Research

Ecological Stoichiometry

We are trying to understand how organisms, mostly heterotrophic bacteria (but we care {just a little less} about other organisms). Specifically, we focus on how growth, temperature and nutrients affect the biomass composition of microbes. We examine changes in biomass composition of organisms in natural systems (soils, freshwater, oceans) as well as in a laboratory setting, with a strong emphasis on chemostats in the latter. We have funding from the Department of Energy's CSP program to characterize the genomes of isolates from Minnesota lakes and to try and understand the genes that are likely to have the biggest effects on the stoichiometry of these organisms.

Effects of stratification on methane and carbon dioxide accumulation. Gray represents periods of stratification

Greenhouse gases in lake: their role in climate change and water quality

Freshwaters are globally significant in terms of releasing greenhouse gases to the atmosphere. The most significant greenhouse gas that they release quantitatively is CO2 which is over 3 Pg per year. Although estimates of CH4 release from freshwaters is much less, only about 160 Tg (0.16 Pg), CH4 warms the atmosphere about 25 times as much as CO2 which means that CH4 from freshwaters has a bigger impact on warming the atmosphere than does CO2 despite much less of it being released. Our work has demonstrated that small lakes and ponds are particularly important when it comes to CH4 release from freshwaters. This is primarily because these systems are very productive, typically eutrophic, leading to low dissolved oxygen levels which is ideal for CH4 to accumulate and eventually be released to the atmosphere. We have also shown that ponds that stratify for longer periods of time tend to accumulate the most CH4 and release it. Currently, we are focusing on the water quality implications of CH4. Most of the methane produced in a lake or pond is oxidized to CO2 before it is released to the atmosphere--oxidation can occur aerobically or anaerobically (with nitrate or sulfate typically). Aerobic oxidation consumes O2 which is important to many organisms at higher trophic levels, including fish. Many lakes, especially deep ones, are losing oxygen which certainly can be driven by our changing climate. Longer periods of stratification due to warming means that hypolimnetic waters can consume more dissolved O2 before mixing occurs and we are exploring the role of CH4 in that process.

Dissolved Organic Matter in Freshwaters

We are interested in the lability, stoichiometry and quantity of DOM in freshwaters. We recently published a paper on DOM in Greenland lakes (Cotner et al. 2022) where we observed that the age of DOM increased with increasing concentrations. Although all the lakes were 'fresh', the lakes with the oldest DOM also had the highest specific conductivities, suggesting that hydrology likely plays a key role. Less salty lakes are more connected to the surrounding landscapes and degradation is primarily P limited but as they get drier and are less connected to the watershed, they likely become more N-limited with less microbial diversity and these factors may more severely inhibit degradation. Furthermore, long residence times means that the DOM is increasingly photo-exposed which also contributes to low degradability.

Lakes that are more connected hydrologically (left) have greater microbial diversity, higher CDOM concentrations with lower light levels and less internal production whereas the opposite patterns are expected in drier, less connected systems.

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