Picophytoplankton Community In Large And Deep Oligotrophic Reservoirs

The picophytoplankton is the fraction of phytoplankton composed by cells between0.2-3μm,including Prochloroccus, picocyanobacteria and picoeukaryotes. It is especially important inoligotrophic water bodies. The effect of nutrients, zooplankton, fish prey and turbulence on thepicophytoplankton community in oligotrophic reservoirs was investigated by annual surveys andenclosure experiments of the picophytoplankton in Liuxihe Reservoir, a large and deepoligo-mesotrophic reservoir in southern China. The field samples were taken every month forseasonal variation, the observations ranging from June2011to April2012. And three enclosureexperiments were performed separately.The first enclosure experiment was designed to check the predation pressure of Daphnia onpicophytoplankton. The experiment had one control without adding Daphnia galeata and threetreatments with adding low, middle and high density of Daphnia galeata, respectively. In thesecond experiment, nitrogen and phosphorus were added to detect the effect of increase ofnutrients on picophytoplankton communites, Hypophthalmichthys molitrix (silver carp),Aristichthys nobilis(bighead carp)and Tilapia nilotica(nile tilapia)were added solely or inmixture to detect their effects on picophytoplankton communites. The vertical mixing experimentwas carried out by adding phosphorous to the bottom of the enclosure and combined with differentvertical mixing.Both picocyanobacteria and picoeukaryotes were found in Liuxihe Reservoir. The annual averagepicophytoplankton cell number was1.44×105cells/mL,of which95%were cyanobacteria, whichcontributed to99.39%of the total phytoplankton abundance. Average biomass was0.23μg/L,contributed0.023%of total. The abundance of picocyanobacteria had peaks in both spring andautumn, when more species were found than in winter. Picocyanobacteria were more abundant inthe riverine zone and the transition zone than in the lacustrine zone except in spring, andpicoeukaryotes were more abundant in the riverine zone all year as well as that ofmicro-nanophyplankton(>3μm). The vertical distribution of picophytoplankton showed a peakabundance at10m depth in all the samples except in spring. The results indicate that thepicophytoplankton was sensitive to light and temperature in Liuxihe Reservoir.The results of the zooplankton adding experiment showed no significant difference between thetreatments. Daphnia galeata increased quickly in the first three weeks and reduced the differencebetween four treatments. The abundance and biomass of picophytoplankton increased whenmicro-nanophytoplankton decreased in the first week and then both markedly descreased. Theresult indicates that the picophytoplankton can assimilate nutrients more efficiently when they arelimited, and they can also avoid from grazing by Daphnia who prefers the larger phytoplankton.But when micro-nanophytoplankton were less abundant, Daphnia may prey on both pico-andmicro-nanophytoplankton.In the second experiment, all fractions of phytoplankton increased in nutrient enriched treatmentswith or without fish. The results showed that the abundance and biomass of picophytoplanktonwere related to phosphorous concentration, while other phytoplankton were influenced by bothphosphorous concentration and zooplankton biomass. The nutrients were higher in the fishtreaments, which coincided with the picophytoplankton abundance. The picocyanobacteria was highest in HAT all the time and picoeukaryotes abundance was highest in H in the end of theexperiment.After adding nutrients in the treatments in the third experiment, the abundance and the biomass ofall phytoplankton increased. The abundance of picocyanobacteria was much higher than that ofothers in the first week and then decreased towards the end. The picoeukaryotes andmicro-nanophytoplankton decreased in the second week then increased later. Picocyanobacteriawas sensitive to nutrients but could not adapt to the high nutrient concentration. In contrast,picoeukaryotes has relatively low assimilation rate but higher affinity to nutrients. Theconcentration of nutrient showed no significant difference between different vertical mixingfrequency, and all significantly higher than in H. The density of picophytoplankton was higher inthe mixing treatments and highest in HP, which shows that vertical mixing can increasepicophytoplankton abundance.In conclusion, nutrient enrichment especially for phosphorous can increase picophytoplanktonabundance and biomass. Picocyanobacteria response more quickly than picoeukaryotes andmicro-nanophytoplankton. Picophytoplankton is not selectively grazed by zooplankton whichprefers the larger phytoplankton. However, zooplankton may graze on both pico-andmicro-nanophytoplankton when micro-nanophytoplankton was less abundant. The mostsignificant impact by the omnivorous fish to picophytoplanton was not the change of predationpressure, but the increase of nutrient concentration. Vertical mixing can increasepicophytoplankton abundance. Among all the possible factors, it seems that the nutrients,especially phosphorous, make up the most influencial factor to picophytoplankton abundance andcommunity structure in Liuxihe Reservoir.