Observation: occasionally stirring the water surface: #1 gets rid of aphids and #2 promotes L. minor species.
Background: I built a dissolved oxygen probe shaker to keep the probes moving in the otherwise slow-moving water column. After running the shaker for several months, I noticed that L. minor duckweed species accumulated around the probe; unlike W. borealis species which out-competed the L. minor species in un-stirred parts of the reactor (i.e. parts with no probe and no disturbance to the water surface). Later when my plants became infested with aphids I noticed that there were no aphids on the plants closes to the probes.
Hypothesis: By gently disturbing the water surface every two minutes then 1) I will promote L. minor species and prevent W. borealis species from taking over the reactors. 2) I will eliminate the aphids.
Results: I replaced the dissolved oxygen probe(s) with larger objects in order to disturb the entire water surface in two cells (0.167 m^2 each) with no disturbance in the third cell (i.e. the control). In less than a week, the L. minor plants re-established themselves and competed with the W. borealis species (prior to installing the larger "disturbing objects" the W. borealis species had already begun out-competing the L. minor species. This mean that when one looks at the water surface they would notice that 90% of the surface area was occupied by Wolffia plants and only 10% by Lemna plants).
In addition, the aphids went away!!! The aphids were always slothful to begin with and would hardly ever be seen moving around on the duckweed mat. I guess they prefer a more sedentary environment than an occasionally turbulent one.
