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Due to global warming, shorter ice cover duration might drastically affect the ecology of lakes currently undergoing seasonal surface freezing. Highmountain lakes show snow-rich ice covers that determine contrasting conditions between ice-off and ice-on periods. We characterized the bacterioplankton seasonality in a deep high-mountain lake ice-covered for half a year. The lake shows a rich core bacterioplankton community consisting of three components: (i) an assemblage stable throughout the year, dominated by Actinobacteria, resistant to all environmental conditions; (ii) an ice-on-resilient assemblage dominating during the ice-covered period, which is more diverse than the other components and includes a high abundance of Verrucomicrobia; the deep hypolimnion constitutes a refuge for many of the typical under-ice taxa, many of which recover quickly during autumn mixing; and (iii) an ice-off-resilient assemblage, which members peak in summer in epilimnetic waters when the rest decline, characterized by a dominance of Flavobacterium, and Limnohabitans. The rich core community and low random elements compared to other relatively small cold lakes can be attributed to its simple hydrological network in a poorly-vegetated catchment, the long water-residence time (ca. 4 years), and the long ice-cover duration; features common to many headwater deep high-mountain lakes.

Planktonic communities are naturally subjected to episodic nutrient enrichments that may stress or redress the imbalances in limiting nutrients. Human-enhanced atmospheric nitrogen deposition has caused profound N:P imbalance in many remote oligotrophic lakes in which phosphorus has largely become limiting. These lakes offer an opportunity to investigate the planktonic community response to nutrient fluctuations in P-limited conditions. The ENEX experiment in Lake Redon (Pyrenees), performed during August 2013, aimed to investigate the structural and stoichiometric effects of pulse nutrient additions on P-limited planktonic communities. We performed P (PO43-), and N (NH4+ or NO3-) additions to the summer epilimnetic community of the ultraoligotrophic lake using self-filling ~100 L enclosures and analysed the response to varying P availability, N:P imbalance, and N source. The nutrient additions were gradients within the range of values seasonally found in the lake and other oligotrophic lakes of the Pyrenees, with a further P level typical of mesotrophic lakes to provided non-limiting conditions.

Seasonal compositional changes in plankton communities are usually considered as species replacements. Given the enormous number of individuals integrating the communities and our limited capacity to count and determine most of them, we likely observe only alternative population peaks of some of the coexisting species. The contemporary coexistence theory addresses coexistence in communities of competing species, considering relative fitness inequalities and stabilising niche differences as components of average long‐term growth rates. Here, we experimentally show that the response patterns predicted by the theory occur when varying nutrient pulses fertilise the planktonic community. We used gently self‐filling 100‐L enclosures to minimise the disturbance of the initial community and different pulse P and N additions to manipulate the apparently species‐poor epilimnetic community of an ultraoligotrophic P‐limited lake. We measured and compared the protist species growth response to gradients of P enrichment and N stoichiometric imbalance. The P and N levels used in most treatments were within the oligotrophic seasonal and inter‐annual variations of the lake and were higher in a few extreme treatments that provided mesotrophic conditions of the remote regions affected by N atmospheric contamination. We alternatively replicated all treatments using ammonium or nitrate as the N source. Most protist species, recorded in this lake across seasons in previous studies, were recovered, indicating a persistent assemblage of species that is seasonally hidden from observation. Recovery included some rare species observed only in the slush layers of the seasonal snow and ice cover. The coexistence‐stabilising mechanisms were indicated by treatment response features, such as frequency‐dependent growth, inverse relationship between fitness inequality and niche differentiation proxies, high‐rank taxonomic levels clustering across the limiting‐nutrient gradient but segregation at the

Planktonic communities are naturally subjected to episodic nutrient enrichments that may stress or redress the imbalances in limiting nutrients. Human-enhanced atmospheric nitrogen deposition has caused profound N:P imbalance in many remote oligotrophic lakes in which phosphorus has largely become limiting. These lakes offer an opportunity to investigate the relationship between the changes in plankton stoichiometry, productivity, and community structure occurring during nutrient fluctuations in P-limited conditions. We performed P (PO3 4 ) and N (NHþ 4 or NO 3 ) pulse additions to the summer epilimnetic community of an ultraoligotrophic lake using self-filling ~100-L enclosures and analyzed the response to varying P availability, N:P imbalance, and N source. Seston C:N:P proportions remained fairly unchanged to P additions that were within the range of values seasonally found in the lake. However, the seston N:P ratio abruptly shifted and approached Redfield’s proportions at P additions typical of mesotrophic conditions that provided non-limiting conditions. N surplus did not affect seston C:N:P proportions. The patterns of seston N:P stability and shift were similar for both N sources. In contrast, productivity was highly sensitive to low and medium P additions and decelerated at high P additions. Phytoplankton biomass dominated particulate organic matter. The autotrophic community differentiated almost linearly across the P gradient. Chrysophytes’ dominance decreased, and diatoms and cryptophytes relative abundance increased. Nonetheless, the stoichiometry stability and non-linear shift involved large biomass proportions of the same species, which indicates that the bulk stoichiometry was related to similar physiological behavior of phylogenetically diverse organisms according to the biogeochemical context. The C:N:P seston stability in P-limited conditions—with loose coupling with productivity, nutrient supply ratios, and species dominance—and the sudden shift t

[EN] The initial presence of humans in the high mountains almost since 8000 years ago shows more elements of domestication than initially estimated. Although the geographic extent of archaeological explorations is still limited, there is evidence of domestic livestock use and, in some cases, likely mixed-cereal agriculture at intermediate altitudes. It has not been possible to verify an impact of this activity on the landscape yet. In the Pyrenees, the distribution of archaeological sites in the high mountain changes associated with climate in the middle of the Holocene. The occupation of shelters and the number of archaeological sites decline. However, the human impact on the landscape is evident in some palaeoecological registers during the Bronze Age and involves the forest opening for grazing, probably using fire, on many occasions. The Middle Ages constitute the definitive transformation of the landscape and an organization of the use of the territory that has remained largely to present. A greater variety of land use, such as mining and metallurgy, added to livestock also with strong impact on forests. In the Sierra Nevada, the development of the irrigation ditch system by the Andalusians modified the hydrology to the present, with other implications on the landscape yet to be determined. Likely, the medieval imprint is still present in the current distribution of the species and the edaphic processes that occur in the high mountain. From the initial mosaic of incipient transformations to the forcefulness and diversity of the Middle Ages, the spatial projection and temporal continuity on how the landscape acculturation of the high-mountain took place still require a deeper exploration throughout the territory., [ES] La presencia inicial de los humanos en la alta montaña desde hace poco menos de 8000 años presenta más elementos de domesticación de lo que inicialmente se había estimado. Aunque la extensión geográfica de las exploraciones arqueológicas es todavía limitada, hay evidencias de uso de ganado doméstico y, en algunos casos, una probable agricultura utilizando mezcla de cereales a cotas intermedias. Por el momento, no se ha podido constatar impacto en el paisaje de esta actividad. En los Pirineos, hay una variación en la distribución de yacimientos ar-queológicos en la alta montaña asociada con el cambio climático de mitad del Holoceno. Decrece la ocupación de abrigos y el número de yacimientos en su conjunto. Sin embargo, el impacto humano sobre el paisaje es evidente durante la Edad del Bronce y supone la apertura de espacios para el pastoreo, probablemente, en muchas ocasiones utilizando fuego. La Edad Media supone la transformación definitiva del paisaje y una organización del uso del territorio que se ha mantenido en buena medida hasta la actualidad. Al uso ganadero se añade una mayor variedad de explotaciones, como la minería y metalurgia, que inciden también en una fuerte presión sobre los bos-ques. En Sierra Nevada, el desarrollo del sistema de acequias por los andalusíes condiciona la hidrología hasta la actualidad, seguramente con otras implicaciones sobre el paisaje todavía por especificar. Es probable que la huella medieval esté todavía presente en la distribución actual de las especies y en los procesos edáficos que se dan en la alta montaña. La proyección espacial y continuidad temporal sobre cómo se dio la culturización del paisaje de la alta montaña, desde el mosaico inicial de incipientes transformaciones hasta la contundencia y diversidad de la Edad Media, todavía requiere de una exploración más profunda a lo largo del territorio.

Los cambios en el clima durante el Holoceno (últimos 11700 años) han tenido importantes impactos en el territorio pirenaico y sus sistemas biofísicos. Particularmente significativos han sido los cambios en la disponibilidad hídrica y en la cubierta vegetal, así como en la dinámica de la criosfera y los procesos superfi ciales de erosión y transporte de sedimentos. A escala milenaria, el Holoceno presenta una estructura tripartita con una fase inicial con progresivo desarrollo del bosque e intensifi cación de los procesos superfi ciales de erosión y transporte hasta hace unos 8000 años, seguida de una con el máximo desarrollo del bosque en el piso montano hasta hace unos 4500 años, descenso de los procesos erosivos y menor disponibilidad hídrica y fi nalmente una relativamente húmeda hasta la actualidad. El impacto en el territorio de las fases más cálidas y secas durante la Anomalía Climática Medieval (900-1300 CE) y más frías y húmedas de la Pequeña Edad del Hielo (1400-1850 CE) está fuertemente modulado por la actividad antrópica. Los cambios recientes muestran las sinergias entre el Calentamiento Global y la Gran Aceleración. Durante el Holoceno (últimos 11700 años) se han producido importantes oscilaciones climáticas que han generado cambios de diferente intensidad y duración en la distribución de la vegetación, los procesos de erosión del suelo y generación de avenidas, la dinámica de los ecosistemas de montaña, los recursos hídricos e incluso de las actividades humanas en los Pirineos (García-Ruiz et al., 2015). En esta sección resumimos los principales impactos en la hidrosfera, criosfera, biosfera y en las sociedades humanas durante este periodo, basados en el estudio de depósitos glaciales, fl uviales, lacustres y de espeleotemas.

Altres ajuts: Spanish Ministry of the Environment project EGALA (OAPN-124/2010), Many phototrophic flagellates ingest prokaryotes. This mixotrophic trait becomes a critical aspect of the microbial loop in planktonic food webs because of the typical high abundance of these flagellates. Our knowledge of their selective feeding upon different groups of prokaryotes, particularly under field conditions, is still quite limited. In this study, we investigated the feeding behavior of three species (Rhodomonas sp., Cryptomonas ovata, and Dinobryon cylindricum) via their food vacuole content in field populations of a high mountain lake. We used the catalyzed reporter deposition-fluorescence in situ hybridization (CARD-FISH) protocol with probes specific for the domain Archaea and three groups of Eubacteria: Betaproteobacteria, Actinobacteria, and Cytophaga-Flavobacteria of Bacteroidetes. Our results provide field evidence that contrasting selective feeding exists between coexisting mixotrophic flagellates under the same environmental conditions and that some prokaryotic groups may be preferentially impacted by phagotrophic pressure in aquatic microbial food webs. In our study, Archaea were the preferred prey, chiefly in the case of Rhodomonas sp., which rarely fed on any other prokaryotic group. In general, prey selection did not relate to prey size among the grazed groups. However, Actinobacteria, which were clearly avoided, mostly showed a size of <0.5 μm, markedly smaller than cells from the other groups.

Winter conditions are rapidly changing in temperate ecosystems, particularly for those that experience periods of snow and ice cover. Relatively little is known of winter ecology in these systems, due to a historical research focus on summer 'growing seasons'. We executed the first global quantitative synthesis on under-ice lake ecology, including 36 abiotic and biotic variables from 42 research groups and 101 lakes, examining seasonal differences and connections as well as how seasonal differences vary with geophysical factors. Plankton were more abundant under ice than expected; mean winter values were 43.2% of summer values for chlorophyll a, 15.8% of summer phytoplankton biovolume and 25.3% of summer zooplankton density. Dissolved nitrogen concentrations were typically higher during winter, and these differences were exaggerated in smaller lakes. Lake size also influenced winter-summer patterns for dissolved organic carbon (DOC), with higher winter DOC in smaller lakes. At coarse levels of taxonomic aggregation, phytoplankton and zooplankton community composition showed few systematic differences between seasons, although literature suggests that seasonal differences are frequently lake-specific, species-specific, or occur at the level of functional group. Within the subset of lakes that had longer time series, winter influenced the subsequent summer for some nutrient variables and zooplankton biomass.

Sharp boundaries in the physical environment are usually associated with abrupt shifts in organism abundance, activity ,and diversity. Aquatic surface microlayers (SML) from a steep gradient between two contrasted environments, the atmosphere and surface waters, where they regulate the gas exchange between both environments. They usually harbor an abundant and active microbial life: the neuston. Few ecosystems are subjected to such a high UVR regime as high altitude lakes during summer. Here, we measured bulk estimates of heterotrophic activity, community structure and single-cell physiological properties by flow cytometry in 19 high-altitude remote Pyrenean lakes and compared the biological processes in the SML with those in the underlying surface waters. Phototrophic picoplankton (PPP) populations, were generally present in high abundances and in those lakes containing PPP populations with phycoerythrin (PE), total PPP abundance was higher at the SML .Heterotrophic nanoflagellates (HNF) were also more abundant in the SML. Bacteria in the SML had lower leucine incorporation rates, lower percentages of “live” cells, and higher numbers of highly-respiring cells ,likely resulting in a lower growth efficiency. No simple and direct linea rrelationships could be found between microbial abundances or activities and environmental variables, but factor analysis revealed that, despite their physical proximity ,microbial life in SML and underlyingwaters was governed by different and independent processes. Overall, we demonstrate that piconeuston in high altitude lakes has specific features different from those of the picoplankton ,and that they are highly affected by potential stressful environmental factors, such as high UVR radiation.

The ELF or fluorescence-labeled enzyme activity (FLEA) technique is a cultureindependent single-cell tool for assessing plankton enzyme activity in close-to-in situ conditions. We demonstrate that single-cell FLEA quantifications based on twodimensional (2D) image analysis were biased by up to one order of magnitude relative to deconvolved 3D. This was basically attributed to out-of-focus light, and partially to object size. Nevertheless, if sufficient cells were measured (25–40 cells), biases in individual 2D cell measurements were partially compensated, providing useful and comparable results to deconvolved 3D. We also discuss how much caution should be used when comparing the single-cell enzyme activities of different sized bacterio- and/or phytoplankton populations measured on 2D images. Finally, a novel method based on deconvolved 3D images (wide field restoration microscopy; WFR) was devised to improve the discrimination of similar single-cell enzyme activities, the comparison of enzyme activities between different size cells, the measurement of low fluorescence intensities, the quantification of less numerous species, and the combination of the FLEA technique with other single-cell methods. These improvements in cell enzyme activity measurements will provide a more precise picture of individual species’ behavior in nature, which is essential to understand their functional role and evolutionary history.

Remote lakes are usually unaffected by direct human infl uence, yet they receive inputs of atmospheric pollutants, dust, and other aerosols, both inorganic and organic. In remote, alpine lakes, these atmospheric inputs may infl uence the pool of dissolved organic matter, a critical constituent for the biogeochemical functioning of aquatic ecosystems. Here, to assess this infl uence, we evaluate factors related to aerosol deposition, climate, catchment properties, and microbial constituents in a global dataset of 86 alpine and polar lakes. We show signifi cant latitudinal trends in dissolved organic matter quantity and quality, and uncover new evidence that this geographic pattern is infl uenced by dust deposition, fl ux of incident ultraviolet radiation, and bacterial processing. Our results suggest that changes in land use and climate that result in increasing dust fl ux, ultraviolet radiation, and air temperature may act to shift the optical quality of dissolved organic matter in clear, alpine lakes.

[EN]High mountain lakes offer research opportunities beyond what could be expected from their quantitative relevance in the Earth system. In this article we present a brief summary of the research carried out in the lakes of the Pyrenees in the last twenty years by the group of limnology of the Centre for High Mountain Research (CRAM) of the University of Barcelona. The studies can be included in three main topics: life in extreme conditions, catchment-lake relationships and environmental changes., [ES]Los lagos de montaña ofrecen oportunidades de estudio que van mucho más allá de lo que su disposición en el territorio y abundancia pudieran sugerir. En este artículo presentamos un breve resumen de los temas que hemos abordado a lo largo de poco más de veinte años de investigación en los lagos de los Pirineos por parte del grupo de limnología del Centro de Investigaciones de Alta Montaña (CRAM) de la Universidad de Barcelona. Los estudios se enmarcan fundamentalmente en tres líneas generales: la vida en condiciones extremas, las relaciones cuenca-lago y los cambios ambientales.

We describe a catalyzed reported deposition-fluorescence in situ hybridization (CARD-FISH) protocol particularly suited to assess the phagotrophy of mixotrophic protists on prokaryotes, since it maintains cell and plastid integrity, avoids cell loss and egestion of prey, and allows visualization of labeled prey against plastid autofluorescence. This protocol, which includes steps such as Lugol’s-formaldehyde-thiosulfate fixation, agarose cell attachment, cell wall permeabilization with lysozyme plus achromopeptidase, and signal amplification with Alexa-Fluor 488, allowed us to detect almost 100% of planktonic prokaryotes (Bacteria and Archaea) and, for the first time, to show archaeal cells ingested by mixotrophic protists.

The ecosystem response of Lake Redó (Central Pyrenees) to fluctuations in seasonal air temperature during the last two centuries was investigated by comparison of reconstructed air temperatures with the sediment record. Fine slicing allowed a resolution of 3–6 years according to the 210Pb dating, although it was still difficult to easily investigate the response to air temperature forcing, since extreme fluctuations in temperature occur on interannual timescales. However, the resolution was sufficient to show responses on decadal and century scales. An overall tendency to warming in mean annual temperature in the Central Pyrenees has been caused by summer and in particular by autumn increases. Many of the measured sediment variables apparently responded to these long term trends, but the significance of the relationships was highly conditioned by the structure of the data. The variables responding most on the finer time scales were the microfossils. For diatoms, chironomids and chrysophytes the main variability correlated to summer and to autumn temperatures. For two planktonic species, Fragilaria nanana and Cyclotella pseudostelligera, we found a link of their variability with temperature fluctuations in their growing months (September and October, respectively). This relationship appeared at a certain point during a general warming trend, indicating a threshold in the response. On the other hand, no significant changes in the dominant species could be linked to temperature, nor in any significant subgroup of the 180 diatom species present in the core. In contrast, for most chironomids (particularly Paratanytarsus austriacus, Heterotrissocladius marcidus and Micropsectra radialis) a negative relationship with summer temperature extended throughout the studied period. This response of the whole group gives chironomids a more robust role as indicators for recording temperature changes on long time-scales (e.g., through the Holocene) and for lake signal inter-comparis