Spring-fed creeks: Is it something in the water?

Wash creek, Southland
Artificial substrates at Wash creek, Southland.
Photo: Fish and Game New Zealand

The failure of didymo to thrive in spring-fed creeks could open research avenues into alternative methods for controlling the algae in a wide range of freshwater environments.

Joint research by Fish and Game New Zealand1 and the National Institute of Water and Atmospheric Research (NIWA)2 has attempted to isolate the reasons why didymo seems to find it difficult to establish in spring-fed creeks – this is despite the creeks being exposed to didymo where they join didymo-affected rain-fed rivers. The work was initiated after Fish and Game New Zealand staff, who were carrying out drift dives for fish surveys, had noticed the phenomenon.

Stu Sutherland and Maurice Rodway of Fish and Game New Zealand presented the findings of their research at the didymo science seminar.

The research involved placing didymo that had been grown on artificial substrates into spring-fed creeks, and placing similar substrates in the adjacent rivers under similar flow conditions. In all cases, the river systems had been invaded by didymo since 2004.

The researchers said that, in all instances, they found that the didymo in the spring-fed creeks died or disappeared from the artificial substrates, while the colonies placed in the main river sites generally stayed healthy.

They considered a number of factors that might help explain why didymo found the creeks difficult to colonise, mainly focusing on water chemistry. The spring-fed creeks generally had higher levels of nitrates, alkalinity (measured as calcium carbonate), calcium and, to a lesser extent, magnesium than the main rivers. Organic carbon, on the other hand, was lower in the creeks.

Maurice Rodway said the water chemistry differences, especially in calcium and alkalinity, may play a part in the discrepancy, but a combination of factors is likely to be responsible for the poor survival of didymo in the spring-fed creeks.

He said another, less likely, factor was grazing by invertebrates but there was no clear evidence from their research to support this. Competition from other algae was also a possibility, but algal levels in the spring-fed creeks tended to be low for other algae, and not just for didymo.

The research report suggests several further lines of enquiry based on the encouraging results from this survey, including:

  • a more detailed look into the chemistry of the water in the spring-fed creeks, testing the effects on didymo of manipulating in-stream alkalinity through use of quarried limestone – an inexpensive and low-risk option
  • controlled experiments in artificial stream systems, varying alkalinity, calcium, magnesium, nitrate and other chemical constituents
  • collating more general information about the water chemistry of spring-fed creeks in different areas, in order to help with risk assessment in various places as more becomes understood about the effect of spring water on didymo.

Report authors:

Stuart Sutherland,1 Maurice Rodway,1 Cathy Kilroy,2 Bill Jarvie1 and Graeme Hughes3

  1. Fish and Game New Zealand, Southland Region
  2. National Institute of Water and Atmospheric Research (NIWA), Christchurch
  3. Fish and Game New Zealand, Central South Island Region

For a full copy of the research report:

www.biosecurity.govt.nz/didymo-research-reports (offsite link to www.biosecurity.govt.nz)

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Page last updated: 30 April 2008