Submerged vegetation in a shallow brackish lagoon does not enhance water clarity but offers substantial refuge for zooplankton

摘要

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 mu mol L-1, TN around 35 mu mol L-1) during the study period. Submerged vegetation was dominated by Ruppia cirrhosa, Stockenia pea:Mara and non-attached Focus 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.