Forest Ditches As Habitat For Aquatic Macroinvertebrate In Boreal Landscapes

Sammanfattning: Large areas of northern Sweden have been ditched to promote forest growth by draining wet soils and peatlands. These ditches have greatly added to length of Sweden’s stream and river network, but the ecological properties of these man-made aquatic environments is poorly understood. The aim of this thesis was to evaluate the extent to which forest ditches support benthic macroinvertebrate communities, test the factors might influence variation in these communities across ditches, and evaluate how these communities differ from those observed in more natural headwater streams. To do this, I analysed macroinvertebrate family richness and community composition from 16 sampling sites distributed across two ditch networks in northern Boreal Sweden. At each site, I also compiled information related to channel structure, water depth, and the upstream distance from more typical headwater streams. I then tested whether variation in community metrics among ditch sites was related to differences in local habitat properties as well in the spatial position of sites within the ditch network.  Finally, I used existing data from four headwater streams within the same broader catchment to evaluate how these communities compare with those observed in forest drainage ditches. Results showed that macroinvertebrate family richness and community composition varied among ditch sampling sites, but that these systems supported a range of aquatic taxa that are typically found in headwater streams.  Overall, family richness tended to increase among ditch sampling locations that had greater water depth at the time of sampling and that were located further downstream in the ditch network. In addition, sampling locations with mineral substrate tended to support communities with a greater representation of more sensitive aquatic orders when compared to sites dominated by peat sediments. Finally, while family richness was not significantly different between ditches and headwater streams, benthic communities in streams were dominated by different set of taxa that reflect greater flow velocity and better overall water quality conditions.  Overall, my results suggest ditches in the boreal forest landscape can support benthic communities that closely resemble what is observed in headwater streams. However, ditch habitats were also highly variable, with several sites indicating poor local habitat conditions for many benthic organisms.  Hydrological patterns, including water depth, but also unmeasured variables like flow velocity and the potential for seasonal drying likely play a key role in influencing the ecological properties of ditches in these landscapes.