A couple of thoughts about dead tadpoles and topping up.
One thing that might be responsible for tadpoles dyng is heat. The lethal temperature for tadpoles is around 35 – 36 C: for young tadpoles, water at this temperature can kill 80% or more. Older tadpoles are more resilient.
We saw mortalities in our (open to full sun) New Pond in the very hot days we had in May; although we weren’t measuring temperature at the time, measurements we’ve been making during the latest warm weather suggest that the pond was at, or very close to, the tadpole-killing temperature in May. And we certainly had quite a lot of dead tadpoles, although some did survive – an outcome which was pretty consistent with the idea that their deaths were due to the high temperatures. We also had dead water slaters and it seems pretty likely to me that this was also due to heat. There are lab studies showing Daphnia are killed by water temperatures around the mid 30s, so it seems quite possible slaters could go the same way.
The possibility raised by Alison is that her dead tadpoles were due to the chemicals added to tapwater to disinfect it – to kill bacteria and viruses. Traditionally, chlorine was used to do this. Chlorine is still used in water treatment but modern processes also include the addition of ammonia to generate chloramines.
Chloramines are used to treat drinking water because they reduce the risk of what the water industry slightly coyly calls ‘disinfection by-products’. These are the chemicals inadvertently formed when chlorine (amongst other things) is added to the water and reacts with organic matter to form by-products which are implicated with causing cancer - obviously not a good thing to have in drinking water.
Unfortunately chloramines are both toxic to aquatic organisms and don’t simply evapoarate – they have to be positively removed.
I haven’t have a chance to look at what data are available on chloramine toxicity to amphibians – beyond seeing lobby group websites which claim they are toxic. So whether adding tapwater equivalent to 10% of the volume of the pond (which is what Alison did) is enough to kill tadpoles I don’t know yet: my suspicion is that it wouldn’t be, since it’s not something a lot of people have reported. But I will dig a bit deeper here.
What all this does point to is that it seems increasingly important to me that the first thing you have to think about with making a pond is not the size, shape, depth, location or plants. It’s how to locate a clean, unpolluted, water supply which is safe for wildlife and which will be available throughout the drier times of the year.
In a world where a lot of the water that comes out of taps contains contaminants, obtaining this water requires a bit of planning.
