Oxygen Invasion! The Discovery of Overnight Reaeration in Small Ponds

Oxygen dynamics in freshwater environments inform a system’s overall productivity, carbon and nutrient cycling, and food web structure.  In small, forested ponds, oxygen levels are much lower than in larger lakes.  Oxygen concentrations decline seasonally, and by mid-summer, oxygen levels are severely depleted.  Many pond organisms, including tadpoles, salamander larvae, macroinvertebrates, and bacteria, have adaptations to tolerate this unique environment.  And it has been largely assumed that as oxygen concentrations decline seasonally, there is little chance for replenishment.  But a recent study shows that’s not the case.

A study led by Meredith Holgerson (a post-doctoral fellow in the Strecker Lab) describes the discovery that oxygen concentrations can increase overnight in small ponds.  On 45% of pond nights sampled, oxygen spikes occurred that transformed the pond environment: oxygen concentrations increased 12-fold (22% saturation) on average.  This is unusual as oxygen increases usually occur during the day and result from photosynthesis.  But here, the oxygen comes from a different source: atmospheric oxygen invasion.  As ponds cool overnight, turbulence is generated at the air-water interface, which increases gas exchange between the water and the air.  As a result, oxygen floods into the ponds.

fig1_diffdotemp

At night (shaded bars), ponds cool off and become warmer than the air. This cooling generates turbulence at the air-water interface that allows oxygen to invade the ponds.

This study has important implications for the ecology and biogeochemistry of small ponds.  First, oxygen spikes may reduce oxygen stress for pond organisms. Secondly, this unique biogeochemical pattern likely means that CO2 and CH4 emissions also increase during these overnight events, thus impacting the global carbon cycle.

Check out the study in Geophysical Research Letters.

 

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