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Abstract Lake Baikal is facing several environmental stressors, including climate change and nearshore eutrophication. To assess recent ecological changes in Lake Baikal and provide a baseline for future comparisons, we sampled spring plankton communities from the pelagic zone of the lake in 2016 and compared these data with unpublished and published historical information going back to 1990. In 2016, one pelagic long-term monitoring station was sampled in early spring (March) during ice cover and 21 long-term monitoring stations located throughout the lake were sampled in late spring (May-June). We measured water chemistry parameters at most stations and the abundance, taxonomic composition and biomass of bacteria, ciliates and phytoplankton at several locations in different areas of the lake. Biotic parameters from 2016 were compared with historical data, showing significant changes in the spring pelagic microbial community since the 1990s. We show increased quantities of small species, mixotrophic ciliates, and the appearance (or increasing number) of small coloured and colourless flagellates. We also show substantially decreased densities of formerly dominant heavily silicified diatoms such as Aulacoseira spp. Since 2007, Synedra acus subsp. radians, a smaller and weakly silicified diatom, has dominated the spring plankton of the lake. These results suggest that Lake Baikal's pelagic plankton community may be changing, with climate likely playing a dominant role in these changes. [ABSTRACT FROM AUTHOR]

Introduction: The lagoon is a component of coastal zones, whose populations of autotrophic picoplankton (APP) remain largely unstudied. These lagoons display high-nutrient productivity and additionally may also be subjected to anthropogenic activities. This study selected Laguna Macapule, located on the eastern shore in the mid-region of the Gulf of California, due to the fact that a drainage network servicing the surrounding agricultural region (>230,000 hectares under cultivation) directs irrigation runoff, shrimp farm effluents, and urban wastewater containing large quantities of nutrients to be discharged into this lagoon. We propose to identify the APP's response to various types of environmental and anthropogenic influence in this highly impacted coastal lagoon. Methods: Two sites (separated by 2.7 km) were monitored from December 2007 to December 2008. One, located at the entrance to Laguna Macapule (oceanic influence) and the other a discharge canal (eutrophic conditions) inside the lagoon at El Tortugón. Results: APP was the numerically dominant phytoplankton fraction (15 × 10 to 620 × 10 cells L) with coccoidal cyanobacteria as the dominant fraction throughout the year. Peak levels were reached in spring-early autumn and they were the second largest contributor to biomass. Abundance of APP cells corresponds to the lagoon's eutrophic status. Maximum numbers and a higher average of APP were recorded at the El Tortugón channel during the warm season (months with SST higher than 24 °C). The general positive relationship of the APP's annual cycle at both sites as well as a negative relationship with heterotrophic nanoflagellates (HNF) abundance, supports the idea that natural forcing, in particular sea surface temperature (SST) is the predominant influences on APP's seasonal variability. Conclusions: Distinguishable significant differences in APP abundances and nutrients were recognizable between the two sites. The interplay of these variables contributed to lower densities of APP in winter and high densities in spring-early autumn. N:P = ~4 suggests that spring-early autumn abundance of the APP autotrophic component was sustained by urea from shrimp farm discharge water. Thus, a total nutrient-based approach is likely the most suitable tool for establishing nitrogen limitation of biological production in Laguna Macapule and similarly impacted ecosystems around the world. [ABSTRACT FROM AUTHOR]

A systematic limnological survey of water bodies of Byers Peninsula (Livingston Island, South Shetland Islands) was carried out during the summer of 2001/02. Abundances of microbial plankton were determined which allowed a delineation of the pelagic food web structure. We also report the nutrient status of these lakes. We demonstrate the occurrence of a trophic gradient that extended from upland lakes (oligotrophic) to the coastal ones (eutrophic). The study shows that a lake's morphology regulates the relative importance of the pelagic and benthic habitats, whereas nutrient loads mainly determine its trophic status. Yet, some of the variability observed could be also a legacy of the landscape. Photosynthetic pigments analyses by high-performance liquid chromatography of the lake waters revealed a major occurrence of chlorophytes, chrysophytes and diatoms. The chlorophyll a concentrations in lakes in the central plateau were consistently lower (< 2.5 μg l-1) than coastal sites, which were one order of magnitude higher. Numbers of both bacterioplankton and autotrophic picoplankton also increased from inland to coastal sites. However, the relative role of autotrophic picoplankton in the total phytoplankton assemblage decreased with the increase in nutrients loads. Our results show that the trophic status clearly plays a significant role in structuring the pelagic communities of these lakes despite climatic constraints. [ABSTRACT FROM PUBLISHER]

1. We examined the effects of nutrients, turbulent mixing, mosquitofish, Gambusia affinis Baird and Girard and sediments on algal composition, algal biomass and autotrophic picoplankton (APP) abundance in a 6-week experiment of factorial design in twenty-four 5-m3 outdoor mesocosms during late autumn 1995. 2. Turbulent mixing decreased surface temperature and increased turbidity, which also was increased by the addition of sediments. Total algal biomass was significantly enhanced by nutrients and mixing, and decreased by the sediment treatment. In the mixing × nutrient treatment, algal biomass increased more than expected from the individual effects, while the fish × mixing and mixing × sediment treatments increased algal biomass less than expected. 3. Cryptomonas (cryptomonad) blooms were observed in the unmixed, high nutrient treatment; Synedra (diatom) blooms were observed in the high nutrient, high sediment treatment; Ulothrix (green algae) blooms were observed in the mixed, high nutrient, low sediment treatment. 4. Eukaryotic APP abundances were increased by sediment addition and by turbulent mixing, and increased synergistically by mixing × sediment and mixing × nutrient interactions. Prokaryotic APP abundances were decreased by nutrient enhancement and by a mixing × nutrient interaction. There were no main effects of fish on APP abundance, but fish were involved in some of the two–way interactions. 5. The large number of significant interaction effects indicates that APP and other phytoplankton are regulated by a complex set of interdependent factors which should be considered simultaneously in studies of phytoplankton population dynamics and community composition. [ABSTRACT FROM AUTHOR]