On the coupling of benthic and pelagic biogeochemical models

Mutual interaction of water column and sediment processes is either neglect ed or only crudely approximated in many biogeochemical models. We have reviewed the approaches to couple benthic and pelagic biogeochemica l models. It is concluded that they can be classified into a hierarchical s et consisting of five levels, differing in the amount of detail given to th e sediment processes. The most complex approach (level 4) fully couples wat er column processes to a vertically resolved biogeochemical sediment model. First simplification is achieved by using a vertically integrated dynamic sediment model (level 3); next is a reflective type of boundary (level 2) w here particulate material arriving at the sediment surface is instantaneous ly transformed into dissolved components. Then comes a set of models in whi ch either the bottom-water concentration of dissolved substances or the sed iment-water exchange is imposed (level 1). Finally, in some biogeochemical models, the bottom is plainly ignored (level 0). We have tested these various approaches in a coupled physical-pelagic-benth ic biogeochemical model for oxygen, nitrogen and carbon cycling in continen tal shelf areas. We discuss the various model approaches with respect to th eir impact on the pelagic system and point out some of the inconsistencies hidden in certain formulations. We conclude that lower boundary types, in w hich sediment fluxes or concentrations are imposed (level 1), are especiall y badly designed because they fail to assure conservation of mass. Finally, we suggest as best choice a level 3 approach in which the evolutio n of sedimentary particulate matter is part of the solution and when the bo ttom fluxes of dissolved constituents are parameterised based on mass budge t considerations. These simplified formulations represent the best balance between computational demand and attained accuracy. (C) 2000 Elsevier Scien ce B.V. All rights reserved.