Eutrophication: Once Happened, Can We Reverse It?

    So what happens if no methods are put in place to prevent eutrophication, or the measures fail, and the water body becomes algae-dominated and turbid? Can the eutrophication and associated impacts be reversed? And can this result in a recovery of water bodies and their ecosystems? Without getting too far into complicated details, some studies on lakes damaged by eutrophication have found a simple diversion of nutrient sources away from water bodies has helped recovery; for example Lake Washington (Edmondson, 1980). However waste pipe diversions took place before extreme effects of eutrophication had materialised.

Lake Washington.

    Contrastingly, a great deal of longer-term research demonstrates that once a threshold is crossed, a 'tipping point', which pushes the system into the low biodiversity, turbid state, there are many factors which prevent recovery to clear water (Kumagai and Vincent, 2003). Occasionally this is due to an insufficient reduction in nutrient levels, i.e. failure to stop agricultural run-off or sewage from reaching the water body. 
  
    However, usually systems appear to recover in tests such as those above, but additional factors later cause a move back to turbid water, even without the addition of more nutrients by humans. For example, leaving piscivorous fish in a recovering water body can cause (smaller) planktivorous fish populations to remain low, leading to decreased predation of algae and returning algal blooms (Kumagai and Vincent, 2003). Fish such as pike (often introduced into lakes by humans) also have to be removed for efficient recovery, as these disturb sediments, increasing turbidity and releasing stored nutrients, making it more difficult for plant communities to recover (Kumagai and Vincent, 2003). 

    Additionally, Sondergaard et al., 1996, demonstrated that grazing waterfowl (such as swans) can prevent lakes recovering after eutrophication. Birds were found to suppress the regrowth of plants in these lakes (required for the return from a turbid water state), negatively impacting water quality; many hypertrophic areas therefore also require enclosures protecting them from grazing. Gulati, 1995,  notes that many lakes have returned to a turbid state as recovery has only focused on reducing the amount of nutrients entering the water; biomanipulation such as the above is generally found to be necessary if water bodies are to improve after eutrophication. A classic case of this is the Norfolk Broads (such as Barton Broad), which has suffered chronic eutrophication, and required huge amounts of additional aid in the form of biomanipulation to even recover small areas.

    Because so many factors are involved in water body recovery, truly reversing eutrophication becomes basically unfeasible - these all need to be continually maintained, and how can we act on all these issues with limited monetary resources?