Plasticity and acclimation to light reflected in temporal and spatial changes of small-scale macroalgal distribution in a stream

The small-scale distribution pattern of macroalgae in the river Ilm, in Germany was monitored. These patterns were then related to abiotic factors and tested to discover whether the distribution of the common macroalgae, Cladophora glomerata (L.) Kütz. and Vaucheria sp., was linked to differences in their photosynthetic plasticity. Cladophora glomerata revealed higher maximum photosynthetic electron transport rates after acclimation to high light (HL) compared with low light (LL) acclimated samples. By contrast, Vaucheria sp. did not acclimate to different growth light conditions. The photosynthetic performance of both algae also varied according to diurnal conditions. High light caused a reversible decrease of the dark-adapted quantum yield (F(v)/F(m)) in C. glomerata and a concomitant reversible decrease of the light-adapted quantum yield (DeltaF/F'(m)). In Vaucheria sp., F(v)/F(m) remained mostly unchanged over the day, whereas DeltaF/F'(m) decreased during the morning at low light. Photosynthetic pigments confirmed acclimational differences between the species. HL C. glomerata showed increased chlorophyll a:chlorophyll b ratios, and higher amounts of xanthophyll-cycle pigments compared with LL samples, whereas Vaucheria sp. did not reveal differences between the light treatments. While preferences for substrate size, water velocity, and depth are similar for C. glomerata and Vaucheria sp., the physiological responses to light conditions are different. It is concluded that light conditions significantly affect the small-scale spatial distribution of macroalgae and that fitness is enhanced in species with a higher plasticity in photosynthetic acclimation in unstable environments.