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Physical-Biological Control of New Production within the Seasonal Thermocline

This project was a NERC responsive-mode project which was co-funded by the Defence Science and Technology Laboratory and ran from 2003 to 2006. The key institutes and scientists involved were:

Research Aim

The main aim of this research was to investigate the generation and dissipation of turbulence in the thermocline, and to quantify how the resulting mixing (supplying nutrients and controlling the light experienced by the algae) affected the growth of phytoplankton within the sub-surface chlorophyll maximum (SCM).

Research Objectives

The objective of the research was to test the general hypothesis that, in seasonally-stratified shelf waters, temporal and spatial variability of the rate of 'new' production is determined by the degree of coupling between physical and biological processes within the seasonal thermocline. In particular it has been suggested that physiological adaptation by phytoplankton making up the subsurface chlorophyll maximum drives a biological pump for extracting nitrate from the bottom mixed layer that is sensitive to internal mixing and to external climatological factors.

Research Approach

The sampling and experimental strategy was based on established methods for obtaining compatible, high-resolution vertical profiles (alternate CTD and FLY) and sections (SeaSoar) of physical, chemical, and biological parameters, allowing quantification of vertical fluxes and primary production on tidal and internal wave time scales. The cruise schedule (with associated mooring deployments) for the project is below.

Cruise Schedule

Moorings

^* = metres above seabed

Project Outcomes

The main products of the research were:

• The first large scale interdisciplinary study of the dynamic processes that determine the properties of the SCM in NW European shelf waters.
• Substantive advances in quantifying internal mixing in NW European shelf seas, and in understanding how primary production is controlled by this small-scale turbulence.
• New parameterisations for internal vertical mixing, and the response of primary production, for use in coupled numerical models.
• Improved capability for estimating primary productivity in stratified waters by satellite remote sensing (ocean colour, sea surface temperature, wind mixing etc.)
• An overall development of our understanding of the dynamics of ecosystems that support important fisheries, and of our abilities in predicting ecological and biogeochemical responses to variations and changes in the climate of the marine environment.