17 Phycologia (2005) Volume 44 (1), 17–25 Published 21 February 2005 The occurrence of the cyanobacterium Cylindrospermopsis raciborskii in Constance Lake: an exotic cyanoprokaryote new to Canada PAUL B. HAMILTON 1 *, LINDA M. LEY 1 ,STUART DEAN 2 AND FRANCES R. PICK 3 1 Research Division, Canadian Museum of Nature, PO Box 3443, Station D, Ottawa, Ontario, K1P 6P4, Canada 2 Surface Water Quality Branch, Ottawa, Ontario, K1J 1A6, Canada 3 Ottawa-Carleton Institute of Biology, University of Ottawa, Ottawa, Ontario, K1N 6N5, Canada P.B. HAMILTON, L.M. LEY, S. DEAN AND F.R. PICK. 2005. The occurrence of the cyanobacterium Cylindrospermopsis raci- borskii in Constance Lake: an exotic cyanoprokaryote new to Canada. Phycologia 44: 17–25. The tropical and subtropical toxin-producing cyanobacterium, Cylindrospermopsis raciborskii, is reported for the first time in Canada. The species is of particular concern because it appears to be spreading throughout temperate North America and produces toxins that can have serious effects on human health. Cylindrospermopsis raciborskii was found in Constance Lake, Ottawa, Ontario, a shallow mesotrophic to eutrophic hardwater lake (summer average total phosphorus 28–48 gl -1 ) where bottom waters can reach sufficiently high temperatures to promote germination of resting stages in the sediments. Peak abundance and biomass occurred in July and August when surface water temperatures were at their maximum. Year to year differences in abundance and dominance over four years of sampling (1998–2001) appeared to be due principally to differences in water temperature rather than nutrients. In the summer of 1998 when biomass was highest over the four years, C. raciborskii contributed 63% of the total phytoplankton biomass and 60% of the cyanobacterial biomass. A warming trend linked to climate change is likely to accelerate the spread and abundance of this potentially toxic species. INTRODUCTION Harmful algal blooms are of increasing concern worldwide (Hallegraeff 1993). Those occurring in freshwater are mostly due to cyanobacteria from a variety of genera. A number of cyanobacterial species produce neurotoxins and hepatotoxins, but at present the toxicity of blooms is difficult to predict (Chorus & Bartram 1999). The filamentous cyanobacterium, Cylindrospermopsis raciborskii (Woloszyn ´ska) Seenayya & Subba Raju, was implicated in one of Australia’s worst cases of human poisoning by cyanobacteria (Falconer 2001). All species described to date of Cylindrospermopsis Seen- ayya & Subba Raju appear to be subtropical or tropical in origin (Koma ´rek & Kling 1991; Padisa ´k 1997). Cylindrosper- mopsis raciborskii was first described (Woloszyn ´ska 1912) from a plankton sample collected from Central Java during 1899–1900 (Padisa ´k 1997). Since the initial description, the species has been recorded from many tropical and subtropical regions and appears to be spreading around the world (Padisa ´k 1997; Briand et al. 2002; Saker et al. 2003). In tropical and subtropical Australia, the species is widespread and can be found in river systems as well as in reservoirs (Fabbro & Duivenvoorden 1996; McGregor & Fabbro 2000). Many Bra- zilian reservoirs are also dominated by this taxon (Bouvy et al. 2000). In North America, Prescott & Andrews (1955) made the first record of C. raciborskii (reported as Anabaen- opsis raciborskii Woloszyn ´ska) from Wooster Lake, Kansas. Since 1955, 14 records from small lakes or reservoirs in Tex- as, Florida and as far north as Minnesota have been reported by Chapman & Schelske (1997). In Florida the species was found in more than 40% of over 165 water bodies surveyed (Williams et al. 2001) and in some lakes the species now * Corresponding author (phamilton@mus-nature.ca). appears to have replaced other bloom-forming cyanobacteria (Chapman & Schelske 1997). St. Amand (2002) more recently mapped at least 48 reports in continental USA outside of Flor- ida. The species can dominate a lake system creating ‘pea- green water conditions’ with bloom densities reaching 10 8 – 10 9 filaments l -1 (Padisa ´k 1997). Until 1979, C. raciborskii was considered simply a nui- sance species in lakes and reservoirs when nutrient rich con- ditions were present for expansive growth. However, in 1979, an outbreak of hepatoenteritis involving 148 people following a copper sulphate algicide treatment of a drinking water sup- ply (Solomon Dam) in Palm Island, Australia, changed the status of C. raciborskii from a nuisance species to a toxic species (Falconer 2001). Cylindrospermopsin, a hepatotoxin, was subsequently identified as the microtoxin in C. raciborskii by Ohtani et al. (1992). Cylindrospermopsin has been detected in surface waters from Australia, Europe and USA (W. Car- michael, personal communication). More recent studies have identified the presence of other toxins including neurotoxins (Lagos et al. 1999; Saker et al. 2003) and other unidentified compounds (Bernard et al. 2003; Saker et al. 2003). The spe- cies has become a public health problem. Until now, C. raciborskii has not been reported from Can- ada. This paper documents the presence of C. raciborskii in a small shallow lake within the city of Ottawa, Canada. We examined the seasonal growth pattern in relation to water tem- perature and water chemistry over four years and compared this with growth patterns described from elsewhere. MATERIAL AND METHODS Study site Constance Lake (45°24'N, 75°59'W) is a shallow 132 ha lake with a maximum depth of 3.3 m and average depth of 2 m.