. The Canadian field-naturalist. Natural history. 442 The Canadian Field-Naturalist Vol. 116 Because of the high nitrogen content, various species are used in agriculture as a green manure or biofertilizer (Lumpkin and Plucknett 1982). Some species, including A. caroliniana, are used as orna- mental plants in aquaria and decorative pools (Lellinger 1985; L. H. Bailey Hortorium 1976). Other uses and applications are summarized by Wagner (1997). In some situations A. caroliniana has been a serious weed disrupting aquatic ecology and interfer- ing with human activities (Thieret 1980). The reduc-
. The Canadian field-naturalist. Natural history. 442 The Canadian Field-Naturalist Vol. 116 Because of the high nitrogen content, various species are used in agriculture as a green manure or biofertilizer (Lumpkin and Plucknett 1982). Some species, including A. caroliniana, are used as orna- mental plants in aquaria and decorative pools (Lellinger 1985; L. H. Bailey Hortorium 1976). Other uses and applications are summarized by Wagner (1997). In some situations A. caroliniana has been a serious weed disrupting aquatic ecology and interfer- ing with human activities (Thieret 1980). The reduc- tion of light (up to 90%) and reduced oxygen levels (50% or more) in the water column under Azolla mats can have such an affect on other vegetation that it has been studied as a means of weed control in rice pad- dies (Wagner 1997). Azolla caroliniana at Ottawa In September of population of A. carolini- ana was found in the westerly pond of Brown's Inlet off the Rideau Canal in Ottawa, Ontario (Figure 1, site 1; Appendix 1, specimen 1). Individual plants and mats in patches up to about m in diameter were common around the perimeter of the pond among other floating and emergent plants. Other vegetation studies in this pond during the previous two years had failed to detect the fern (personal observations).. Figure 1. Map of the area of the Rideau and Ottawa Rivers where Azolla caroliniana populations occurred in 1997 and 1998. Numbers correspond to approxi- mate locations of collections listed in Appendix 1. The pond, being connected to the Rideau Canal system, is completely drained in the autumn (usually October) of each year. Draw-down of water in the Canal commenced on 15 October 1997. Drainage of the pond was complete by 18 October and mats of A. caroliniana were left stranded on the wet mud. On 19 and 20 October, daily minimum temperatures were about -3°C. The first significant snowfall occurred on October 21 (9 cm) and then again on October 26 (18 cm); however,
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