Scientists characterize terrestrial and aquatic ecosystems quite differently, and even use opposite criteria to define ecosystem health. For a terrestrial system, productivity (lots of nutrients and biological activity) is deemed to be a positive feature. Consider a forest versus a desert: the greater the productivity, the greater the ranking of the ecosystem on an intuitive scale. There are more species present, more standing biomass of organisms, more of everything in the forest than in the desert. Yet, in freshwater ecosystems, we think of it quite the other way. Systems with high productivity generally are considered out of balance, degraded, and low quality, whereas lakes and streams that have low productivity are more highly valued, “clean,” and often have higher biodiversity. This is because if there are ample nutrients in aquatic systems, then bacteria and other microorganisms that reproduce very rapidly can bloom to dominate the system, using up oxygen and burdening the water with waste products or toxins. This will kill larger animals, such as aquatic insects, amphibians, and fish. A muddy, green farm pond has high productivity, but it is mostly bacteria, algae, and other microorganisms that are undesirable. Sometimes, only a small number of species will dominate and drive out, suffocate, or poison the others that were present before the bloom. This is what we find when nutrients (waste or fertilizer) from human activities enters our rivers and lakes.
Click here for an underwater video of Powdermill Run, a healthy headwater stream.
Or, take a walk up the creek by drone here.
By contrast, systems with low productivity typically have little bacterial or algal load, greater dissolved oxygen, and support a greater diversity of insects feeding as specialists in the habitat, and thus a more stable and complex food chain passing up through fishes and amphibians and even into birds that rely on insects that emerge from the stream or lake. In fact, water quality of streams is generally determined by monitoring the insects that live in the stream, with healthier streams producing a greater variety of specialist insects. Where can you find a good example of a healthy stream? Generally, headwater streams flowing out of forested highlands are a good bet, with clear, fast flowing water, rocky bottoms (not muddy), and a closed canopy of trees overhead. The fast flow keeps the water well-oxygenated and scours out sediment, lack of full sunlight inhibits algae.
A standard method to assess the quality of a stream is to collect the insects that live there. If the assemblage of species is dominated by those that are sensitive to habitat degradation, then the stream must be healthy all year long. If those that tolerate poor quality dominate, then the stream is challenged in some way for at least part of the year (there is an occasional flush of nutrients or toxins, or it gets hot, etc.) Because the insects spend their whole lives in the stream, they are a better indicator than is a chemical sample that can only describe the stream at the moment the sample was taken.
In order to facilitate accurate identification of stream insects, we worked with a broad team of collaborators to create a reference web site that provides images and lessons for either aspiring habitat evaluators or as an easy reference for experienced ones. Funded by the National Science Foundation, the site is free and easy to use (and many of the insects are pretty!). Go to Macroinvertebrates.org, and see for yourself!