Lake Hayes is probably one of the most photographed lakes in New Zealand.
Underneath its alpine beauty the 276ha, 33m-deep lake holds highly polluted water and lake bed smothered with historical sediments and nutrients from predominantly pastoral catchment, mostly dumped by Mill Creek.
Over the summer the lower lake water column becomes oxygen deficient, which is hostile to aquatic life. The depth of the oxygen deficient water column has been increasing steadily over the past two decades and rendering 75% of the lake water oxygen deficient during summer.
The Otago Catchment Board recognised the issue in the 1960s. In the 1990s, its successor Otago Regional Council (ORC), in collaboration with Queenstown-Lakes District Council (QLDC), launched its ambitious Lake Hayes Management Strategy.
There were signs of water quality improvements but the lake health remained stubbornly poor.
With rising pollution episodes, local community (for example the Friends of Lake Hayes) interest also grew with the view of restoring the lake.
ORC has responded by committing to restoration in consultation with the community. This is welcome, and an opportunity to showcase effective restoration, given many lake restorations around the world have become lifelong white elephants.
My literature research indicates copycat lake restoration is risky, given wide-ranging lake dynamics. I attribute many of the failures to a lack of sound planning/rationale and critical information including nutrient budgets.
Mill Creek, despite its very good water quality grade, could still dump 6 tonnes of nitrogen and 0.3 tonne of phosphorus annually into Lake Hayes.
The lake holds more phosphorus than nitrogen needed for algal blooms which is more dangerous than having more nitrogen and less phosphorus. This is because cyanobacteria, also known as blue-green algae, thrive on phosphorus and can introduce atmospheric nitrogen into the lake to satisfy its nitrogen needs.
Adding more nitrogen into the lake can make the lake worse. To date, the key algal nutrient driving poor lake water quality has been phosphorus.
Thus, the priority should be to reduce phosphorus from entering the lake from the catchment and to remove historical phosphorus stored in the lake. I am confident catchment phosphorus runoff losses can be reduced by currently available mitigation measures.
I believe the most effective long-term restoration rationale is by permanent removal of phosphorus accumulated in the lake system. I say this because there are restoration methods which can stabilise or immobilise lake phosphorus and store it at the bottom of the lake. The fundamental issue with the above approach is, with time, stabilised phosphorus can be mobilised by bacterial activity and released back into the water for algal blooms.
Let us look at the restoration methods available to do the job. They can be classified as physical, chemical and biological.
Despite the extensive use of many methods, I would not consider any as silver bullet or cost effective.
Physical methods (for example flushing, aeration or dredging sediments), while intrusive, are less risky. Dredging can remove phosphorus permanently, however, it needs sufficient land to reuse the nutrient-rich material. Aerating the lake with numerous air diffusers (similar to wastewater treatment systems) can provide well-needed oxygen but permanent phosphorus removal is poor. Hence aeration may have to be permanent.
Flushing with good water can dilute and oxygenate polluted water and induce phosphorus flushing. The intrusive stand-alone flushing projects may succeed if adequate and correctly located lake inlets and outlets are engineered to avoid dead lake zones. If the discharge effects can be managed, flushing can be used in combination with dredging to promote phosphorus exit.
Ideally, we stay clear of chemicals. Applying tonnes of phosphorus binding agents such as alum can reduce phosphorus from water temporarily. Unfortunately, regular dosing is required to stabilise re-released phosphorus. This means a permanently installed aluminium dosing facility nearby. After all the hard work, we are stuck with large amounts of phosphorus and aluminium at the lake bottom. There are also eco-toxicity issues with the use of aluminium.
Iron-based chemicals cause fewer issues but phosphorus will continue to remain in the lake. Biological or bio-manipulation methods offer limited success but can be used as a final polishing process to reduce algae or to improve biodiversity.
This costly and timely community project is definitely not in the ''suck-and-see'' category. Project planning to restore alpine lakes must be based on precautionary principles and sound scientific rationales with little or no unintended outcomes.
In addition to sound planning and being precautionary, patience and perseverance are also essential for ORC and its community to succeed.
If so, I am positive we can get there with best bang for the buck.
-Dr Selva Selvarajah is an Resource Management Act consultant and researcher (Enviroknowledge). He is also a former senior ORC staff member.
