There were deer hoof prints in the light dusting of snow on the ground during my visit to Cassop on 2nd February. They led right up to the edge of the pond but the surface, at least around the edges, was frozen solid. I had to break the ice with the heel of my boot before I could get to the water to collect a sample. Underneath the ice, there was a bright green floc of algae suspended just above the sediment, apparently thriving despite the freezing conditions. I used the end of a plastic pipette to hook some of this and drop it into a bottle, then skimmed the end of the pipette over the surface of the sediment to hoover up some of the fine material.
Under the microscope, the green floc resolved itself into tangled filaments of Spirogyra, a very common genus, instantly recognizable from the characteristic ribbon-shaped chloroplast wrapped in a spiral just inside the cell wall and slippery feel. Over fifty different species of Spirogyra have been recorded from Britain and Ireland but this one was impossible to identify because you need the reproductive bodies in order to do this and, as is very often the case, these were not present in the population I had collected. The slippery texture Spirogyra and its relatives is caused by mucilage exuded by the cells.
The filaments in the photograph each have a diameter of about 35 micrometres. One micrometre is a thousandth of a millimetre so, to put this measurement in context, you could lay about thirty of these filaments side by side across a full stop.
The material I hovered up from the sediment consisted mostly of tiny calcite crystals although, mixed in with these I could also see tiny needle-like diatom cells growing in clusters, all radiating out from a single point. These seemed to be the same species, Fragilaria rumpens, that had I noticed on the underside of the duckweeds in my January 26th post. This time they appeared to be growing directly on the sediment surface or, in one case, piggybacking on a chain of cylindrical cells of another diatom, Melosira varians. The Spirogira appears to be immune to epiphytes, partly due to the mucilage which has been shown to contain chemicals which inhibit the growth of other algae. There were also a few cells of Nitzschia and Navicula, motile genera which glide in and around the calcite crystals.
I have attempted to convey an impression of this microscopic world in the diorama: the sediment surface with a filament of Melosira and its epiphytes in the left foreground, a few clusters of Fragilaria rumpens and, hanging above them, the tangle of Spirogyra filaments. A single cell of Nitzschia glides between the calcite crystals in the foreground.