Cyanobacteria blooms are characteristic of eutrophic lakes and are often a threat to water consumption (Millie et al. 1999). Lakes of the prairie ecozone of Canada are particularly prone to cyanobacteria blooms harbouring nitrogen-fixers (Aphanizomenon, Anabaena, Gloeotrichia spp.), in part because of phosphorus (P)-rich soils and high irradiance regimes (Barica 1987). However, despite the prevalence of cyanobacteria in phytoplankton of some lakes, there is little agreement on whether nitrogen fixed from the atmosphere (N_2) is quantitatively incorporated into the aquatic food web. On one hand, their colonial nature and potential toxicity make het-erocystous cyanobacteria poor food for zooplankton (Ghadouani et al. 2004). On the other hand, historical correlations between cyanobacterial abundance and stable isotope signals in plankton and lake sediments (Talbot 2001) suggest that atmospheric N2 fixed by cyanobacteria may be an important source of nitrogen (N) to lake ecosystems. Little is known of how the importance of N sources may vary as a function of lake position within the greater landscape. Many chemical properties of lakes are strongly influenced by the position of a lake within the hydrologic or topographic landscape, such that headwater lakes tend to exhibit lower nutrient concentrations and algal densities than do downstream sites (Soranno et al. 1999). Based on these patterns, we hypothesized that the intensity of N_2 fixation by cyanobacteria should increase with distance from headwaters, and that fixed N_2 should be more completely incorporated into the primary and secondary consumers of downstream lakes.
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