Composition of filamentous
algae present in California
rice fields-04

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Project Leader and Principal UC Investigators

David Spencer, ecologist, USDA/ARS Exotic and Invasive Weeds Research Unit, Dept. of Plant Sciences, UC Davis


Algae have been a long-running problem for California rice growers, primarily affecting the early stages of rice production.  It is a “weed” problem that has been difficult to manage in some fields even with the use of copper sulfate (bluestone).

Rice seedlings can become entangled in floating algal mats and are dislodged from the soil. Large algal mats can cause the loss of an entire stand.  This new project was initiated to develop a greater understanding of the algae problem and what are the best options for control.

In 2004 eight rice fields were sampled every two days from May 1 to June 1 for 22 water quality parameters and algal biomass.  Samples of algal material were collected for identification. Soil samples were collected prior to flooding the fields with water and analyzed for nitrates, Olsen extractable phosphorus, exchangeable potassium, total copper, and extractable copper.

"Black algae" seen here at 40 times magnification has become more prevalent in grower fields.

Researchers observed a shift in abundance from predominantly green algae (Sphaeroplea, Tribonema, Ankistrodesmus, Tetraspora) and diatoms (Navicula) early in May to dominance by blue-green algae (almost totally Nostoc, with Anabaena and Phormidium also present) in late May to early June. The same species occurred in all the fields at about the same time. 

The most abundant species that may commonly be referred to as “black algae” collected from these fields is Nostoc.  This and related species can tolerate relatively high levels of copper due to its production of mucilage. Thus, it is likely that copper sulfate treatments have been selecting for Nostoc.  For example, Nostoc was not reported in a 1972 survey of algae in rice fields.

There was considerable variation in algal biomass and nearly all 22 water quality parameters measured among the eight sampled fields. However, results from this limited sampling showed that algal abundance was greater in fields with higher concentrations of ammonium, sodium, and calcium in the water. This information may help identify areas expected to have more severe algal problems, but it requires additional validation.

The fate of copper sulfate used to control algae and other pests is also of interest. Researchers found that total copper varied from 56.6 to 85.6 mg / kg, with a mean of 69.6 mg / kg for soil samples. The fraction of total copper that is potentially available to plants, animals, and microbes, ranges from 15% to 31% of the total copper in the soil of the sampled rice fields.

Further research should focus on developing control methods that are more effective at controlling Nostoc and related species, methods that may lead to reduced copper inputs into the soil, and additional evaluation of field characteristics that can be used to predict abundant algal growth.


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