Date: 3/31/18 8:51 am From: George Hammond <worldsmith...> Subject: [birders] OT: tadpoles, oxygen, "winter kill"
> On Mar 30, 2018, at 10:43 PM, Ron Gamble <rongamble...> wrote:
> Interesting non-bird note………….at least a couple hundred large tadpoles (either green or bull frog tadpoles) seen coming to water surface! Perhaps to “gulp” air?; in sedges right at road edge.
> And ideas or confirmation from folks about what these tadpoles were doing? I got down on ground at road edge and caught 4 quite easily with my hand. The soft but plentiful noise they made is what caught my attention first!
Just an idea, and I haven’t seen the site, but for what it’s worth:
Ponds and small lakes in our area sometimes develop very low oxygen levels in the water over the winter. If there is a lot of decomposing vegetation in the water, (as is typical of shallow ponds and lakes) the decomposers (fungi, bacteria, other microbes) continue to live and consume oxygen all winter long, but if the pond is covered with ice, oxygen cannot easily diffuse into the water from the air, so the amount of oxygen dissolved in the water gets very low. Algae and underwater plants growing in the pond can contribute some oxygen through photosynthesis, but if there is snow on top of the ice, that cuts off light, and so little photosynthesis can occur.
The aquatic species that winter in ponds and small lakes are adapted to deal with some low oxygen conditions, but sometimes it is too low for them and they die. Occasionally whole schools of fish and masses of frogs die all in one winter. When a large number of fish or frogs die from anoxia, it’s called “winter kill”, and it’s a pretty well-known phenomenon. It can be a natural event, a result of a dry fall (creating low water levels) followed by an early and then long-lasting winter (so ponds freeze over early, and then remain “sealed” for a long time). Human activity that increases nutrient levels in the water (erosion, waste water disposal, fertilizer run off, etc.) can make it worse: more nutrients, more plants, more decomposing plant material, less oxygen.
So, maybe, those tadpoles were coming to the surface in the shallows because there was more oxygen there. Tadpoles have gills, and get their oxygen from the water, but “gulping” might help them get oxygen all the same.
Obligatory bird connection: hmm, not easy. I imagine that winter kills could be valuable food sources for bald eagles and maybe other scavengers that are present in early spring. Fish and other animals that die from winter kill decompose very slowly in the cold water, so when a pond or small lake first starts to melt out, there could be a sudden bounty. I wonder if other fish-eaters like loons and mergansers might eat dead fish if they were sufficiently fresh.