PS 47-37
“Oxygen Probe”: Using planar optodes to quantitatively measure oxygen concentrations and to describe oxygen dynamics in salt marsh sediments over time
Salt marshes, located between coastal ocean and coastal watersheds, are extraordinarily productive ecosystems found in estuaries worldwide, and are often heavily influenced by human activities: many receive high nitrate input from land, degrading water quality and in some cases causing harmful algal blooms and low oxygen zones. Northeastern saltmarshes are dominated by the cordgrass Spartina alterniflora; its roots are a conduit for oxygen to otherwise anoxic sediment. The availability of oxygen belowground could influence the fate of pollutant nitrate, making quantification of oxygen over time important. We hypothesized that sediment oxygen dynamics occur on and around roots of Spartina, and that changes in oxygen availability occur as a result of aboveground changes in light conditions. In lab, we maintained Spartina in three sediment blocks taken in-situ from Plum Island Ecosystem Long Term Ecological Research site, and used oxygen-sensitive planar optodes to image their root systems within the sediment, with aboveground light conditions changing every 12 hours. We took images every 3-4 minutes, with each block imaged for two ‘light on’ (sunrise) events separated by a ‘light off’ (sunset) event. We used a Licor6400 gas exchange system to measure aboveground light, stomatal conductance, and photosynthetic activity of leaves every 30 seconds.
Results/Conclusions
Analyzing the images produced using planar optodes, we quantified oxygen concentrations through time for the three blocks of sediment and their associated root systems. Using an ANCOVA-based statistical analysis, we found thousands of regions where mean slope of oxygen concentration change during the two hours before a light change was significantly different from mean slope of oxygen concentration change during the two hours after. These areas fell around the roots, demonstrating that light changes aboveground affect oxygen concentration around roots, even deep in sediment. In one sediment block, in which roots grew rapidly over the course of imaging, patterns of oxygen concentration changes correlated with location along the root: areas around mature sections showed the opposite change as areas around root tips, where tissue is not yet mature. For example, in response to ‘sunrise’, mature root sections exhibited a shift towards oxygen supply while root tips exhibited a shift towards oxygen demand. The opposite pattern emerged in response to ‘sunset’. The overall pattern for all sediment blocks showed that significant changes in oxygen concentration are correlated with light changes aboveground, and these changes are associated with locations on or around roots and not randomly distributed in the saltmarsh sediment.