Attachment and short-term stalk development of Didymosphenia geminata: effects of light, temperature and nutrients

Abstract

We measured cell attachments and stalk lengths of the stalked diatom Didymosphenia geminata in situ, using in vivo staining, under different nutrient (phosphorus and nitrogen) and light treatments. Our aims were: (1) to investigate the effects of light and temperature (season) on attachment and stalk growth, including the effect of ultraviolet radiation (UVR), which has been suggested as a possible factor favouring D. geminata proliferation; and (2) to test the hypothesis that enrichment with dissolved phosphorus (DRP) inhibits the initial attachment of D. geminata cells. Although low concentrations (below ∼ 2 mg m−3) of DRP appear to be responsible for D. geminata proliferation, its general absence where DRP>2 mg m−3 is not understood. The experiments were conducted in outdoor channels over 14 months, spanning a range of water temperatures and light intensities. Increasing visible light intensity usually had a positive effect on attachment densities and stalk length, but both were depressed by very high intensities. Exposure to UVR generally led to lower attachment rates and shorter stalks. Increasing water temperature with season had a positive influence on the proportion of cell attachments producing stalks. Elevated nutrients (up to 6.6 mg m−3 P, and up to 115 mg m−3 N above ambient) did not affect D. geminata cell attachment unless the treatment channels contained previously colonised substrata. Nutrient enrichment negatively affected stalk length. Earlier findings of a positive role of light and a negative effect of nutrients on stalk length in D. geminata were corroborated, except that photoinhibition was demonstrated at very high visible light intensities. There was no evidence of a positive effect of UVR on D. geminata proliferation at the initial stages of attachment and growth. The results indicated that the absence of D. geminata from rivers with high DRP concentrations is not the result of nutrient interference with initial cell attachment.