Sediment-Water Column Exchange of Nutrients in Coastal and Shelf-Sea Waters

Lead Research Organisation: University of Portsmouth
Department Name: Sch of Earth & Environmental Sciences

Abstract

Over the last 50 years concentrations of the nutrient elements nitrogen, phosphorus and silicon entering coastal seas such as the North Sea have increased due to Human influences. Coastal seas are very important in controlling the cycling of these elements essential to life on Earth, particularly as nearly a third of the world's ocean primary production takes place in these regions. Primary production is the growth of phytoplankton - floating microscopic plants also known as algae. Algal growth, which is fuelled by these nutrient elements, takes up carbon dioxide from the atmosphere, thus helping to regulate atmospheric concentrations of this greenhouse gas. Until recently, it was thought that the North Sea receives enough nutrients from rivers and the atmosphere to sustain the large phytoplankton blooms that are observed there. At the end of spring; the phytoplankton have used up nearly all the available nutrients in the water column and they die and sink to the sea bed. It was thought the regeneration of nutrient levels occurring over the proceeding winter period mainly from rivers and the oceans was sufficient to start the cycle again in the spring. However, it has been discovered that the actual amount coming in from rivers and surrounding seas is less than that needed to start the cycle again in the spring. It has now been hypothesised that a sediment source released during the stirring up of sediments (i.e. through resuspension of the surface layers of the sea bed) elevates the concentration of nutrients in the water to levels high enough to stimulate large phytoplankton blooms in the spring when light levels also favour algal growth. Sediment resuspension can be caused by strong winds, waves, tides and also by trawling gear used in fishing. In this proposed study, we will investigate the impact of sediment resuspension on nutrients and determine if it is either a source (puts nutrients from the sediment into the water) or a sink (takes nutrients out of the water into the sediment). To do this study we will take part in research cruises to 3 areas of the North Sea where we will place equipment on the sea bed to simulate the stirring of the sediments. We will collect data on sediment resuspension and nutrient release as well as make measurements of background transfer of nutrients without resuspension. We will also collect sediment to study the same processes in the laboratory under more controlled conditions. The data will be used with computer models of this system to test the models and help to provide better predictions of the impact of resuspension events on nutrients in the water column. The results of the proposed project will provide us with a better understanding of the role of nutrients and sediment interactions in coastal and shelf seas and the influence this has on primary production. Consequently, this work will contribute to quantifying the role of resuspension in controlling the fate of nutrients in regional and global budgets. The gathered data will give modellers who produce mathematical descriptions of these systems, additional tools and information to help effectively manage the quality of coastal waters.
 
Description This project was undertaken in collaboration with Cefas and funded by NERC and Defra as part of the Marine Ecosystem Connections (MEC, ME3205) project. In this project we have demonstrated the importance of resuspension events for evaluating nutrient fluxes and for changing the thinking that resuspension fluxes both anthropogenic (trawling) and natural need to be incorporated into future biogeochemical models. The findings from this work have been disseminated at a number of international conference (ALSO/AGU) and published in a special issue of biogeochemistry (Couceiro et al. 2012). Other key findings which are currently in the process of being submitted to Limnology and Oceanography as a manuscript were found to be that resuspension leads to net release of phosphate to the water column while diffusive fluxes are into the sediments; important in P limited eutrophic areas. The release of phosphate and dissolved silicon during resuspension greatly exceed diffusive inputs to the water column and removal of nitrate is increased by resuspension. The increase in water column phosphate and particularly dissolved silicon, exceed that predicted for release of pore waters - inferring important water column release processes are occurring. This study has shown that resuspension is potentially an important pathway for resupplying the water column with nutrients before and during phytoplankton blooms and should therefore be considered along with diffusive fluxes in future ecosystem models.
Exploitation Route Important for Cefas and Defra to further understand nutrient fluxes in UK coastal waters and ultimately health of UK seas and sea-bed habitats. This research can be used by biogeochemical modellers to start to model the benthic exchange of nutrients in cohesive sediments which currently relies on diffusive fluxes and does not take into account resuspension, thus missing a large flux both in and potentially out of the water column. This will then help further understand nutrient biogeochemical models for example understanding eutrophication.

The main users of this research are Defra through Cefas. Marine Ecosytem Connections was a project run by Cefas on behalf of Defra, our data has feed into this larger project which subsequently has been used by Cefas to feed through to Defra in terms of milestone reports when assessing trawling nutrient resuspension versus ambient fluxes. One of the reports submitted by Defra is:

Clare Coughlan, Johan van der Molen, John Aldridge, Ruth Parker, Dave Stephens (2011) Marine Ecosystem Connections: essential indicators of healthy, productive and biologically diverse seas - Impact of climate variability and trawling on ecosystem dynamics: GIS and ERSEM model analysis. Defra Milestone Report ME3205 pp22.
Sectors Environment

URL http://www.cefas.defra.gov.uk/our-science/ecosystems-and-biodiversity/marine-ecosystem-connections.aspx
 
Description The data published in the Biogeochemistry 2012 article has been used by Cefas in their reports to Defra and is being used in future decisions regarding funding directions for MSFD. Information from project (and publication's) was used by Cefas to inform Defra through their feeder and final reports for MEC's.
First Year Of Impact 2012
Sector Environment
Impact Types Societal

 
Description Small plants may hold big answers 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Media (as a channel to the public)
Results and Impact Press release

Impact was further interest from non-peer reviewed publications
Year(s) Of Engagement Activity 2008