An extensive series of sediment cores from many connected lakes, wetlands, and upland tundra and forests in northern Manitoba, Canada will be collected and examined for several indicators of ecosystem composition and function over the past 8000 years. Although ecological responses to Arctic warming have been shown in previous research, such as increases in lake productivity, permafrost thaw, shrub expansion, and northward shifts in the subarctic tree line, most of this work involves the study of a single ecosystem type, such as terrestrial forests, wetlands, or lakes. An objective of the new research is to examine how interactions among these ecosystems in the past have affected their responses to past climate change. Changes in one ecosystem may affect the response of others to climate change, such as when soil runoff from forests and wetlands alters the chemistry and productivity of the lakes into which they drain.
This project will be significant for determining how Arctic ecosystems may change in the future as a result of rapid warming. A better understanding of the linkages among these ecosystems will be essential for understanding ecological impacts on productivity, nutrient cycles, or biodiversity in the Arctic and will help inform the scientific community and the public about climate change, the direct impacts of this climate change on the people of the Arctic and extended global impacts on the carbon cycle and climate warming.
Climate warming is occurring rapidly in high-latitude ecosystems, such as boreal forest and tundra. Areas that are currently colder than freezing on average may experience very different conditions in the future. Vast stretches of permanently frozen soil (permafrost) may thaw. Deep reserves of soil carbon, accumulated over thousands of years in wetlands, may decompose rapidly, releasing greenhouse gases to the atmosphere. Vegetation will migrate northward as cold temperatures and short growing seasons yield to more hospitable climatic conditions and greater nutrient availability. Changes in nutrient or carbon runoff to lakes, for instance, can alter critically important communities of phytoplankton and other organisms living in the water column, including fisheries. To assess overall change in northern landscapes, it is important to take an integrated approach that examines terrestrial, wetland, and aquatic ecosystems and their potential linkages. Our research focused on how terrestrial and aquatic ecosystems in a low-Arctic region of Manitoba, Canada may change with warming. Rather than attempt large-scale warming experiments, we looked to past warm periods as analogs for future warming. Over the past 10,000 years, many northern regions were affected by a gradual warm-to-cool climate transition. We extracted sediment cores from eight lakes and adjacent peatlands; cores act as natural archives preserving a host of clues including pollen, diatoms, magnetics, charcoal, carbon, and cations. We combined this historical work with descriptions of chemistry and biology of modern lakes, wetlands, and soils. The modern landscape was surprisingly naturally acidic, with half the lakes having a pH (measure of acidity) of less than 5.5 compared to a more normal range of 6.5-8.5 for North American lakes. The underlying bedrock (e.g., granite) is largely devoid of mineral nutrients that buffer aquatic ecosystems (i.e., maintain a high pH) and are critical to plant growth. As expected, areas with higher abundance of wetlands had higher export of dissolved carbon, but there was no relationship between dissolved carbon and pH in lakes, likely because this landscape is naturally acidic. This contrasts with the results of previous work from the Arctic and elsewhere, where bedrock is more alkaline (e.g. limestone). , These results add important new context for how acidic Arctic landscapes function and will respond to climate change. Second, lakes, peatlands, and terrestrial vegetation in this region experienced significant change with past climate change. Warming (~6500-2500 years ago) resulted in lakes became more productive, forests establishing, and aquatic ecosystems gradually became more acidic. Peatlands accumulated slightly more carbon, possibly a result of greater plant growth. With the transition to cooler/moister conditions ~2500 years ago, forest cover diminished slightly, lakes were less productive, and carbon accumulation declined in peatlands. However, there was a mismatch between the initiation of peatland decline and lake ecosystem changes, suggesting that peatlands had less impact on aquatic systems than did climate. Third, more recent periods of climate change including the Little Ice Age (LIA) and modern warming also significantly impacted this landscape. Lakes responded to the end of the LIA with biological shifts indicating greater plankton productivity and the initiation of tree stand development in the watersheds. Impacts of modern warming show as more subtle but unified biological shifts in lakes in conjunction with recruitment declines in tree stands beginning around 1900 AD. Even more dramatic is the loss (40%) of small surface ponds in the last half century that we documented using comparisons of modern and historical imagery. We originally hypothesized that development of peatlands over the past several thousands of years would be a primary constraint on lake chemistry because previous studies have shown that export of dissolved carbon from watersheds (leading to the dark tea color of surface waters) acidifies lake water. Overall, our results show that acidic Arctic landscapes have been responsive to past climatic changes, but development of wetlands, and increased in dissolved carbon export, appear to be a minor constraint on aquatic ecosystem productivity and species composition compared to climatic changes. Increased carbon accumulation in peatlands during past warming suggests that future warming may enhance plant growth to a greater extent than soil decomposition, causing these landscapes to become potential sinks of carbon rather than sources. Unlike other areas of the Arctic underlain by limestone, the acidic nature of this landscape suggests that low nutrient levels may greatly constrain both terrestrial and aquatic productivity and hence rates of carbon accumulation. In addition to these outcomes, this research has been significant in terms of training of undergraduate and graduate students as well as postdoctoral researchers. A total of 53 students have been mentored on this project, providing a talented pool of future scientists and a citizenry better educated about potential climate warming impacts in northern regions. Elements of this research have been incorporated into university-level courses, and several public presentations have been given by the researchers.
