Comparative ecology of submersed freshwater macrophytes in acidic environments: Responses to dissolved inorganic carbon availability

Submersed macrophyte species from acidic and circumneutral lakes were grown in the greenhouse at low and high dissolved inorganic carbon (DIC) concentrations at pH 6 to determine (a) if species vary in their response to DIC enrichment, (b) if response to DIC enrichment was related to the DIC source used for photosynthesis (free CO2, both bicarbonate (HCO3 -) and free CO2, or sediment CO2 and free CO2), and (c) if sediment type influenced response to DIC enrichment. A survey of acidic lakes was also undertaken to characterize variability of DIC spatially and temporally within the growing season as well as to characterize sediment properties that could influence macrophyte growth and community structure.;Variability in species response to DIC enrichment occurred at two levels---among individual species and among groups of species based on form of DIC used in photosynthesis. Bicarbonate users showed a significantly greater response to DIC enrichment than either species restricted to free CO2 or species also able to use sediment CO2. Bicarbonate users may therefore behave as a functional group in their response to DIC enrichment.;In contrast to response to DIC enrichment, submersed macrophyte species were largely unresponsive to sediment resources. This may partly be a function of a plant's ability to obtain resources from the water column as well as from the sediment.;Lake chemistry data show that substantial spatial variation in DIC and sediment characteristics exists not only among lakes, but within lakes, often at a relatively small scale. This was particularly true of DIC, which differed along a vertical gradient within a particular site as well as among sites within the same lake. Because submersed macrophytes respond differently to carbon availability, variations in the DIC regime have the clear potential to influence community composition in the manner predicted by our greenhouse studies.;Environmental variation among acidic habitats suggests that effects of acidification on macrophyte growth and community structure will not be uniform. Understanding patterns of response to environmental variability can help us predict the types of macrophyte communities that may develop in different lake systems as environmental conditions change due to acidification.