To bugs in streams, fine sediment is not so fine

Original article: Blöcher, Johanna & Ward, Matthew & Matthaei, Christoph & Piggott, Jeremy. (2020). Multiple stressors and stream macroinvertebrate community dynamics: Interactions between fine sediment grain size and flow velocity. Science of The Total Environment. 717. 137070. 10.1016/j.scitotenv.2020.137070.

Agricultural development and stream bugs

Converting land for agriculture can often drastically change the above-ground plant and animal communities, increase tilling (i.e., loosening) of soil, and redirect water flow. These alterations to the landscape contribute to soil erosion and less water in nearby streams and other bodies of water; in turn, the eroded material often ends up in streams, and flow velocities in the shrunken streams decrease.

A stream passing through a dairy pasture: its crumbling banks will lead to large additions of fine sediments to the water.

The maintenance of current or development of further agricultural land under modern practices has the potential to exacerbate these impacts on streams and their inhabitants. Flow and substrate conditions in streams both have strong influence over other physical conditions and the structure of the community of aquatic organisms, in particular macroinvertebrates (“bugs,” like insects, worms, crustaceans, and mollusks), and many of these organisms are tolerant of only certain conditions: too much fine sediment, importantly, can clog the stream bed, reduce shelter and habitat variety, and damage insects’ respiratory organs.

These impacts, however, can be variable in strength, and different organisms vary in their tolerance of these impacts; their responses can thus be all over the place. Drs. Blöcher, Ward, Matthaei, and Piggot set out to determine how different combinations of flow velocity and sizes of fine sediment impact abundance and diversity, in addition to other metrics, of stream macroinvertebrates, insects in particular. While also critically important intermediates in streams’ food webs, aquatic insects help link aquatic and terrestrial environments and food webs. Healthy streams need healthy surroundings and healthy bugs!

Keeping things real

In order to ensure environmental conditions in their experiment were as natural as possible, the team of scientists set up mesocosms, a sort of isolated and controlled patch of natural environment where smaller experiments can be run, along the Kauru River in New Zealand. Here the scientists were able to assign each mesocosm one of three flow conditions and one of four sediment treatments so that each possible combination was tested in some mesocosm. For the first half of their two-month study, the scientists added sediment of the chosen size, anywhere from 0-2mm, and allowed the river’s organisms to move in and get settled. For the second half, the scientists manipulated flow conditions, letting water flow through quickly, at a medium speed, or slowly. To finish, they collected all of the bugs living in their sites and identified them to build an idea of who their stream community was composed of.

Who cares about fine sediment?

Drs. Blöcher, Ward, Matthaei, and Piggot aimed to test hypotheses about how sediment and flow conditions affect the stream macroinvertebrates. Some bugs, like mayflies, stoneflies, and caddisflies, often prefer strongly rapidly flowing streams with minimal fine sediment, so the scientists expected to see fewer of them in their samples when they added fine sediment and slowed water flow. This hypothesis was supported in their study and provided just one of their many results that strengthened the idea that not only are fine sediments detrimental to in-stream bugs but the effects are made worse when flows are slow. Even without changes to flow velocity, any addition of fine sediment decreased the abundance and diversity of many important groups of bugs! The same was true in the opposite treatment: without changes to sediment, slower water left those important bugs worse off.

A mayfly nymph that would be severely affected by deposition of fine sediments from soil erosion: it prefers to cling to rocks that would be buried, and its delicate gills would easily get damaged.

Some organisms were able to benefit from many of the different experimental changes: burrowing worms thrived by having more prime burrowing material, and bugs that do not rely on large objects like boulders or logs to cling to or feed from were relatively unaffected by the experiment and ended up with less competition from different species.

In addition to measuring the abundance and diversity of bugs living in the stream bed, the scientists examined insect drift, an important method of travel and dispersion where the bug allows itself to get swept downstream. The smaller the sediment size, the more likely bugs were found to utilize drift, but when flow velocity was low, drift also dropped: it’s hard to get carried away by slow water!

Rocking out

Drs. Blöcher, Ward, Matthaei, and Piggot’s study provided strong evidence that fine sediment, like that from soil erosion, can take a severe toll on many stream bugs, especially when water flow velocity is reduced. These two conditions are common in streams whose landscape is or is becoming utilized for agriculture as cultivating this land can often lead to increase soil erosion and runoff of these fine particles into streams. If we want to protect healthy waters while our influence expands, there is at least one clear lesson to be learned from this study: preventing eroding soil from entering streams will go a long way!


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Garrett Frandson

I'm a master's student at the University of Missouri studying stream macroinvertebrates and the substrate they inhabit. I'm broadly interested in bugs, streams, drivers of change including climate change and other anthropogenic disturbances, and communicating the value and beauty of natural systems and the need to protect them to the general public and those who can enact policy changes. When not working, I'm probably poking around and photographing these systems and their inhabitants! IG: @frandsong

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