Eva Willén

Diversity predicts stability and resource use efficiency in natural phytoplankton communities.

The relationship between species diversity and ecosystem functioning has been debated for decades, especially in relation to the “macroscopic” realm (higher plants and metazoans). Although there is emerging consensus that diversity enhances productivity and stability in communities of higher organisms; however, we still do not know whether these relationships apply also for communities of unicellular organisms, such as phytoplankton, which contribute ≈50% to the global primary production. We show here that phytoplankton resource use, and thus carbon fixation, is directly linked to the diversity of phytoplankton communities. Datasets from freshwater and brackish habitats show that diversity is the best predictor for resource use efficiency of phytoplankton communities across considerable environmental gradients. Furthermore, we show that the diversity requirement for stable ecosystem functioning scales with the nutrient level (total phosphorus), as evidenced by the opposing effects of diversity (negative) and resource level (positive) on the variability of both resource use and community composition. Our analyses of large-scale observational data are consistent with experimental and model studies demonstrating causal effects of microbial diversity on functional properties at the system level. Our findings point at potential linkages between eutrophication and pollution-mediated loss of phytoplankton diversity. Factors reducing phytoplankton diversity may have direct detrimental effects on the amount and predictability of aquatic primary production.

Regional species pools control community saturation in lake phytoplankton

Recent research has highlighted that positive biodiversity–ecosystem functioning relationships hold for all groups of organisms, including microbes. Yet, we still lack understanding regarding the drivers of microbial diversity, in particular, whether diversity of microbial communities is a matter of local factors, or whether metacommunities are of similar importance to what is known from higher organisms. Here, we explore the driving forces behind spatial variability in lake phytoplankton diversity in Fennoscandia. While phytoplankton biovolume is best predicted by local phosphorus concentrations, phytoplankton diversity (measured as genus richness, G) only showed weak correlations with local concentrations of total phosphorus. By estimating spatial averages of total phosphorus concentrations on various scales from an independent, spatially representative lake survey, we found that close to 70 per cent of the variability in local phytoplankton diversity can be explained by regionally averaged phosphorus concentrations on a scale between 100 and 400 km. Thus, the data strongly indicate the existence of metacommunities on this scale. Furthermore, we show a strong dependency between lake productivity and spatial community turnover. Thus, regional productivity affects beta-diversity by controlling spatial community turnover, resulting in scale-dependent productivity-diversity relationships. As an illustration of the interaction between local and regional processes in shaping microbial diversity, our results offer both empirical support and a plausible mechanism for the existence of common scaling rules in both the macrobial and the microbial worlds. We argue that awareness of regional species pools in phytoplankton and other unicellular organisms may critically improve our understanding of ecosystems and their susceptibility to anthropogenic stressors.

Dominance patterns of planktonic algae in Swedish forest lakes

Dominance patterns of phytoplankton during a late phase (August) in the seasonal sequence of species are presented from 15 Swedish forest lakes with little or no local anthropogenic impact. The main question to elucidate is if predominance of a small number of species (1–3) occurs during a mature phase of the annual succession i.e. if there is an evident competitive exclusion of species in favour of a few dominants. Ten August months per lake are used to illustrate interannual variations caused mainly by differences in weather conditions. In general, 1–3 dominant taxa do not reach more than 60% of the total phytoplankton volume. Only lakes exposed to some stress factors exhibit a more pronounced dominance pattern with 1–3 species occupying >80% of the phytoplankton biomass. Stress factors are harsh climate (arctic lake), light deficiency (very brown water), acidification and occurrence of the invasive raphidophycean species Gonyostomum semen. The variation in phytoplankton assemblages in relation to environmental variables and years was tested by classification and ordination methods (TWINSPAN, CCA). The consistency of the species/lake groupings and the set of explanatory environmental variables was checked in a discriminant function analysis. Species associations during investigated years and environmental variables show a very good consistency and 75% of the lakes was classified in the same species group irrespective of year, indicating stable species assemblages from summer to summer.

Cyanotoxin production in seven Ethiopian Rift Valley lakes

Abstract We hypothesized that unusual deaths and illnesses in wild and domestic animals in lake areas of the Rift Valley south of Addis Ababa were caused by toxic cyanobacteria. In the first cyanotoxic analyses conducted in samples from Ethiopia, we found lakes Chamo, Abaya, Awassa, Chitu, Langano, Ziway, and Koka all had concentrations of microcystins (MC) ranging from trace to hazardous, whereas only traces less than limits of detection (LOD) of cylindrospermopsin (CYN) were found. In the December 2006 dry season we sampled the lakes for analyses of MC, CYN, species structures, and calculations of cyanobacteria biomass. We used the Utermöhl technique to analyse cyanobacterial biomass and monitored MC toxins using HPLC-DAD, LC-ESI-MS-MRM, and ELISA-test and CYN with HPLC-DAD and ELISA. The various toxicity tests coincided well. In 4 of the lakes (Chamo, Langano, Ziway, and Koka), the inter-lake range of total MC concentration was 1.3–48 μg L−1; in 3 (Abaya, Awassa, and Chitu), we found only traces of MC. Microcystis aeruginosawas the dominant species, with Microcystis panniformis, Anabaena spiroides, and Cylindrospermopsisspp. as subdominants. The MC concentration, especially in Lake Koka, exceeded levels for serious health hazards for humans, cattle, and wildlife.

Bioaccumulation of microcystins in the food web : a field study of four Swedish lakes

Abstract The transfer of microcystins (MC) up the food chain was measured in 4 lakes in central Sweden; Ekoln, Lilla Ullfjärden, Valloxen, and Storsjön. In lakes Ekoln and Valloxen, Microcystis aeruginosa was the dominant cyanobacterium, while the oscillatorian species Planktothrix prolifica form dense blooms in Lake Lilla Ullfjärden. The cyanobacterial composition in Lake Storsjön was more diverse with several Microcystis and Dolichospermum species. All dominant taxa are well-known producers of hepatotoxic MC. The highest recorded MC in the water samples from Lake Lilla Ullfjärden was measured in the bloom of P. prolifica (35 μg L−1). The highest MC content was measured in invertebrates; however, the MC concentration was usually low in fish. Maximum levels were 9 μg g−1 dry weight (dw) in zooplankton, 10 μg g−1 dw in benthos, and 2.7 μg g−1 dw in fish (smelt) liver. In fish muscle the highest recorded levels were 0.10–0.18 μg g−1 dw in bleak and smelt from Lake Lilla Ullfjärden and in pike-perch and roach from Lake Storsjön. Based on the World Health Organization’s tolerable daily intake value of 0.04 μg kg–1 body weight, we conclude that any risk related to MC from human consumption of fish from these lakes is minimal.