Lake Ellesmere / Te Waihora - State of The Lake and Future Management

State of The Lake and Future Management

In a 2010 report on lake water quality Lake Ellesmere / Te Waihora was deemed the second most polluted lake in New Zealand in terms of nutrient content and algal growth. Algal blooms are a regular summer occurrence and toxic algae bloomed in the lake in 2009. Furthermore most tributaries to the lake exceed the contact recreation guidelines for faecal coliforms and levels of faecal coliforms in Boggy Creek and Doyleston Drain frequently exceed the stock-drinking water guideline value.

Results for E. coli levels are also poor with 42% of sites associated with the lake failing national recreational guide standards for water quality. No sites achieved the shellfish/food gathering standard or were fit for drinking. However there is some debate over the definition of the trophic status of Lake Ellesmere / Te Waihora. Although the lake has high nutrient and phytoplankton values that place it in the category of hypertrophic (extremely enriched) the lake does not exhibit many of the characteristics of such a classification. For example it does not regularly undergo severe oxygen depletion, nor does it produce unsightly toxic algal blooms or fish kills, unlike other lakes in the area with the same trophic status. Furthermore it supports abundant fish and bird communities.

Recently it has been recognised that the combination of abstraction and climate was causing adverse effects on groundwater levels that in turn adversely affected the spring fed stream discharge into Lake Ellesmere / Te Waihora. The dependency of lake on the ground water system in its catchment cannot be reduced, but it is to be hoped that by managing groundwater abstractions during times when the inputs to the aquifer system are low the output system to the lake will be maintained at a level that ensures the protection of Lake Ellesmere / Te Waihora. Being a low land lake Lake Ellesmere / Te Waihora not only receives inputs within the immediate vicinity of the lake but also from the wider catchment across the plains to the foothills. The quality of lake tributaries reflects the intensive land use surrounding them, with elevated nutrients and bacteria found in many sites. This has implications for the scale of management issues. The dependency of Lake Ellesmere / Te Waihora on groundwater sourced input has resulted in local authority Environment Canterbury implementing a restorative programme for lowland streams. Riparian protection around the lake margin and tributaries will greatly help reduce some contaminant inputs such as sediment and phosphorus, but catchment wide nutrient and water allocation management will be needed to reduce nitrates and improve freshwater inflows to the lake.