CONTRASTS IN DENSITY OF BENTHIC INVERTEBRATES BETWEEN MACROPHYTE BEDS AND OPEN LITTORAL PATCHES IN EAU-GALLE LAKE, WISCONSIN

Small areas that are barren of plants commonly occur amidst dense littoral zone vegetation of lakes. This study compared the benthic invertebrate communities of nonvegetated patches to those in the surrounding vegetated sediments in a Wisconsin lake. Mean benthic invertebrate densities in the sediments of Ceratophyllum demersum and Potamogeton nodosus beds were 13 and seven times, respectively, those of nearby nonvegetated areas within the littoral zone. The contrast in densities was especially striking for the gastropods Amnicola limosa and Gyraulus parvus, which were collected below Ceratophyllum at 162 times and 48 times, respectively, their densities in open areas. The conjecture that these density differences were due to a lack of a detrital food base in the nonvegetated areas was not supported by our investigation. The most likely explanations for such a contrast in invertebrate densities are: (1) disturbance effects in areas lacking plants, and (2) increased predator efficiencies in the open zones. Removal of macrophytes by herbicides, drawdown or mechanical means creates large expanses of open areas; our data indicate that macrophyte removal would result in a less diverse and much less abundant invertebrate fauna in littoral zone sediments. More than 90% of the invertebrates in 0-15 cm cores were found within the top 5 cm in nonvegetated areas as well as in sediments below a rooted macrophyte (Potomageton nodosus) and a nonrooted macrophyte (C. demersum). Despite the presence of its root system, which might oxidize the sediments, proportional abundances of invertebrates deeper in the sediments below Potamogeton were not greater than those below Ceratophyllum or in the open zones.