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By Cathy Kilroy

Ever since the alga Didymosphenia geminata was discovered growing prolifically in a South Island river, scientists have puzzled over how “blooms” (unusually large algal growths) of this alga can form. Rivers below lakes and dams in the South Island have been especially severely affected. Massive, persistent mats of didymo in these rivers have changed their character, reduced their appeal for angling, and have sometimes caused problems such as clogging irrigation intakes. So far didymo has not been detected in any North Island river.

Didymo in the upper Ohau River.

Didymo cells and stalks.

D. geminata (commonly known as didymo, or “rock snot”) is a stalked diatom. Cells colonise rivers by attaching to rocks or other surfaces and then growing out into the water column on stalks. The stalks are exuded from the cells and are made largely of carbohydrate. The cells divide while still attached to their stalks, forming thick mats of intertwined stalks with cells mostly at the surface.
Didymo is unusual because its blooms appear only in rivers with very low levels of nutrients. Often these are pristine rivers, like the Mararoa, Hurunui and Buller Rivers. But algal blooms are almost always linked to high nutrients. How does didymo do so well in low nutrients?

Early on in the New Zealand didymo outbreak we assumed that didymo possessed some special mechanism for sucking up nutrients. International researchers have proposed a couple of ways in which this might happen, both linked to processes on the stalks. For example, see here. However, research in New Zealand has revealed an alternative explanation.

The experimental channels set up at the confluence of the Otiake Spring Creek and a braid of the Waitaki River.

Since 2008 I have collaborated with Canadian scientist Dr Max Bothwell in running experiments in streamside mesocosms beside the didymo-affected Waitaki River. An early outcome of the research was development of a method to determine cell division rates in didymo. High division rates in most algal blooms lead to large numbers of cells, which form the blooms.

To our surprise, we found that cell division rates in didymo in the Waitaki River were extremely low. Nutrient addition experiments showed that the cells did not have enough phosphorus (P).

Further experiments demonstrated that stalk length in didymo was inversely related to cell division rates: the lower the cell division rates, the longer the stalks. This suggested that a well-documented property of diatoms was operating in didymo in low-nutrient rivers. The property is release of excess carbohydrates into the water when nutrients (especially P) are in short supply. In the case of didymo the carbohydrate is released as stalks.

In a survey of South Island rivers we found that cell division rates in didymo blooms were always very low. The blooms were present only in rivers where average dissolved P was very low. Didymo in higher nutrient waters had higher cell division rates, shorter stalks, and did not form blooms.

All this indicated that didymo blooms are not driven by any mechanism for obtaining additional nutrients. On the contrary, the blooms are caused by low nutrients in the overlying water, which promotes excessive stalk production. Subsequent surveys, experiments and observations in New Zealand have all been consistent with low nutrients (specifically low P) driving the blooms.

Explanations for didymo blooms involving additional supplies of P did not consider the responses of didymo cells and stalks. Furthermore, these explanations require water chemistry characteristics that are inconsistent with observed patterns of didymo bloom occurrence in New Zealand.


The New Zealand research has been published in three papers in international journals:

Bothwell, M.R.; Kilroy, C. (2011). Phosphorus limitation of the freshwater benthic diatom Didymosphenia geminata determined from the frequency of dividing cells. Freshwater Biology 56: 565-578.

Kilroy, C.; Bothwell, M.L. (2011). Environmental control of stalk length in the bloom-forming, freshwater benthic diatom Didymosphenia geminata. Journal of Phycology 47: 981-989.

Kilroy, C.; Bothwell, M.L. (2012). Didymosphenia geminata growth rates and bloom formation in relation to ambient dissolved phosphorus concentration. Freshwater Biology 57: 641-653.

A further paper examines the evidence for one of the alternative explanations for didymo blooms:

Bothwell, M.L.; Kilroy, C.; Taylor, B.W.; Ellison, E.T.; James, D.A.; Gillis, C-A.; Bladon, K.D.; Silins, K.D. (2012). Iron is not responsible for Didymosphenia geminata bloom formation in phosphorus-poor rivers. Canadian Journal of Fisheries and Aquatic Sciences 69: 1723-1727.

This research has been funded by NIWA capability funds/core funding, Department of Conservation, NZ Fish & Game, and Meridian Energy.


Dr Cathy Kilroy is a freshwater ecologist specialising in freshwater algae.