Nutrient removal processes in saltmarsh and adjacent habitats

The saltmarsh habitats across England, and the remainder of the UK, can remediate nutrients such as ammonium, nitrate and phosphate that may otherwise pollute the marine environment. Multiple processes can contribute to this remediation, across varying timescales, including sediment burial and plant/microbial uptake.

Nitrate, that can also lead to marine dead zones when in excess, can be microbially-processed through denitrification, which transforms it to the environmentally benign di-nitrogen gas (N2).

The rates at which denitrification proceeds can vary due to sediment context, and over seasonal cycles of vegetation/microbial production and loss. The Environment Agency (EA) is particularly interested in quantifying variation in rates across contexts and season in saltmarshes under their remit i.e. in England.

Extending a pilot study in 2023/2024, based on one marsh in one estuary in two seasons, and working with Bangor University and their Wetland Hydroperiod Simulator, we aim to provide key data on denitrification rates to understand this important nitrogen removal process. This understanding can inform extensions to the Centre for Fisheries and Aquacultural Sciences’ (CEFAS) Combined Phytoplankton and Macroalgae (CPM) model.

1. Extend the pilot study to a national survey of six estuaries, either with muddy or sandy sediment, and two marshes within each estuary, to understand spatial variation in denitrification rates and the role of sediment.
2. Extend the pilot study to an annual survey of one marsh in each of one muddy and one sandy estuary to understand seasonal variation in denitrification rates across environments.

• Extraction of paired sediment cores and porewater concentrations from multiple locations across saltmarsh vegetation zones – upper, mid and lower shore.
• Co-located vegetation characterisation and analysis sediment characteristics.
• Incubation of paired cores (with and without acetylene blocking) within the Wetland Hydroperiod Simulator to estimate potential denitrification rates under realistic tidal conditions.

These key data on denitrification could contribute to proposed extensions to the UK Saltmarsh Code, and help inform policies such as Nutrient Neutrality. Additional PhD research through a recently funded SuMMeR proposal led by Plymouth University, and co-supervised by UKCEH and Natural England, will help understand how nutrient remediation is valued in different contexts and whether or not it trades-off with biodiversity values, given a recent review that shows nutrient impacts on saltmarsh vegetation. Together with other highlighted research we will contribute to a broader national picture of the contributions saltmarshes make to human well-being.

Our Partners include Christian Dunn and Dan Aberg from Bangor University, John Aldridge from CEFAS, Lucy Stainthorpe from the Environment Agency. With thanks to collaboration with Jo Preston and Andy Van Der Schatte Olivier from the University of Portsmouth, who lead a tandem project on de-nitrification rates in Seagrass and Oyster beds.

The pilot project and national scale project were both funded by the Environment Agency.