We have collected data from Broadoak Nature Reserve as well as Langton pond. We found a larger number of species at Broadoak. There are two reasons for this:

• It takes a while for species numbers to build up in a pond. Detritivores and herbivores move in first, followed by carnivores when there is enough food for them. The results from Langton pond back this up, with new species being found every two or three weeks.
• Many pond invertebrates need water with high levels of oxygen. As Langton pond didn’t have any vegetation to start with, the numbers of oxygen loving species is still relatively low.

In both Broadoak and Langton pond we found more herbivores and detritivores than carnivores. This is what we expected for three reasons:

• Herbivores are often smaller than their predators, so the predators have to eat quite a few to get the energy they need e.g. Water fleas are smaller than Phantom midges.
• Predators don’t always eat all of their prey. They won’t eat a hard exoskeleton for example.
• Predators can’t digest all of their prey, so some is wasted.

Looking at Langton ponds results, the first question that we have to ask is how the pond invertebrates got there. Most of the species were probably brought in on the feet of birds that had been to other ponds. In the first weeks after the pond was filled a pair of Mallard were seen around the pond quite a lot and we get a lot of smaller birds visiting the site. Other species would have been brought in by the adults flying in to lay eggs and the eggs hatching. Biting and non-biting midge larvae and dragonfly nymphs arrived this way, for example.

It is not surprising that the first invertebrates to move into the pond were bloodworms. They live in the mud at the bottom of ponds feeding on detritus (decaying plant and animal matter). There would have been quite a lot of detritus in the mud when the pond was filled because the soil that was used to cover the liner came from land that had been used to grow crops a couple of years before. Bloodworms can also cope with low oxygen levels, because they have a chemical similar to haemoglobin. There wasn’t much oxygen in the pond to start with because there weren’t any plants in it. To start with there wasn’t anything eating the bloodworms and they didn’t have much competition for food, so they did really well. By the end of our study their numbers were dropping. We think this is partly because predators like water mites were increasing. It could also be because they’d eaten most of the detritus in the mud, or because there were more organisms competing with them for food.

Water fleas arrived shortly after the bloodworms. They were probably carried in on bird’s feet. They could survive because they feed on algae. The algae spread quickly because there were a lot of nutrients in the soil and because the summer was quite warm. Water fleas reproduce really quickly (without having to mate), which is why their numbers went up so quickly. Most of the time we found more water fleas in the deep water. This is what we expected. Algae grows in the summer and is found mainly in open water, so this is where the water fleas were. In the winter when there isn’t as much algae around we’ll probably find more water fleas in the shallow water, because they will have to go to the margins to find food. One reason why we found a lot of water fleas in the shallow water in August could be because the wind was blowing the algae towards the shallow platform, so the water fleas were moving to follow it.

There are lots of different species of water flea. The first ones we found were red, which must mean that they had a chemical like haemoglobin, so they could live in the low oxygen. By the middle of July most of the ones we were catching were clear. This shows that the amount of oxygen was going up. Another reason that we know oxygen levels have been going up is because we have been finding more species that need clean water with a lot of oxygen, like mayfly and waterlice. As the plants grow we should find more and more species like this. By the beginning of September more and more of the bloodworms that we were catching were brown instead of red. This is another reason why we know oxygen levels are going up, because it means that the bloodworms could get away with having less of their oxygen trapping chemical.

If you plot a graph of the results you can see that soon after water flea numbers rise, the number of Phantom midge goes up. When water flea numbers drop, Phantom midge numbers drop. This is because water fleas are Phantom midge’s favourite food. The midges are like stealth hunters. They are see through so that the fleas can’t see them and have air pockets to raise and lower themselves in the water. They hover above the fleas and then pounce on them. In very shallow water the midges probably find it difficult to hunt because there isn’t much room for them to hover above their prey. This might be why we found more midges in the deep water. Once water flea numbers have risen the graph shows that it takes a while before Phantom midge numbers go up. This happens even with bigger predators and their prey, because the predators wait until they are fairly sure that prey numbers are going to stay high for a while before they start to reproduce. It also takes a while for the predators to reproduce. The bigger the predator the longer it normally is.

We were surprised at how many Lesser Water Boatmen we found, particularly in July. Some adults were probably brought in by birds and then laid eggs. When you walked around the pond you can see young ones teeming around every bit of vegetation. We probably had so many because there aren’t many top predators yet. Greater Water Boatmen, water beetles and water mites will eat them, but there aren’t many of them yet. We haven’t been getting many large ones though, which probably means that not many of them are surviving to adulthood. This could be because they are struggling to find food with so many of them. We think that the numbers of Lesser Water Boatmen started to go down in September because water mites were on the increase and attacking them. In nearly every pond dip that we did in August and September we saw Lesser Water Boatmen with water mites attached to them. Following the drop in boatmen, water mite numbers also fell as the mites found it harder to find food and became food for other insects.

As we said earlier, we are starting to see species that need clean water with a lot of oxygen, like waterlice and mayflies. The latest oxygen loving species to arrive are the Ramshorn snail and stick cased caddis. This is a good sign because it means that our pond is developing well. It will probably take at least another year for it to settle down completely. When the number of species has stopped increasing the number of each species that we find should also start to find a balance. We will never find a large number of top predators like dragonfly nymphs though, as the pond isn’t large enough to support a lot of them.

November 2005

The surge in mayfly in October was not entirely unexpected. Research had shown us that mayfly tend to increase during the colder months. It is possible that an increase in duck weed has also given them an improved food supply. As we are currently finding few predators there is little to keep their numbers in check.