I was back at Smallhope Burn last week (see “Nitzschia and a friend…”), albeit a few kilometres further downstream from the site I discussed back in September. Despite my visit being in mid-November, many of the stones I picked up here had tufts of young, healthy-looking Cladophora glomerata with, between them, apparently bare rock surface on which I could see the tiny, almost-black shells of Ancylidae snails. These have rasping mouthparts and move across the stone surface grazing on the microscopic algae (probably mostly diatoms) that inhabit it. Though the surface of the rock looked bare and felt rough to the touch, there is almost certainly a thin “sward” of diatoms and other algae here. Otherwise, the Ancylidae (the “cows” of my underwater pastures) would not be here.
A boulder from Smallhope Burn covered with tufts of Cladophora glomerata interspersed with Ancylidae snails with (right) a shell of an Ancylidae snail on my fingertip. The shell is about two millimetres across and the boulder is about 30 centimetres across.
Under the microscope,I can see several different types of diatom, though most look as if they live on or around the Cladophora rather than in the patches between the tufts. Cocconeis pediculus and Rhoicosphenia abbreviata are both common epiphytes (see “Cladophora and friends”) whilst Diatoma vulgare and Melosira varians form chains that are loosely-attached to the substrate and, if detached, will easily become entangled within the Cladophora tufts. The Navicula species are both motile forms that can glide in and around the filaments in search of light. I suspect that my standard method of sampling diatoms (brushing the tops of stones vigorously with a toothbrush) is a little too coarse to get a true indication of how the diatoms differ between the bare patches and the Cladophora tufts.
Diatoms from Smallhope Burn, Low Meadows, November 2014. a. Cocconeis pediculus; b.,c. d. Rhoicosphenia abbreviata; e., f. Diatoma vulgare; g. Ulnaria ulna; h. Melosira varians; i. Navicula tripunctata; j. Navicula gregaria. Scale bar: 10 micrometres (1/100th of a millimetre).
Our thinking on the ecology of Cladophora has changed over the past few years. If you consult literature from the 1970s, you’ll see a general agreement that Cladophora prefers waters that are rich in nutrients. This is, indeed, my own observation but, at the same time, you can find a lot of Cladophora in rivers with quite low levels of nutrients and, sometimes, no or little Cladophora in rivers that are nutrient-rich. A recent paper from Ireland helps put these observations into perspective, by demonstrating that the quantity of Cladophora is strongly influenced by the density of grazers as well as the quantity of nutrients. My suspicion is that grazing invertebrates can keep Cladopora under control until a “tipping point” is reached when either the density of grazers drops or production of Cladophora acceleratesers to an extent that the grazers can’t keep up with the rapid growth of the alga, at which point the bed becomes smothered. It may even be an example of the “alternative stable states” theory developed by Brian Moss and colleagues for the Norfolk Broads, though that would take some more work to confirm.
Moss, B. (2010). Ecology of Freshwaters. A View for the Twenty-First Century. 4th Edition. Wiley-Blackwell, Oxford.
Sturt, M.M., Jansen, M.A.K. & Harrison, S.S.C. (2011). Invertebrate grazing and riparian shade as controllers of nuisance algae in a eutrophic river. Freshwater Biology 56: 2580-2593.
Whitton, B.A. (1970). The biology of Cladophora in freshwaters. Water Research 4: 457-476.