Plankton dynamics in a river-lake system – on continuity and discontinuity

Abstract

We studied the plankton dynamics in the shallow, rapidly flushed lake Neuendorfer See and an adjacent reach of the river Spree. During summer/fall, zooplankton and phytoplankton densities increased exponentially in the lake and decreased exponentially in the river without major changes in species composition. Both parts of the system can be described as tubular plug flow reactors that differ markedly in the growth rates of plankton. Whereas reproduction in rotifers was constant in the whole system, the mortality was about 10 times higher in the river compared to the lake caused by the filtration activity of abundant mussels. Physiological conditions can be regarded as continuous in the river-lake system. According to trophic conditions, the river-lake system is divided in two contrasting subsystems with the lake classified as `autotrophic' and the river as `heterotrophic' and the site of discontinuity was located sharply at the lake-river transition. At very low discharges, the differences between the subsystems were reduced and intra-pelagic mechanisms became more important. A model of plankton dynamics in the river-lake system is presented.