Didymo: New model helps build likely growth picture
Just how serious could didymo growth be if this invasive alga spread throughout New Zealand? That rather unsettling question was considered by National Institute of Water and Atmospheric Research (NIWA) scientist Cathy Kilroy during her presentation at the MAFBNZ didymo science seminar in July this year.
Commissioned by MAF Biosecurity New Zealand, Cathy and her NIWA colleagues* have been working on a new model to predict the coverage and thickness of didymo in all New Zealand waterways should didymo be introduced and establish there.
She explained that an earlier analysis, which resulted in the 'Likely Environments Map' (LEM), was an assessment of the relative likelihood of establishment and growth of didymo in New Zealand rivers. The analysis indicated that didymo would be more at home in South Island rivers than in the North Island, largely because of the cooler temperatures in the South. All known didymo-infected rivers to date are in the top two high-risk categories on the LEM, suggesting that the LEM assessment was appropriate – at least for the South Island.
This picture may have changed somewhat with the availability of further data as didymo has spread within the South Island, and North American data showing that didymo is spreading into warmer areas. The NIWA team surveyed the thickness and percentage cover of didymo in affected rivers, then combined this data with river environment data to develop a predictive model. Eighteen environmental variables were included, covering factors that influence didymo establishment, such as hydrology, geology, flooding frequency, light availability, water chemistry, substrate composition and lake effects.
New model for likely didymo growth
Cathy said the new model for likely didymo growth (Didymo Predictive Maps – DPM) again predicts, like the earlier LEM map, that North Island rivers may be generally less susceptible to didymo than rivers in the South Island.
This does not mean, however, that river users should be complacent about freshwater hygiene in the North Island. The North Island still has areas likely to be susceptible to medium levels of didymo mat development, chiefly the central plateau and higher-altitude rivers draining the main divide.
In contrast to the LEM, the DPM predictions are not heavily influenced by temperature. The predicted didymo growth patterns involve a wider range of variables, including the influence of lakes (there is less flooding in lake-fed rivers, therefore less opportunity for didymo to be washed out by high flows), the stability and hardness of the substrate, and flow variability. Some areas of the North Island appear to be at lower risk of profuse didymo development because of the presence of soft mudstone and siltstone in their catchments, she noted.
Large parts of South in higher growth categories
Large parts of the South Island are, by contrast, predicted to be in the higher growth categories, particularly in the eastern and southern regions, as well as scattered rivers in Fiordland, Westland and the Nelson–Marlborough region. Indeed, the already-infested rivers in the South Island are generally in the new DPM's higher categories for didymo growth. Many rivers on the west coast of the South Island are likely to be somewhat less vulnerable to high growth levels because they are frequently flooded, and some also carry relatively high loadings of dissolved organic material – both believed to be relatively unfavourable conditions for didymo growth.
Cathy said that clarifying the role of temperature in the distribution of didymo was complex because the large overseas and New Zealand datasets available are air temperatures rather than water temperatures. Water temperatures are more significant in didymo distribution, but their relationships with air temperature are not straightforward. She said a comprehensive set of water temperature data from New Zealand waterways, taken over time, would provide invaluable data, essential to improving our understanding of the distribution and growth levels of didymo and other aquatic species. Setting up a national database of river water temperatures should be a priority, and stakeholder groups may be able to assist in this in the future.
Distribution in lakes not yet understood
She said that although didymo has appeared in several lakes, including Te Anau, Wakatipu and Manapouri, the factors that influence its distribution in lakes are not yet understood. She noted that it appears to prefer rocks to sandy areas, and will readily settle on wharf structures and other objects such as trailing ropes. "We hope to do more work on didymo in lakes, to help identify whole-lake-scale factors favouring growth, and determine the triggers for blooms within lakes."
*Cathy Kilroy, John Leathwick, Katie Dey, Neil Blair, Helen Roulston, Julian Sykes and Donna Sutherland, NIWA, Christchurch
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Page last updated: 24 June 2008