Hemmings, John C.P., Meric A. Srokosz, Peter Challenor and Michael J.R. Fasham

Southampton Oceanography Centre, European Way, Southampton SO14 3ZH, UK, Tel: +44 23 8059 7793, Fax: +44 23 8059 6400, E-mail: J.Hemmings@soc.soton.ac.uk

Parameterizing the microbial loop: an alternative to a size-structured model

The structure of the plankton food web in the upper layer of the ocean has important implications for the export of material from the euphotic zone. While the action of the microbial loop causes material to be recycled in the upper layer, the activity of mesozooplankton leads to a significant downward flux of material. The balance between these pathways must be properly represented in climate models to predict carbon export. However, the number of biogeochemical compartments available to represent the food web is limited by the need to couple biogeochemical and physical models. A parameterization is explored which allows both pathways to be represented in a 4 compartment model. The model's key feature is a recycling pool comprising all zooplankton grazers, bacteria and dissolved organic nitrogen. The fraction of this pool representing mesozooplankton is modelled as a function of the other state variables (dissolved inorganic nitrogen, phytoplankton and detritus), formulated to reflect the likely availability of suitable phytoplankton and/or detritus for mesozooplankton grazers. Its value, increasing as conditions change from oligotrophic to eutrophic, affects the dynamics of the recycling pool.

The extent to which the 4 compartment model is able to reproduce annual cycles from a 10 compartment size-structured model with an explicit microbial loop is investigated. The models are run with external forcing data representing oligotrophic and non-oligotrophic regions of the North Atlantic. Free parameters in the simpler model are optimized to reduce the discrepancy between output variables common to both models and the remaining differences are examined.