Temperature as a driver for the expansion of the microalga Gonyostomum semen in Swedish lakes

Gonyostomum semen (Ehrenb) Diesing is a bloom-forming and noxious phytoplankton species, that usually occurs in brown-water lakes and which is often referred to as an invasive species. The aim of our study was to analyze changes over time in the occurrence and distribution of blooms, and to find possible drivers of this change. We also performed spatial analyses to identify environmental factors coupled to Gonyostomum's distribution. The effect of temperature on key processes in the Gonyostomum life cycle was further investigated experimentally to determine potential mechanistic causes. Our results show that G. semen has expanded in Swedish lakes since 1988. At the turn of the Millennium it was present in more than a quarter of the lakes included in the Swedish national lake monitoring program. Gonyostomum-lakes have significantly higher DOC, higher nutrient levels, and lower pH than non-Gonyostomum lakes. Trend analyses show a significant increase in the number of lakes with Gonyostomum, as well as in biomass and occurrence in samples. One explanation is that we more often find water temperatures exceeding 6 degrees C, which is also the threshold for positive growth in our laboratory experiments. Moreover, according to our partial least square regression model (PLS) analysis in one lake, we find that the increase in biomass is a function of temperature in combination with other factors. Thus, we conclude that an increase in water temperature resulting in longer growth season may be a driver of the expansion of Gonyostomum. However, temperature alone cannot explain why the species has expanded to new lakes within the same climatic region. Possibly an interplay between DOC and temperature can explain the patterns observed. (C) 2012 Elsevier B.V. All rights reserved.