Experimental warming promotes phytoplankton species sorting towards cyanobacterial blooms and leads to potential changes in ecosystem functioning.

A positive feedback loop where climate warming enhances eutrophication and its manifestations (e.g., cyanobacterial blooms) has been recently highlighted, but its consequences for biodiversity and ecosystem functioning are not fully understood. We conducted a highly replicated indoor experiment with a species-rich subtropical freshwater phytoplankton community. The experiment tested the effects of three constant temperature scenarios (17, 20, and 23 °C) under high-nutrient supply conditions on community composition and proxies of ecosystem functioning, namely resource use efficiency (RUE) and CO2 fluxes. After 32 days, warming reduced species richness and promoted different community trajectories leading to a dominance by green algae in the intermediate temperature and by cyanobacteria in the highest temperature treatments. Warming promoted primary production, with a 10-fold increase in the mean biomass of green algae and cyanobacteria. The maximum RUE occurred under the warmest treatment. All treatments showed net CO2 influx, but the magnitude of influx decreased with warming. We experimentally demonstrated direct effects of warming on phytoplankton species sorting, with negative effects on diversity and direct positive effects on cyanobacteria, which could lead to potential changes in ecosystem functioning. Our results suggest potential positive feedback between the phytoplankton blooms and warming, via lower net CO2 sequestration in cyanobacteria-dominated, warmer systems, and add empirical evidence to the need for decreasing the likelihood of cyanobacterial dominance.

Nutrients and not temperature are the key drivers for cyanobacterial biomass in the Americas.

Cyanobacterial blooms imperil the use of freshwater around the globe and present challenges for water management. Studies have suggested that blooms are trigged by high temperatures and nutrient concentrations. While the roles of nitrogen and phosphorus have long been debated, cyanobacterial dominance in phytoplankton has widely been associated with climate warming. However, studies at large geographical scales, covering diverse climate regions and lake depths, are still needed to clarify the drivers of cyanobacterial success. Here, we analyzed data from 464 lakes covering a 14,000 km north-south gradient in the Americas and three lake depth categories. We show that there were no clear trends in cyanobacterial biomass (as biovolume) along latitude or climate gradients, with the exception of lower biomass in polar climates. Phosphorus was the primary resource explaining cyanobacterial biomass in the Americas, while nitrogen was also significant but particularly relevant in very shallow lakes (< 3 m depth). Despite the assessed climatic gradient water temperature was only weakly related to cyanobacterial biomass, suggesting it is overemphasized in current discussions. Depth was critical for predicting cyanobacterial biomass, and shallow lakes proved more vulnerable to eutrophication. Among other variables analyzed, only pH was significantly related to cyanobacteria biomass, likely due to a biologically mediated positive feedback under high nutrient conditions. Solutions toward managing harmful cyanobacteria should thus consider lake morphometric characteristics and emphasize nutrient control, independently of temperature gradients, since local factors are more critical - and more amenable to controls - than global external forces.

Effects of space and environment on phytoplankton distribution in subtropical reservoirs depend on functional features of the species.

Environmental factors and dispersal can influence the structure of biological communities. Their effects can depend on the functional features of the species in the community. Since species belonging to the same trophic level, such as phytoplankton, may show functional differences, we investigated whether the effects of environment and dispersal differ among phytoplankton species from different functional groups. We analyzed data from a rainy and a dry period in 30 reservoirs in a subtropical region. In both periods, the environment as well as high and limited dispersal influenced the metacommunity structure. The functional groups had a low correspondence in their response to both dispersal and environment. Our results showed that the influence of the processes underlying the structure of the metacommunities, such as species sorting (environment influence), mass effect (high dispersal), and neutral dynamics (limited dispersal), depended on the functional characteristics of the organisms and could vary even among species of the same trophic level. These findings suggested that species at the same trophic level could not be considered ecological equivalents. This paper includes a Portuguese and Spanish version of the abstract in the online resources. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s00027-021-00837-0.

Warming and top predator loss drive direct and indirect effects on multiple trophic groups within and across ecosystems.

The interspecific interactions within and between adjacent ecosystems strongly depend on the changes in their abiotic and biotic components. However, little is known about how climate change and biodiversity loss in a specific ecosystem can impact the multiple trophic interactions of different biological groups within and across ecosystems. We used natural microecosystems (tank-bromeliads) as a model system to investigate the main and interactive effects of aquatic warming and aquatic top predator loss (i.e. trophic downgrading) on trophic relationships in three integrated food web compartments: (a) aquatic micro-organisms, (b) aquatic macro-organisms and (c) terrestrial predators (i.e. via cross-ecosystem effects). The aquatic top predator loss substantially impacted the three food web compartments. In the aquatic macrofauna compartment, trophic downgrading increased the filter feeder richness and abundance directly and indirectly via an increase in detritivore richness, likely through a facilitative interaction. For the microbiota compartment, aquatic top predator loss had a negative effect on algae richness, probably via decreasing the input of nutrients from predator biological activities. Furthermore, the more active terrestrial predators responded more to aquatic top predator loss, via an increase in some components of aquatic macrofauna, than more stationary terrestrial predators. The aquatic trophic downgrading indirectly altered the richness and abundance of cursorial terrestrial predators, but these effects had different direction according to the aquatic functional group, filter feeder or other detritivores. The web-building predators were indirectly affected by aquatic trophic downgrading due to increased filter feeder richness. Aquatic warming did not affect the aquatic micro- or macro-organisms but did positively affect the abundance of web-building terrestrial predators. These results allow us to raise a predictive framework of how different anthropogenic changes predicted for the next decades, such as aquatic warming and top predator loss, could differentially affect multiple biological groups through interactions within and across ecosystems.

As interações interespecíficas dentro e entre ecossistemas adjacentes dependem fortemente das mudanças de seus componentes abióticos e bióticos. Entretanto, pouco se sabe sobre como mudanças climáticas e a perda de biodiversidade em um ecossistema específico pode impactar as múltiplas interações tróficas de diferentes grupos biológicos dentro e entre ecossistemas. Nós utilizamos micro ecossistemas naturais (bromélias-tanque) como sistema modelo para investigar os efeitos individuais e interativos do aquecimento e da perda de predadores aquáticos (simplificação trófica) nas relações tróficas em três compartimentos integrados da teia alimentar: i) micro-organismos aquáticos, ii) macroorganismos aquáticos e iii) predadores terrestres (via efeito entre ecossistemas). A perda de predadores de topo aquáticos afetou substancialmente os três compartimentos da rede trófica. No compartimento da macrofauna aquática, a simplificação trófica aumentou a riqueza e abundância de filtradores, direta e indiretamente, por meio de um aumento da riqueza de espécies de detritívoros, provavelmente através de uma interação de facilitação. Para o compartimento da microbiota, a perda de predadores de topo aquáticos teve um efeito negativo sobre a riqueza de espécies de algas, provavelmente por meio da diminuição da entrada de nutrientes provenientes das atividades biológicas dos predadores. Além disso, os predadores terrestres mais ativos responderam mais à perda de predadores de topo aquáticos, por meio de um aumento de alguns componentes da macrofauna aquática, do que predadores terrestres mais estacionários. A simplificação trófica aquática alterou indiretamente a riqueza e abundância de predadores cursoriais terrestres, mas esses efeitos tiveram direção diferente de acordo com o grupo funcional aquático, filtradores ou outros detritívoros. Os predadores construtores de teias foram indiretamente afetados pela simplificação trófica aquática devido ao aumento da riqueza de filtradores. O aquecimento aquático não afetou os micro ou macro organismos aquáticos, mas afetou positivamente a abundância de predadores terrestres construtores de teias. Esses resultados nos permitem levantar um quadro preditivo de como diferentes mudanças antropogênicas preditas para as próximas décadas, como o aquecimento e a perda de predadores de topo aquáticos, podem afetar diferencialmente vários grupos biológicos por meio de interações dentro e entre os ecossistemas.

Main predictors of periphyton species richness depend on adherence strategy and cell size.

Periphytic algae are important components of aquatic ecosystems. However, the factors driving periphyton species richness variation remain largely unexplored. Here, we used data from a subtropical floodplain (Upper Paraná River floodplain, Brazil) to quantify the influence of environmental variables (total suspended matter, temperature, conductivity, nutrient concentrations, hydrology, phytoplankton biomass, phytoplankton species richness, aquatic macrophyte species richness and zooplankton density) on overall periphytic algal species richness and on the richness of different algal groups defined by morphological traits (cell size and adherence strategy). We expected that the coefficients of determination of the models estimated for different trait-based groups would be higher than the model coefficient of determination of the entire algal community. We also expected that the relative importance of explanatory variables in predicting species richness would differ among algal groups. The coefficient of determination for the model used to predict overall periphytic algal species richness was higher than the ones obtained for models used to predict the species richness of the different groups. Thus, our first prediction was not supported. Species richness of aquatic macrophytes was the main predictor of periphyton species richness of the entire community and a significant predictor of the species richness of small mobile, large mobile and small-loosely attached algae. Abiotic variables, phytoplankton species richness, chlorophyll-a concentration, and hydrology were also significant predictors, depending on the group. These results suggest that habitat heterogeneity (as proxied by aquatic macrophytes richness) is important for maintaining periphyton species richness in floodplain environments. However, other factors played a role, suggesting that the analysis of species richness of different trait-based groups unveils relationships that were not detectable when the entire community was analysed together.

Planktonic heterotrophic nanoflagellate abundance along a trophic gradient in subtropical reservoirs: influence of a bottom-up or top-down control mechanism? / Abundância de nanoflagelados heterotróficos ao longo de um gradiente trófico em reservatórios tropicais: influência do mecanismo de controle bottom-up ou top-down?

The aim of the current study was to assess the variability in heterotrophic nanoflagellate abundance attributable to variables responsible for the influence of bottom-up and top-down control mechanisms in the plankton food web along a trophic gradient in subtropical reservoirs (Paraná State, Brazil). In particular, we hypothesised that food resources were the main determinant for the abundance of heterotrophic nanoflagellates (HNF) along a trophic gradient. Results showed that HNF communities were weakly influenced by the bottom-up mechanism. Moreover, there was evidence of a slight influence from a top-down control mechanism on this community. Therefore, the results of the present study did not corroborate the hypothesis initially proposed, and further studies are needed to elucidate the complex outcomes of trophic gradients on planktonic HNF communities, including other predictor variables related to abiotic interactions, such as morphometrics, hydrology, priorities uses and climate conditions.

O objetivo do presente estudo foi investigar a variabilidade da abundância de nanoflagelados heterotróficos (NFH) planctônicos atribuída a variáveis responsáveis pela influencia dos mecanismos de controle bottom-up e top-down na cadeia alimentar planctônica ao longo de um gradiente trófico em reservatórios subtropicais (Estado do Paraná, Brasil). Nós avaliamos a hipótese de que a abundância de NFH aumenta ao longo de um gradiente trófico principalmente devido ao efeito bottom-up. Os resultados mostraram importantes relações presa-predador entre NFH e bactéria e fioplâncton. Entretanto, a ausência de um padrão claro de aumento da abundância de NFH em direção aos reservatórios eutróficos sugere que o controle bottom-up não foi relacionado ao gradiente trófico. Além disso, apesar de significante, um fraco efeito bottom-up e um sinal do mecanismo de controle top-down sugerem que o recurso alimentar não foi o principal fator regulador da dinâmica de NFH nos reservatórios estudados. Assim, os resultados do presente estudo não corroboraram a hipótese inicialmente proposta e futuros estudos são necessários para elucidar a complexidade da influencia de gradientes tróficos em NFH, incluindo outras variáveis preditoras relacionadas às interações abióticas como morfometria, hidrologia, prioridade de uso e condições climáticas.

Spatial and temporal fluctuation of phytoplankton functional groups in a tropical reservoir / Variação espacial e temporal dos grupos funcionais do fitoplâncton em um reservatório tropical

Along the horizontal axis of reservoirs are generally recognized three zones (fluvial, transition and lacustrine) with distinct hydrodynamic and physical, chemical and biological properties. Quarterly samplings were conducted in 2002, in the limnetic region from each zone, at different depths of a tropical reservoir. To test the hypothesis that highest biomass (biovolume) of phytoplankton are found in the transition zone, the PERMANOVA analysis was realized. 106 taxa were recorded. Significant differences between biovolume values of the three zones were not verified (pseudo F = 0,89; p = 0,55). Higher values of biomass were obtained in lacustrine and transition zones of the reservoir. The functional group (FG) N (Cosmarium spp.) was dominant in these zones, in rain period, and related to low phosphorus concentration, high transparency and water column stability. Low biovolume values in reservoir characterized oligotrophic conditions in all zones of the reservoir, most of the period. The dominance of FGs Y, Lo, E, P and A, as evidenced by the CCA, was associated with low light availability and higher nutrients concentrations.

Ao longo do eixo horizontal de reservatórios são, em geral, reconhecidas três zonas (fluvial, transição e lacustre) com distinta hidrodinâmica e propriedades físicas, químicas e biológicas. Foram realizadas amostragens trimestrais no ano de 2002, na região limnética de cada zona, em diferentes profundidades, em um reservatório tropical. Para testar a hipótese de que os maiores valores de biovolume fitoplanctônico ocorrem na zona de transição do reservatório, foi realizada a Análise PERMANOVA. Foram registrados 106 táxons. Não foram registradas diferenças significativas entre os valores de biovolume fitoplanctônico das zonas do reservatório (pseudo F = 0,89; p= 0,55). Maiores valores de biovolume foram obtidos na zona lacustre e na zona de transição do reservatório. O grupo funcional N (Cosmarium spp.), dominante nestas zonas, no período chuvoso, esteve relacionado às baixas concentrações de fósforo, alta transparência e estabilidade da coluna de água. Os baixos valores de biovolume caracterizaram condições oligotróficas em todo o reservatório, na maior parte do período de estudo. A dominância dos FGs Y, Lo, E, P e A, como evidenciado por meio da CCA, esteve associada à baixa disponibilidade de luz e concentrações mais altas de nutrientes.