Diel Pulses of O_2 and CO_2 in Sandy Lake Sediments Inhabited by Lobelia Dortmanna

Abstract

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 O2 and CO2 in the pore—water of vegetated sediments between light and darkness. Pulses of O2 and CO2 were extensive in the upper sediment layers of high root density, because Lobelia roots absorb CO2 and release O2 during photosynthesis but absorb O2 and release CO2 during respiration, whereas fluxes across leaf surfaces are impeded by a cuticle of low gas permeability. Release of most O2 from the roots during Lobelia photosynthesis leads to O2 concentrations close to saturation in the root zone and O2 penetration to great sediment depth because of low microbial O2 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 O2 concentrations in the root zone. Cycles of O2, CO2 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 O2 from the roots should ensure efficient aerobic degradation or organic matter, lead to oxidized forms of Fe, Mn, and N, and can supply O2 to mycorrhizal fungi and an aerobic fauna associated with the roots.