Habitat recovery and restoration in aquatic ecosystems: current progress and future challenges

1. Aquatic ecosystems are degraded by a variety of pressures as a result of the growing human population. Global-scale impacts include homogenization of biological communities, removal of top predators and ecosystem engineers, chemical pollution by excess nutrients and contaminants as well as deteriorating structural diversity, connectivity and process dynamics. There is a pressing societal need to reverse the decline in biodiversity and replace lost ecosystem functioning and services in aquatic ecosystems by enabling natural recovery or by active restoration. 2. Common concepts and approaches for conservation, recovery and restoration in freshwater and marine ecosystems, aided by recent advances in ecological theory, include decision criteria on priorities for conservation, harnessing natural recovery by cessation of impacts, restoring connectivity and meso-habitat diversity as well as the geomorphological structural template including hydrodynamic processes. Re-oligotrophication at catchment or regional sea-scale benefits from integrating freshwater and marine restoration. Species or assemblages that convey biogenic structure or act as ecosystem engineers and keystone species should be given priority. Top-down control can be reinstated in closed systems. 3. Differences between freshwater and marine ecosystems include the greater spatial restriction of many species in fresh water, the importance of rooted vegetation and insects in freshwater, and the much greater dispersal and connectivity in marine systems. These differences dictate different approaches, with more scope for active restoration work in fresh water and harnessing natural recovery in marine systems. 4. Restoration schemes need clearly defined target states. They should generally take a process-oriented and stepwise adaptive management approach judging success against reference or control sites. Societal and political expectations need to be managed and restoration schemes should not promise too much. Even minor rehabilitation of degraded ecosystems can put back some biodiversity and key services. Sometimes 'Ersatz'-ecosystems are better than nothing and the best that can be achieved, especially in urban settings. Copyright (C) 2016 John Wiley & Sons, Ltd.