Abstract
While reduction in nutrient loading is a prerequisite for mitigation of harmful cyanobacterial blooms in nutrient-enriched waters, in certain surface waters eutrophication control is not always feasible due to practical and economic constraints or might be effective only in the long run. Yet, the urgent need to control cyanobacteria in water for drinking, irrigation, aquaculture, industry and recreation has spurred the development of a plethora of alternative methods that claim to be fast acting. Here, we provide a critical overview of several of these end-of-pipe measures: effective microorganisms (EM®), golden algae (Ochromonas), plant/tree extracts, ultrasound and artificial mixing of non-stratifying waters. Most of the end-of the pipe measures claim to provide sustainable control of harmful cyanobacterial blooms, while at best only targeting symptom relief rather than eutrophication relief. Support for “effective” microorganisms, golden algae, plant extracts, ultrasound and artificial mixing of non-stratifying waters to diminish eutrophication problems such that the resulting water quality meets societal and legislation demands is limited, and several proposed underlying mechanisms are doubtful. None of these curative measures seem the desired wide applicable solution to cyanobacterial nuisance; they should not be considered Columbus’s egg. A critical evaluation of end-of pipe measures is crucial for water authorities in their choice for mitigating measures.
Highlights
Excessive nutrient loading is the major cause of water blooms of cyanobacteria, i.e., elevated densities throughout the water column (Conley et al 2009; Smith et al 1999)
The reduction in external nutrient loading is a prerequisite for improvement, but lake recovery can be delayed for decades due to internal phosphorus loading (Søndergaard et al 1999)
Hazardous cyanobacterial blooms may remain in these waters for many years
Summary
Excessive nutrient loading is the major cause of water blooms of cyanobacteria (blue-green algae), i.e., elevated densities throughout the water column (Conley et al 2009; Smith et al 1999). In a study with spent medium of Stratiotes aloides in which two strains of M. aeruginosa were grown, a delayed lag phase was observed, but as exponential growth rates were equal or slightly higher than the controls (Mulderij et al 2005), these exposed cyanobacteria apparently caught up rapidly pointing toward rapid decline of the active compounds It remains to be seen whether field applications of plant extracts or allelochemicals can produce a window of clear water long enough for submerged and presumably allelopathic active macrophytes to establish. We advise against artificial mixing of shallower waters, as sediment release of phosphorus may fuel cyanobacterial blooms rather than mitigate nuisance by cyanobacteria
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