GROWTH AND NUTRIENT ECOLOGY OF DIDYMOSPHENIA IN BRITISH ISLES AND OTHER EUROPEAN COUNTRIES

The study raises a number of practical matters for management. The failure of water management organizations, including those in the UK and Ireland, to include organic phosphate in surveys is a hindrance to interpreting the wider literature. This means that general surveys on diatoms already made on rivers of some European countries are of limited use for understanding the ecology of Didymosphenia. The following are possible approaches to controlling nuisance growths. 1. Catchment management should aim to minimize changes likely to enhance the organic phosphate concentration of the water. Possible factors which might do so include deforestation and soil disturbance, especially on peaty or other organic-rich soils, and deliberate release to the river of liquids or other materials with a high organic content. 2. Understanding of the distribution of the diatom within the catchment is needed to avoid the risk of repeat re-inoculation from upstream. 3. As it seems likely that Didymosphenia (and perhaps other stalked diatoms) compete(s) for organic P more effectively than most non-stalked diatoms, enhancing inorganic:organic P should favour non-stalked diatoms. This should be done in early spring at the time when Didymosphenia is just starting to form stalks. Once the colonies have started to form, it is probably be too late to achieve much success in that particular year. It might be necessary to continue phosphate addition for several months each year. The site chosen for phosphate addition should be towards the upper end of the distribution range within the river, though it may be impractical to include small tributaries. Experimental studies are needed to establish the ratio of phosphate forms needed for non-stalked diatoms to outcompete Didymosphenia, but inorganic P would probably need to be well in excess of organic P. Phosphate addition would be most effective, if the inflow were matched to the river flow. 4. It might be possible to enhance the competitive success of algae less dependent on organic phosphate by using agents selectively inhibitory to PMEase and PDEase. For instance, Durrieu et al. (2003) showed that the heavy metals Cr, Ni, Cu, Zn, Cd, Hg and Pb were all highly inhibitory to Chlorella vulgaris surface PMEase activity. However, the present authors would discourage such an approach, because of the wider implications for the ecosystem. In addition, upland streams with elevated Zn and Cd are often dominated by metal-tolerant strains of other organic phosphate-utilizing algae (Whitton et al., 2005), so Didymosphenia might also evolve strains tolerant to these and perhaps other metals. The authors suggest that the increases in Didymosphenia in Europe and elsewhere, sometimes reaching nuisance proportions, are probably the result of enhanced breakdown of peat or other organic-rich soils leading to an increased export of organic phosphate to drainage streams. Atmospheric N deposition, climatic warming and catchment disturbance are all known to be factors enhancing peat degradation. If climatic warming proves the most important, the effect on D. geminata may be due to an increased and extended period of high organic phosphate concentrations in spring.