Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Biology

Supervisor

Creed, Irina

2nd Supervisor

Trick, Charles

Abstract

There is a collective prediction among ecologists that climate change will enhance phytoplankton biomass in temperate lakes. Yet there is noteworthy variation in the structure and regulating functions of lakes to make this statement challengeable and, perhaps, inaccurate. To generate a common understanding on the trophic transition of lakes, I examined the interactive effects of climate change and landscape properties on phytoplankton biomass in 12,644 lakes located in relatively intact forested landscapes. Chlorophyll-a (Chl-a) concentration was used as a proxy for phytoplankton biomass. Chl-a concentration was obtained via analyzing Landsat satellite imagery data over a 28-year period (1984-2011) and using regression modelling. The most common lake trophic state was oligotrophic (median Chl-a < 2.6 μg L-1), while the least common was hyper-eutrophic (median Chl-a > 56 μg L-1). Lake volume was the most important factor in determining the present trophic state of the lakes. The majority of the lakes (91.6%) did not show a change in trophic state over an almost 3-decade long sampling period; only 4.0% of the lakes became more eutrophic, and 4.4% of the lakes became more oligotrophic. Lakes with smaller volumes were further responsive to temperature (warmer lakes were more eutrophic), while lakes with larger volumes were more responsive to precipitation (wetter lakes were more oligotrophic). Early warning indicators of change in trophic state were examined in the patterns of the residuals of the time series of Chl-a once non-stationary and stationary trends were removed. Remarkably, the majority (56.5%) of the lakes showed patterns in the residuals that were not defined by a single trophic metric but fluctuated among different trophic states. There was an unexpected instability among some lakes as they switched between oligotrophic and eutrophic states (12.5%) or were transitioning from eutrophic towards oligotrophic states (23.4%), or from oligotrophic towards eutrophic states (20.6%). The complex responses of phytoplankton biomass to climate change suggests that our ability to predict the future trophic state of lakes will be limited but enhanced if we recognize that lakes and their catchments will be both impacted by climate change.

Summary for Lay Audience

The impact that climate change will have on Canadian temperate lakes remains poorly understood. There are several reasons for our lack of confidence in describing the effect of climate change. First, many lakes that are near human populations are also impacted by the anthropogenic pressures – direct or indirect use of surface waters for consumption and the use of lakes for the intended or unintended deposits of wastes. Second, our ability to predict the changes in lake ecology is hampered by our slow observance of changes that are currently taking place. It appears that lakes may be changing from clear water states to turbid productive water states with an increased incidence of potentially harmful algal blooms. Although undesirable, these changes can be either gradual (i.e., linear), or small and non-linear, and the latter is much harder to identify. Finally, since lakes are of many shapes and sizes (i.e., they have different morphometry), they will not be impacted by climate change equally. Thus, reports on climate change about the functioning of lakes might be too general. This thesis attempts to avoid these problems by studying over 12,000 lakes in the temperate forest region of Canada. Using satellite records of lake chlorophyll-a (a proxy measure of algal biomass in lakes) over 28 years, I have determined that climate change affects ~44 % of the lakes, with ~21 % of the lakes becoming more productive and ~23 % of the lakes becoming less productive. The remaining lakes either do not respond to the changing climate or oscillate between low and high productivity. The trends documented in this thesis indicate how the lakes might look like in the future (as climate change continues) and if they can be used as a healthy water supply for the next generations.

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