Dynamics of fecal coliform and culturable heterotroph densities in an eutrophic ecosystem: Stability of models and evolution of these bacterial groups.

Time series of a population of fecal coliforms and a community of total viable counts were recorded during years 5 and 6 after the "birth" of an eutrophic aquatic ecosystem (sewage treatment lagoons). These time series were used to re-examine models, previously published, describing their temporal dynamics as well as the relationships between bacterial and environmental variables. The dynamics of the fecal coliforms and their relationships to the environment were unchanged; the fecal coliform abundances displayed an annual cycle with maximum reduction in numbers during the summer, which would be due at least partly to environmental variables (hypotheses of control by irradiance and pH, which have a seasonal behavior, are supported by the data). On the contrary, the total viable count dynamics moved towards a closer dependence on phytoplankton, from a situation of relative independence with respect to other biotic components of the ecosystem. Indeed during the first two years, only one of the abiotic variables in the model (the biological oxygen demand, which is an indicator of available organic matter) seemed to have an effect on the total viable counts. The behavior of these bacterial groups, measured during 1980-1982 and 1984-1986, shows that demographic and ecological laws founded on the observation of other organisms also apply to heterotrophic bacteria. A population, such as the fecal coliforms in the present study, has a limited ecological amplitude and is then more likely to react to environmental variables such as irradiance, pH, and phytoplanktonic metabolic products, whose bactericidal action is highest during the summer months and lowest during winter. On the other hand, a community such as that detected by the total viable counts of the present study is composed of many species and thus has a larger ecological amplitude. This makes it easier for the species to occupy the various available habitats and to maintain themselves through ecological succession and endogenous rhythms.