Recruitment of resting stages may induce blooms of Microcystis at low N:P ratios

Many phytoplankton species form resting stages when environmental conditions are harsh and these can survive for a long time in the sediment (Livingstone and Jaworski, 1980; Lampert, 1995). When environmental conditions are favourable again, they recruit to the water phase and continue growing (Hansson et al., 1994; Hansson, 1996a). Many species of cyanobacteria, for example Microcystis, Anabaena and Aphanizomenon, form resting stages and are, in addition, the most frequent bloom-forming cyanobacteria (Willén and Mattsson, 1997). Some studies have shown that recruitment of algae from sediment may be important for the pelagic populations (Reynolds and Rogers, 1976; Preston et al., 1980; Forsell and Pettersson, 1995; Hansson, 1996a; Head et al., 1999) and may even be responsible for bloom formation (Reynolds and Walsby, 1975; Hansson et al., 1994; Boero et al., 1996; Perakis et al., 1996). Blooms are sometimes formed rapidly (within days) and can often not be explained by growth of the extant planktonic population alone (Reynolds and Walsby, 1975; Hansson, 1996a), suggesting that the rate of recruitment from sediment to water may be a process of importance in the initiation of algal blooms. In many urban areas, leaching of nutrients from land to water is a significant and growing problem. Since dense cyanobacterial growths are dependent on high amounts of nutrients, they are favoured by nutrient input from land, and this leads to increasing intensity and frequency of blooms in lakes and estuarine waters (Paerl, 1988). Such algal blooms have become a considerable threat to the quality of surface waters, thereby limiting potential uses, such as drinking water, recreation, fishing, and, more fundamentally, ecological function. To improve the value of water resources, a variety of actions have been undertaken. These include measures on land (e.g. changed agricultural practices and improved sewage treatment) and in aquatic systems [e.g. biomanipulation by fish removal (Shapiro et al., 1975; Hansson et al., 1998) and constructions of wetlands (Mitsch, 1992; Annadotter et al., 1999; Zedler, 2003)] to decrease nitrogen and phosphorus loading. However, the mechanisms and processes involved in algal bloom formation are not well known, and to provide society with useful decision-support tools, further knowledge about what initiates blooms is crucial. Thus, our study aims to address the following questions. Are resting stages important for the development of algal blooms? Do the resting cells need the same environmental