Pond-dipping at Pallet's Pond
Earlier in the year, David Barnard and Linden attended a course at Mop End near Amersham, on identifying, and surveying, amphibians and reptiles. These courses, run by the Field Studies Council, are aimed at providing amateur observers with enough information about identification and surveying methods to enable them to contribute to a growing body of knowledge. It is encouraging to know that these records can be used by scientists to add to their understanding of the range and distribution of different species.
We were encouraged to put our experience to the test and on a still, cloudy evening in July, went to Pallet's Pond to see what we could find. This visit was partly an exploration of practicalities and possibilities. As you probably know, the pond has been covered with black plastic in an attempt to eradicate New Zealand Pygmyweed. This highly invasive plant, if left untreated, will completely smother the water body, crowding out all native plants and organisms. Pallet's Pond is quite large and deep, and this has made it very difficult to cover all the parts of the pond that have been affected by this rampant plant, and indeed, we were very disappointed to learn that the weed is still colonising part of the pond. Fortunately, the majority of the weed has been smothered, but the Society will be investigating alternative measures to deal with the problem. We have had plenty of rain during the winter and spring, and the plastic is mostly underwater and barely visible; we think that this now acts like a liner to the pond and that it probably will not affect the creatures that live in the water.
Back to the practicalities of the pond-dipping: Sue Philips and Linden, dressed in chest-height waders, found it quite hard to keep their balance on the billowing plastic sheet, but managed to stay upright! The next problem was that there is a lot of duckweed floating on the surface of the water, so we tried to sweep it aside in order to dip in clear water. It is important not to disturb the silt at the bottom of the pond. This was difficult, as the plastic billows in various places and quite a bit of the substrate came up with the net, clouding the water and making it difficult to see the creatures. However, once we had mastered this, we managed to empty our net into white plastic trays so that we could observe them.
The amphibian element of our course in April focussed on identifying the three most prevalent native newt species, as well as frogs and toads. We did not expect to find these in the pond, because by July they have finished breeding and will be spending much of their time on land where they find shelter among dense foliage and can forage for food. However, we did hope to find some tadpoles of frogs and toads, but were unable to do so. What we did see took us into new territory, but it confirmed that the pond is alive with myriads of tiny life-forms which we all felt ill-equipped to identify. Although we had magnifying glasses and identification books, it was very difficult to see the tiny details of these incredibly active organisms. Some were like transparent tubes, almost invisible. Others were deep black, elongated as they moved forward and retracted to rounded blobs when at rest, but no more than 6mm long when extended. Were they leeches or planarian flatworms? We could not decide. Tiny pink worm-like creatures moved about on the bottom of the dish, while other insect-like organisms whirled about in the water, moving so fast that it was often hard to focus on them. We felt that we were looking at many different forms of insect larva and may have found some damsel-fly larvae with a three-pronged tail, but again, we were not sure. There are several species of insect larvae that have this kind of tail.
One creature that we felt we could identify was the larval form of a newt, about 1cm long. Under the magnifying glass, we could see that it had feathery gills and black eyes. Even our expert at Mop End had not been able to identify the species of newt when the larva was at this stage in its development. Only two creatures were easily identified: the Water Boatman, which swims on its back, and the Great Pond Snail, both large enough to see the details.
We felt that the only way to familiarise ourselves with these myriads of creatures would be to look at them through a microscope. The water teemed with delicate and fragile life-forms, able to move easily in the supportive water with the apparently feeble means of cilia or lashing hairs, or whip-like flagella. Notwithstanding our struggle against NZPW, the abundance of life suggests that there is a healthy ecosystem at work in Pallet's Pond, which must have enough oxygen in it to support a wide variety of creatures.
Further pond-dips are planned, but we would ideally be looking for the larger amphibians which would be within our expertise to identify and record using approved survey methods.
When we had finished looking at everything, we tipped the contents of the trays back into the pond. Next time, we will bring a supply of water to wash all the creatures out, as some of them adhered to the base of the dish and were difficult to remove. All the equipment that we used had to be cleaned with a safe disinfectant or weak bleach solution. It is important not to transfer any pathogens that might be present to other water bodies, and the Pygmyweed could be transported inadvertently.