|dc.description.abstract||Though delimited by readily observed boundaries, lakes are not isolated systems but integrate and respond to processes that occur throughout their watersheds. These systems are currently undergoing rapid changes on regional to global scales brought about by drivers such as changing climate and associated changes such as increased loading of terrestrial dissolved organic matter (DOM). These changes are not only occurring at rapid speeds, but they have the potential to alter many fundamental lake processes such as provisioning of habitat for aerobic organisms, internal loading of nutrients, and release of potent greenhouse gases such as methane. The studies in this dissertation were designed to investigate how lake ecosystems respond to and interact with climatic drivers, with the ultimate goal of understanding how these sweeping changes will alter lake ecosystems. Long-term increases in concentrations of DOM are well documented in lakes in many regions of the world. In many cases, these changes are related to decreasing atmospheric sulfur deposition. However, changes are also related to changing precipitation patterns and DOM is predicted to continue to increase in many regions with projected precipitation increases. Although increases are now a well-established phenomenon, it is not well understood how the quality of this material changes concurrent with these increased concentrations. Further, drivers of changes in DOM quality are even less understood. The first study in this dissertation uses a long-term dataset of lake parameters from the North Temperate Lakes Long-Term Ecological Research Site in Northern Wisconsin to investigate changes in DOM quality in seven lakes since approximately 1990. I tested for intra-lake correlation in DOM quality metrics to evaluate whether drivers of DOM quality are regional or lake specific. I found significant correlation across lakes in DOM quality metrics, suggesting that drivers of DOM quality across these lakes are regional and not lake specific. Further, DOM quality metrics were also correlated with indicators of soil moisture, suggesting that the degree of overall moisture in the environment may be one regional driver of DOM quality.
Along with changes in DOM quantity and quality, lake surface temperatures are known to be increasing. The second study in this dissertation examined the interactive effects of DOM quality and temperature on ecosystem respiration (ER). ER is the process whereby organic carbon is mineralized to CO2 through the respiratory activity of aquatic organisms. Rates of ER influence several lake processes such as carbon cycling and consumption of dissolved oxygen. The carbon quality temperature hypothesis states that rates of ER should increase more with temperature in systems dominated by complex, refractory DOM. I tested this hypothesis using a mesocosm experiment where mesocosms contained either a refractory DOM treatment or a labile DOM treatment. I placed a high frequency temperature and dissolved oxygen sensor in each mesocosm and after 27 days, calculated daily rates of ER in each mesocosm. In accordance with the carbon quality temperature hypothesis, I found that rates of ER were significantly different in the two treatments and that ER did increase more with temperature in the refractory DOM treatment.
Widespread long-term increases in lake surface temperatures have been documented across several studies. Dissolved oxygen (DO) in lakes responds to temperature through multiple mechanisms. Solubility of DO decreases with temperature, rates of ER increase with temperature resulting in increased demand for DO from the water column, and warming induced increases in stratification stability and duration may potentially reduce deep-water DO concentrations. In the fourth chapter of this dissertation, I compiled a database of temperature and DO profiles having an average of 24 years of data from over 400 globally distributed lakes to examine long-term changes in surface and deep-water DO. I found that DO has been declining in both surface and deep waters since 1980. In contrast, surface percent saturation has remained relatively unchanged in the face of increasing surface temperatures suggesting that surface DO declines are driven by solubility. However, deep water DO percent saturation has declined on average despite overall unchanging deep-water temperatures, suggesting that deep-water DO declines are not driven by the direct effects of temperature. Combining a random forest algorithm and multiple regression approach, I found that changes in water clarity and increases in water column density differences were the primary drivers of changing deep-water DO. In addition, though water clarity affected deep-water DO, more DO declines were related to increases in density differences than declines in clarity.
The final study in this dissertation further explored changes in lake DO, focusing specifically on changes in anoxia/hypoxia. Low levels of DO can exclude aerobic organisms from large volumes of lake habitat. Traditionally, 2 mg/L has been considered hypoxic and the threshold that results in fisheries collapse. However, some highly sensitive species, such as economically important salmonids, may exhibit behavioral changes at levels as high as 5 mg/L. Anoxia itself excludes the majority of aerobic organisms and results in chemical changes such as storage of methane and internal loading of limiting nutrients. Here, I examined trends in the proportion of the water column below DO thresholds ranging from 5 mg/L to 0.5 mg/L. Further, I calculated changes in the volume of water below the thresholds 5 mg/L and 0.5 mg/L. Finally, I calculated changes in the seasonal rate of DO depletion through time and changes in the duration of stratification to understand what better explains the observed trends in DO. I found that for all thresholds, lakes where the proportion of the water column below the threshold increased exceeded those where it decreased by more than two times. Similarly, the volume of lake water below thresholds increased in more than twice as many lakes as it decreased. Changes in the seasonal rate of DO depletion were relatively evenly scattered around zero, whereas the majority of lakes had increases in the duration of stratification. This suggests that observed increases in water column hypoxia/anoxia are related more to increased stratification durations than changes in the seasonal rates of DO depletion.||