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Monday, March 02, 2009

Urban Vernal Pool

A vernal pool is a temporary pool of water almost always devoid of fish due to the pool drying up seasonally. As rain falls and the vernal pool fills, creatures which have laid dormant or encysted in the mud-cracked bottom spring into life, others fly or hop in and take advantage of the watery resource with a lack of large fish predators, and the ephemeral pool becomes a hub of activity for a while just long enough for its denizens to complete their life cycles.

Does the top of my whiskey barrel plant stand qualify as a vernal pool of sorts? On the cold morning of Feb 8 I noticed it was teeming with live things and bent over to have a closer look. To my horror I realized the shallow pool was loaded with mosquito larvae (wrigglers). Still, they weren't mosquitoes yet so I decided to observe the pool for awhile. I was surprised, for one thing, the mosquitoes were breeding since to me it seemed quite cold. The first day of observation the water temperature was 50F; the following morning it was 40F. The next several nights were as cold or colder. There were two sizes of larvae, basically, and one pupa on the first day. After five days, though, the number of larvae that had changed to pupae (charmingly described as tumblers) was substantially greater, so the cold temperature was not keeping the larvae from maturing. Finally on the seventh day, I dumped out the experiment in the interest of public safety, prematurely ending the life of the infernal pool.

Following is a chart of rainfall and min/max temperatures for the month preceding the observation of the mosquito larvae. It's likely the first eggs were laid on Jan 23, the fortuitous day when a rain event followed a long period of very warm weather which must have roused the wintering-over adult mosquitoes. If so, the larger larvae were about 14 days old when I first observed them. The second set of eggs were likely laid on Feb 4, making the small larvae just 2 days old when I noticed them first. (This assumes the most likely mosquito being a species of culex, and ruling out aedes which could have laid eggs on the damp surface of the whiskey barrel, waiting for rain to fill it.)

There were a few other creatures in my pool: some tiny worms I think, maybe roundworms. Springtails jumping on the surface. A spider that found itself trapped on a floating but soggy leaf. There were undoubtedly plenty of microorganisms floating around in the pool, providing fodder for the mosquitoes. But I did not see those with my naked eye.

In southern CA, it's probably best to never assume it's too cold for mosquitoes to breed.

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