Dissolved oxygen dynamics in less-than-ideal environments
Citation:
Stecher, J., Ted DeWitt, AND D. Marois. Dissolved oxygen dynamics in less-than-ideal environments. Oregon Chapter of American Fisheries Society, Bend, OR, March 04 - 08, 2019.
Impact/Purpose:
Dissolved oxygen (DO) has a fundamental effect on both the health and the biogeochemical function of aquatic ecosystems, but measuring it over the temporal and spatial scales necessary to understand its dynamics presents some challenges. Specifically, sedimentary DO has a direct effect on denitrification, a major process in the removal of active nitrogen from the environment; this is particularly relevant to systems at risk from nitrate pollution. The present work details a new method for collecting long-term (weeks to months) oxygen time series data in tidal wetland soils without the expense and disturbance associated with frequent trips to the field. This work will be presented in a session on innovative uses of new technologies at the annual meeting of the Oregon Chapter of the American Fisheries Society in Bend, OR, in March 2019.
Description:
Dissolved oxygen (DO) has a fundamental effect on both the health and the biogeochemical function of aquatic ecosystems. In estuaries, changes in DO relevant to biotic systems often occur over a wide range of temporal and spatial scales, forced by tidal inundation and rainfall, organic matter degradation, benthic and water-column photosynthesis and respiration, and air-water exchange. A mechanistic understanding of DO dynamics and the interplay among these drivers often requires time series data collected in very targeted (and sometimes not easily accessible) places. The advent of relatively inexpensive, programmable optical DO loggers allows generation of high-resolution time series over weeks to months without the expense and disturbance associated with multiple trips to the field. However, challenges remain in affixing sensors where they are a) in scientifically relevant locations, b) safe from both environmental and vandalism damage, and c) reasonably practical to deploy and recover. In this work, we adapted a commercial DO logger (the Onset U26) to measure both sediment porewater and pore-gas oxygen dynamics as a function of tidal inundation, rainfall, and distance-to-channel in a mesohaline marsh in central Oregon. The intent of the work was to quantify the effect of water table changes on oxygen supply (both dissolved in pore water and in the gas phase) to the marsh soil in order to map the likely spatial extent of denitrification. The loggers were buried in the high marsh at various depths below grade by removing an intact block of soil, coring horizontally into the soil wall, installing the loggers, and then replacing the sediment block. The sensor end of each logger was placed distal to the sediment block which minimized any disturbance effect. Loggers were deployed for 6 months, allowing assessment of a wide range of tidal and precipitation effects. As expected, sites deeper in the sediment and further away from the channel were below the water table for a larger fraction of time, spent more time anoxic or sub-oxic, were less affected by tidal and precipitation forcing, and were slower to respond to that forcing. Implications for application of this method to other difficult environments will be discussed.