Changes in submerged macrophyte colonization in shallow areas of an oligo-mesotrophic lake and the potential role of groundwater

This study aimed at investigating whether enhanced epiphyton growth at sites with groundwater discharge may have contributed to a significant change in shallow littoral macrophyte abundance reported from oligo-mesotrophic Lake Stechlin during the last 50 years. In the 1960s, shallow littoral areas were dominated by small charophyte species such as Chara aspera, C. filiformis and C. rudis. Recent mappings indicated a strong decline of this shallow water charophyte community from 42 ha to 3 ha and a shift to the occurrence of macrophyte species typical of eutrophic lakes such as Potamogeton perfoliatus, P. pectinatus and Myriophyllum spicatum. We analyzed the nutrient content of macrophytes, and measured epiphyton growth at sites with different groundwater influence. Water column nutrient enrichment may have increased the abundance of eutrophic species, but this did not explain the decrease of charophytes. Our data suggest that enhanced epiphyton growth in shallow littoral areas with groundwater influx could impair the development of small charophytes by shading, increasing drag forces and the charophytes' sensitivity to herbivory.