Report: TR06/15

Authors: Carolyn L. Faithfull, David P. Hamilton, David F. Burger and Ian Duggan (Centre for Biodiversity and Ecology Research)

Abstract

This report was commissioned by Environment Waikato to examine the available methods for internal (bottom sediment) nutrient removal and their suitability for application in the Waikato peat lakes. Lakes Ngaroto, Kainui, Rotomanuka and Cameron were chosen as focus lakes for the study, based on existing restoration objectives and recreational and conservation values.

A range of methods designed to reduce internal nutrient loading was reviewed, including hypolimnetic aeration, hypolimnetic withdrawal and fish removal, with sediment removal, and additions of alum, Phoslock, zeolite, iron making slag and carbon examined in detail. Experimental and observational work was carried out to assess the feasibility of these methods in the target lakes:

• Alum was determined to be the least desirable of the options examined; as there is a high probability soluble toxic forms of alum will be expressed or released due to the low alkalinity and pH of the target lakes.
  
  
• Sediment removal is the most expensive option, but may be appropriate in lakes without high ecological values. However, the external nutrient load must be reduced substantially to prolong benefits of dredging.
  
  
• Phoslock and zeolite both effectively reduced phosphate concentrations in the incubation cores. Zeolite was more effective than Phoslock in preventing nutrient release from the sediments following anoxia. However, Phoslock was more effective than zeolite in removing nutrients from the water column. Both substances should be applied with care and with careful documentation of chemical and biological effects as there may be hitherto undocumented effects on aquatic biota.
  
  
• The use of iron making slag to reduce internal nutrient load is not recommended, as nutrient concentrations were not reduced in the iron treated cores in the incubation study.
  
  
• Dissolved organic carbon addition may be useful in the naturally dystrophic peat lakes, and can be added naturally if measures are taken to conserve the surrounding peat substrate (e.g. riparian vegetation, maintenance of lake levels). Further study is required to assess how humic substances would affect primary productivity and the availability of phosphate for algae growth.
  
  
• Hypolimnetic withdrawal is not a suitable nutrient reduction technique, as none of the Waikato peat lakes possesses a large anoxic hypolimnion, a long period of anoxia or a consistently large lake inflow.
  
  
• Hypolimnetic aeration may be useful in Lake Rotomanuka, as this the only peat lake known to regularly stratify and become anoxic during summer. Hypolimnetic aeration can break down stratification and potentially reduce the abundance of cyanobacteria by disrupting their buoyancy.

Before a pilot study is carried out to reduce internal nutrient load, it is recommended the external nutrient load of the target lake(s) is reduced substantially, to the order of 50 % for most lakes where the catchment is highly modified and effects of restoration measures are not evident. On the basis of the evidence presented in this report, we recommend using fish removal and macrophyte re-establishment as part of an integrated strategy to improve water quality and transparency. Use of a flocculent could also be considered as part of an integrated strategy for reducing internal nutrient loads. The latter strategy should be undertaken under circumstances of mutual cooperation and communication amongst environmental managers, scientists and material suppliers, as optimising the timing and quantity of the flocculent will be an iterative process requiring adjustments for each waterbody application.

Waikato Peat Lakes Sediment Nutrient Removal Scoping Exercise [PDF, 710 KB]