No abstract

Using the macrophyte Potamogeton nodosus, we investigated variability in abundance of plant-dwelling invertebrates among individual plants. Plants were collected from three Potamogeton beds in Eau Galle Lake, Wisconsin, in June and August 1987. Invertebrate abundance on P. nodosus and the amount of plant surface area were positively correlated in both June and August. In August the amount of leaf damage (plant condition) was another important predictor of invertebrate abundance. Plant surface area and plant condition were responsible for most to almost all of the variability in invertebrate abundance on P. nodosus (R2 = 0.66 in June; R2 = 0.83 in August). The correlation between invertebrate abundance and plant condition in August and the lack of such a correlation in June indicated that plant age, rather than plant condition per se, was a causal mechanism for increased invertebrate abundance. Some plants were heavily colonized by invertebrates; a single plant collected in June held a total of 555 invertebrates, which included 177 chironomid larvae and 143 naidid worms. We estimate that the P. nodosus in a 20 × 60 m Potamogeton bed supported about 33 million invertebrates in June and approximately 30 million invertebrates in August. The use of lake management techniques in which plants are eliminated would therefore markedly reduce invertebrate abundance in the littoral zone, and would, in turn, deny fishes and waterfowl an important and abundant food resource.

We investigated the distribution of benthic macroinvertebrates in four aquatic habitat types in the lower Mississippi River: dike fields, a natural bank, a secondary channel, and an abandoned channel over a high flow (flood stage), two moderate, and two low flow periods. The biotas present in the natural bank, the secondary channel, and the abandoned channel showed only minor changes in composition over the various flow regimes. The natural bank was consistently dominated by the burrowing mayflies Tortopus incertus and Pentagenia vittige2.a and hydropsychid caddisflies; the consistently most common taxa in the secondary channel were the sand-dwelling chironomids Robackia cZaviger and Chemovskiia orbicus; phantom midges, tubificid oligochaetes, and fingernail clams were always the most abundant macroinvertebrates in the abandoned channel. The dike fields, however, showed large changes in biotic composition over the different flow regimes. These compositional changes correlated with changes in river stage and resultant alterations in current and substrate. This study indicates that the distribution of macroinvertebrates in the lower Mississippi River is a function of the physical characteristics of the system, notably current velocity and substrate composition.

No abstract