Spatial variation in benthic macroinvertebrate community structures in tributaries of Verdal river : effects of biotic and abiotic environmental factors and restoration measures

Rivers and streams are one of the most degraded ecosystems in the world, including Norway. Human activities have long impacted rivers and streams, both directly, and indirectly, and this ultimately affects the streams health. Benthic macroinvertebrates and fish are the most common biological variables to use as bioindicators for ecological condition. Macroinvertebrates as a group play an important role in stream ecosystems, they can affect important stream processes, such as nutrient cycling, primary production, decomposition, and translocation of materials. In addition, they are important part of stream food webs. Verdal watershed located in Trøndelag county in Norway is considered an important Atlantic salmon (Salmo salar) and anadromous brown trout (Salmo trutta) river. Runoff from agriculture and migration barriers are one of the main threats to the biodiversity of Verdal river. To prevent further degradation of the Verdal watershed and further loss of biodiversity, restoration measures, mainly connectivity and addition of spawning gravel, were conducted in 2016 and 2017 in several of the streams to improve area and productivity of fish. The aim of this study is to examine the responses of benthic macroinvertebrates to restoration measures conducted in the tributary streams of Verdalselva, and shed light on the research questions: 1) Did the in-stream measures affect the macroinvertebrate community, and what determines the variation in macroinvertebrate assemblages? 2) Does the reintroduced presence of salmonids affect macroinvertebrate community structure? And 3) Which variables determines the variation in the organic stressor metric ASPT index scores? The collection of macroinvertebrate data was obtained by kicksampling method in 12 tributaries of Verdal river. Analysis of the data was undertaken with ordination analysis. The results indicate that the restoration measures have no effect on macroinvertebrate assemblages, as to yet. It is likely that the macroinvertebrates need longer time to respond. Allochthonous input of the streams appears to have a significant effect on macroinvertebrates, shifting towards a pollution-tolerant community structure. Density of salmonids had a significant effect on the macroinvertebrate community structure. There was a small, but insignificant, difference in diversity between the upper and lower reaches of the streams. ASPT index scores were mostly determined by distance to the fjord, distance to the main river and fish densities. However, the positive effect fish densities have on the ASPT index is most likely due to covariance where both groups respond similarly to favourable conditions. In conclusion, long-time monitoring of the benthic macroinvertebrate community is needed to detect long-time responses to the recently conducted habitat measures.