Vertical niche separation of phytoplankton: large-scale mesocosm experiments

Vertical distribution patterns of Heterocapsa triquetra, Mesodinium rubrum, Aphani-zomenon sp., Anabaena lemmermannii, Monoraphidium contortum and chl orophyll a were analyzed in large-scale mesocosms extending throughout the euphotic layer and manipulated with low and high levels of mineral nutrient additions. Using an ordination method, quantitative comparisons of the var iability in the vertical distribution due to species differences, to experi mental treatments, and to inter-annual variation were made. The inter-speci fic variance component was largest. The significance of inter-specific diff erences was verified by a randomization test. There was also significant in ter-annual variability in the species- specific vertical distribution patte rns, but the inter-annual effect was less pronounced than the differences b etween species. Differences in the nutrient loads to the mesocosm did not h ave significant effects on species-specific vertical distribution patterns. The results demonstrate that the vertical segregation patterns of phytopla nkton are species specific and robust, not overly sensitive to changes in e nvironmental conditions, but appear to be persistent biological phenomena, The implications of these findings to co-existence, competition, and divers ity in the plankton are discussed.