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We examined changes in G. semen occurrence and bloom incidence in 146 boreal lakes in Sweden sampled at least once between 1992 and 2010, and used a time-by-space model to assess the environmental variables that best explain patterns in G. semen distribution and bloom formation. We showed that G. semen has become more common, although there were no significant shifts in its geographical distribution during the study period. In particular, G. sem en was spreading into new lakes in the Central Plains ecoregion (southern Sweden), whereas its occurrence and biomass usually remained low in the Borealic Upland and Fennoscandian Shield ecoregions. G. semen biomass and the incidence of blooms did not increase significantly during the study period, but fluctuated among years and reached a maximum in 2003. The occurrence of G. semen was mainly explained by temperature and the length of the growing season, whilst local-scale variables, such as pH and water color, were the best predictors of blooms. Analysis of bloom formation at three different levels of G. semen dominance: G. semen >50%, >75%, and >90% of total phytoplankton biomass revealed a wide range of responses to environmental variation. For example, pH, water color and to a lesser extent temperature explained bloom formation at the 50% level, whereas lake morphometry was important at the 90% level. These results suggest that with ongoing brownification and climate warming boreal systems will likely become more susceptible to invasions of G. semen . |
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