Lake restoration studies: Failures, bottlenecks and prospects of new ecotechnological measures

This paper critically reviews the published works on lake restoration in north-western Europe, with the aim to highlight the causes of failures of lake biomanipulation, and to identify the main bottlenecks that have impeded progress. More importantly, we explore the prospects of applying new ecotechnological measures to lakes with a focus on shallow lakes. These complementary measures are: (1) reduction of sediment resuspension; (2) water-level management; and (3) the use in shallow lakes of bivalves as effective grazers on lake seston, especially when cyanobacteria are dominant. If the sustainability of the positive effects of biomanipulation is considered over a decade, there are probably more cases of failures than successes. The failures can be ascribed to several bottlenecks that include: (1) inadequate reduction of allochthonous phosphorus (P) and an increase in autochthonous P inputs, i.e. release of P from the lake sediments following reductions of external P inputs; (2) poor edibility of filamentous and colonial cyanobacteria to daphnids; (3) inadequate coverage of the lake area by macrophytes partly due to foraging on the macrophytes by both fish and birds; (4) ineffective reduction of planktivorous fish biomass and our inability to maintain the fish mass to a ‘low level’ for longer periods; and (5) failure of northern pike ( Esox lucius) after its transplantation to the lakes to develop a population level that can control planktivorous fish to desired low levels. Three potentially complementary ecotechnological measures are discussed. The first such measure concerns prevention of sediment resuspension in lakes by creating islands in order to minimise the wind fetch to reduce the wave amplitude. The second measure involves allowing greater water-level fluctuations (WLF) in lakes as planned in lowland countries like the Netherlands; WLF are likely to allow more space for water , and may lead to improved water quality and higher biodiversity. The third ecotechnological measure relates to grazer populations that complement herbivorous zooplankton to regulate phytoplankton, particularly to control cyanobacterial blooms. For this, the bivalve Dreissena polymorpha appears to be a good potential candidate for grazing on phytoplankton, especially in shallow eutrophic lakes that are dominated by filamentous and toxic cyanobacteria (e.g. Planktothrix agardhii and Microcystis aeruginosa).