A gravel pit pond that I thrashed with Matt on my work experience was almost completely choked by New Zealand Pygmyweed, or Australian Swamp Stonecrop (Crassula Helmsii), an invasive species of pondweed that came in from abroad, and has taken over numerous British ponds, especially in south and midland England and most of Wales. The pondweed is particularly adaptable to it’s environment, being able to survive in both acidic and alkali conditions without detrimental effects, as well as surviving in semi-saline areas. As such it is able to grow in almost any body of still or slow moving freshwater it is introduced too, and often destroys the local subaquatic flora. This often leads to oxygen depletion within ponds, and invertebrates, fish and amphibians can quickly die out.
The Crassula in cannot create viable seeds in Britain, meaning that it must reproduce entirely through cuttings. The Pygmyweed can grow from a piece of leaf or stem as small as 5mm, which means that you must be very careful when fishing or surveying in a pond with Crassula in, so that you do not bring pieces of it to another pond which is not infested. This trait also makes it incredibly difficult to remove from habitats it has already taken over, and to this day it has not been successfully removed from any area.
Matt and I made care not to bring any of the New Zealand Pygmyweed with us when we went to the next pond, cleaning our nets, and the tray we used to search for invertebrates. Even a small piece of Crassula could have taken over another pond.