No abstract

Shoe Lake and East Graham Lake, part of a small chain of lakes in southeastern Michigan, USA, differ in nutrient loading and in the structure and productivity of their aquatic plant communities. A comparative study of species frequency and biomass distributions, nutrient contents, and responses to experimental nutrient enrichment and shading, was conducted to determine the principal factors controlling the macrophyte dynamics. A central objective was to address the question of why rooted macrophyte growth declines with eutrophication, and to test existing models designed to explain this phenomenon. In the more eutrophic Shoe Lake, diversity and productivity of rooted macrophytes were relatively low, restricted primarily by combined shading of phytoplankton, periphyton, and non-rooted macrophytes (principally Ceratophyllum demersum, along with Utricularia vulgaris and Cladophorafracta). In the less eutrophic East Graham Lake, lower nitrogen availability restricted the growth of all of these shading components, resulting in clearer water and higher productivity and diversity of rooted macrophytes. The macrophytes did not allelopathically suppress the phytoplankton in East Graham Lake. The results supported a direct relationship between nutrient loading, increasing growth of phytoplankton, periphyton and non-rooted macrophytes, and decline of rooted macrophytes.