Invertebrate Life Cycle Patterns in the Benthos of a Floodplain Lake in Minnesota

This was a study of seasonal distribution of dominant benthic macroinvertebrates in Pickerel Slough, a Mississippi River floodplain lake in Minnesota. The lake was shallow–0.75 m deep during the summer–and had a relatively uniform benthic habitat with no rooted aquatic plants. The total number of benthic macroinvertebrates was greatest in the late fall and winter; they decreased rapidly in the spring owing to emergence of dipterans and decrease in number of oligochaetes. Congeneric species demonstrated three patterns of life cycle interrelationships. Complete seasonal replacement was found for Tanypus: T. stellatus produced two summer generations and probably overwintered as eggs, whereas T. neopunctipennis overwintered as larvae. Overlapping seasonal replacement was shown by Procladius: the largest P. culiciformis preceded P. bellus in both emergence and larval growth. Although both populations were present in the sediments at the same time, they always maintained larval size difference. Overlapping seasonal replacement combined with annual substitution was demonstrated by Chironomus. The largest species, C. plumosus, was the first to have growing larvae in the fall, and this was followed by the growth of the two smaller species: C. near staegeri, and C. decorus. C. plumosus and C. decorus predominated in alternate years. There was a population of growing dipteran larvae at all times during the year. The dominant populations (Chaoborus punctipennis, Chironomus spp., Procladius spp., and Tanypus stellatus) had the growing larval stages of their life cycles spaced so that they did not overlap, thus reducing the competition between them.