Physiological changes of submerged macrophytes in response to a floating filamentous green algae bloom in clear-water conditions

The aim of this study was to evaluate the physiological responses of a submerged macrophyte to a floating filamentous green algal bloom in clear-water conditions. Elodea nuttallii was grown with floating Cladophora sp. at four different levels (0, control; 140, 280, 560 g FW/m(2)) in an outdoor experimental system, and its photosynthetic and antioxidant systems were evaluated. The presence of floating Cladophora sp. significantly changed the water environment by decreasing light intensity and increasing dissolved oxygen and the pH value. The photosynthetic parameters of E. nuttallii (e.g. Delta F/F-m',F-v/F-m, total chlorophyll) were higher in the presence of floating Cladophora sp. than in the control at the beginning of experiment. Because of the increasing dissolved oxygen concentration and pH value, the values of these indicators decreased (except for photosynthetic pigments) during the experiment. Compared with E. nuttallii in the control, E. nuttallii growing in the presence of floating Cladophora sp. showed higher malondialdehyde content, catalase activity, and peroxidase activity. The biomass of E. nuttallii was decreased by about 30% in the presence of high biomasses of floating Cladophora sp. (280 and 560 g FW/m(2)). These results suggest that floating Cladophora had complex effects on the biomass of E. nuttallii and that changes in water quality resulting from floating Cladophora sp. may be more important than its direct shading effect.