The Role and Interactions of Submersed Macrophytes in a Shallow Wisconsin Lake

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Abstract
Growth and community organization of submersed macrophytes were studied in a southern Wisconsin impoundment in relation to macroinvertebrates, fishes, and phytoplankton. Macrophytes covered 50 to 70% of the lake bed in summer, growing to a water depth of 3.5 m. Total biomass peaked in late July at 130 to 200 g dry wt/m2. Undisturbed macrophyte beds were stratified into canopy, midwater, and basal layers. Narrow-leaved pond-weeds (Potamogeton spp.) dominated among 13 vascular plant species. But mechanical harvesting, in the fourth and fifth years of the study, disrupted vertical stratification and exposed the lake bed to monotypic growths of water stargrass (Heteranthera dubia (Jacquin) MacMillin). Three-fourths of macroinvertebrate species were found on or beneath macrophytes during summer. Few species comprised more than 5% of individuals in plant or bottom samples. Most macroinvertebrates consumed algae and detritus derived from macrophyte decay. Fishes under 120 mm total length were sheltered by plant beds denser than 200 g dry wt/m2. Larger bluegills (Lepomis macrochirus Raf.) and largemouth bass (Micropterus salmoides (Lacepede)) penetrated looser foliage or remained offshore. Macrophyte decay and plant harvesting created channels used by bass as cruising lanes. Plant-dwelling fishes grazed mainly aquatic insect larvae or microcrustaceans. Bass shifted to fry, while bluegills turned to macrophyte tissue, as summer progressed. Blue-green algal blooms developed in July when curly-leaf pondweed (P. crispus L.) decayed or other plants were mechanically harvested. Secchi disk visibility concomitantly dropped from over 3.5 to 0.6 m.