SummaryA typical marl lake of the Upper Great Lakes region has very few quantitatively important aquatic macrophytes. The macrophytes, however, dominate the total primary production of the lake. Submersed vegetation is extremely sparse on the shallow (less than I m) marl bench that characterizes the littoral of these lakes, and is completely dominated by one. little‐known species (Scirpus subterminalis Torr.) between 1 and 7 m.A detailed investigation of the spatial and seasonal distribution of macrophytic species and biomass showed that S. subterminalis strongly dominated the lake (79% of total biomass). S. suhterminalis represented an almost pure stand (to 200 g m−2mean annual ash‐free dry weight) at all times of the year at intermediate depths of macrophytic growth (1–6 m). Two species of Chara (of eight varieties and forms) were present in significant quantities (12% of total biomass; to 100 g m−2) but were severely limited to shallow depths (0‐S‐l m) and protected areas. Several annual submersed angiosperms were present (9% of total biomass), but only two species were quantitatively important. Potamogeton illinoensis Morong. and P. praelongus Wulfen formed brief summer peaks (less than 100 g m−2) at 3 and 4–6 m, respectively.A striking feature of the seasonal biomass distribution of Scirpus subterminalis was the higher, viable biomass (to 150g m−2) throughout the winter under ice cover. Cyclic fluctuations of the S. subterminalis populations were discerned at different depths, each with different periodicities. The population at 2 m exhibited a fall peak; that at 4 m had a summer maximum. The lowest overall biomass ofS. subterminalisoccurred in the 2 m population in June.Chara populationsat 0–2 m also exhibited a relatively constant biomass throughout the year. The appearance of Nitella at 7 m in July‐October and ofCharaat 5 m in September‐October was interpreted as an interaction between light, thermal, and carbon stratification.Estimates of macrophytic productivity of perennial (‘evergreen’) species populations whose biomass remains relatively constant throughout the year were made employing several different methods of calculation and turnover factors. All methods resulted in productivity estimates in good agreement with the conservative value of 178 g m−2year−1for the entire lake. In comparison to the other components (phyto‐planktonic, epiphytic and epipelic algae) of the primary production of Lawrence Lake, the aquatic macrophytes constituted a major portion (anuual mean 82·77 g C m−2year−1or 48·3 %) of the total production of the lake.The low diversity but relatively high quantitative importance of macrophytes in marl lakes is attributed to an adverse dissolved inorganic and organic chemical milieu which inhibits phytoplanktonic production and allows only certain adapted macrophytes to develop strongly. The phenomenon of perennial biomass levels throughout the year is believed to be much more common than previously suspected and has iikely resulted from adaptations of submersed macrophytes to ameliorated conditions of water and temperatures relati