Modelling bacterial biomass and activity in the River Meuse (Belgium)
The phytoplanktonic biomass (chlorophyll-a) and primary production (incorporation of 14CO2), as well as the bacterial biomass (enumeration by epifluorescence microscopy after acridine orange staining) and bacterial production (3H-thymidine incorporation) were followed for two years (1983 and 1984) in the section of the River Meuse located between the Belgian-French border and Dinant (figure 1). All these parameters show seasonal fluctuations with minimum values during winter and maximum values during summer (figure 2). The analysis of these data show the connection between primary production and bacterial activity in this area. Despite recent progresses in microbial ecology, most models describing organic matter degradation in aquatic ecosystems still basically derive from the STREETER and PHELPS's (1925) model of organic matter pollution, in which the rate of decomposition is assumed to be first order with respect to the organic load. Although these geochemical approaches often yield quite good predictions, they consider organic matter degradation as a chemical property of the organic matter itself, without taking into account the activity of the micro-organisms. Based on a study of the successive stages involved in the interaction between organic matter and heterotrophic bacteria, a microbiological model of bacterial degradation of organic matter in aquatic systems has been developed (BILLEN and SERVAIS, 1988). The main features of this model, called the H3SB model, can be summarized as follows (figure 3) : organic matter is mostly supplied by primary production and allochtonous inputs in the form of macromolecular biopolymers (H) which cannot be taken up directly by bacteria. Only some small molecules, recognizable by the bacterial permeases, can be used by bacteria. These compounds called direct substrates (S) are produced by exoenzymatic hydrolysis of the high molecular weight compounds. Among these compounds, we distinguish three classes : H1, compounds which are easily and rapidly hydrolyzed into direct substrates; H2, compounds which are slowly hydrolyzed, and H3, the refractory organic matter. Once taken up by bacteria, direct substrates can be either catabolized and respired or used for the production of bacterial biomass (B). Once formed the bacterial biomass is subject to a mortality process (grazing by heterotrophic microzooplankton or lysis). This view of the interactions between bacteria and organic matter is certainly idealized. Its usefulness however lies in the fact that excellent methods have been developed for directly measuring the rate of most of the basic processes listed above (exoenzymatic hydrolysis, uptake of direct substrates, bacterial production and bacterial mortality) and for studying their kinetics and control mechanisms. Mathematical relationships have been proposed for the H3SB model. Equations (1), (2), (3), (4) and (5) describe respectively the time fluctuations of H1, H2, H3, S and B. Exoenzymatic hydrolysis and bacterial uptake of direct substrates are characterized by MICHAELIS-MENTEN kinetics, bacterial production is proportional to substrate uptake and bacterial mortality is represented by a first order term. This basic model of biodegradation has been used to build a model of heterotrophic activity, in the section of the River Meuse studied, taking into account a simplified form of the hydrodynamical processes. Three types of organic matter inputs to the river have been considered : phytoplankton Lysis, domestic loads and soil leaching. The values of the parameters involved in this model have been determined by different experiments performed on the natural population of bacteria from the River Meuse (figure 4), or by fitting experimental data of biodegradation experiments performed on organic matter from various origins : for example, River Meuse organic matter (figure 5) or phytoplanktonic material (figure 6). This model of heterotrophic bacterial activity in a river has allowed the seasonal fluctuations of a bacterial biomass and production in the river to be assessed, giving the known hydro-dynamical parameters (wet section and variations of discharge), temperature, domestic load (calculated from the population density) and phytoplanktonic biomass. The data of the calculations of this determinist model of heterotrophic activity agree well with the experimental date (figures 7, 8, 9, 10). This kind of bacterioplankton model could in the near future be used to construct a complete ecological mode of rivers leading to studies on the impact on mater quality.