Effects of Experimental Sediment Resuspension on a Coastal Planktonic Microbial Food Web

Effects of sediment resuspension on microbial food web dynamics in the water column of microcosms consisting of sediment cores with the overlying water have been studied. These microcosms were obtained from Rassower Strom, coastal Southern Baltic, and incubated for 108 h under in situ light and temperature conditions. They were resuspended with oscillating grids at intensities that reproduced the range of naturally occurring seston concentrations during resuspension events in these waters. After 12 h of resuspension, the abundances of pico- and nanoautotrophs, rhizopods (naked and testate amoebae) and heterotrophic nanoplankton had increased and were significantly positively correlated to seston concentration, indicating that resuspension of benthic representatives directly affected their water column abundances. Similar trends were found for benthic diatoms and ciliates. In addition, resuspension enhanced population growth of pico- and nanoautotrophs and resuspended diatoms over the whole course of the experiment. This enhancement was independent of a direct transport of cells from the sediment and was most likely caused by reduced nitrogen limitation due to resuspension. The abundance of heterotrophic nanoplankton was tightly correlated to those of their main food organisms, pico- and nanoautotrophs and bacteria. It was also significantly increased by resuspension over the whole course of the experiment. This is consistent with decreased bottom-up control of heterotrophic nanoplankton due to resuspension. These results indicate that sediment resuspension may strongly affect the planktonic microbial food web of coastal waters, both by a direct transport of cells from the sediment into the water column and by indirect effects mediated by dissolved nutrients and trophic interactions.