Are Northern Lakes in Relatively Intact Temperate Forests Showing Signs of Increasing Phytoplankton Biomass?

Abstract

Increased reports of phytoplankton blooms in remote pristine lakes provide the perception that climate-driven fundamental changes may be occurring in lake ecosystems. There is a crucial need for detailed surveys of lakes covering large spatial and temporal scales to determine whether phytoplankton biomass is on the rise in northern forested landscapes. To characterize spatial and temporal variation in chlorophyll-a (Chl-a) as a proxy of phytoplankton biomass in lakes located in the relatively intact northern temperate forest in the Great Lakes-St. Lawrence forest region, we developed a regression model that relates Landsat 4–5 TM and 7 ETM\u2009+\u2009optical reflectance in blue, green, and red bands to Chl-a from 45 lake water samples. Reflectance from the Landsat three-band algorithm ((B1-B3)/B2) showed the strongest correlation with in situ data explaining 78% of the variance in Chl-a. We applied this model to produce a 28-year time series of Chl-a in 12,644 lakes within the northern temperate forest. By applying a two-way analysis of variance, we were able to quantify the different components of variation in Chl-a. We found that space and time components combined explained 55.9% of the total variation in Chl-a (35.6% for space and 20.3% for time), while the space\u2009×\u2009time interaction component explained 44.1% of the variation. The spatial pattern revealed relatively low Chl-a in headwater lakes to higher Chl-a in lower reaches of watersheds. Only a minority of lakes were found to have a temporal trend in Chl-a; some lakes displayed positive trends while others displayed negative trends. The trends were substantial enough to indicate changes in trophic states (that is, oligotrophic lakes becoming eutrophic, or vice versa). These spatial and temporal patterns indicate the complex interactions of local and regional catchment characteristics and lake-specific properties that filter the response of northern lakes to climate change. The large proportion of the space\u2009×\u2009time interaction component shows the importance of processes occurring within lakes that affect water mixing and nutrient concentrations and therefore availability of the nutrients to phytoplankton.