Light-enhanced oxygen respiration in benthic phototrophic communities

Abstract

Two microelectrode studies demonstrate the effect of light intensity and photosynthesis on areal oxygen respiration in a hypersaline mat at Guerrero Negro, Mexico, and in an intertidal sediment at Texel, The Netherlands. The hypersaline mat was studied in the laboratory at light intensities of 0, 10, 25, 50, 190, 200, and 509 μE m-2 s-1 using the light-dark shift technique to measure gross photosynthesis rates. Areal gross photosynthesis increased from 0 to 31.3 nmol O2 cm-2 min-1 and areal net photosynthesis increased from -3.9 to 16.7 nmol O2 cm-2 min-1 with increasing light intensities Areal respiration, calculated from the difference between areal gross and areal net photosynthesis, increased from 3.9 to 14.4 nmol O2 cm-2 min-1 with increasing surface irradiance. This light-enhanced areal respiration was related to an increase in oxygen penetration depth from 0.2 to 2.0 mm, thus expanding the volume of sediment involved in oxygen respiration beneath the mat surface. The mean rate of oxygen respiration per volume of mat remained constant at a rate of ~100 nmol O2 cm-3 min-1. Oxygen profiles for the intertidal sediment were recorded in situ during the day at prevailing light intensities. A 1-dimensional diffusion-reaction model was used to estimate gross photosynthesis and oxygen respiration per volume of sediment, as well as the euphotic depth and the sediment-water interface concentration of oxygen. Areal gross photosynthesis ranged from 9.7 to 26.3 nmol O2 cm-2 min-1 and areal net photosynthesis ranged from 1.1 to 81 nmol O2 cm-2 min-1, both showing maximum values in the early afternoon. Areal respiration ranged from 8.1 to 18.2 nmol O2 cm-2 min-1 and showed a good positive linear correlation with areal gross photosynthesis (R = 0.97, n = 9). The enhanced areal respiration in the intertidal sediment was due to an increase in mean respiration per volume of sediment from 18.5 to 52 nmol O2 cm-3 min-1. The oxygen penetration depth only varied between 3.5 and 4.3 mm. Both studies thus indicate that areal respiration is enhanced significantly with increasing light intensities due to increased areal gross photosynthesis. By assuming that areal respiration in darkness is equal to areal respiration in the light, bell jar incubations would have underestimated areal respiration in the light 3.7-fold and areal gross photosynthesis 1.5-fold for the hypersaline mat and 10.7- and 2.6-fold for the intertidal sediment.