REPRESENTATION OF EMILIANIA-HUXLEYI IN PHYTOPLANKTON SIMULATION-MODELS - A 1ST APPROACH

Emiliania huxleyi (Lohmann) Hay et Mohler is represented as a state variable in a general phytoplankton model that also includes one diatom group and one phytoplankton group encompassing the 'other flagellates' than E. huxleyi. Furthermore, three nutrient variables (nitrogen-, phosphorus-, and silicon-nutrients) are included. The main features of the model are that E. huxleyi has been given a higher growth affinity for orthophosphate than the two other groups, and that diatoms, besides being dependent on silicon, have been given a higher maximal growth rate than the two flagellate groups. The output from the simulation model is compared with observations made in mesocosm experiments loaded with different amounts of nitrate and orthophosphate. The simulated E. huxleyi is not able to grow as well as the real E. huxleyi, and this applies especially to the experiments low in orthophosphate. It is difficult to account for the high observed numbers of E. huxleyi in terms of the available inorganic othophosphate in these experiments. The literature, however, gives some evidence that E. huxleyi is able to utilise organic phosphorous sources. By assuming an organic phosphorus source that adds phosphorus to the orthophosphate pool, however, the fit between the model and the observations is improved. Furthermore, simulations indicate that the phosphorus originating from organic substances should be more available for the E. huxleyi than for the other groups. In simulation models this is achieved by defining a higher E. huxleyi uptake affinity for orthophosphate, but it may also be assumed that the orthophosphate originating by enzymatic activity on organic substances are spatially (at the scale of the uptake process) more close to the E. huxleyi cell than to the other phytoplankters present in the water body.