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Highlights • High water transparency in a macrophyte-dominated, shallow brackish lagoon. • Submerged vegetation reaches high cover, but only low height. • No strong effects of vegetation on suspended matter, nutrients and phytoplankton. • Zooplankton generates locally and temporarily high grazing pressure. • Refuge function of submerged plants is restricted to the lower vegetation layer. • Zooplankton performs vertical rather than horizontal diurnal migrations. Abstract The small-scale impact of submerged vegetation (locally enhanced water transparency) on abiotic / biotic parameters was studied in a shallow (< 1 m mean water depth) brackish lagoon at the German Baltic Sea coast. Suspended matter, nutrients, chlorophyll, light, and zooplankton composition and biomass were compared between dense and sparse submerged vegetation at two sites five times during the vegetation period of 2013. Water transparency was relatively high (light attenuation around 1 m−1), and nutrient concentrations moderate (TP around 1.1 μmol L−1, TN around 35 μmol L−1) during the study period. Submerged vegetation was dominated by Ruppia cirrhosa , Stuckenia pectinata and non-attached Fucus vesiculosus , and reached high cover in the "dense vegetation" plots, but only low PVI (= plant volume inhabited) values due to low height. Contrary to our expectations and numerous investigations in freshwater lakes, we did not find significant differences between dense and sparse vegetation for any parameter investigated. During all sampling occasions, copepods dominated zooplankton biomass. Zooplankton biomass was higher at night than at daytime both in dense and sparse vegetation during most sampling occasions. We conclude that PVI was too low to allow for strong plant impact on the environment, and that high water exchange rates in this shallow coastal lagoon may override the impact of submerged macrophytes on transparency. Our results indicate that phytoplankton was not limited by light, but by nutrients and zooplankton, and that zooplankton performs diurnal vertical migration and uses the dense vegetation layer straight above the sediment surface as daytime refuge. [ABSTRACT FROM AUTHOR]

Long-term and interannual changes in composition of submerged vegetation, diaspore reservoir and germination were investigated in the lagoon system Westrügensche 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. [ABSTRACT FROM AUTHOR]