Allelopathic effects of Baltic Sea spring bloom dinoflagellates on co-occurring phytoplankton

Abstract

Dinoflagellate-dominated spring blooms are globally uncommon, but they regularly occur and are even increasing in the Baltic Sea, varying interannually in importance with diatom- dominated blooms. The success of dinoflagellates in the spring phytoplankton community has remained poorly understood, as they are expected to be inferior competitors due to their low growth rates and nutrient uptake capacities under nutrient-replete spring conditions. To prevail in the phytoplankton community, dinoflagellates must either be favored by specific environmental settings or possess adaptations, such as allelopathy, to compensate for their competitive disadvan- tage. Using batch cultures, we studied the allelopathic effects of 3 dominant vernal dinoflagel- lates — Biecheleria baltica, Gymnodinium corollarium and Scrippsiella hangoei — on 5 typical spring bloom diatoms and 1 cryptophyte. We also tested the effects of the dinoflagellates on each other. Three of the 5 diatoms — Melosira arctica, Skeletonema marinoi and Thalassiosira baltica — were significantly inhibited by cell-free filtrates or live cells of all dinoflagellates. Chaetoceros cf. wighamii and Diatoma tenuis were suppressed by G. corollarium, and D. tenuis was also sup- pressed by live cells of S. hangoei. In contrast, the cryptophyte Rhodomonas sp. was stimulated by all dinoflagellate species. The effects of dinoflagellate filtrates on other dinoflagellate species were mostly positive, but co-culturing tended to inhibit the growth of the respective target dino - flagellates. As some of the major players of the diatom spring bloom can be suppressed by co- occurring dinoflagellates in culture, we conclude that allelopathy may be one mechanism by which vernal dinoflagellates frequently outcompete diatoms and form intense spring blooms.