Mean Seagrass Meadow Metabolism by Time of Day in Virginia, USA

The study examined benthic O2 fluxes over a seagrass meadow using the aquatic eddy covariance technique, while measuring variables that have been shown to affect metabolism (PAR, temperature, O2 concentration, current velocity).
Accurate daily metabolic estimates of respiration, gross primary production, and net ecosystem metabolism are widely used to assess ecosystem health and blue carbon sequestration and storage in vegetated coastal ecosystems. Aquatic eddy covariance allows direct measurements under naturally varying in situ conditions of oxygen (O2) fluxes between a benthic substrate and the water above. Here, we used hourly O2 fluxes measured with this approach to examine how respiration for a Zostera marina seagrass meadow varies through night and day, and how this affects commonly performed metabolic estimates. Fitting our database of 2,115 hourly benthic O2 fluxes for a seagrass meadow revealed that respiration decreased linearly by 29% through the night. We primarily attribute this to consumption of highly labile compounds that are formed by photosynthesis and accumulate during daytime. Furthermore, a corresponding linear increase in respiration through the day coupled with photosynthetic production described by a standard photosynthesis irradiance curve provided an accurate prediction of measured O2 fluxes (R2 = 0.993). These results document that night- and daytime respiration vary significantly in a seagrass meadow, and that both can be described accurately by a piecewise linear relationship. Many studies have questioned the widely used assumption in metabolic estimates that night- and daytime respiration are constant and equal. However, if night- and daytime respiration can be approximated as we found here by piecewise linear relationships, these standard means for calculating daily metabolic numbers remain valid. It is, however, important that such estimates are based on full 24-h records of benthic flux data.