Padrões geográficos da diversidade taxonômica e funcional do fitoplâncton em ecossistemas aquáticos do litoral ao semiárido do Rio Grande do Norte

Abstract

Patterns in biodiversity are the result of multiple interacting processes at both local and regional spatial scales. Microorganisms, considered highly dispersed, have their communities expected to be predominantly structured by the local environment through species sorting. The functional approach has increasingly been used to understand patterns of biodiversity and its consequences to the ecosystem. Increasing scientific effort has been devoted to testing patterns and mechanisms regulating functional-taxonomic diversity relationship. The spatial scale has the potential to mediate this relationship because both the number of species and environmental heterogeneity are expected to increase with spatial scale. Consequently, a great number of functional traits may be selected by environmental filtering at larger spatial scales leading to a positive functional-taxonomic diversity relationship. Nevertheless, trait variation over an environmental gradient can be due to either species turnover or intraspecific trait variation. We aimed to evaluate how the spatial scale influences the taxonomic and functional phytoplankton diversity of freshwater environments distributed along an arid gradient from the (humid region) coast to the countryside (semiarid region) of Rio Grande do Norte. Likewise, we also intended to verify (1) which factors (local or regional) determines taxonomic and functional β-diversity at two spatial extensions (watershed and landscape) (Chapter 1); (2) how the relationships between functional and taxonomic metrics (richness, evenness, diversity, and functional redundancy) vary according to spatial scale grain (lake or watershed) (Chapter 2); and (3) how environmental gradients explain the size variation of phytoplankton community and its functional groups (Chapter 3). We collected samples in the subsurface of the limnetic region of 98 lakes distributed along 14 watersheds, between September and October 2012. The functional metrics were based on mixed traits, as surface, size, presence of mucilage and pigment composition (green, blue, brown and mixed) and type of life forms (colonial, filamentous or unicellular). Our results show that phytoplankton community was mainly explained by species sorting at both spatial extensions, with decreasing importance with the scale. However, functional β-diversity was better predicted by species sorting than taxonomic one. In addition, the approaches were spatial scale dependent, while functional approach was significant at the watershed level, taxonomic weakly significant at the landscape level. When observing the functional-taxonomic metrics relationship, we found a positive linear relationship with almost all metrics measured. Only richness was spatial scale dependent, changing from a visually asymptotic relationship to a linear one with increasing spatial scale grain. Consequently, functional redundancy decreased at both spatial scale grain with changes in the metrics responsible for it. At the lake level, functional redundancy was correlated with an increase in functional diversity, while at the watershed level with richness, taxonomic and functional ones. Overall, the phytoplankton community increased in its average size along with Cyclopoida biomass. Species turnover was predominant as the pattern of size variation, and repeated itself within each of the functional groups, except for the non-toxic unicellular algae with hard cell walls and the toxic filamentous algae for which intraspecific variation was more important along Calanoida biomass and eutrophication gradient, respectively. This study contributes to the species-sorting model of metacommunity mechanism for phytoplankton communities reinforced by the functional trait-based approach. Species turnover had a strong influence on determining phytoplankton size responses to broad gradients. Finally, this study highlights a substantial role of functional complementary for structuring phytoplankton communities and the importance of including distinct spatial scales in metacommunities studies.