Effect of aquatic plants upon planktonic and periphytic organisms: a microcosm-based approach

Aquatic plants have a major influence upon other aquatic organisms, by altering both water chemistry and spatial structure of the habitat in shallow water bodies. Some of them, such as Stratiotes aloides L., may suppress algal growth. But how aquatic plants would ultimately influence the heterotrophic community and the aquatic ecosystem as a whole is far from clear. Our microcosm-based study demonstrated that even a modest density of S. aloides caused a decline in phytoplankton chlorophyll concentration and periphytic algae abundance, including cyanobacteria, whereas diatoms appeared to be immune to the plant influence. Photosynthetic rate remained unaltered despite decreased chlorophyll concentration. While bacterial counts remained largely unchanged, more bacteria were observed forming microcolonies as well as associating with particulate organic matter. Numbers of periphytic heterotrophic organisms did not differ significantly between the planted and plant-free control microcosms. Zooplankton diversity increased and cladocerans assumed a more prominent position within the microcosms with macrophytes. We assume that the presence of plant's leads to increased importance of bacteria and protists in the functioning of the food webs. Therefore, decreasing of algal abundance does not involve reducing the number of heterotrophic planktonic and periphytic organisms.