Department Aquatic Ecology

Climate change is reshaping freshwater ecosystems by warming waters and modifying nutrient dynamics. These combined environmental changes exert novel gradients of selection on phytoplankton populations and communities. Temperature and phosphorus availability are individually critical determinants of growth in phytoplankton and can have interactive impacts on population and community dynamics. Although we understand how interspecific variation in thermal and resource-use traits of phytoplankton can affect community composition in response to changing environments, the extent of local adaptation in these responses among strains of the same species remains poorly understood. In this study, we examined the local adaptation in the temperature- and phosphorus-dependent growth in the freshwater diatom Fragilaria crotonensis. We predict that the growth of this diatom is locally adapted to the environmental conditions of the lakes of origin. To test this, we isolated strains from eight Swiss lakes (one strain per lake) with distinct historical temperatures and nutrient status. We estimated the growth rate of each strain under combined gradients of temperature and phosphorus availability. We fitted Monod curves to the growth rate data, and we quantified the minimal phosphorus requirements (P*), half-saturation constants (K_s) and maximum growth rates (μ_max) for each strain as a function of temperature. We also fitted thermal performance curves and quantified activation energies (E_a) and cumulative performance across the thermal gradient as a function of phosphorus availability. We observed large variation among strains in the dependence of P* on experimental temperature, with strains from phosphorus-rich lakes showing stronger increases in phosphorus requirements with warming. These patterns imply local adaptation to phosphorus availability. Our findings highlight a potentially critical role for local adaptation in shaping phytoplankton responses to global change and call for a greater recognition of this trait variation in making predictions of community-level responses to future climate.

Ecosystems are warming alongside shifts in other abiotic factors, leading to interactive effects on populations and communities. This underscores the importance of studying how organisms respond to multiple environmental changes simultaneously. In pelagic ecosystems, as surface waters warm, longer and stronger periods of thermal stratification lead to changes in resource (light and nutrient) availability. We investigate the combined effects of temperature and resource availability on the growth rates of 19 populations (comprising 17 species) of freshwater phytoplankton in order to examine how temperature influences the minimum resource requirements (and Monod parameters) for light, nitrogen, and phosphorus. We also evaluate how resource availability affects each population's thermal traits (i.e., thermal performance curve - TPC - parameters). When averaged across all populations, the requirements for light and phosphorus tended to display a U-shaped relationship along temperatures. Individual populations varied greatly in their responses to temperature, leading to shifts in the identity of the best competitor across the thermal gradient, particularly for nitrogen and phosphorus. TPC responses to resource limitation were highly variable, but thermal optima and maxima of individual populations often decreased with resource limitation, and thermal breadths (range where growth is 80% or more of its maximum) often increased due to a flattening of TPCs. Across all populations and resource types, the maximum optimum temperature across resource levels (maximum T_opt) tended to be positively correlated with the temperature at which populations had the lowest resource requirements (minimum R*). However, the temperature at which populations were the best competitors tended to be ~5°C colder on average than the temperature at which they grew the fastest. The populations with the highest thermal optima also had the lowest minimum resource requirements. Our findings reveal trait associations suggesting that some taxa already exhibit trait combinations that would support high performance under future warm and resource-limited conditions.

Aim: Global change can impact the stability of biological communities by affecting species richness and synchrony. While most studies focus on terrestrial ecosystems, our research includes both terrestrial and aquatic realms. Previous works measure overall community synchrony as co-variation among co-occurring species, ignoring the tail dependence—when species fluctuate together at extreme abundance levels. We used community time-series data to test two hypotheses across realms: a positive relationship between diversity (richness) and stability, and a negative relationship between synchrony and stability. Additionally, we explored how tail-dependent synchrony contributes to variations in community stability.
Location: Global.
Time Period: 1923–2020.
Major Taxa Studied: 7 taxa across freshwater (fish, plants, invertebrates) and terrestrial (birds, plants, invertebrates, mammals) realms.
Methods: We synthesised 20+ years of species abundance/biomass data from 2668 communities across seven taxonomic groups. Using a variance-ratio approach and copula models, we measured overall and tail-dependent synchrony. Hierarchical linear mixed-effects models in a Bayesian framework were used to assess the effects of richness and both synchrony types on stability.
Results: We found a positive diversity–stability relationship in terrestrial but not in freshwater communities, with terrestrial stability being nearly three times higher. A negative synchrony –stability relationship was found in both realms. The best model explaining stability included realm differences, richness and both types of synchronies. For freshwater, only overall synchrony significantly impacted stability, while richness and both synchrony types were key predictors for terrestrial stability. Notably, the model overestimates terrestrial stability when tail-dependent synchrony is excluded.
Main Conclusions: Richness strongly enhanced terrestrial stability, offering the most extensive support for this relationship to date. In addition, tail-dependent synchrony provides key insights into stability differences across ecosystems. As extreme environmental events increase, incorporating tail-dependent synchrony in future stability studies is crucial.

Freshwater ecosystems support a disproportionate share of global biodiversity. Freshwater macroinvertebrates are frequently used as bioindicators to assess the ecological status of streams and rivers. However, ongoing climate change is altering the compositions of these communities, highlighting the need for tools to detect climate-related shifts. We propose and evaluate an ecological index based on the SPEcies At Risk (SPEAR) approach to quantify the proportion of species that are sensitive to higher temperatures within Ephemeroptera, Plecoptera and Trichoptera communities. We calculated this index using six different data sources for thermal preferences: experimentally measured thermal tolerances, the mean temperature from global occurrence records (GBIF), preferred temperatures obtained from multivariate species distribution models (SDM), the central temperature tendency derived from Swiss monitoring data according to the KLIWA approach, and two different sources of expert knowledge from a curated trait database. For all sources, we designated the species comprising the lower 50th percentile of thermal preference values as sensitive and the upper half as insensitive to warmer temperatures. To evaluate the performance of the six versions of this SPEAR_temperature on a geographic scale, we leveraged data from two Swiss wide monitoring programs including samples from 570 sites for the period 2010–2022 and from 183 sites for 2018–2023. While all six versions recapitulated a declining proportion of sensitive species with increasing water temperature, the relation was strongest when using the KLIWA-based thermal preference classification. On average across all sites, we observed a significant decline of the SPEAR_temperature index over the 2010–2022 period, independent of the source for the classification. This is consistent with a signal of warming-related community turnover, or ‘thermophilisation’. These results indicate that the index is suitable to document temperature-associated variation in EPT communities over both time and space. The attempt to derive a similar SPEAR_temperature index for fish was not successful.

Recent research has shown that climate change can both induce and modulate the expression of plastic traits but our understanding of the role of phenotypic plasticity as an adaptive response to climate change is limited. In this review, we dissect the mechanisms and impact of phenotypic plasticity as a response to accumulating climatic pressures on the individual, species and community levels. (i) We discuss how plasticity can affect individuals, populations and community dynamics and how climate change can alter the role of plasticity. We hypothesise that some pathways to phenotypic plasticity such as irreversible and anticipatory organismal responses will be reduced under increasing climate change. (ii) We then propose an integrated conceptual framework for studying phenotypic plasticity to advance our understanding of the feedbacks between the different levels of biological organisation. (iii) By formulating as yet unaddressed research questions within and across levels of biological organisation, we aim to instigate new research on phenotypic plasticity and its role in climate change responses.

In aquatic ecosystems facing climate change, higher temperatures often co-occur with alterations in resource availability. The metabolic theory of ecology uses activation energy to assess the sensitivity of biological processes to temperature, but neglects how resource availability might modify temperature sensitivities. To understand the impacts of resource limitation on temperature sensitivities, we performed experiments manipulating temperature and three key resources (nitrogen, phosphorus, and light) in six species of freshwater phytoplankton. We measured activation energies of population growth rates and how they were modulated by resource limitation. We find that the interaction of temperature and resource limitation is dependent on the resource type. Phosphorus limitation did not modify the temperature sensitivity of growth rates, light limitation reduced the sensitivity of growth rates to temperature in all species, and nitrogen limitation had species-dependent effects. We explore how stoichiometry and cell physiology may explain the complex responses of phytoplankton to multiple environmental changes.

Phytoplankton play a key role in biogeochemical cycles, impacting atmospheric and aquatic chemistry, food webs, and water quality. However, it remains challenging to reconstruct changes in algal community composition throughout the geologic past, as existing proxies are suitable only for a subset of taxa and/or influenced by degradation. Here, we investigate if compound-specific hydrogen isotope ratios (δ²H values) of common algal lipids can serve as (paleo)ecological indicators. First, we grew 20 species of algae – representing cyanobacteria, diatoms, dinoflagellates, green algae, and cryptomonads – in batch cultures under identical conditions and measured δ²H values of their lipids. Despite identical source water δ²H values, lipid δ²H values ranged from −455 ‰ to −52 ‰, incorporating variability associated with chemical compound classes and taxonomic groups. In particular, green algae synthesized fatty acids with higher δ²H values than other taxa, cyanobacteria synthesized phytol with relatively low δ²H values, and diatoms synthesized sterols with higher δ²H values than other eukaryotes. Second, we assessed how changes in algal community composition can affect net δ²H values of common algal lipids in 20 experimental outdoor ponds, which were manipulated via nutrient loading, and the addition of macrophytes and mussels. High algal biomass in the ponds, which was mainly caused by cyanobacterial and green algal blooms, was associated with higher δ²H values for generic fatty acids, relatively stable δ²H values for phytol and the dinoflagellate biomarker dinostanol, and lower δ²H values for the more cosmopolitan sterol stigmasterol. These results are consistent with expectations from our culture-based analyses, with both datasets indicating large taxon-specific changes that are unlikely to be driven by bacterial heterotrophy. This suggests that measuring δ²H values of multiple lipids from sediment and calculating ²H-offsets between them can resolve changes in algal community composition from changes in source water isotopes. With an appropriate availability of sedimentary lipids, this approach could permit the reconstruction of both taxonomic variability and hydroclimate from diverse sedimentary systems.