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Growth and voltinism in the aquatic insects of a regulated river subject to groundwater inflows

The water temperature and discharge of the Vassbygdelvi River, Aurland watershed, was altered after regulation due to transfer of cold water from the higher parts of the catchment. The lower part of the river receives a considerable amount of groundwater, which affected the water temperature due to reduced discharge. Compared to a closely situated reference station, the temperature increased by 9% on a yearly basis. The temperature differences were most pronounced during the winter when average January water temperature at the reference and the stretch influenced by groundwater was 0.8 and 3.3°C, respectively. Studies of benthic insects in the two habitats showed that the temperature differences had little effect on the length of the life cycle. Although some species show significant differences in growth pattern during parts of the year, with more rapid development in the winter‐warm locality, there was a trend for synchronous growth at both localities. Compared to the situation before regulation, two species of stoneflies became extinct and one mayfly species colonized the area, probably as a result of the increased number of annual degree‐days. Copyright © 2008 John Wiley & Sons, Ltd.