One-dimensional Modeling of Macrobyte growth : part of the study Macrophytes and nutriments Dynamics in the upperreaches of the Scheldt Bassin

Abstract: Massive macrophyte development in the Flemish running waters in the northern part ofBelgium is a result of the improved water quality regarding organic loads in combination witha continuous high nutrient input. Since the macrophytes, higher aquatic plants with roots andstems, are contributing to the flooding problem in the Flemish regions by raising the waterlevel in rivers where the water level already is high, the macrophyte management is animportant issue in the Flemish areas. There are few existing studies of the influence of macrophytes on the nutrient dynamics inrunning waters. Several studies indicate however that macrophytes act as a biological filter,lowering the nutrient concentrations in the flowing waters. This would mean that themacrophyte communities would contribute to a biological cleaning of the water masses,playing a part in the solution of the eutrophication problem in the estuary. This study is a part of a project stretching over 3 years, which in the end will result in apredictive tool for macrophyte management, taking into account the risk of flooding as well asthe possible filtering effect of the macrophytes. The Mike 11 software from DHI in Denmarkis used to simulate the longitudinal hydrodynamics and transport properties of a small riverreach of the Aa-river. An integrated process modeling tool in the Mike software, the Ecolabmodeling tool, is used to build up the biogeochemical model. This will result in a realisticdescription of the macrophyte growth in the near future, in particular regarding its influenceon the nutrient dynamics. At this stage, it is possible to describe the hydrodynamics well for a typical winter period,whereas the hydrodynamic description is more complex during the summer period due tomacrophyte growth. More data are required to have a precise description of thehydrodynamics in the summer, and for the validation of the hydrodynamics in the winter. Thetransport properties of the river reach should be further validated using conductivity data andresults from tracer studies for high and low flow discharges, and the influence of the deadzones will be further investigated using a stochastic one-dimensional model. The biochemicalmodel set-up is only in the starting phase, and process studies of the nutrient uptake by themacrophytes have to be undertaken in order to further develop the biochemical model.