SUMMARY. 1. The River Suså is a small, nutrient-rich stream situated in an open landscape with clayish subsoil under intensive cultivation. Discharge was variable daily and seasonally due to low groundwater input. Above-ground development of submerged macrophytes was restricted to late May to November, when water velocity and depth were low. Dominant macrophytes were rooted Potamogeton pectinatus and Sparganium emersum and unrooted Cladophora. Biomass development was closet) related to light availability.
2. Growth rates of macrophytes were linearly related to light availability when self-shading was accounted for. Potamogeton pectinatus grew rapidly m May-June, concentrated the biomass at the water-surface during July-August, and then declined exponentially when the shoots became basally senescent. Sparganium emersum had linear, flexible leaves that were continuously replaced from a basal meristem. Sparganium emersum was less susceptible to high water velocities than Potamogeton pectinatus and the biomass declined later and at lower rates during autumn. Sparganium emersum also regrew after culling that left its meristem intact in the sediment. Unrooted Cladophora developed a high biomass during sunny periods and subsequently disappeared at high discharges. The summer biomass of rooted macrophytes was greater in years with high summer discharge, whereas the biomass of Cladophora and of the epiphytic microbial community was lower due to scouring.
3. Submerged macrophytes played a key role in structure and functioning of the ecosystem. They reduced water velocities two to four fold during summer and promoted extensive organic sedimentation. The biomass of benthic diatoms declined parallel to increased macrophyte shading and sedimentation. In addition, submerged macrophytes formed a large substratum for macroinvertebrates and for a microbial community.
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