Freshwater food web biomanipulation: A powerful tool for water quality improvement, but maintenance is required

Experiences with lake food web biomanipulation have involved three basic biomanipulation types, repeated in two freshwater environments with distinctly different physio-chemical characteristics. The manipulation types are planktivore additions, complete planktivore removals and planktivore control through piscivore addition. The efficacy of these manipulations is strongly dependent on whether the aquatic environments comprise deep lakes that stratify, or shallow lakes and ponds that are not subject to extended periods of summer stratification. The general results of these six biomanipulation categories are that planktivore additions to stratified lakes almost invariably result in reduced grazer biomasses and increased algal densities. Planktivore additions in shallow lakes or ponds also result in more algae, with the modes of action including: increased bioturbation, decreased grazer biomass and reductions in macrophytes. Planktivore removals in deep stratified lakes frequently result in initial food web-mediated reductions in algal standing stocks, but given sufficient time, ungrazable algae often dominate and total algal biomasses return to premanipulation conditions. Planktivore removals in shallow lakes or ponds tend to be quite stable due to initial decreases in bioturbation and increases in macrophyte cover and zooplankton grazing pressure. However, long-term studies suggest that even these initially very successful manipulations may be subject to deterioration due to macrophyte species substitutions, reinvasion by planktivore-benthivores, and algal species substitutions. The emerging consensus is that in most cases ‘some maintenance is required’. Piscivore additions to deep lakes that stratify, seldom result in more than short periods of algal control. Species substitutions among invertebrate predators, the proliferation of ungrazable algae and changes in nutrient recycling, frequently produce new restructured food webs capable of sustaining algal biomasses that rival those observed during premanipulation periods. Piscivore additions in shallow lakes and ponds have been more successful, but only as long as planktivore-benthivore biomass remains low. Success is further enhanced when macrophyte communities remain intact. However, long-term studies suggest that piscivore densities often decline and that piscivores seem unable to maintain the planktivore-benthivore community at the very low levels that are required. Again the emerging consensus is that in most cases, maintenance is required. None of this invalidates the biomanipulation principle. Many biomanipulations have produced quite spectacular initial results and a few have maintained low algal biomasses for several years. Monitoring and maintenance seems a small price to pay. Given that all of the alternatives to biomanipulation (i.e. sewage treatment in all its forms) are much more management-intensive and expensive, and given that biomanipulations can be applied to non-point source situations that cannot be treated with traditional engineering-based methods, the prospect of having to maintain biomanipulations remains, on balance, quite acceptable. In conjunction with nutrient abatement and with the implementation of well planned maintenance programmes, the cost benefits of biomanipulation seem unassailable.