Vernal endgame

About this time last year, our vernal pool near East Middlebury had been reduced to a puddle by some warm and dry weeks in May. According to our data logger, it was completely dry on June 16. 2020. This year the pool might be dry a little earlier than last year.

Vernal pool NEW370 on June 6, 2021. The pool was reduced to a puddle 4 m by 2 m and about 8 cm deep.

I visited yesterday to swap the SD card and batteries in the water depth data logger and barely got my feet wet doing that. The remaining puddle was full of wood frog tadpoles but not as many as I expected based on the dead tadpoles I found last year after the pool was dry. There might have been 200 tadpoles this year and many hundreds of corpses last year.

We saw lots of wood frog eggs in April, so I assume these are wood frog tadpoles. They are about 30 mm long.

I didn’t find any dead tadpoles and didn’t see any live ones under wet leaves. It seems they had all migrated to the final puddle as the pool shrank. There were also a few salamander larvae swimming in the puddle. These were smaller and had external gills. I saw only a dozen of these.

I assume this is the larva of a spotted salamander. It is about 18 mm long, about half the size of the tadpoles.
There were dozens of spotted salamander egg masses in April, and yesterday many of these were still holding water despite being exposed for a week or two. There did not seem to be any eggs or larvae in the egg masses so maybe they all hatched. But there were not many salamander larvae in the puddle, so maybe they did not hatch in time.
Several of these aquatic insect larvae were swimming around in the vernal puddle. They are about 15 mm long. I don’t know what order of insects this is. Update: This is the larva of a predaceous diving beetle (Dytiscidae).
Results from the water depth data logger in NEW370. Data were collected every 30 minutes. May was a month of decreasing water depth and increasing temperature. Weather Underground was refusing to show me historical data from local stations so Rutland is used as a crude proxy for local rainfall.

Without regular rainfall, this vernal pool loses water quickly. Water depth decreased steadily throughout May. Rain events in late May were sufficient to lower water temperature but not to halt the decline in water depth. Water depth was about 9 cm when I visited on June 6. Some rain is possible on June 8, so the amphibians could survive for several more days. Their fate seems to be in the hands of thunderstorms.

We did a poor job redeploying the HOBO Pendant temperature logger a few weeks ago. It did not get slid all the way down the pole. It became exposed to the air several days ago when the water level was about 5 cm higher. The pool may not be completely dry for a week or two after that date. In the background is a white plastic container with a cabled temperature sensor recording the water temperature displayed above. A big steel bolt weighs the container down to the pool bottom.

Above is a short video of the tadpoles in the 9 cm deep puddle on June 6, 2021. No salamander larvae are visible in the scene, but some of the Dytiscid larvae are swimming around. The last part of the video is a beaver pond I passed on the way to the pool. Green frogs, bullfrogs, and gray tree frogs were calling around the pond. You can hear gray tree frogs in the video. This observation of bullfrogs is 0.7 miles from Ripton where there is no record of bullfrogs according to the Vermont Reptile and Amphibian Atlas. My last observation of bullfrogs was 0.9 miles from the Ripton town line, so progress is being made either by me or the frogs.

This was a good day for Osmunda. In addition to the royal fern (Osmunda regalis) in the vernal pool (see first photo above), cinnamon fern (left, O. cinnamomea), and interrupted fern (right, O. claytoniana) were displaying their sori next to the beaver pond.

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