| dc.description.abstract | Spring is an ecologically important period for phytoplankton communities, as environmental stressors such as acid deposition and recovery, climate warming, and shoreline development may exert strong influence at this time of year. However, few studies have examined trends in spring phytoplankton community composition and biomass, and it is not known if changes in the spring phytoplankton are associated with trends in phytoplankton communities during the summer and fall periods. To determine long-term changes in phytoplankton community composition and biomass during the spring period, we examined spring phytoplankton samples, water chemistry samples, and meteorological data from the mid-1970s to 2011 at five lakes (Blue Chalk, Dickie, Harp, Plastic, and Red Chalk) in the Muskoka-Haliburton region of south-central Ontario, Canada. Trends in spring water chemistry and climate included significant increases in alkalinity, dissolved organic carbon concentration and fall mean air temperature, and significant decreases in concentrations of calcium, magnesium, and sulphate and the number of ice-on days. Significant increases in the relative and absolute biovolume of chrysophytes were observed at one lake (Blue Chalk), with coincident declines in the relative biovolume of diatoms and chlorophytes (P < 0.05). Significant increases in the absolute biovolume of chlorophytes, cryptophytes, and cyanophytes were observed at Dickie Lake (P < 0.05). Inter-annual variability of spring phytoplankton biovolume was high within each lake. Environmental variables that explained a significant portion of the spring phytoplankton variation differed for each lake, but generally included conductivity, nitrate/nitrite concentration, fall and/or winter mean air temperatures, and number of ice-on days. Variation partitioning analysis indicated that spring water chemistry variables, independent of climate, explained a significant proportion of the temporal variation in composition of spring phytoplankton communities at four of the five lakes (12%–28%, mean = 18.9%), while climate variables, independent of water chemistry, explained a significant proportion of the variation in one lake only (P < 0.05; 15%). High unexplained variation (55%–77%, mean = 68.7%) indicates that other factors play an important role in explaining the observed inter-annual variation in the spring phytoplankton community composition. A comparison of variation in spring phytoplankton with ice-free season composite phytoplankton revealed that chrysophytes, cryptophytes, and dinoflagellates generally have higher relative biovolumes in spring phytoplankton samples than in ice-free season composite phytoplankton samples. However, changes observed in the ice-free season composite phytoplankton in previous studies were not observed to the same extent in spring phytoplankton communities. | en |
