| Net annual productivity and annual carbon budgets were determined for populations of Littorella uniflora var. americana and Isoetes macrospora, in a mesotrophic and oligotrophic lake in northern Wisconsin, for the purpose of describing the annual patterns of Crassulacean Acid Metabolism (CAM) activity, and of determining the contribution of CAM to annual productivity in the natural environment. Nocturnal carbon accumulation (CAM), daytime uptake of external CO(,2) via the C(,3) mechanism, and refixation of endogenously generated CO(,2) from daytime respiration were the sources of carbon income. CAM activity was estimated based on the diurnal acid rhythms, which reached maxima of 133 to 182 (mu)eq (.) g('-1) leaf fresh weight for the various populations. Daytime uptake of external CO(,2) was estimated with a ('14)C methodology, where the plants were uprooted and the root compartment was labeled (since the majority of photosynthetic carbon is obtained from the sediments).; CAM activity was related to temperature, light and perhaps CO(,2) availability. CAM, C(,3) and dark respiration followed similar annual patterns, and were of similar magnitude, with maxima occurring near the time of maximum temperature.; Net annual production was measured for each population by following leaf turnover. Turnover rates for the Littorella populations were 1.56 and 1.72 (.) yr('-1), and for the Isoetes populations, 0.85 and 1.00 (.) yr('-1). Measured net annual productivity and calculated net annual productivity (based on CAM, C(,3), respiration etc.) agree with 12% for the four populations. CAM activity was greater for the more productive population of each species. The carbon gain via CAM, divided by total carbon gain, showed CAM contributed 45 to 55% of the annual carbon gain for the study populations. The results further suggest that the contribution of CAM to annual productivity is greater for the less productive population of each species.; The role of CAM, utilization of sedimentary CO(,2), nutrient dynamics, evergreen habit, growth form and habitat preference are evaluated in relation to the identification of isoetids as specialists on infertile sites. |
