Short-term analysis of the phytoplankton structure and dynamics in two ponds with distinct trophic states from Cierva Point (maritime Antarctica)

Phytoplankton communities dominating Musgos and Papúa ponds with differing trophic states were sampled over 3 days enabling the detection of the physiological and population responses of microalgae to short-scale changes in biotic and abiotic factors, rather than frequently analyzed changes in community composition responses to long-scale environmental changes. We hypothesized that both environments undergoing diel changes would be dominated by phytoplankton with generalist strategies, while community structure would be mostly dictated by the trophic state of each water body. The phytoplankton biovolumes of both ponds were strongly dominated by euplanktonic nanoflagellated Chlorophyta, while phycocyanin-rich picocyanobacteria dominated the picophytoplankton. Parallel diel cycles of air and water temperatures were more pronounced on a sunny, warm day which prompted algal photosynthesis, revealed by strong increases in dissolved oxygen and pH. Nutrient and phytoplanktonic chlorophyll a confirmed the hypertrophic condition of Papúa pond. This accounted for the distinct community composition encountered in each pond, which remained stable throughout the study, as revealed by the SIMI index. The inverse relationship between the chl a/abundance ratio and the abundances of dominant species together with varying net growth rates (k′) showed algal reproduction, yet densities remained rather stable in both cases. In Musgos pond, fluctuations in k′ for small and median ciliates shadowed those of pico- and nanophytoplankton, respectively, strongly suggesting that they can control algal growth in these 2-level trophic chains.