Eutrophic Freshwater Ecosystems Research Articles

Abundance and feeding of microheterotrophic flagellates from a eutrophic lake

The growth of planktonic bacteria from a eutrophic lake was evaluated with microflagellate predators present and absent. Differential filtration (50 and 8 µm filters) was used to exclude ciliates and larger zoo-and phytoplankton from replicate experimental cultures. Additional filtration (1 µm filter) excluded heterotrophic microflagellates from a second set of experimental cultures, producing cultures that contained either bacteria and microflagellates or only bacteria. Growth of bacteria and microflagellates was evaluated by epifluorescent microscopy from repeated sampling over approximately 200 h. Bacterial numbers were reduced in the presence of microflagellates, and microflagellates were observed to contain bacterial prey. However, microflagellate numbers were high (about 106 cells ml-1) and were less than an order of magnitude lower than bacterial numbers. Bacteria growing in the presence of microflagellates did not show predator-prey population oscillations but had in-phase oscillations in numbers, suggesting that microflagellate predation in freshwater may not control numbers of planktonic bacteria. Clearance rates of heterotrophic microflagellates, estimated to be only 30 body volumes hr-1, were insufficient to maintain flagellate growth, suggesting that other energy sources may be needed to maintain microflagellates in eutrophic freshwater ecosystems.

A minimal model of eutrophication in freshwater ecosystems

Abstract A very simple model of eutrophication is presented. It includes several quite aggregated parameters and can be studied by analytical methods. Nevertheless, its dynamic behaviour reflects well real reservoir evolution as observed under the impact of increasing biogenic pollution. Such models, simple in construction but sensitive to the main trends of an ecosystem, are termed ‘minimal’. The state variables in the model are the concentrations of: (1) phytoplankton; (2) biogenic elements (nutrients); (3) detritus; and (4) dissolved oxygen. Transformations among the substances are described by a system of four ordinary differential equations. Steady-state dynamics are studied (the so-called quasi-stationary process). The qualitative analysis undertaken shows that the total amount of substances in a reservoir (phytoplankton + detritus + nutrients) is a very important ecosystem control parameter. In fact, it is this parameter that determines the rate and the degree of eutrophication. It also turns out that the relations between some observed characteristics of the ecosystem may define beforehand the future behaviour of the reservoir.