Didymo round-up

As of October, didymo had been detected in 56 South Island rivers. NIWA has recently wrapped up several major didymo studies commissioned by MAF Biosecurity New Zealand (MAFBNZ) to better understand its likely spread, its ecology, and impact, and test potential control methods.

Predicting didymo’s distribution and growth

The latest model on habitat suitability for didymo growth, based on new survey data from 145 South Island sites, shows a similar general pattern to the 2005 didymo Likely Environments Map (LEM), with South Island rivers much more susceptible than North Island rivers. The new models predict that about 5% of South Island’s larger river reaches could be severely affected by didymo and over 50% could be visibly affected. Both the LEM and colour-coded maps from the new models will help managers assess the risk of impacts from didymo in currently unaffected rivers, and identify and manage high-risk sites. The models and maps can be updated as new data become available. For the full report, see www.biosecurity.govt.nz (Didymo - Research Reports).

Ecological impacts

Understanding didymo’s effects on river ecosystems may help to identify species and ecological processes at high risk, and prioritise conservation efforts. Quantitative surveys of invertebrates over a 13-month period in three Southland rivers showed that thick didymo growth is associated with overall increased invertebrate abundance and diversity but a decrease in proportional abundance of species favoured by fish (mayflies, stoneflies, and caddisflies). This study was conducted by NIWA, Fish & Game New Zealand, the Cawthron Institute, and the University of Otago. Further research is needed on the long-term ecological effects of didymo.

GemexTM field trial results

Final results of field trials of GemexTM, a chelated copper product, in a Southland stream show that GemexTM could possibly control localised early-stage didymo infestations with minimal impact on other aquatic life. No live didymo cells were found after 6 weeks at one location 0.3 km downstream of the treatment site, and didymo growth on thin mats was suppressed for up to 4 km downstream. Overall, effects on non-target species, particularly invertebrates and native galaxiid fish, were minimal. However, some juvenile trout mortality occurred at localised downstream sites. Further investigation into impacts on trout and other nontarget species is required, as well as the long-term effects of  GemexTM.

Research subject: Algae