Opposite pondweed

"I collected one small piece to confirm the ID and to photograph. This species has declined drastically in Britain in the last couple of decades and is now extremely rare with only a handful of sites left in Hampshire. It likes clean calcareous water, so it is suprising that it has even disappeared from most of our chalk streams. When I checked the records later I discovered (not surprisingly) that it had been previously recorded from this spot by one of the past vice-county recorders, Paul Bowman, in 1990, but no-one had reported it since."