Diel Pulses of O_2 and CO_2 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 CO 2 in the pore—water of vegetated sediments between light and darkness. Pulses of O 2 and CO 2 were extensive in the upper sediment layers of high root density, because Lobelia roots absorb CO 2 and release O 2 during photosynthesis but absorb O 2 and release CO 2 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 2 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 CO 2 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 CO 2 stimulated the development of higher O 2 concentrations in the root zone. Cycles of O 2 , CO 2 and nutrients are, therefore, intimately coupled in the Lobelia—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 or 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.