Back in early April I was looking into Kilmartin Burn, in Argyllshire, and noticing the water milfoil (Myriophyllum alterniflorum) smothered in epiphytes. Having looked at the various components separately under the mciroscpe (see post of 5 April), the next step was to put the various components together again, in an attempt to visualise the community of algae growing in and around the finely-divided leaves of the water milfoil. It took longer than expected but here, at last, is the finished product.
The epiphytic life on the finely-divided leaves of Myriophyllum alterniflorum from Kilmartin Burn in April 2013. The long needle-like cells of Ulnaria ulna are about one tenth of a millimetre in length.
The picture shows the finely-divided leaflets of Myriophyllum, each bearing numerous needle-like cells of Ulnaria ulna which, at one tenth to almost a quarter of a millimetre in length, are giants amongst the other diatoms. The Ulnaria cells generally occur in small clusters, radiating out from a mucilage pad at their base. There are also many fan-shaped colonies of another, smaller, diatom, Meridion circulare, some of which stood erect on the surface of the Myriophyllum leaflets whilst others were lying flat. Finally, you can also see a small number of elliptical cells of Cocconeis placentula, which grow flat on the surface of the Myriophyllum, and a few cells of Achnanthidium minutissimum. You can see photographs of Ulnaria, Meridion and Cocconeis cells from Kilmartin Burn in the 5 April post, and we also met Cocconeis placentula growing on the underside of duckweed in Cassop Pond back in the 26 January post. Achnanthidium minutissimum is an extremely common diatom, particularly in clean waters, and there are several illustrations of this on my website, www.martynkelly.co.uk.
All of these algae are thriving in the relatively-sheltered conditions created by the finely-divided Myriophyllum leaves. Had I hunted harder, I would also have found many invertebrates living here too, so the Myriophyllum creates a self-contained world within the wider stream environment. But this has a cost as the mass of diatoms intercept the sunlight before it reaches the Myriophyllum leaves. People have suggested that one way that eutrophication – the artificial enrichment of lakes and rivers by nutrients – can disrupt ecosystems is by increasing the mass of these epiphytic algae and ultimately “shading” the host plant until it can no longer survive. There is also some evidence of submerged plants secreting chemicals which attract invertebrate grazers which, in turn, stop the epiphytes attaining such damaging quantities.