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Keywords:

  • ecosystem function;
  • eutrophication;
  • nutrient spiralling;
  • stream management

Summary

1. Lakes in the Rotorua region of New Zealand are affected by eutrophication from urbanisation and agricultural land use. Some lake tributaries contain geothermally influenced waters, and it is currently unknown whether geothermal tributaries are active sites of nutrient cycling or represent point sources of nutrients to the lakes.

2. Using government data sets, we characterised the physicochemical conditions of geothermal and non-geothermal streams. We then measured ecosystem metabolism and reach-scale uptake of nitrate (inline image), ammonium (inline image) and phosphate (inline image) in summer 2010 (n = 8 streams). Finally, we used government data to compare annual nutrient flux from geothermal and non-geothermal surface water inputs to Lake Rotoiti.

3. As expected, geothermal streams had higher temperature, conductivity and nutrient concentrations and lower pH. However, primary production, community respiration and inline image uptake rates in geothermal streams were not different from those in their non-geothermal counterparts. Uptake rates of inline image were higher in geothermal streams, and inline image uptake was below detection in geothermal streams, probably due to the saturation by naturally high inline image concentrations.

4. A comparison of Lake Rotoiti inputs suggested that geothermal streams are not significant sources of inline image and inline image, while geothermal inputs of inline image represent an average of 46% of total inline image flux from Lake Rotoiti tributaries.

5. Despite their high temperature and low pH, geothermal streams are active sites of photosynthesis, respiration and inline image and inline image cycling, indicating dynamic biofilm communities.

6. Management options for geothermal streams, if any, should focus on inline image retention (e.g. uptake or coupled nitrification and denitrification) but could prove challenging given the persistent, naturally occurring high inline image flux.