Productivity, growth, and photosynthesis of two small Isoetid plants, Littorella uniflora and Isoetes macrospora
(1) The annual productivity and tissue nutrient contents were determined for populations of the isoetids Littorella uniflora var. americana and Isoetes macrospora in an infertile and a moderately fertile lake in northern Wisconsin, U.S.A. These species have crassulacean acid metabolism (CAM). (2) Annual leaf turnover, determined by biomass marking, ranged from 0-85 to 1 72 leaves per leaf for the study populations. Population annual production, including below- ground biomass and allocation to vegetative spread, ranged from 63 to 332 g C m-2 yr-1. Leaf turnover rates were significantly greater for the Littorella populations than for the Isoetes populations and compared well with published values. (3) The Littorella population in the infertile lake had similar tissue nutrient concen- trations and a significantly greater areal productivity than the population in the more fertile lake. The Isoetes population in the infertile lake had significantly greater tissue concentrations of N, P and K than the population in the more fertile lake. However, it received 40\% less light (PAR) because it grew at greater depth. Leaf turnover was greater for the Isoetes population in the more fertile lake, but population productivities in the two lakes were similar. (4) These data along with data for similar plants suggest that the way in which these plants grow is well-adapted to low fertility sites with waters having a low inorganic carbon content.