A MODEL OF DITCH VEGETATION IN RELATION TO EUTROPHICATION

A functional model of a ditch ecosystem has been developed, aimed at d escribing the relation between nutrient input and water quality and do minant vegetation in drainage ditches. Its aim is the derivation of th e 'critical nutrient loading' for a shift from submerged vegetation to duckweed dominance. The model, called PCDitch, describes the competit ion between several functional groups of macrophytes, as well as algae . The macrophyte groups were defined according to the layer(s) in whic h they grow: submerged, floating or emergent, rooted or non-rooted. Th e model also includes the cycling of nutrients within the water, the s ediment top layer and the vegetation. The model has been applied to th e data of 8 experimental ditches located at Renkum (The Netherlands), which received different levels of nutrient loading during 4 years. Th e controls and the low-and medium-loaded ditches remained dominated by submerged plants, while in the high-loaded ones a dense cover of duck weed developed. In the sand ditches, submerged biomasses were lower th an in the respective clay ditches. An optimization study has been perf ormed for a number of sensitive parameters, minimizing the total sum o f squared differences between simulated and measured values for all di tches, resulting in a set of parameter values that gives the best over all fit. The parameters included the maximum growth rates, the minimum phosphorus contents and the overwintering fraction of the plant group s. The model simulations by PCDitch were grossly comparable to the fie ld observations, with duckweed in the high-loaded ditches and submerge d plants in the other ones, The fit for algae and charophytes remained poor. Further calibration as well as testing the model in field situa tions are recommended to improve the model's predictive value. (C) 199 8 IAWQ. Published by Elsevier Science Ltd.