DIEL PULSES OF O-2 AND CO2 IN SANDY LAKE-SEDIMENTS INHABITED BY LOBELIA-DORTMANNA

Lobelia dortmanna is a common representative of the small isoetid plants dominating the vegetation on sandy sediments in nutrient-poor softwater lakes in North America and Europe. We report here the discovery of pronounced depth profiles and pulses of dissolved O-2 and CO2 in the pen-water of vegetated sediments between light and darkness. Pulses of O-2 and CO2 were extensive in the upper sediment layers of high root density, because Lobelia roots absorb CO2 and release O-2 during photosynthesis, but absorb O-2 and release CO2 during respiration, whereas fluxes across leaf surfaces are impeded by a cuticle of low gas permeability. Release of most O-2 from the roots during Lobelia photosynthesis leads to O, concentrations close to saturation in the root zone and O-2 penetration to great sediment depth because of low microbial O-2 consumption rates. The CO2 concentrations in the root zone of the most nutrient-poor sediments examined here (0.37-0.71 mmol/L) were below the levels required to saturate Lobelia photosynthesis (>3 mmol/L), and experimental enrichment with CO2 stimulated the development of higher O-2 concentrations in the root zone. Cycles of O-2, CO2 and nutrients are, therefore, intimately coupled in the Lobelin-sediment system, in contrast to terrestrial and other aquatic plants, where gas and nutrient fluxes are physically separated between leaves and roots and their respective surrounding media. The release of O-2 from the roots should ensure efficient aerobic degradation of organic matter, lead to oxidized forms of Fe, Mn, and N, and can supply O-2 to mycorrhizal fungi and an aerobic fauna associated with the roots.