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Phosphorus controls improve Rotorua water quality

Phosphorus controls improve Rotorua water quality

Dr Mike Scarsbrook, DairyNZDairyNZ

Bay of Plenty Regional Council recently released a long-awaited report from Waikato University that summarises water quality data, assesses the effects of alum dosing in Puarenga and Utuhina streams and models a range of possible management scenarios for Lake Rotorua.

Overall, the report provides a very valuable summary of the best available knowledge on Lake Rotorua and its inflows. Based on evidence in the report there are three main conclusions:

  • Water quality in Lake Rotorua has shown an improving trend since 2003
  • Availability of P is controlling algal growth in the lake
  • There are a range of management options available for maintaining Lake Rotorua’s water quality, including continued alum dosing and controls on catchment inputs of N and P.

There are three key figures that are at the heart of the report:

The first summarises the Trophic Level Index (TLI) for Lake Rotorua over the last 30-40 years. A trend of improving state of the lake (reducing TLI) is evident from 2003. The TLI in 2012 was the lowest on record. The lake is currently meeting its TLI target of 4.2.

Alum graph 1

TLI is a composite index. It is often more revealing to look at individual elements that contribute to it. In the case of Lake Rotorua the graphs below show that availability of dissolved P (DRP) has been very low since around 2008 coinciding with the period of alum dosing in Puarenga and Utuhina (upper), there has been an increase in nitrates available for plant growth over the last few years (mid) and levels of algae in the lake surface waters have trended down, particularly when you look at annual maximum values (lower). This evidence shows that available P is limiting the growth of algae in Lake Rotorua, with alum dosing affecting P availability. The increased availability of nitrate is most likely the result of reduced algal uptake – a pattern often seen in other lakes around the world when P is controlled.

Alum graph 2 Alum graph 3 Alum graph 4

The final element of the report looks at a number of modelling scenarios and assesses the combined effects of reductions in nitrogen loads from the catchment, P mitigation and alum dosing on the water quality in the lake. What these scenarios predict is that a range of management approaches could achieve desired water quality (i.e. TLI = 4.2). The greatest improvement in TLI was predicted to occur from maintaining current N loads from the catchment (642 T/yr vs 435T/yr target) and driving P down even further through mitigation action on the land and continued alum dosing. However, it is recognised that significant catchment P mitigation and continued alum dosing pose significant challenges.

Alum graph 5