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We isolated phytoplankton assemblages from five tropical lakes differing in environmental conditions and incubated them under different extreme growth conditions. Conditions were: under light with nutrient addition (L þ N), under light with nutrient and organic carbon addition (L þ N þ OC), and in the dark with nutrient and organic carbon addition (N þ OC). These were intended to simulate phytoplankton succession during early, intermediate and late eutrophication stages, respectively. Decreases in species diversity and richness in N þ OC assays from all lakes, except the most eutrophic one, suggested consistent diversity and richness declines during late eutrophication stages. Declines in species diversity were generally caused by a stronger reduction in species richness than evenness, indicating that species exclusion is a more probable response to eutrophication than co-existence and dominance of strong competitors. Under N þ OC conditions, the osmotrophic chlorophyte Chlamydomonas was generally dominant, indicating that this flagellate may prevail in late eutrophic communities in the tropical lakes studied. The L þ N and L þ N þ OC assays did not differ from each other in species diversity, but exhibited differences in species composition and richness. This suggests that the changing role of the phytoplankton community in whole-lake matter fluxes during early eutrophication stages could be a more important research question, than predicting the community’s diversity response in tropical lakes.

We investigated how the lipid composition (fatty acids and sterols) of benthic microbial mats, which represent an important basal food resource for stream food webs, differs between tropical streams located in protected pristine and agricultural Cerrado savannah areas. The total microbial biomass and lipid composition differed significantly between pristine and agricultural streams in parallel with differences in water quality and hydrodynamic characteristics. Agricultural streams exhibited lower total biomass of benthic microbial mats than pristine streams. However, the higher concentrations of essential polyunsaturated fatty acids, such as linoleic acid (LIN, 18:2ω6), α-linolenic acid (ALA, 18:3ω3), and eicosapentaenoic acid (EPA, 20:5ω3), that were observed in agricultural streams suggest enhanced lipid complexity and a higher nutritional quality of the microbial community relative to pristine streams. Meanwhile, pristine stream microbial communities had higher total concentrations of saturated fatty acids and cholesterol than those of agricultural streams, reflecting their heterotrophic microbial communities. Moreover, stream morphotype and associated differences in the hydrodynamic characteristics affected the community composition and thereby also the lipid composition of microbial mats. Land-use-induced changes in the total biomass and lipid composition of microbial communities may affect the trophic transfer of energy in stream food webs, leading to changes in the composition and productivity of primary consumers and their predators, and thereby affecting stream ecosystem functioning.

We analysed diel cycles of total protein, lipid and carbohydrate content of seston in four seasonal sampling campaigns in a tropical hypereutrophic reservoir. Physical and water chemical variables and taxonomic composition of phytoplankton were measured in parallel. Seston lipid and carbohydrate contents exhibited highest values during the day, while protein content was highest at night. Carbohydrate content was negatively correlated with nitrate and nitrite concentrations. Lipid content was negatively correlated with temperature and positively with soluble reactive phosphorus (SRP) concentrations. Protein content was positively correlated with temperature and negatively with SRP concentrations. In all sampling campaigns, the biochemical composition was correlated to the biovolume of phytoplankton. However, relationships differed between seasons. In the dry season, the relative biovolume of non-dominant groups such as cyanobacteria and bacillariophyceae was strongly correlated with all biochemical compounds of seston. During the rainy season, significant correlations were detected between biochemical compounds of seston and biovolumes of the dominant groups, chlorophyceae and euglenophyceae. We conclude that diel fluctuations in environmental conditions as well as changes in the community composition of phytoplankton affected seston biochemical composition during the diel cycles investigated. However, the type of interaction was clearly influenced by seasonality in this tropical reservoir.

The biochemical factors that determine the food quality of protists for rotifers are poorly understood. We evaluated population growth rates and egg production of the rotifer Keratella quadrata fed with four protist species growing on either an algal or a bacterial diet. The cryptomonad Cryptomonas phaseolus, considered as a good quality prey, and assays without prey served as controls. Population growth rates and egg numbers of K. quadrata were correlated with single biochemical compounds (fatty acids, amino acids and sterols) of the protists. Feeding on the alga C. phaseolus or the algivorous ciliates resulted in enhanced population growth rates and high egg production by K. quadrata, whereas feeding on bacterivores supported only moderate egg production but no population growth. The rotifers' egg production was correlated with the protist biochemical composition, including polyunsaturated fatty acids (PUFAs), the sterol desmosterol, ergosterol, stigmastanol and the amino acid leucine. No significant relationships were observed between population growth rates of the rotifers and the protists' biochemistry, suggesting that population growth and reproduction of K. quadrata may have different nutritional requirements. To our knowledge, this is one of the first studies to analyze a large variety of biochemical compounds to determine the food quality of protists for a zooplankton predator and the first study to analyze the biochemical quality of protists to a rotifer species.