Turbulence as a control on the microbial loop in a temperate seasonally stratified marine systems model

Abstract

The European Regional Seas Ecosystem Model (ERSEM) has been coupled with the General Ocean Turbulence Model (GOTM) to create a 1-D representation of a seasonally stratified site in the North Sea. This model has been validated and shown to reproduce biomass and production measurements successfully. The model was then used to investigate the role of turbulence in transporting nutrients across the thermocline. It was found that the turbulence characteristics control the pumping of nutrients into the mixed layer and the export of carbon into the deeper layers. Hence primary production in the thermocline is driven by the import of nutrients and inhibited by the export of carbon. Furthermore it is demonstrated that the temporal variability of production is strongly influenced by fluctuations in solar irradiance. The effect of tidal mixing upon nutrient transport leads to a 23% increase in primary production compared to a simulation without tidal mixing. On a spring tide, nutrient pumping enhances phytoplankton growth. As the tide moves from springs towards neaps, the phytoplankton above the thermocline become nutrient stressed and undergoes lysis. The resultant release of dissolved organic carbon drives bacterial production in this region. The optimal position for grazers is found to lie where the peaks of bacterial and phytoplankton biomass overlap. External physical forcing is found to indirectly drive both the microbial loop and secondary production, thus demonstrating that changes in the stratification of the water-column influences the development of the microbial loop. Zooplankton biomass and grazing is substantially enhanced in the tidally forced model leading to an increase in the production of fast sinking POM which in turn impacts substantially on the flux of carbon to the seabed. Hence the degree of stratification ultimately influences the benthic pelagic coupling. Additionally, we present a simple scheme to classify both the physical and biological properties of the system.