Studies have been carried out on the phytoplankton of two freshwater lakes-eutrophic Rostherne Mere (Cheshire, UK) and mesotrophic Lake Glubokoe (Moscow region, Russia). Diversity was investigated in relation to two major aspects : (a) species composition, with indices of diversity being estimated from phytoplankton cell and colony counts, and (b) intraspecific diversity, looking at variation in elemental composition within selected species. The major genera of phytoplankton in Rostherne Mere included Stephanodiscus and Asterionella (spring diatom bloom), Chlamydomonas and Sphaerocystis (clear water phase), and a species assembly dominated by Microcystis (1997) or Ceratium (1999) in the late summer/autumn bloom. The sequence of major algal groups recorded at Lake Glubokoe were: chrysophytes, dinoflagellates and diatoms in spring, chlorophytes and blue-greens in the clear water phase, and an assembly of diatoms, blue-greens, dinoflagellates and chrysophytes in the late summer/autumn bloom. Indices of species diversity were determined in relation to species richness (Richness index, species count), general diversity (Shannon index) and species dominance (Simpson index). In both lakes, species richness and general diversity were greatest in the late summer bloom, a period of maximum phytoplankton stability and high productivity, characterised by K-selected species. Species dominance in Rostherne Mere was highest at extremes of productivity, including times of low algal biomass (clear water phase, 1997 and 1999) and very high biomass (late summer bloom, 1999). In Lake Glubokoe species dominance was maximal at the end of the summer/autumn bloom, a period of intermediate productivity and dominance of a limited number of species. Overall diversity (maximum seasonal Shannon index) decreased with increase in productivity, being highest in Lake Glubokoe (1998, low productivity), intermediate in Rostherne Mere 1997, and lowest in Rostherne Mere 1999 (highest chlorophyll-a levels). Phytoplankton diversity within the water column (mid-day) was determined at the key phases in lake stratification in Rostherne Mere (1999) and at the period of late summer bloom in Lake Glubokoe (1998). At each phase of the seasonal growth cycle, differences in the vertical distribution of individual species and algal groups reflected positioning of algae in relation to ambient conditions (nutrients, light, temperature, density gradients) or sedimentation within the water column. Phytoplankton diversity in species richness closely corresponded to the overall phytoplankton population (biomass profile and individual counts), usually having the maximum values in the epilimnion. Profiles of species dominance varied markedly in some of the depth analyses. High species dominance reflected either the growth of individual species (surface populations of chlorophytes in the clear water phase -Rostherne Mere) or the accumulation of populations at a particular depth due to sedimentation (hypolirrmion peak of diatoms in Lake Glubokoe). Phytoplankton statistical groups and assemblages were analysed within the water column of Rostherne Mere at the time of markedly high algal productivity (late summer bloom in 1999). Intraspecific diversity was examined on the basis of elemental composition of three main species (Ceratium hirundinella, Anabaena flos-aquae and Microcystis aeruginosa), sampled within the same water column in Rostherne Mere. The elemental composition of single cells within a mixed phytoplankton preparation was determined using Electron Probe X-ray Microanalysis. Intraspecific variation was investigated in relation to differences in elemental composition within the water column, variation within micropopulations, and pattern of elemental correlation. X-ray analysis detected a similar range of elements in the populations of Ceratium, Anabaena and Microcystis: magnesium, silicon, phosphorus, sulphur, chlorine, potassium, calcium, with P as the major anionic element, K as the major cation. Si was present at substantial levels. Elemental composition varied considerably within micropopulations of each species. Variation in the concentration of most elements was unimodal. Bimodal distribution was shown by K in Ceratium cells, and by Si in Anabaena and Microcystis cells, indicating particular subpopulations, within these species. Distinct subpopulations of high-Si and low-Si cells in Anabaena and Microcystis were attributed to the presence of Si as a surface layer associated with Al in high-Si cells. Low-K cells in Ceratium were seen as part of the population undergoing senescence. The pattern of elemental correlations revealed a major statistical association between Mg, P, S and K in each species throughout the water column. The XRMA results suggested an underlying homogeneity in the elemental composition of Ceratium, Anabaena and Microcystis within the top 8 m of the water column.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:488134 |
| Date | January 2001 |
| Creators | Levado, Eugenia Semjonovna |
| Publisher | University of Manchester |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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