Settling blooms of filamentous cyanobacteria as food for meiofauna assemblages

Summer blooms of filamentous nitrogen-fixing cyanobacteria in the Baltic Sea are normally dominated by Aphanizomenon sp. and the toxin-producing Nodularia spumigena. In a 2-week laboratory experiment, we followed the uptake by representative benthic meiofauna species of 14C-labeled organic carbon from blooms, each dominated by one of these cyanobacteria. Natural bloom material was collected and labeled by incubation with NaHCO3. Uptake of cyanobacterial carbon was recorded for the major meiofauna taxa living in the firstcentimeter layer, namely ostracods, harpacticoids, and nematodes. The uptake rates were within the range found for diatoms in other studies, indicating that cyanobacteria may be an important food resource for the meiobenthos. The uptake of cyanobacterial carbon varied significantly among species, even within the same class. The ostracod Candona neglecta showed the highest uptake values, whereas two other ostracod species took up very little of the label. There was no significant difference in utilization of carbon from Aphanizomenon sp. and N. spumigena and no reduction in the abundance of the meiofaunal taxa analyzed compared to unexposed controls, indicating that Baltic meiofaunal assemblages in general experience no mortality when exposed to settled cyanobacteria, even the hepatotoxic N. spumigena. The study of food webs and the trophic relationships that characterize them are essential for understanding an ecosystem. Marine soft-bottom sediments constitute the second largest ecosystem on Earth and are mainly found below the photic zone. Benthic communities inhabiting these sediments must feed primarily on allochthonous organic matter derived from settling and seasonal phytoplankton blooms (Elmgren 1978; Graf 1992; Ólafsson and Elmgren 1997). This energy supply to the benthic community and the processes that decompose the settled organic matter are among the fundamental factors shaping the