Climate change scenarios predict a doubling of the atmospheric CO2 concentration by the end of this century. Yet, how rising CO2 will affect the species composition of aquatic microbial communities is still largely an open question. In this study, we develop a resource competition model to investigate competition for dissolved inorganic carbon in dense algal blooms. The model predicts how dynamic changes in carbon chemistry, pH and light conditions during bloom development feed back on competing phytoplankton species. We test the model predictions in chemostat experiments with monocultures and mixtures of a toxic and non-toxic strain of the freshwater cyanobacterium Microcystis aeruginosa. The toxic strain was able to reduce dissolved CO2 to lower concentrations than the non-toxic strain, and became dominant in competition at low CO2 levels. Conversely, the non-toxic strain could grow at lower light levels, and became dominant in competition at high CO2 levels but low light availability. The model captured the observed reversal in competitive dominance, and was quantitatively in good agreement with the results of the competition experiments. To assess whether microcystins might have a role in this reversal of competitive dominance, we performed further competition experiments with the wild-type strain M. aeruginosa PCC 7806 and its mcyB mutant impaired in microcystin production. The microcystin-producing wild type had a strong selective advantage at low CO2 levels but not at high CO2 levels. Our results thus demonstrate both in theory and experiment that rising CO2 levels can alter the community composition and toxicity of harmful algal blooms.

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Internal phosphorus loading has become a major problem in many shallow freshwater lakes over the past decades due to the build-up of phosphorus stocks in the sediment. Iron is a natural capping agent which can enhance sediment P binding capacity, thus reducing P availability and shifting a lake from an algal to a macrophyte dominated state. Iron could, however, also impose toxic effects on the biota. We therefore provide a synopsis of iron toxicity studies and lake restoration measures using iron addition. Iron toxicity studies revealed that, even though iron is an essential nutrient for growth, when added in excess, it can negatively affect aquatic organisms. We found 13 studies testing the effect of iron addition as a restoration measure in the field (10) or using sediment from lakes and reservoirs in the laboratory (3). Twelve of the studies reported increased P retention after iron addition, which depended on the iron salts used and the concentrations added in two studies, whereas one study found no effect on P retention. Eight out of the nine field studies that reported biotic responses found reduced chlorophyll concentrations in the water column, whereas toxic effects of iron on organisms remained absent. Iron addition was most successful when external P loading, and concentrations of organic matter and sulphate were low as well as densities of sediment disturbing fish and crayfish. We conclude that iron addition can be a successful restoration method when these conditions are met.

Aim Aegagropila linnaei is a freshwater macroalga that is generally regarded as a rare species. It is apparently absent from large but seemingly suitable areas of the Northern Hemisphere, implying a limited dispersal potential and an imprint of Pleistocene glaciations in its biogeography. However, despite the popularity of its enigmatic lake ball-form, detailed biogeographical studies of A. linnaei have never been conducted. The main means of reproduction of A. linnaei is fragmentation and akinetes are not formed, supporting the assumption of limited dispersal capacity. The aim of this study was to reconstruct the biogeography of A. linnaei, and to identify possible refugia during glaciations, as well as to evaluate dispersal potential by quantitative desiccation experiments.Location Palaearctic.Methods The current distribution of A. linnaei was inferred from herbarium specimens, literature data and recent field observations. All herbarium specimens were morphologically re-examined. Desiccation experiments were performed with vegetative filaments of three isolates of A. linnaei, as no specialized resistant stages are known. For comparison, the widespread freshwater algae Cladophora glomerata and Rhizoclonium sp. were included. Internal transcribed spacer (ITS) ribosomal DNA sequences were generated and a ribotype network was constructed.Results Aegagropila linnaei was recorded from 283 locations in freshwater and brackish environments. The majority of locations were in central and northern Europe in previously glaciated areas. Desiccation experiments showed that A. linnaei is very susceptible to desiccation. Based on ITS sequences of 34 samples, five different ribotypes were identified. Four of these ribotypes had a restricted distribution. Aegagropila linnaei represents a single species with little genetic variation (0.1-0.5%).Main conclusions This is the most comprehensive study of this species so far, reporting many new locations and tackling several taxonomic problems. Few additional finds were made from North America, and the origin of A. linnaei is inferred to be in Asia. The highest density of its present-day locations is in previously glaciated areas in Europe, where glacial ice-dammed lakes might have functioned as refugia. Low effective long-distance dispersal capacity is inferred, based on high susceptibility to desiccation and its modes of dispersal.

Aegagropila linnaei, a freshwater green macroalga, had been abundant in several locations in The Netherlands before the 1960s. Both the 'lake ball' form of this alga and dense unattached mats floating over the sediment have been described from these locations. After 1967, this species has not been recorded anymore from The Netherlands. In 2007, several historical collection sites were surveyed for extant populations of A. linnaei. All habitats have changed drastically during the last 50 years and were affected severely by eutrophication. Populations of A. linnaei seem to have become extinct in all but one location (Boven Wijde, province Overijssel), where we found very small amounts of attached filaments. The attached form had not been reported previously from The Netherlands. Environmental conditions do not seem suitable anymore to maintain extensive unattached growth forms including the enigmatic lake balls, and the species must be regarded as threatened in The Netherlands and we propose to include A. linnaei in a national red list. The decline of populations elsewhere is reviewed and discussed in this paper. In addition to morphological identification of the attached filaments, partial sequences of the nuclear large subunit rDNA were generated and compared with different growth forms and habitats from several other locations outside The Netherlands. The sequences confirm the identity of the Dutch material and indicate very little divergence both between populations in different locations and between different growth forms.