As with everything, my insatiable curiosity leads me to adventures.  This past week I went on a hike to one of the remote areas of Trinity River Audubon Center to view a pond that I hadn't seen before (because you have to hack your way through a lot of brush to get there, and I'd never gotten around to it.)  It's been hot here in Texas (you may have noticed) but the sight of the pond itself was eerie.  The shores had receded nearly 40 feet in some areas and there were dead carp (skeletons) strewn on the shoreline and at least 40 large dead mussels.  Wading birds were wading in the water, but it wasn't very deep water.

What struck me most, though, was the heat.  I had the impression that when I walked off the grass (where it felt over 100 degrees) and onto the dried and cracked clay of the mudbank, the temperature rose quite a bit.

So, a few days ago, I armed myself with a digital thermometer (with probe) and a friend and I went back out there to check the pond. 

It had shrunk further.

My impression that the dried verges of the pond constituted a micro heat island was confirmed (see my other blog for pictures and data --  What was most interesting was that the Audubon Vegetation Volunteers had planted fast-growing sedges around two smaller ponds that had dried up during the period of low rainfall in 2009 -- and in these planted ponds, the sedges and other plants (including longleaf pondweed) kept the shallow end of the pond somewhat covered in vegetation. 

There was a 6 degree difference (Fahrenheit) in the air temperature above the two ponds, and over ten degrees difference in the temperature of the water in the shallows of both ponds.

From what I can see, cattails don't seem to protect the ponds as well (although I didn't do a direct measurement on the ponds where the primary vegetation is cattail... all but one of those are dry now.)

So a working hypothesis is that:
a) some fast-growing vegetation along the verges of any body of water help keep the water cooler and reduce evaporation.
b) Certain types of pondweeds also have a similar function.
c) small bodies of water without sedges or pondweed can develop into micro heat islands, increasing the rate of water loss and habitat loss.

I'll investigate further at the end of this week -- if any of you has additional data to contribute, that would be of interest.