At the end, I am at home in the woods
After 12 nights of recording bat calls near an Indiana bat maternity roosting colony, we deployed the AudioMoths for a week at the vernal pool where we recorded bat calls in August. Instead of putting two AudioMoths at the vernal pool, we put one by the pool and two in the forest surrounding the pool. One of the forest AudioMoths recorded nothing (a battery was inserted backwards), so we got data from only one non-pool AudioMoth.
Part of the Vermont Center for Ecosystem Studies Vernal Pool Monitoring Project includes audio recordings to document the first calls of frogs at the pools. This year they started using AudioMoths to make the recordings. AudioMoths are open-source data loggers with a low-power sleep mode, real-time clock, microSD card slot, MEMS microphone, and circuitry to support audio capture. AudioMoths can be configured to automatically save audio recordings on a custom schedule. The AudioMoth software and hardware are well thought out and the audio quality is very good (see audio file below). The AudioMoths were deployed at about 50 Vermont vernal pools in weather-tight cases (Figure 1) and recorded for a few short sessions every night for several weeks in the spring.
A water depth data logger has been running unattended at a vernal pool (MLS619) at Snake Mountain in Bridport since April 10. The water was a meter deep when I waded out then to change the batteries and swap the microSD card. I have not been in a hurry to do the swap again assuming that the longer I wait the less wet I will get doing it. But I didn’t know how long the batteries would power the logger. The previous record was 11+ weeks, although a similar logger has been running in my office for more than a year on the same batteries. That one only saves data when certain conditions are met but it has been waking up every 30 minutes and sensing its environment for almost 13 months. I decided 16 weeks would make a good record for field deployment at MLS619 and was pleased to find that data had been written to the microSD card once every 30 minutes during those 3.7 months.
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.
In addition to the HOBO temperature logger in each vernal pool, we have a water depth data logger installed in the two pools we are watching this year. We have looked at the data from late January to early April and the loggers are continually recording new data every 30 minutes.
Ned and I have been monitoring a second vernal pool this year. The new pool is in Bridport on the southern extension of Snake Mountain. It is right in the middle of the Champlain Valley, but up on a rocky ridge. The pool is almost twice a big as our other one and a foot deeper. It is lower in elevation at 570 feet above sea level (the other one is in the foothills of the Green Mountains at 1260 feet a.s.l.).
The most important difference between the two pools might be chemical. The new pool sits between two ledges of Middle Cambrian dolostone or dolomitic quartzite, limey rocks which enrich the soil with calcium and magnesium. The old pool is surrounded by Cheshire quartzite and the vegetation there (red oak, beech, birch) suggests that the soils are not rich in calcium.
The vernal pool we have been monitoring has been dry since about June 17. There were a couple of rainy days in late June, but the puddles formed did not last more than a day. The pool was dry when I visited on July 10, September 29, and October 23. That is, there was no standing water, but the soil under the leaves was always damp. New data from the water depth datalogger indicate that only two rainfall events between June 30 and October 23 produced standing water in the pool.
It has been three weeks since I last visited the vernal pool and installed the Version 4 (Ultrasonic) water depth data-logger. I was curious to learn whether the new logger was working and decided to collect the data and replace the batteries in both loggers — the Version 3 logger (laser rangefinder) had also been running since the last visit.
Daytime high temperatures in the first half of June in Middlebury are historically in the 70s F, but so far this year half of the days in June have been in the 80s F. Historically in June, Burlington gets 2.5 inches of rain by the 20th, but so far has gotten only 0.4 inches (last year they got 3.8 inches by now). So maybe I should not have been so surprised on Friday to find that our vernal pool was completely dry. Ned and I visited to collect data from the loggers and install a new version of the DIY water depth and water temperature logger. The new logger might not have much work to do for a while.
Six weeks ago I installed version 3 of a water depth data logger in the vernal pool we are monitoring. I liked that logger more than the earlier versions because the parts cost $35 instead of $50. After posting about it I noticed that many Arduino hobbyists measure distance with an ultrasonic rangefinder instead of the laser rangefinder I had used, so I ordered a few HC-SR04p rangefinders.
On May 3 we made the official third visit to the vernal pool we are monitoring. During this visit we are supposed to collect the frog call recorder and do the final surveys for amphibian eggs and aquatic invertebrates. At the second visit we failed to find any fairy shrimp, so I was looking forward to adding them to my crustacean life list. I was also looking forward to collecting the microSD card from the data logger we had installed on April 17 and examining two more weeks worth of data. And at the last minute I decided to bring a 24 foot-long pole and get some “aerial” photos from above the pool. By the time we left the pool to hike back to the road, I was dismayed at my lack of success at most of these goals.
Vernal pools are little ponds which have no permanent stream entering or exiting. They fill with snowmelt, rain, and groundwater, but in droughty years they often dry out before summer’s end. These characteristics prevent fish from living in vernal pools (fish can’t get to them and fish couldn’t escape when they dry out). This allows animals with little defense against hungry fish to prosper in vernal pools, especially if they can also survive when the pool dries out.
