Resilience of Pelagic and Benthic Microbial Communities to Sediment Resuspension in a Coastal Ecosystem, Knebel Vig, Denmark

The effects of resuspension on benthic and pelagic photosynthesis, bacterial production, denitrification and other aspects of C- and N-cycling were studied in a Danish coastal water system. Experiments were done in two 3•6–3•8 m deep mesocosms where resuspension was generated by stirring of the water. Six millimetres of the sediment was suspended, causing a reduction of Secci-depth in the overlying water to 10 cm. The system studied was N-limited and had considerable populations of benthic diatoms and cyanobacteria with photosynthetic activity of the same magnitude or larger than the pelagic.The algae created a distinct mat on the sediment surface and effectively controlled the sediment–water fluxes of dissolved inorganic nitrogen. During resuspension, the O2content of the water in the two mesocosms decreased by 5%, and chlorophyll acontent in the water increased by 28%, corresponding to a transport of only 2% of the benthic algal chlorophyll to the water phase. Primary production and nutrients in the pelagic were not affected, but bacterial production increased for 1 day. After resuspension, the average benthic chlorophyll was unchanged, but was more unevenly distributed. Benthic primary production was affected for 1 week after the event, mostly due to water turbidity; dark respiration and fluxes of N-species were unchanged. Denitrification rates were stimulated in resuspended mesocosms relative to undisturbed controls. The system studied showed remarkable stability with some short-term responses to resuspension and almost all measured parameters returned tostatus quowithin 1 week.