Agricultural drains doing double duty: water drainage and habitat for biota
June 13, 2019
Photo credits: brook trout, northern pike, blacknose dace (left, top to bottom) by Katie Stammler; maintained and unmaintained drains by Belinda Ward-Campbell.
Landscapes and ecosystems have increasingly been modified by human activities. Agriculture, for instance, is a familiar feature on the landscape with approximately 50% of global land area being crop or pasture land. One of the defining features of human activities on the landscape is that certain ecosystem services can be favoured over others leading to what has been called “domesticated ecosystems”. In agriculture, this would mean creating conditions that favour food crop production over maintaining ecosystem features necessary for species diversity.
In order for crops to grow and thrive, agricultural lands often need to be drained of excess water. Drains are created for this purpose either by digging out a new channel on the landscape or by channelizing an existing headwater stream. Over time, drains can require maintenance in order to ensure efficient movement of water off the landscape. Maintenance can involve digging out the channel with an excavator and cutting back riparian vegetation. Although drains are completely or partially constructed features, they still support biota such as fish, benthic invertebrates, and aquatic vegetation and maintenance activities may disrupt these ecosystems.
Left photo - before drain maintenance; Right photo - following drain maintenance (by Belinda Ward-Campbell)
Ward-Campbell et al. (2017) used drain maintenance as an opportunity to understand the resilience and resistance of ecosystems to disruptive human activity. A resistant ecosystem avoids changing in the face of disturbance while a resilient ecosystem will change as a result of disturbance but can readily recover. For their study, they examined whether drain ecosystem features that support fish are altered due to maintenance, and if yes, do they recover and how quickly.
The study involved eight pairs of maintained and unmaintained reference drains plus unmaintained reference sites at downstream locations of the maintained drains. Paired drains were within the same sub-watershed all of which were located within southern Ontario. Measurements and sampling took place prior to fall/early winter drain maintenance and 10 to 12 times following maintenance over a period of 2 years. The first sampling post-maintenance was at 5 months.
The researchers measured seven physical features that can influence the number and kinds of fish that inhabit watercourses: channel width, water depth and velocity, substrate size, bank and in-stream vegetation, and overhead cover. They also considered the benthic macroinvertebrate assemblage as an indicator of food availability for fish. Metrics included the number of taxonomic groups, overall abundance, and changes in the relative densities of chironomid larvae.
Over the course of the study, Ward-Campbell et al. (2017) observed changes in some physical features of the drains but not in the assemblage of benthic macroinvertebrates. Bank and in-stream vegetation and overhead cover decreased after maintenance but returned to pre-maintenance condition within two years while water depth and velocity results were inconsistent. Changes in stream width and substrate type were not observed possibly due the approach used when excavating and presence of fine substrates, respectively. The lack of change in benthic macroinvertebrates could be due to a number of reasons: invertebrates may have taken refuge in the hyporheic zone during excavation or returned to the drain from the removed materials or the community may have been able to bounce back over the five months prior to the first post-maintenance sampling.
This study is important since it helps address some concerns expressed by fishery managers with respect to the effects drain maintenance may have on fish and fish habitat. Although some drain features that provide habitat for fish were resistant to change, others were altered to such an extent that negative impacts on fish populations cannot be ruled out without further investigation.
Read the full article (journal subscription required):
Ward-Campbell B, Cottenie K, Mandrak N, and McLaughlin R (2017) Maintenance of agricultural drains alters physical habitat, but not macroinvertebrate assemblages exploited by fishes. Journal of Environmental Management, 203:29-39.