Long-term and interannual changes of submerged macrophytes and their associated diaspore reservoir in a shallow southern Baltic Sea bay: influence of eutrophication and climate

Long-term and interannual changes in composition of submerged vegetation, diaspore reservoir and germination were investigated in the lagoon system Westrugensche Boddenkette, Baltic Sea, north-east Germany. Comparison with a survey from 1932, showed vegetation cover is similar to the past, maintaining high cover to depths of 2.8 m despite a period of eutrophication between about 1960 and 1990. Species dominance shifted, however, from small charophytes to larger species like Potamogeton pectinatus. We explain interannual vegetation changes by weather conditions. Such changes were observed in several species, most notably in Chara canescens. This annual species seems to be favoured by extensive winter ice cover. The diaspore reservoir and the germination success of submerged macrophytes do not mirror their frequency in the vegetation, but rather reflect life form strategies. Small oospores, mainly of annual charophytes, represented >97% of all diaspores but very few Chara oospores germinated. The numerous Tolypella oospores probably originated from a discrete period with high abundance during the 1950s and have completely failed to germinate. Angiosperm seeds are larger and less frequent but have higher germination success, especially Ruppia seeds. In conclusion, charophytes are outcompeted by larger angiosperms due to the combined effect of moderate eutrophication and climate change.