Vertical niche separation of phytoplankton: Large-scale mesocosm experiments

Abstract

Vertical distribution patterns of Heterocapsa triquetra, Mesodinium rubrum, Aphanizomenon sp., Anabaena lemmermannii, Monoraphidium contortum and chlorophyll 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 variability in the vertical distribution due to species differences, to experimental treatments, and to inter-annual variation were made. The inter-specific variance component was largest. The significance of inter-specific differences was verified by a randomization test. There was also significant inter-annual variability in the species-specific vertical distribution patterns, but the inter-annual effect was less pronounced than the differences between species. Differences in the nutrient loads to the mesocosm did not have significant effects on species-specific vertical distribution patterns. The results demonstrate that the vertical segregation patterns of phytoplankton are species-specific and robust, not overly sensitive to changes in environmental conditions, but appear to be persistent biological phenomena. The implications of these findings to co-existence, competition, and diversity in the plankton are discussed.