Plankton communities of an acidic,polymictic, brown-water lake

The plankton of an acidic, polymictic, brown-water lake was investigated over a one-year period. The phytoplankton community was dominated by Chlamydomonas sp., Melosira varians and Peridinium pusillum during different times of the year. Densities were abnormally low, with a maximum of only 13,781 individuals per liter during the March bloom. The water column was nearly void of phytoplankton during the fall. These low densities were attributed to several of the physical and chemical characteristics of the water, including low pH, low levels of nutrients and light limitation. The latter factor was important because of low light transmission into the highly colored water, shading from macrophytes and the nearly continuous transport of the phytoplankton into the aphotic zone due to mixing of the water column by sea breezes.The zooplankton was dominated by Diaptomus floridanus, Polyarthra vulgaris, Keratella cochlearis and Daphnia ambigua. The observed densities were typical of southeastern oligotrophic lakes. The zooplankton community did not exhibit a decrease in density that corresponded with the observed low phytoplankton densities, suggesting the possibility that they relied heavily on bacterial and detrital food sources.     

Comments on the antarctic Rotifera

The Southern and Western water of L. Kyoga is characterised by low mean alkalinities, 1.23±0.08 m eq 1^-1; conductivities, 106±5 µS cm^-1; pH, 7.2±0.2 and high transparencies, 1.7±0.3 m. In the east, comparatively high alkalinities, 1.96±0.34 m eq 1^-1; conductivities, 168±21 µS cm^-1; pH of 7.4±0.2 and low transparencies, 0.8±0.2 m are encountered. Hardness varies as closely as alkalinity from 0.6±0.1 m eq 1^-1 in the west to 1.0±0.2 m eq 1^-1 in the east. SiO₃·Si and SO₄·S decrease westwards from the eastern lake arms, while NO₃·N and PO₄·P vary seasonally in the lake, which is mostly saturated with oxygen (86–120%) from the surface to the bottom. Some evidence of polymixis was encountered.A rich plytoplankton flora occurred with the major groups well represented. Chlorophyceae (37%), Bacillariophyceae (36%) and Cyanophyceae (26%). Synedra spp. (20%). Anabeana spp. (16°10), Nitzschia spp. (10%) were the dominant species groups during July–August 1981. The macrophyte genera Vallisneria, Potamogeton, Nymphaea, Ceratophyllum, Hydrilla occur prominently in the clearer southern lake water. Rotifers and Crustacean Zooplankters found are also recorded. Meroplanktonic crustaceans, chironomids and insects often occur in large numbers.     

Consequences of changes in thermal regime for plankton diversity and trait composition in a polymictic lake: a matter of temporal scale

1. Changes in plankton species diversity and community structure as a result of global warming are of growing concern in ecological studies, as these properties contribute substantially to key ecosystem processes. 2. We analysed the effect of short‐term temperature rise and changes in the thermal regime during summer on plankton diversity of the eutrophic and polymictic Müggelsee in Germany, from 26 years of summer records (1982–2007). We tested for changes in community properties, such as species richness, evenness and population size of phyto‐ and zooplankton, during alternating periods of thermal stratification and mixing, which were between 2 and 8 weeks long. Moreover, we tested for overall long‐term temporal trends in annual averages of the community properties during stratified and mixed events. 3. We found that the overall number of stratification events increased significantly across the study period. When the lake was stratified, consistently higher surface water temperatures and lower epilimnetic nutrient concentrations were found. As the length of thermal stratification increased, the phytoplankton shifted towards a higher proportion of buoyant cyanobacteria capable of N‐fixation (Aphanizomenon, Anabaena). Diatoms were at a disadvantage because of high temperature, exceeding their upper lethal limit and sedimentation losses. Zooplankton species with high thermal tolerances (i.e. Thermocyclops oithonoides, Thermocyclops crassus) and/or those that grow quickly at high temperatures (i.e. rotifers) became more common. 4. During periods of continuous mixing, the community remained largely unchanged, except for some minor increase in the biomass of diatoms. 5. While a noticeable shift towards N‐fixing cyanobacteria was observed with increasing length of stratified events, and rotifers and copepods became the main predators, there were minimal changes in diversity, except for an increase in cyclopoid copepods and a decrease in diatom diversity. As for cyanobacteria, the net short‐term effect on their diversity was neutral as a result of species replacements. In the long term, however, the diversity of cyanobacteria and cladocerans declined while that of rotifers increased. 6. Overall, our study presents a cautionary example of how we might fail to foresee the impact of climate‐induced changes on ecosystem processes if we restrict our studies to seasonal or yearly temporal scales, thus neglecting the impact of substantial changes operating at smaller temporal scales.     

Distribution and dynamics of bacterioplankton production in a polymictic tropical lake (Lago Xolotlán,Nicaragua)

From 1988 to 1993 we assessed the variability of bacterioplankton production and biomass in Lake Xolotlán (L. Managua), Nicaragua via [3H]thymidine incorporation into DNA and cell counting. Bacterial production ranged from 3 to 8 μg C l-1 h-1, and since production was equal throughout the water column, areal production was high (≈ 600–1200 mg C m-2 d-1). Bacterial abundance in Lake Managua was extremely high, 7–30 × 109 cells l-1. Thus, specific rates of bacterial production were low. There was a strong correlation between production and number and the specific rate of bacterial production was constant. Comparable measurements of production via [3H]leucine incorporation into proteins indicated that bacteria were experiencing ‘balanced growth’. We conclude that bacterioplankton in Lake Xolotlán had reached its carrying capacity and a significant correlation between bacterial production and concentration of phaeophytin implied that dead or dying algae was the limiting substrate for bacterioplankton. The majority of bacterial number and most of bacterial production (up to 75%) were associated with particles in the >3-μm fraction, probably lysing algal cells to which bacterioplankton were ‘attached’. Grazing on bacterioplankton must be low and bacteria should be a ‘sink’ for organic matter in Lake Xolotlán. This revised version was published online in July 2006 with corrections to the Cover Date.     

Factors Controlling the Planktonic Community in the Shallow-Lake of Créteil,France

A two-year study on the seasonal pattern of plankton was performed in a shallow, polymictic, eutrophic lake. Autogenic successions typical of stratified lakes did not occur. Only small edible algae developed throughout the two years and Cyanophyceae rarely appeared, and did not bloom. Algal sequences depended on wind-induced periodic mixing of water. The seasonal pattern of Rotifers varied greatly from year to year and some species depended on sporadic increases of algae. In contrast, the seasonal pattern of microcrustacea was more regular. Resting stages played an important role in zooplankton development in this variable environment. Successful fish reproduction depended a great deal on climatic conditions. The changes from year to year in predation pressure from young fish appeared to be an important factor controlling zoo-plankton structure.