Daphnia HQ

The swarm of water fleas (Daphnia obtusa) featured in the 10th October blog were photographed in a dune slack just across the road from Cresswell Lagoon in 2008. Their pond has changed rapidly and conspicuously. The left hand photograph shows the slack in 2007, largely bare mud, with a tuft of bulrush (Schoenoplectus lacustrus) in the middle and grasses around the edge. To the right is the same field edge in winter 2011, now thickly carpeted with grasses, spike rush (Eleocharis palustris) and silver weed (Potentilla anserina). The Daphnia are more abundant when the substrate is bare mud. Perhaps they are wiped out as the plants create a climbing frame for the many predators which favour the Daphnia as food, or the planktonic algae that the Daphnia themsleves need do less well amongst the thicker stems. If you spend some time watching the twitching shoals of Daphnia you will notice they steer clear of plants or other underwater architetcure. The slack is routinely inundated when Cresswell lagoon overflows across the dune road. The water is saline, which does not cause too much of a problem for the plants and animals in the slack, but cars surfing through the backwash may end up rather rustier rather more quickly than expected. The slack is at its best when largely bare of vegetation. There is a string of thse little slacks along the fence line adjacent to the road, easily overlooked but strikingly beautiful when ochre black mud combines with the flash of shallow water, lurid green of early colonisers such as celery leaved buttercup (Ranunculus scelatatus) and fluted blue-green arches of bulrush.