Phytoplankton dynamics in a shallow lake dominated by common water milfoil

Phytoplankton temporal fluctuation and vertical distribution were studied by seasonal and close interval siphon sampling (May-Sep) in a shallow lake dominated by common water milfoil (Myriophyllum sibiricum). Factors potentially regulating phytoplankton primary production were investigated in situ in 2 enrichment bioassays. The results suggest that the macrophyte vegetation provided an unfavourable habitat for large colonial chlorophytes. Small species, mainly cryptophytes but also small chlorophytes and cyanobacteria, characterised the summer phytoplankton. Phytoplankton abundance as well as primary production were considerable and remained in the mesotrophic range. The total nitrogen to total phosphorus ratio during the growth season, and the enrichment bioassays, showed that phosphorus was a significant regulator of primary production. Small-celled species had competitive advantages in the dense vegetation, and the canopy structure created by dense stands of common water milfoil allowed phytoplankton growth. Hence, although the macrophytes altered the physical and chemical conditions in the lake, coexistence of small-celled algae and macrophytes was possible

Temperature Effects Explain Continental Scale Distribution of Cyanobacterial Toxins

Insight into how environmental change determines the production and distribution of cyanobacterial toxins is necessary for risk assessment. Management guidelines currently focus on hepatotoxins (microcystins). Increasing attention is given to other classes, such as neurotoxins (e.g., anatoxin-a) and cytotoxins (e.g., cylindrospermopsin) due to their potency. Most studies examine the relationship between individual toxin variants and environmental factors, such as nutrients, temperature and light. In summer 2015, we collected samples across Europe to investigate the effect of nutrient and temperature gradients on the variability of toxin production at a continental scale. Direct and indirect effects of temperature were the main drivers of the spatial distribution in the toxins produced by the cyanobacterial community, the toxin concentrations and toxin quota. Generalized linear models showed that a Toxin Diversity Index (TDI) increased with latitude, while it decreased with water stability. Increases in TDI were explained through a significant increase in toxin variants such as MC-YR, anatoxin and cylindrospermopsin, accompanied by a decreasing presence of MC-LR. While global warming continues, the direct and indirect effects of increased lake temperatures will drive changes in the distribution of cyanobacterial toxins in Europe, potentially promoting selection of a few highly toxic species or strains.Peer reviewe

Spatial distributions of phytoplankton in rock pools : metacommunities to molecules

Ecological studies on how populations and communities assemble and function traditionally focussed on the effects of local environmental conditions and species interactions. Still, no species, the consequences of its actions or interactions, are confined to a single spatial scale. The importance of spatial processes, such as dispersal, immigration, emigration, habitat connectivity and availability, was eventually realised through metapopulation and -community ecology. Accordingly, species distributions are deciphered at large scales, whereas processes at small scales reveal the mechanisms behind the observed patterns. Spatial distributions of microorganisms are generally considered to be wide, even global. This implies that fundamental patterns and processes, such as food web structures, primary production, or carbon cycling governed by these organisms are structured by local processes. Applying the metacommunity concept on microorganism assemblages has loosened the perception of them as closed systems strongly structured by local environmental conditions. However, uncertain species identities and dispersal modes impede the comprehension of their patterns and processes. Moreover, an anthropocentric scale is often applied on microorganisms. The scale of fundamental microorganism assemblage forming processes is generally difficult to perceive, leading to inappropriate extrapolations of small-scale patterns. The papers summarised in this thesis assess spatial distributions of phytoplankton in rock pools. Rock pools form natural metacommunities. They have well-delineated boundaries, occur in high numbers within a limited area, and their species assemblages are connected by dispersal. The main aim of the thesis was to explore the mechanisms underlying observed spatial patterns. Descriptions of phytoplankton species, functional groups and traits were included. Variables structuring phytoplankton assemblages were studied at regional and local scales. Centimetre scale vertical distributions of phytoplankton were considered, which added a spatial dimension ecologically relevant for phytoplankton. Molecular properties of a species isolated from the rock pool habitat were characterised. Their potential ecological implications were considered, as molecular processes cannot be neglected in a mechanistic understanding of phytoplankton patterns. Finally, the importance of an ecologically meaningful sampled scale in phytoplankton studies was emphasised. The phytoplankton assemblages in the studied rock pool metacommunity were characterised by acclimatising and colonising species adapted to a light abundant and well-mixed habitat. An overarching structuring effect of habitat stability on the phytoplankton assemblages was discovered. The effects of pool stability were not explained by mere local environmental conditions, as these were similar among the pools. Regionally, the observed distributional patterns suggested competition-colonising trade-offs. At this metacommunity scale, assemblages in stable pools were likely influenced by monopolising and priority effects, whereas effective outcompeting and colonising were important for assemblages among unstable pools. Vertical distributions of phytoplankton implied a local structuring effect of turbulence, diverging between stable, deep and unstable, shallow pools. Despite well-mixed water columns, the vertical species-specific distributions were structured. Thus, by influencing fundamental community forming processes, small-scale turbulence non-randomly formed the local species assemblages, with implications at the metacommunity scale. At the molecular level, the chemotypic characterisation of a rock pool species revealed production of bioactive compounds. The molecular effects, inhibition of enzymes present in both prokaryotes and eukaryotes, implied consequences beyond the individual scale. An ambiguous morphologic and genetic species identification exemplified the bottleneck in disentangling microorganism patterns. Furthermore, the unknown biological and ecological functions of the molecules demonstrated the difficulties in comprehension of their large-scale impacts. Unexpectedly structured species-specific centimetre distributions in the well-mixed rock pools were discovered by precision sampling. Generalisations of such small-scale patterns with conventional phytoplankton sampling methods might lead to an underestimation of their effects at large scales. Overall, the results in this thesis highlight the ineffectiveness of imposing a single spatial scale on microorganisms. A meaningful understanding of microorganism patterns and processes requires a widening of the traditional local scale, but also a comprehension that small-scale processes have importance for large-scale microorganism patterns. Throughout, the sampled scale should be non-arbitrarily considered. The findings in this thesis also exemplified the vulnerability of ambiently influenced rock pool ecosystems. The results can be considered in phytoplankton research and monitoring, as well as nature conservation and management

Morphologic, Phylogenetic and Chemical Characterization of a Brackish Colonial Picocyanobacterium (Coelosphaeriaceae) with Bioactive Properties

Despite their cosmopolitan distribution, knowledge on cyanobacteria in the family Coelosphaeriaceae is limited. In this study, a single species culture of a coelosphaeran cyanobacterium isolated from a brackish rock pool in the Baltic Sea was established. The strain was characterized by morphological features, partial 16S rRNA sequence and nonribosomal oligopeptide profile. The bioactivity of fractionated extracts against several serine proteases, as well as protein-serine/threonine phosphatases was studied. Phylogenetic analyses of the strain suggested a close relationship with Snowella litoralis, but its morphology resembled Woronichinia compacta. The controversial morphologic and phylogenetic results demonstrated remaining uncertainties regarding species division in this cyanobacteria family. Chemical analyses of the strain indicated production of nonribosomal oligopeptides. In fractionated extracts, masses and ion fragmentation spectra of seven possible anabaenopeptins were identified. Additionally, fragmentation spectra of cyanopeptolin-like peptides were collected in several of the fractions. The nonribosomal oligopeptide profile adds another potential identification criterion in future inter- and intraspecies comparisons of coelosphaeran cyanobacteria. The fractionated extracts showed significant activity against carboxypeptidase A and trypsin. Inhibition of these important metabolic enzymes might have impacts at the ecosystem level in aquatic habitats with high cyanobacteria densities

Stratification strength and light climate explain variation in chlorophyll a at the continental scale in a European multilake survey in a heatwave summer

To determine the drivers of phytoplankton biomass, we collected standardized morphometric, physical, and biological data in 230 lakes across the Mediterranean, Continental, and Boreal climatic zones of the European continent. Multilinear regression models tested on this snapshot of mostly eutrophic lakes (median total phosphorus [TP] = 0.06 and total nitrogen [TN] = 0.7 mg L-1), and its subsets (2 depth types and 3 climatic zones), show that light climate and stratification strength were the most significant explanatory variables for chlorophyll a (Chl a) variance. TN was a significant predictor for phytoplankton biomass for shallow and continental lakes, while TP never appeared as an explanatory variable, suggesting that under high TP, light, which partially controls stratification strength, becomes limiting for phytoplankton development. Mediterranean lakes were the warmest yet most weakly stratified and had significantly less Chl a than Boreal lakes, where the temperature anomaly from the long-term average, during a summer heatwave was the highest (+4 degrees C) and showed a significant, exponential relationship with stratification strength. This European survey represents a summer snapshot of phytoplankton biomass and its drivers, and lends support that light and stratification metrics, which are both affected by climate change, are better predictors for phytoplankton biomass in nutrient-rich lakes than nutrient concentrations and surface temperature.Peer reviewe

Stratification strength and light climate explain variation in chlorophyll a at the continental scale in a European multilake survey in a heatwave summer

To determine the drivers of phytoplankton biomass, we collected standardized morphometric, physical, and biological data in 230 lakes across the Mediterranean, Continental, and Boreal climatic zones of the European continent. Multilinear regression models tested on this snapshot of mostly eutrophic lakes (median total phosphorus [TP] = 0.06 and total nitrogen [TN] = 0.7 mg L−1), and its subsets (2 depth types and 3 climatic zones), show that light climate and stratification strength were the most significant explanatory variables for chlorophyll a (Chl a) variance. TN was a significant predictor for phytoplankton biomass for shallow and continental lakes, while TP never appeared as an explanatory variable, suggesting that under high TP, light, which partially controls stratification strength, becomes limiting for phytoplankton development. Mediterranean lakes were the warmest yet most weakly stratified and had significantly less Chl a than Boreal lakes, where the temperature anomaly from the long-term average, during a summer heatwave was the highest (+4°C) and showed a significant, exponential relationship with stratification strength. This European survey represents a summer snapshot of phytoplankton biomass and its drivers, and lends support that light and stratification metrics, which are both affected by climate change, are better predictors for phytoplankton biomass in nutrient-rich lakes than nutrient concentrations and surface temperature