Browsing by Author "Cotner, James B"

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    High Frequency Oxygen Data from Eight Shallow Prairie Pothole Lakes, 2009-2013
    (2020-10-15) Rabaey, Joseph S; Cotner, James B; Zimmer, Kyle D; Domine, Leah M; Rabae005@umn.edu; Rabaey, Joseph S; University of Minnesota Cotner Lab
    Dissolved oxygen controls important processes in lakes, from chemical reactions to organism community structure and metabolism. In shallow lakes, small volumes allow for large fluctuations in dissolved oxygen concentrations, and the oxygen regime can greatly affect ecosystem-scale processes. This data includes high frequency dissolved oxygen measurements that we used to examine differences in oxygen regimes between two alternative stable states that occur in shallow lakes. We compared annual oxygen regimes in four macrophyte-dominated, clear state lakes to four phytoplankton-dominated, turbid state lakes by quantifying oxygen concentrations, anoxia frequency, and measures of whole-lake metabolism. Oxygen regimes were not significantly different between lake states throughout the year except for during the winter under-ice period. During winter, clear lakes had less oxygen, higher frequency of anoxic periods, and higher oxygen depletion rates. Oxygen depletion rates correlated positively with peak summer macrophyte biomass. Due to lower levels of oxygen, clear shallow lakes may experience anoxia more often and for longer duration during the winter, increasing the likelihood of experiencing fish winterkill. These observations have important implications for shallow lake management, which typically focuses efforts on maintaining the clearwater state.
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    Upper Midwest lakes are supersaturated with N2
    (2020-06-22) Loeks-Johnson, Brianna M; Cotner, James B; cotne002@umn.edu; Cotner, James B
    Little is known about the exchange of gaseous nitrogen (N2) with the atmosphere in freshwater systems. Although the exchange of N2, driven by excess or deficiencies relative to saturation values, has little relevance to the atmospheric N2 pool due to its large size, it does play an important role in freshwater and marine nitrogen (N) cycling. N-fixation converts N2 to ammonia, which can be used by microbes and phytoplankton, while denitrification/anammox effectively removes it by converting oxidized, inorganic N to N2. We examined N2 saturation to infer net biological nitrogen processes in 34 lakes across 5° latitude and of varying in trophic status, mixing regime, and bathymetry. Here, we report that nearly all lakes examined in the upper Midwest (USA) were supersaturated with N2 (>85% of samples, n = 248), suggesting lakes are continuously releasing nitrogen to the atmosphere. The traditional paradigm is that freshwaters compensate for N-limitation through N-fixation, but these results indicate that lakes in this region arewere constantly losing N to the atmosphere via denitrification and/or anammox, suggesting that terrestrial N inputs are needed to balance the internal N cycle.