STEPHEN JANE
Limnology - Freshwater Ecology - Ecosystem Science - Global Change
RESEARCH
Some published projects
With surface warming, the length of the warm season is increasing. During this warm season, many lakes experience horizontal layers of distinct water temperature. Warm, less dense water at the surface overlies cool, more dense water at
depth. Just like oil and vinegar, these different layers don't readily mix. As the warm season lengthens, this layering period becomes extended. Processes that oxygenate lakes happen near the surface, meaning that deep waters are cut off from new sources of oxygen for longer periods during the warm season. We find that this longer stratification is contributing to increases in the amount of water having critically low levels of dissolved oxygen. This implies reductions in habitat for most aerobic organisms and fundamental changes to water chemistry that affect algal growth, drinking water quality, toxicity of metals, and cycling of carbon and nutrients.
In addition to long-term surface warming, many lakes
are experiencing increased loading of dissolved organic matter from the surrounding landscape. This 'browning'
expands cool water, potentially helping to mitigate warming impacts for cold-water species like brook trout. However,
browning also favors deep-water oxygen depletion. Cold-water species like brook trout often retreat to deep water during warm periods because it remains cool. However, our study shows that deoxygenation associated with browning outpaces expansion of cool water in most cases. The figure to the left shows how most of the water column in one particularly 'brown' lake has oxygen levels too low to support brook trout. This low-oxygen water intersects surface waters that are too warm. In summary, we find that browning exacerbates the effects of surface warming for cold-water species in most cases.
Dissolved oxygen is a fundamental regulator of lake ecology and biogeochemistry. Its concentration in lakes is influenced by a variety of mechanisms. The most direct is via solubility, where concentrations decrease at higher temperatures. Deep-water dissolved oxygen responds indirectly to temperature as temperature influences stratification; stronger density gradients limit transport of oxygen from surface waters where oxygenation occurs. In this study, we sought to understand whether and how oxygen was responding to warming surface temperatures. Using long-term data from roughly 400 lakes across multiple continents, we found that dissolved oxygen is declining in both surface and deep waters. Surface declines are associated with declining solubility with warmer temperatures. Deep water declines are not associated with changes in solubility because deep water temperatures are unchanging on average. Deep-water oxygen declines appear to be more closely associated with increasing density gradients as surface temperatures warm at a more rapid rate than deep water temperatures.