PME miniDOT® Logger Facilitates Major Findings in Small System

Holgerson had initially measured dissolved oxygen (DO) in a series of ponds at dusk (when DO should be highest following daytime photosynthesis) and at dawn (when DO should be lowest) in a series of forested ponds. In contrast to her expectations, Holgerson found that ponds often had higher DO at dawn relative to dusk in the summer. To follow up on these surprising results, Holgerson borrowed an oxygen logger and found that the ponds often showed an overnight increase in DO, which she thought could be due to reaeration from the atmosphere. Holgerson noted that the spikes in DO often occurred after the ponds had been cooling and speculated that DO may be increasing when the ponds water column turned over (or mixed). The other possibility was that benthic (or bottom-water) DO may have been high, and water column mixing could bring up this water. To test between these hypotheses, Holgerson deployed a PME miniDOT® Logger 10 cm above the sediment, recording DO and temperature levels every 5 minutes from June 26^th to July 21^st 2014.

With data from the miniDOT® and findings from a previous study on the same pond, Holgerson found that bottom waters had consistently low DO and exhibited some of the same nighttime spikes in oxygen, thereby eliminating the hypothesis that benthic waters explained the nighttime increase in DO in the surface waters. This left convection-driven turbulence as the most likely cause of the nocturnal reaeration events. Convective cooling mixes the surface and bottom waters, creating turbulence at the surface that allowed for reaeration events to rapidly occur. Holgerson’s observations were the first of their kind in these systems and highlight the importance of investigating otherwise accepted paradigms across a wide range of settings.