Large-scale parasitic infection of diatoms in the Northfrisian Wadden Sea

In August 1993 during a synoptic evaluation of the plankton of the Northfrisian Wadden Sea, the infection of two diatom species (Guinardia delicatula, Guinardia flaccida) with parasitic protists was observed and quantified. The morphological appearance of infected cells suggests that nanoflagellates of the genera Pirsonia and Cryothecomonas were responsible for the infection. Guinardia delicatula dominated the phytoplankton community in the northern part of the Northfrisian Wadden Sea with densities of up to 520 cells cm−3. A second population was found in the outer Elbe estuary. In the whole area infection of G. delicatula could be observed. The percentage of infected cells was highest in the inner part of the northern Wadden Sea, reaching up to 35%. Guinardia flaccida occurred in lower densities (up to 3.6 cells cm−3) but with a higher fraction affected (up to 65%). The large-scale occurrence of heavy infection indicates that in the Wadden Sea phytoplankton parasites may at times be successful competitors to zooplankton in controlling energy flow and food web dynamics and that they may play a regulating role in phytoplankton succession due to their pronounced species selection.