Mixotrofia do fitoplâncton em um gradiente de luz e nutrientes

Abstract

Precipitation events and water level fluctuations are environmental factors that affects aquatic ecosystem functioning influencing phytoplankton dynamic and their resources. Nutrient availability and trophic state of arid and semi-arid regions are controlled by quantity and rain periodicity. Future climate scenario predicts an increase in intensity and frequency of droughts in semi-arid regions. Drought leads to water level reduction and consequently increase nutrients concentrations, turbidity, salinity and conductivity, favoring cyanobacteria blooms. However, recent studies shows that mixotrophic algae can be better competitors under more extreme conditions, such as prolonged periods of droughts. Mixotrophic organisms play important role as producers and consumers reflecting in the structure of food webs. In this work, we evaluate the effect of precipitation gradient in semi-arid region on the structure of phytoplankton community based on a trait-based approach. We confirm the hypothesis that in dry period the main phytoplankton traits are related to a high cyanobacteria biomass (nitrogen fixation, filaments, coloniality), however, in extremely drought periods with critical water level, cyanobacteria collapse and shifts the dominance to mixotrophic algae. To confirm that the reduction on light availability caused by sediment resuspension was the main factor on phytoplankton pattern, we performed laboratory experiments with competition between cyanobacteria and a mixotrophic species, manipulating light and sediment addition in systems with high levels of nutrients. Besides this, we also performed grazing experiments to estimate bacterivory by flagellate algae in distinct light and nutrients conditions and propose new methodologies to facilitate ingestion rates quantification. Our study shows the importance of mixotrophic algae in eutrophic environments, such as semi-arid systems affected by hydric deficit, and compare methodologies in order to facilitate bacterivory rates quantification, allowing a better knowledge about this kind of mixed nutrition. Therefore, research about mixotrophy implies in paradigmatic changes in how we understand aquatic food webs nowadays, in particular this is even more critic when it links to shifts in environmental conditions in a changing climatic world.