We present a descriptive overview of summertime transport and mixing in two ~50-m-long temperate ponds. Observations were motivated by examination of the spatial and temporal distribution of Environmental DNA, but also have broader ecological and biogeochemical implications, because ponds are characterized by high biodiversity and intense biogeochemical processing.  Two distinct layers were observed, with very different patterns of transport and mixing. In the upper “Surface Mixed Layer” (SML), currents reached a few cm/s, flowing downwind near the surface and upwind near the SML base. Vertical turbulent mixing was sufficient to homogenize this layer in about 0.5 hours, comparable to the time required for currents to transport water across the pond. Below the SML, in a “Stratified Layer” (SL), mm/s currents usually reversed several times per hour, gravity held light warm water above cold dense water, and vertical turbulent mixing was negligible. Nightly free convection increased SML depth to >1 m, whereas daytime heating often reduced SML depth to <0.5 m, with the shallowest depths observed near the upwind shore. Implications for biogeochemical transport are discussed. Observations suggest that the deeper (>1.6 m) regions of observed ponds were not ventilated through most of the summer, although creation of bottom water by preferential cooling in shallows may complicate this theory. The small SML thickness, and the similarity of timescales for vertical mixing and lateral advection, both suggest that applicability of popular one-dimensional vertical parameterizations for surface fluxes may be limited to shallow depths in small water bodies. A simple parameterization for SML depth is shown to be useful for distinguishing the upper region of rapid transport and mixing from the lower, quiescent layer.

Primary Presenter: Stephen Henderson, Washington State University (Vancouver) (steve_henderson@wsu.edu)

Authors:

Stephen Henderson, Washington State University (Vancouver)  (steve_henderson@wsu.edu)

Jeffrey Nielson, Utah State University  ()

Sandra Mayne, Washington State University  ()

Jeffrey Manning, Washington State University  ()

Caren Goldberg, Washington State University  ()