Effect of Flow Fluctuations on Patch Dynamics and Chironomid Distribution in a Medium-Sized Lowland River

Abstract

ABSTRACT The response of macroinvertebrates to changes in flow was studied in the Drzewiczka River at the downstream end of a whitewater slalom-canoeing track located in the tailwater of a dam. From 1980 to 2002, surface water was released daily, except in the winter, from the reservoir for two-three hours to enable training of canoeists. Discharge ranged from 2.1 to 12.0 m3 s−1 daily during this high discharge fluctuation period (DI), when measured in 2000–2001. The reservoir was emptied for dredging in February 2002, after which the river returned to its natural discharge of about 2 m3 s−1, a time we called the temporary renaturalization period (DII). Monthly samples of environmental variables and macroinvertebrates were collected over the year during DI and DII from distinct visible patches of the bottom including pool, stagnant, submersed macrophytes, bank, and riffle habitats The most obvious changes in the environment in DII were the redeposition of fine mineral particles on the river bottom, increased amounts of fine particulate organic matter, and a transition of submersed macrophytes from the middle part of the river channel to bank habitat. Macroinvertebrate assemblages adapted very quickly to these changes, especially chironomids, which were the prevalent taxa. During DI at each habitat (except the stagnant patch), the Orthocladiinae (periphyton scrapers)/Chironomini density ratio was higher than 1, ranging from 1.2 (at M) to 12.1 (at R); during the DII, this index diminished to 0.7 (at M) to 4.5 (at R). Thus, low water velocity and high amounts of organic matter in DII benefited gathering collectors (Chironomini).