. Signs of excessive erosion like undercut banks and over compacted, plant-less soil can be signs that an ecosystem isn’t doing so well. Water also cannot infiltrate into soils as easily when they are too compacted – which may cause floodwaters to pool rather than seep back into the ground. To help prevent impacts to soils, you can stay on designated trails and recreation areas when visiting a local stream, pond, or wetland.
Ways we can all help promote healthy aquatic ecosystems!
- Stay on trails, boardwalks, and already-impacted surfaces when visiting local streams, ponds, and wetlands.
- Pack out your trash or dispose of it properly in a trash can or dumpster.
- Consider bringing an extra baggie or container on hikes to pick up litter.
- Help prevent the spread of invasive plant species by checking your shoes and clothing for burs, seeds, or other plant materials.
- Help prevent the spread of invasive animal species by not releasing aquatic pets into local ecosystems.
- Consider reducing the amount of chemicals you use in your lawn or garden to prevent chemical run-off into nearby wetlands, ponds, or streams. More about this in my post on improving you backyard wildlife habitat, if you haven’t read it yet!
- Consider joining a volunteer group in your area that helps pull up invasive plant species in parks and on public lands.
- Teach your family about healthy aquatic ecosystems. I’ve made a free aquatic ecosystem scavenger hunt for you to download, shown at the bottom of this page! Or purchase my Pond Ecology Bundle on Etsy – which includes activities, flashcards, and posters for learning about ecosystems!
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References and Further Reading
- Dennehy, K. (2016). Road salt can change sex ratios in frog populations, study says. Yale News. Available: https://news.yale.edu/2016/11/22/road-salt-can-change-sex-ratios-frog-populations-study-says
- Environmental Protection Agency. (2020). Impacts of Mismanaged Trash. Available: https://www.epa.gov/trash-free-waters/impacts-mismanaged-trash
- Environmental Protection Agency. (2016). Indicators: Benthic Macroinvertebrates. Available: https://www.epa.gov/trash-free-waters/impacts-mismanaged-trash
- Frey, D. (2018). Study shows Yellowstone wolves’ impact on streams. The Wildlife Society. Available: https://wildlife.org/study-shows-yellowstone-wolves-impact-on-streams/
- National Ocean Service. (n.d.). What is nutrient pollution? Available: https://oceanservice.noaa.gov/facts/nutpollution.html
- National Park Service. (2017). Elk and moose exclusion fence. Available: https://www.nps.gov/articles/elk-and-moose-exclusion-fence.htm
- Natural Resource Defense Council. (2015). The Story of Silent Spring. Available: https://www.nrdc.org/stories/story-silent-spring
- U.S. Fish and Wildlife Service. (2019). Midwest Region – Bald and Golden Eagles Fact Sheet. Available: https://www.fws.gov/midwest/eagle/Nhistory/biologue.html