Recovery of the Velky Bolevecky pond (Plzen, Czech Republic) via biomanipulation – Key study for management

Abstract

Promotion of successful examples of biomanipulation and long-term monitoring is important for encouragement of similar activities, which are still relatively scarce. A project to improve water quality in the Velky Bolevecky pond (Plzeň, Czech Republic) began in 2006. The main activities in this project were: large scale fish reduction; stocking of predatory fish; usage of phosphorus binding coagulants; and reintroduction of macrophytes. During five years the impact of a eutrophic system with massive cyanobacterial blooms with low water transparency transformed into a clear water system with high water transparency and well developed macrophyte cover. All aspects relating to the ecological status of the water body such as total phosphorus concentration, chlorophyll-a concentration and cyanobacterial and other phytoplankton density improved significantly during or immediately after the project realization. For example, water transparency increased three fold. Zooplankton density also increased, however the difference was not significant. Oligotrophication of the system became evident also in the species composition of zooplankton and phytoplankton. Species adapted to nutrient reduction and macrophyte cover developed. Seven years after the project was over, the fish stock remained on the low densities naturally. A good ecological stage was achieved with low nutrient levels, high water transparency, very low densities of cyanobacteria and other phytoplankton together with a well-developed macrophyte cover. All of which has been maintained without any expensive impact. The highest expenditure is, paradoxically, due to macrophyte harvesting because their massive explosion in the pond is not in accordance with recreational and sporting utilization. It seems, that reduction of nutrient loading and switching of a formerly eutrophic system to an oligotrophic one is possible and sustainable over a relatively long time period in a shallow temperate pond. Subsequent good management of the system will, of course, be necessary.