My miniature shampoo bottle full of brown gunge from the bed of Lago di Maggiore made it back to the UK with no mishaps and it gives me the opportunity to see if some of the big ideas I talked about in my post of 13 June actually work in practice. So, on the morning of the day after my return, I dipped a Pasteur pipette into to suspension and put a drop onto a microscope slide, lowered a coverslip onto this and slid it under my microscope.
The principle of an ecological level playing field is that we all share a common view of the condition of a water body. In practice, this does not necessarily mean that we could all analyse each other’s samples because the scale of biogeographical variation across Europe is so huge that we would all find too many species we couldn’t recognise. However, for the diatoms, we have a better chance of achieving this than for most groups of organisms. Indeed, until relatively recently, most diatoms were thought to be cosmopolitan. Since then, we have learnt that this is often not the case but, usually, the implications for ecology are relatively small.
I had travelled over 1200 kilometres from the top left hand corner of Europe to Lago di Maggiore but when I looked down my microscope, I had little problem identifying most of the organisms that were in the sample we scraped from the rocks at Ispra. Some of the more distinctive forms are illustrated below. The identifications are tentative because they are made from the live samples, without the cleaning process most diatomists use to prepare diatoms for accurate identification. Nonetheless, most were very similar to organisms I find in UK lakes and streams.
Diatoms from the littoral zone of Lago di Maggiore, June 2013. A. Encyonema (probably E. caespitosum) living in a mucilage tube; b. a chain of Fragilaria capucina; c. Diatoma ehrenbergii; d., Cocconeis pediculus growing epiphytically on dead cells of the green alga Oedogonium. The scale bar is 25 micrometres (1/25th millimetre) long.
The most abundant diatom, however, was not one of those illustrated above. It was a much smaller diatom, Achnanthidium minutissimum or a close relative. Seen from above, these have a lanceolate outline whilst from the side, they have a distinctively “bent” shape. Over half of all the diatoms in the sample belonged to this species although, as it is so small, it probably does not represent half the biomass. These are fast-growing species that can quickly cover bare surfaces and can also thrive in the presence of grazing invertebrates (we also found some small caddis larvae clinging to the edge of the stone).
Members of the Achnanthidium minutissimum complex from the littoral zone of Lago di Maggiore, June 2013. The first two images show it from the side and above; the next two images show side views each with a short stalk by which it attaches to surfaces. The scale bar is 10 micrometres (1/100th of a millimetre).
More importantly, from the point of view of the exercise I was conducting, Achnanthidium minutissimum and its relatives are abundant throughout Europe in lakes that are at high or good status. You can find them in more impacted lakes too but rarely in such large numbers, so the presence of so many cells of A. minutissimum is a positive sign.
The final twist in this tale was to see how I would have classified this lake, had it been in the UK rather than Italy. Chemically, Lago di Maggiore is similar to Windermere, in the Lake District, having water that is neither particularly hard nor particularly soft, and it is also a lake scoured out by glacial activity, though Windermere is tiny by comparison. Had I found this set of diatoms in Windermere, the UK classification system would return the answer as “high status”. The Italians are still developing their classification system, so I could not apply that, but I did also calculate a pan-European index that we use for international comparisons, and got the same answer. It is only a very rough exercise, with just a single lake, but to be able to travel so far across Europe and still feel that our classification systems are broadly comparable made me feel that the whole exercise had been worthwhile.