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The roles of inorganic carbon and light availability in the quantitative distribution and photosynthetic productivity of submersed aquatic macrophytes were investigated in two lakes in a stream‐connected hard‐water lake chain where light availability and pH increase and total inorganic carbon decreases as water flows from a turbid productive lake through progressively clearer less productive lakes. Bicarbonate‐using species dominated the macrophyte communities of both lakes, but species requiring free CO2 were present, primarily in the more turbid lake. In photosynthesis experiments, CO2‐requirer Najas flexilis was significantly competitive when light limitation reduced the efficiency of HCO3−‐user Potamogeton pectinatus. The results suggest that inorganic carbon availability can be important in macrophyte growth even within relatively small ranges of total alkalinity and that light availability is a potentially important interacting factor.

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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.

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