Carla Kruk

Environmental Warming in Shallow Lakes: A Review of Potential Changes in Community Structure as Evidenced from Space-for-Time Substitution Approaches

Abstract Shallow lakes, one of the most widespread water bodies in the world landscape, are very sensitive to climate change. Several theories predict changes in community traits, relevant for ecosystem functioning, with higher temperature. The space-for-time substitution approach (SFTS) provides one of the most plausible empirical evaluations for these theories, helping to elucidate the long-term consequences of changes in climate. Here, we reviewed the changes at the community level for the main freshwater taxa and assemblages (i.e. fishes, macroinvertebrates, zooplankton, macrophytes, phytoplankton, periphyton and bacterioplankton), under different climates. We analyzed data obtained from latitudinal and altitudinal gradients and cross-comparison (i.e. SFTS) studies, supplemented by an analysis of published geographically dispersed data for those communities or traits not covered in the SFTS literature. We found only partial empirical evidence supporting the theoretical predictions. The prediction of higher richness at warmer locations was supported for fishes, phytoplankton and periphyton, while the opposite was true for macroinvertebrates and zooplankton. With decreasing latitude, the biomass of cladoceran zooplankton and periphyton and the density of zooplankton and macroinvertebrates declined (opposite for fishes for both biomass and density variables). Fishes and cladoceran zooplankton showed the expected reduction in body size with higher temperature. Life history changes in fish and zooplankton and stronger trophic interactions at intermediate positions in the food web (fish predation on zooplankton and macroinvertebrates) were evident, but also a weaker grazing pressure of zooplankton on phytoplankton occurred with increasing temperatures. The potential impacts of lake productivity, fish predation and other factors, such as salinity, were often stronger than those of temperature itself. Additionally, shallow lakes may shift between alternative states, complicating theoretical predictions of warming effects. SFTS and meta-analyses approaches have their shortcomings, but in combination with experimental and model studies that help reveal mechanisms, the “field situation” is indispensable to understand the potential effects of warming.

Steady-state assemblages of phytoplankton in four temperate lakes (NE U.S.A.)

For four temperate lakes (Northeast U.S.A.) we identify periods of persistent phytoplankton assemblages and investigate the ecological conditions that correlate to these persistent assemblages. Periods of persistent assemblages, here considered as steady-state phases, were defined according to equilibrium criteria (two or three coexisting species, contributing to 80% of the standing biomass, for at least 2 weeks) defined by Sommer et al. (1993, Hydrobiologia 249: 1–7). For all four lakes, samples were taken weekly during the ice-free season and phytoplankton attributes (biomass, assemblages, diversity, species richness, change rates) and abiotic variables (temperature, I* – as light mean in the mixing zone – zmix, and nutrients) were analysed. Chodikee (CH), an eutrophic and rapidly flushed lake, did not show any persistent phase. The remaining three lakes showed single steady-state phases that occurred at varying times during the ice-free season. Steady-state phases occurred during early stratification in late spring in the stably stratified oligotrophic Mohonk Lake (MO), in the late summer stratification in the meso-eutrophic Stissing Lake (ST), and during spring mixing in Wononscopomuc Lake (WO). MO showed a 3-week period with dominance of F assemblage (Botryococcus braunii, Willea wilhelmii and Eutetramorus planctonicus), characteristic for clear epilimnia, tolerant to low nutrient and sensitive to high turbidity. For three weeks, ST had a stable assemblage with dominance of Lo(Woronichinia sp.), common assemblage in summer epilimnion of mesotrophic lakes and sensitive to prolonged or deep mixing; and P, assemblage able to live in eutrophic epilimnia with mild light and sensitive to stratification and silica depletion. In contrast, the mesotrophic Wononscopomuc Lake (WO) showed persistent assemblages during a 4-week period of spring circulation, when a dinoflagellate (Lo) was co-dominant with Nitzschia acicularis (C). The latter species is characteristic for mesotrophic lakes, tolerant to low light and sensitive to stratification and silica depletion. Both Lo and P assemblages, among seven others, had before been quoted, in literature, as dominant in maturing stages. We could not find consistent statistical differences between the periods classified as steady-state and non-steady-state. However, the data demonstrated that prolonged period of both mixing and stratification can maintain dominant assemblages. Although, historically sensed as opposite mechanisms, both mixing and stratification, if persistent, were observed maintaining dominant assemblages because both scenarios are characterized by environmental constancy.

Emergent neutrality drives phytoplankton species coexistence

The mechanisms that drive species coexistence and community dynamics have long puzzled ecologists. Here, we explain species coexistence, size structure and diversity patterns in a phytoplankton community using a combination of four fundamental factors: organism traits, size-based constraints, hydrology and species competition. Using a ‘microscopic’ Lotka–Volterra competition (MLVC) model (i.e. with explicit recipes to compute its parameters), we provide a mechanistic explanation of species coexistence along a niche axis (i.e. organismic volume). We based our model on empirically measured quantities, minimal ecological assumptions and stochastic processes. In nature, we found aggregated patterns of species biovolume (i.e. clumps) along the volume axis and a peak in species richness. Both patterns were reproduced by the MLVC model. Observed clumps corresponded to niche zones (volumes) where species fitness was highest, or where fitness was equal among competing species. The latter implies the action of equalizing processes, which would suggest emergent neutrality as a plausible mechanism to explain community patterns.

Competition Drives Clumpy Species Coexistence in Estuarine Phytoplankton

Understanding the mechanisms that maintain biodiversity is a fundamental problem in ecology. Competition is thought to reduce diversity, but hundreds of microbial aquatic primary producers species coexist and compete for a few essential resources (e.g., nutrients and light). Here, we show that resource competition is a plausible mechanism for explaining clumpy distribution on individual species volume (a proxy for the niche) of estuarine phytoplankton communities ranging from North America to South America and Europe, supporting the Emergent Neutrality hypothesis. Furthermore, such a clumpy distribution was also observed throughout the Holocene in diatoms from a sediment core. A Lotka-Volterra competition model predicted position in the niche axis and functional affiliation of dominant species within and among clumps. Results support the coexistence of functionally equivalent species in ecosystems and indicate that resource competition may be a key process to shape the size structure of estuarine phytoplankton, which in turn drives ecosystem functioning.

A trait-based approach to summarize zooplankton–phytoplankton interactions in freshwaters

Assessing zooplankton grazing on phytoplankton is crucial to understand, model, and predict the structure and dynamics of pelagic communities. Our hypothesis is that phytoplankton consumption by zooplankton in freshwater lakes can be well represented by clustering phytoplankton species into morphology-based functional groups (MBFG) and zooplankton species into broad taxonomic units: cladocerans, calanoid copepods, and rotifers. We characterized zooplankton potential grazing on MBFG based on an extensive literature review of experimental data including clearance and ingestion rates. Rotifers show greater potential grazing upon small- and medium-sized species (MBFG I and IV) and presented a Type III trait-based functional response. Cladocerans also show greater potential impact upon MBFG IV but a Type II response. Both groups maintained their respective feeding response regardless of the type of food available, indicating poor food selectivity. Copepods consumed different MBFGs, but a clear Type II pattern was observed when feeding on MBFGs V and VI. Prediction intervals indicated a greater variability in cladocerans’ and copepods’ response. This approach is a step to summarize and characterize grazing to the future quantification of ecosystem models. Further efforts should be done to include information about different larval stages and phytoplankton traits not directly related to morphology.

Comunidades de macroalgas en puntas rocosas de la costa de Rocha, Uruguay

Macroalgal communities are fundamental for marine ecosystems and very sensitive to environmental changes (e.g temperature, salinity). The Uruguayan Atlantic coast represents a transition between subtropical and subantarctic phycoflora and therefore has a key role to the regional biogeography of macroalgae. The objective of this study was to analyze the macroalgal community richness and biomass, its spatial distribution and its changes along diverse environmental gradients in rocky outcrops of Rocha coast (33°S; 54°W), Uruguay. Two samplings were carried out (summer and winter) in rocky shores of Cerro Verde (La Coronilla), Cerro Rivero (Punta del Diablo) and El Cabito (La Paloma). Transects were used to cover different microhabitats and depths, environmental variables were measured and samples collected. A total of 36 taxa were identified (21 Rhodophyta, 11 Chlorophyta and 4 Ochrophyta). The species richness and biomass were higher in summer for every site, with communities dominated by subtropical species. Temperature was a key modulating factor. For all the species, 50 % were previously registered in previous literature for Uruguay and Ulva spp. complex (Chlorophyta) was the most frequent. Ochrophyta were less frequent being associated with cold and clear waters. Present results provide a basis for the macroalgae management of the Uruguayan coast, reopening a line for future research in the subject.

Morphology-based differences in the thermal response of freshwater phytoplankton

The thermal response of maximum growth rate in morphology-based functional groups (MBFG) of freshwater phytoplankton is analysed. Contrasting an exponential Boltzmann–Arrhenius with a unimodal model, three main features were evaluated: (i) the activation energy of the rise (Er), (ii) the presence of a break in the thermal response and (iii) the activation energy of the fall (Ef). The whole dataset (N = 563) showed an exponential increase (Er ∼ 0.5), a break around 24°C and no temperature dependence after the breakpoint (Ef = 0). Contrasting thermal responses among MBFG were found. All groups showed positive activation energy (Er > 0), four showed no evidence of decline in growth rate (temperature range = 0–35°C) and two presented a breakpoint followed by a sharp decrease in growth rate. Our results evidenced systematic differences between MBFG in the thermal response and a coherent response significantly related to morphological traits other than size (i.e. within MBFG). These results provide relevant information for water quality modelling and climate change predictions.