Clumpy coexistence in phytoplankton: The role of functional similarity in community assembly

Emergent neutrality (EN) suggests that species must be sufficiently similar or sufficiently different in their niches to avoid interspecific competition. Such a scenario results in a multimodal distribution of species abundance along the niche axis (e.g., body size), namely clumps. From this perspective, species within clumps should behave in a quasi-neutral state, and their abundance will show stochastic fluctuations. Plankton is an excellent model system for developing and testing ecological theories, especially those related to size structure and species coexistence. We tested EN predictions using the phytoplankton community along the course of a tropical river considering (i) body size structure, (ii) functional clustering of species in terms of morphology-based functional groups (MBFG), and (iii) the functional similarity among species with respect to their functional traits. Considering body size as the main niche axis, two main clumps (clump I and II) were detected in different stretches of the river and remained conspicuous through time. The clump I comprised medium-sized species mainly from the MBFG IV, while the clump II included large-bodied species from the groups V and VI. Pairwise differences in species biovolume correlated with species functional redundancy when the whole species pool was considered, but not among species within the same clump. Within-clump functional distinctiveness was positively correlated with species biovolume considering both seasons, and also at the upper course. These results suggest that species within clumps behave in a quasi-neutral state, but even minor shifts in trait composition may affect their biovolume. In sum, our findings point that EN belongs to the plausible mechanisms explaining community assembly in river ecosystems.