University of Bath

The influence of breathing sediment on oxygen dynamics and the marine benthos

We study in situ O2 dynamics to understand seasonality in sediment & water column processes in a coastal environment and coinciding effects on aquatic systems.

The primary goal of this study is to examine rates and controls of benthic O2 fluxes for shallow-water permeable sediments using EC measurements under field conditions that include naturally varying light and current flow. When possible, these EC measurements will be paired with simultaneous microprofiling. Ultimately, the influence of seasonal variations in organic matter (OM) on sediment-water O2 fluxes, and corresponding O2 distributions in the marine benthic region, will be characterised. In regard to water-column O2 dynamics and O2 budgets, this study will focus on

1) elucidating and quantifying the influence of algal blooms and subsequent precipitation to the sediment on a seasonal basis 2) assessment of benthic fluxes linked to historical water column data at a shelf location off the coast of Plymouth.

Objectives 1) Develop methodology to establish EC and microprofiling techniques based on technical requirements to allow for temporal and spatial scale analysis. 2) Characterise the influence of seagrass on coastal O2 distributions as a function of both EC and microprofile data. a. The EC captures the flux over a footprint, while the microprofiler is a point measurement, so this placement is ideal in determining the specific influence of the seagrass. 3) Incorporate environmental time series data with EC data to develop a cohesive picture of O2 dynamics, via assessment of turbulent O2 fluxes within the benthic region) linked to historical water column data at a designated coastal study site (L4) off the coast of Plymouth.

Project outline

Key biogeochemical processes at the sediment-water interface, including decomposition of precipitated algal matter and bio-irrigation, have been shown to have a significant influence on sediment-water oxygen (O2) uptake. Yet, this seasonal component of water-column O2 dynamics is frequently overlooked in assessments of O2 budgets. The main objective of this research is to comprehend in situ dissolved O2 dynamics on seasonal scales. This study focuses on elucidating and quantifying the sediment-water interface O2 budgets at a coastal site off the coast of Plymouth (UK) by providing a comprehensive analysis of the water column O2 budget, with particular focus on the influence of seasonal algal blooms (Fig. 1) and sedimentary biochemical on sediment O2 uptake. An eddy covariance system is used to assess turbulent oxygen fluxes from the bed, accompanied by supporting laboratory experiments evaluating the influence of bio-irrigation on sediment O2 uptake. All equipment is deployed over one seasonal cycle at the Western Channel Observatory (WCO), which is a long-term coastal monitoring site with extensive historical datasets for the water column (> 20 years) and benthos (> 50 years). The novel O2 data obtained via this project will be combined with the pelagic and benthic datasets to gain important understanding of the O2 dynamics at the WCO (Fig. 2).

Science

The main objective of this research is to understand in situ O2 dynamics with aim to improve understanding of seasonality in sediment and water column processes in a coastal environment and corresponding effects on aquatic systems. The primary project will provide a comprehensive analysis of water column O2 budgets, with particular focus on the influence of seasonal algal blooms on sediment O2 uptake in a historically significant coastal environment off the coast of Plymouth (in coordination with Plymouth Marine Laboratory). Studies the team have already conducted show different seasonal dynamics. This suggest that any processes consuming O2 within the sediment have to compete with water column processes (this has been looked at minimally in the literature and certainly not on a seasonal basis).

One of the more novel and effective methods of measuring benthic sediment-water O2 fluxes is the eddy correlation (EC) technique. This method allows for undisturbed, comprehensive assessment of O2 transport into and out of the sediment over a ~20 m2 footprint, taking into account sediment heterogeneity, bioirrigation, and various chemical reactions in the sediment. EC flux measurements are based on paired O2 concentrations obtained with O2-specific microsensors (Unisense A/S) and current velocities at a set location ~10 cm above the sediment. A major contribution to this project is the state-of-the-art aquatic Eddy Covariance kit (~£150,000) which is currently on loan to Dr Bryant from a collaborator.

Impact

This project assesses the key biogeochemical processes at the sediment-water interface, including decomposition of precipitated algal matter and bio-irrigations. I aim to measure the significance of influence on sediment-water oxygen (O2) uptake using the eddy covariance technique. The eddy covariance system is used to assess turbulent oxygen fluxes from the sea floor.