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.

Experimental warming promotes CO2 uptake but hinders carbon incorporation toward higher trophic levels in cyanobacteria-dominated freshwater communities.

Shallow freshwaters can exchange large amounts of carbon dioxide (CO2) with the atmosphere and also store significant quantities of carbon (C) in their sediments. Current warming and eutrophication pressures might alter the role of shallow freshwater ecosystems in the C cycle. Although eutrophication has been widely associated to an increase in total phytoplankton biomass and particularly of cyanobacteria, it is still poorly understood how warming may affect ecosystem metabolism under contrasting phytoplankton community composition. We studied the effects of experimental warming on CO2 fluxes and C allocation on two contrasting natural phytoplankton communities: chlorophytes-dominated versus cyanobacteria-dominated, both with a similar zooplankton community with a potentially high grazing capacity (i.e., standardized density of large-bodied cladocerans). The microcosms were subject to two different constant temperatures (control and +4 °C, i.e., 19.5 vs 23.5 °C) and we ensured no nutrient nor light limitation. CO2 uptake increased with warming in both communities, being the strongest in the cyanobacteria-dominated communities. However, only a comparatively minor share of the fixed C translated into increased phytoplankton (Chl-a), and particularly a negligible share translated into zooplankton biomass. Most C was either dissolved in the water (DIC) or sedimented, the latter being potentially available for mineralization into DIC and CO2, or methane (CH4) when anoxic conditions prevail. Our results suggest that C uptake increases with warming particularly when cyanobacteria dominate, however, due to the low efficiency in transfer through the trophic web the final fate of the fixed C may be substantially different in the long run.

Socio-economic or environmental benefits from pondscapes? Deriving stakeholder preferences using analytic hierarchy process and compositional data analysis.

Ponds occupy a large share of standing water worldwide and play an important role in providing various ecosystem services. There are concerted efforts of the European Union either to create new ponds, or to restore and preserve existing ponds as nature-based solutions to provide benefits to ecosystem and human well-being. As part of the EU PONDERFUL project, selected pondscapes (i.e. landscapes of ponds) in eight different countries - hereafter "demo-sites", are studied to comprehensively understand their characteristics and their efficiency to provide ecosystem services. In addition, the needs and knowledge of stakeholders who own, work, research, or benefit from the pondscapes are also important, because of their capabilities to create, manage and develop the pondscapes. Therefore, we established connection with stakeholders to study their preferences and visions on the pondscapes. Using the analytic hierarchy process, this study shows that in general stakeholders in the European and Turkish demo-sites prefer environmental benefits to economic benefits, while stakeholders in the Uruguayan demo-sites rank the economic benefits higher. More specifically, in the European and Turkish demo-sites, the biodiversity benefits, i.e. life-cycle maintenance, habitat and gene pool protection, receive the highest ranking among all groups. On the other hand, stakeholders at the Uruguayan demo-sites rank provisioning benefits as the most important, because many ponds in Uruguayan demo-sites are being used for agricultural purposes. Understanding those preferences helps policy makers to address the needs of stakeholders more correctly, when considering any action or policy for the pondscapes.

Periphyton biomass and life-form responses to a gradient of discharge in contrasting light and nutrients scenarios in experimental lowland streams.

Climate-induced changes in precipitation and land-use intensification affect the discharge of streams worldwide, which, together with eutrophication and loss of riparian canopy, can affect periphyton biomass and composition, and therefore, ultimately the stream functioning. We investigated the responses of periphyton biomass and life-forms (i.e., high profile, low profile and motile) to these changes applying an experimental approach by modulating nutrients (nutrient diffusion substrates enriched with 0.5 M NH4NO3 + 0.031 M KH2PO4 and without nutrient enrichment) and light availability (50% shade and full light) along a gradient in discharge ranging from 0.46 to 3.89 L/s (0.7 to 6.5 cm/s) in twelve large-sized (12- m long) outdoor flumes resembling lowland streams. We also analysed the potential effects of other environmental variables including macroinvertebrates on the responses of periphyton to discharge, nutrients, and light. Light and nutrient availability drastically affected periphyton biomass and composition responses to discharge. Periphyton biomass decreased with increasing discharge when shaded but this did not happen when exposed to full light. Under full light conditions, nutrient enrichment mediated an increase in the periphyton biomass with increasing discharge, possibly reflecting an increased metabolism, but this did not happen under non-enriched conditions. Enrichment further affected the compositional responses of periphyton to discharge, with an increase in the biomass of motile, fast-growing, small-sized flagellated at low discharge conditions, and mitigating a loss of high profile periphyton under higher discharges. Light did not affect periphyton composition, and the abundance or feeding-group composition of the macroinvertebrates did not affect biomass or composition of the periphyton either. Our results suggest that nutrient enrichment and light play an important synergistic role in the responses of the periphyton biomass and composition to discharge and emphasize the relevance of riparian canopy conservation and eutrophication control to avoid periphyton growth under increased discharge scenarios in small lowland streams.

Small-sized omnivorous fish induce stronger effects on food webs than warming and eutrophication in experimental shallow lakes.

Warming, eutrophication, and increased omnivory by small-sized fish are global change processes that induce major effects on the food web structure and primary producers of shallow lakes. Despite the key relevance of phytoplankton and periphyton in freshwaters, the combined and potential synergistic effects of fish omnivory, warming and eutrophication, especially on periphyton, remains little addressed, particularly for subtropical shallow lakes. We experimentally tested the food web effects on phytoplankton and periphyton induced by small visually feeding omnivorous fish (Rhodeus ocellatus), high nutrient enrichment and warming (+4.5 °C) in thirty-two 1000 L-mesocosms simulating littoral conditions of subtropical shallow lakes. We aimed at analysing the mechanisms and responses of periphyton and phytoplankton to these experimental factors. All mesocosms included the submerged macrophytes Vallisneria denseserrulata and Potamogeton lucens and artificial plants at 50% plant volume inhabited, plankton and macroinvertebrates. Small-sized visually feeding omnivorous fish enhanced phytoplankton dominance and periphyton loss. These changes coincided with a decrease in zooplankton biomass and a diversity loss of both zooplankton and macroinvertebrates as well as an increase in snail abundance. Fish presence led to a collapse of cladocerans, thereby releasing the grazing pressure on phytoplankton, and predator and collector macroinvertebrates were replaced with small snails (Radix peregra < 0.5 cm) resulting in enhanced grazing on periphyton. Eutrophication reinforced the fish effects, while warming had weak or no effects. Our results indicate that omnivory by small-sized visually feeding fish may induce stronger effects on the food webs of shallow lakes, towards phytoplankton-dominated states, than the combined effect of nutrient enrichment and warming under the present experimental conditions.

Glyphosate-based herbicide exposure affects diatom community development in natural biofilms.

Glyphosate herbicide is ubiquitously used in agriculture and weed control. It has now been identified in aquatic ecosystems worldwide, where numerous studies have suggested that it may have both suppressive and stimulatory effects on diverse non-target organisms. We cultured natural biofilms from a hypereutrophic environment to test the effects on periphytic diatoms of exposure to a glyphosate-based herbicide formulation at concentrations from 0 to 10 mg L-1 of active ingredient. There were clear and significant differences between treatments in diatom community structure after the 15-day experiments. Diversity increased more in low glyphosate treatments relative to higher concentrations, and compositional analyses indicated statistically significant differences between glyphosate treatments. The magnitude of change observed was significantly correlated with glyphosate-based herbicide concentration. Our results show that glyphosate-based herbicides have species-selective effects on benthic diatoms that may significantly alter trajectories of community development and therefore may affect benthic habitats and whole ecosystem function.

Seasonal and social factors associated with spacing in a wild territorial electric fish.

In this study, we focused on the seasonal variation of the determinants of territory size in the weakly electric fish Gymnotus omarorum. This species is a seasonal breeder that displays year-round territorial aggression. Female and male dyads exhibit indistinguishable non-breeding territorial agonistic behavior and body size is the only significant predictor of contest outcome. We conducted field surveys across seasons that included the identification of individual location, measurements of water physico-chemical variables, characterization of individual morphometric and physiological traits, and their correlation to spatial distribution. G. omarorum tolerates a wide range of dissolved oxygen concentration, and territory size correlated positively with dissolved oxygen in both seasons. In the non-breeding season, territory size was sexually monomorphic and correlated only with body size. In the breeding season, territory size no longer correlated with body size but differed between sexes: (i) the overall spatial arrangement was sexually biased, (ii) territory size depended on gonadal hormones in both sexes, which was expected for males, but not previously reported in females, (iii) female territory size showed a positive relationship with gonadal size, and (iv) females showed relatively larger territories than males. This study demonstrates seasonal changes in the determinants of territory size and thus contributes to the understanding of the mechanisms underlying the behavioral plasticity natural territorial behavior.

Environment not dispersal limitation drives clonal composition of Arctic Daphnia in a recently deglaciated area.

One of the most prominent manifestations of the ongoing climate warming is the retreat of glaciers and ice sheets around the world. Retreating glaciers result in the formation of new ponds and lakes, which are available for colonization. The gradual appearance of these new habitat patches allows us to determine to what extent the composition of asexual Daphnia (water flea) populations is affected by environmental drivers vs. dispersal limitation. Here, we used a landscape genetics approach to assess the processes structuring the clonal composition of species in the D. pulex species complex that have colonized periglacial habitats created by ice-sheet retreat in western Greenland. We analysed 61 populations from a young (<50 years) and an old cluster (>150 years) of lakes and ponds. We identified 42 asexual clones that varied widely in spatial distribution. Beta-diversity was higher among older than among younger systems. Lineage sorting by the environment explained 14% of the variation in clonal composition whereas the pure effect of geographical distance was very small and statistically insignificant (Radj2 = 0.010, P = 0.085). Dispersal limitation did not seem important, even among young habitat patches. The observation of several tens of clones colonizing the area combined with environmentally driven clonal composition of populations illustrates that population assembly of asexual species in the Arctic is structured by environmental gradients reflecting differences in the ecology of clones.

Global effects of agriculture on fluvial dissolved organic matter.

Agricultural land covers approximately 40% of Earth's land surface and affects hydromorphological, biogeochemical and ecological characteristics of fluvial networks. In the northern temperate region, agriculture also strongly affects the amount and molecular composition of dissolved organic matter (DOM), which constitutes the main vector of carbon transport from soils to fluvial networks and to the sea, and is involved in a large variety of biogeochemical processes. Here, we provide first evidence about the wider occurrence of agricultural impacts on the concentration and composition of fluvial DOM across climate zones of the northern and southern hemispheres. Both extensive and intensive farming altered fluvial DOM towards a more microbial and less plant-derived composition. Moreover, intensive farming significantly increased dissolved organic nitrogen (DON) concentrations. The DOM composition change and DON concentration increase differed among climate zones and could be related to the intensity of current and historical nitrogen fertilizer use. As a result of agriculture intensification, increased DON concentrations and a more microbial-like DOM composition likely will enhance the reactivity of catchment DOM emissions, thereby fuelling the biogeochemical processing in fluvial networks, and resulting in higher ecosystem productivity and CO2 outgassing.

Monitoring fish communities in wadeable lowland streams: comparing the efficiency of electrofishing methods at contrasting fish assemblages.

Electrofishing is considered a reliable tool to assess the assemblages and biodiversity of fish in wadeable streams. The most widely used electrofishing techniques (point [P], single-pass [S-P], and multiple-pass [M-P]) vary as to the effort needed for sample collection, and this may potentially influence the degree of accuracy. Moreover, little is known about the comparability of the methods and their specific performance in streams with different fish assemblages. The aim of this investigation was to validate (using M-P sampling as reference) the use of P and S-P electrofishing techniques to accurately assess the richness, density and size distribution of fishes in small streams at both regional and global scale independently of fish assemblages and geographical region. We sampled 50-m-long reaches in a total of 33 lowland stream reaches that were located in different climatic and biogeographical regions (Uruguay and Denmark) and hosted different fish assemblages. Subtropical fish communities exhibited higher richness (Uy: 12-32, Dk: 1-9) and densities (Uy: 1.3-5.2, DK: 0.1-4.9 in. m(-2)) than temperate streams. We applied both "global models" using the entire database (33 sites) and "local models" including the same number of sites but using the climatic region as a model variable. Regression analyses revealed that the P, S-P and M-P methods all provided an adequate picture of the species composition and size distribution, and transfer equations for comparison between methods are thus not required. Conversely, richness was better predicted by S-P and by P techniques for regional and global models, respectively. Transfer equations obtained for abundance revealed that the P and S-P models can accurately transform catch data into M-P estimations. The transfer equations provided here may have great relevance as they allow relatively reliable comparisons to be made between data obtained by different techniques. We also show that less intensive sampling techniques may be equally useful for monitoring purposes as those requiring more intensive efforts (and costs). We encourage validation of our developed transfer equations on data from other regions of the world.

Effects of temperature, salinity and fish in structuring the macroinvertebrate community in shallow lakes: implications for effects of climate change.

Climate warming may lead to changes in the trophic structure and diversity of shallow lakes as a combined effect of increased temperature and salinity and likely increased strength of trophic interactions. We investigated the potential effects of temperature, salinity and fish on the plant-associated macroinvertebrate community by introducing artificial plants in eight comparable shallow brackish lakes located in two climatic regions of contrasting temperature: cold-temperate and Mediterranean. In both regions, lakes covered a salinity gradient from freshwater to oligohaline waters. We undertook day and night-time sampling of macroinvertebrates associated with the artificial plants and fish and free-swimming macroinvertebrate predators within artificial plants and in pelagic areas. Our results showed marked differences in the trophic structure between cold and warm shallow lakes. Plant-associated macroinvertebrates and free-swimming macroinvertebrate predators were more abundant and the communities richer in species in the cold compared to the warm climate, most probably as a result of differences in fish predation pressure. Submerged plants in warm brackish lakes did not seem to counteract the effect of fish predation on macroinvertebrates to the same extent as in temperate freshwater lakes, since small fish were abundant and tended to aggregate within the macrophytes. The richness and abundance of most plant-associated macroinvertebrate taxa decreased with salinity. Despite the lower densities of plant-associated macroinvertebrates in the Mediterranean lakes, periphyton biomass was lower than in cold temperate systems, a fact that was mainly attributed to grazing and disturbance by fish. Our results suggest that, if the current process of warming entails higher chances of shallow lakes becoming warmer and more saline, climatic change may result in a decrease in macroinvertebrate species richness and abundance in shallow lakes.

Climate change effects on runoff, catchment phosphorus loading and lake ecological state, and potential adaptations.

Climate change may have profound effects on phosphorus (P) transport in streams and on lake eutrophication. Phosphorus loading from land to streams is expected to increase in northern temperate coastal regions due to higher winter rainfall and to a decline in warm temperate and arid climates. Model results suggest a 3.3 to 16.5% increase within the next 100 yr in the P loading of Danish streams depending on soil type and region. In lakes, higher eutrophication can be expected, reinforced by temperature-mediated higher P release from the sediment. Furthermore, a shift in fish community structure toward small and abundant plankti-benthivorous fish enhances predator control of zooplankton, resulting in higher phytoplankton biomass. Data from Danish lakes indicate increased chlorophyll a and phytoplankton biomass, higher dominance of dinophytes and cyanobacteria (most notably of nitrogen fixing forms), but lower abundance of diatoms and chrysophytes, reduced size of copepods and cladocerans, and a tendency to reduced zooplankton biomass and zooplankton:phytoplankton biomass ratio when lakes warm. Higher P concentrations are also seen in warm arid lakes despite reduced external loading due to increased evapotranspiration and reduced inflow. Therefore, the critical loading for good ecological state in lakes has to be lowered in a future warmer climate. This calls for adaptation measures, which in the northern temperate zone should include improved P cycling in agriculture, reduced loading from point sources, and (re)-establishment of wetlands and riparian buffer zones. In the arid Southern Europe, restrictions on human use of water are also needed, not least on irrigation.