Deployment of ENEX enclosures in high‐mountain lake Redon (Spain)

The ENEX experiment in Lake Redon (Pyrenees) investigated the relationship between the changes in plankton stoichiometry, productivity, and community structure occurring during nutrient fluctuations in P‐limited conditions. Columnar enclosures were used for treatments varying P availability, N:P imbalance, and N source. C:N:P seston ratios were stable in P‐limited conditions, with loose coupling with productivity, nutrient supply ratios, and species dominance. The stoichiometric ratios shifted to Redfield proportions in P‐repleted conditions. The results suggest a complex regulation of P scarcity in planktonic communities that goes beyond immediate acclimation growth responses and might include alternative physiological and biogeochemical states

Episodic nutrient enrichments stabilise protists coexistence in planktonic oligotrophic conditions

1. Seasonal compositional changes in plankton communities are usually considered as species replacements. Provided the enormous number of individuals integrating the communities and our limited capacity for counting and determining a substantial proportion of them, it may happen that we would only be observing alternative population peaks of a large number of coexisting species. The contemporary coexistence theory addresses coexistence in communities of competing species considering simultaneously relative fitness inequalities and stabilising niche differences as components of average long-term growth rates. Here, we experimentally show that response patterns predicted by the theory occur when varying nutrient pulses fertilise the planktonic community. 2. We used gently self-filling 100 L enclosures to minimise 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 protist species growth response to a gradient of P enrichment and another of N stoichiometric imbalance. The P and N levels selected were within the oligotrophic seasonal and interannual variation of the lake, plus some extreme treatments providing mesotrophic conditions of remote regions affected by N atmospheric contamination. All treatments were replicated using ammonium or nitrate alternatively as N source. 3. Most protist species ¿ recorded across seasons in previous studies in this lake ¿ were recovered, indicating a persistent assemblage of species that is seasonally-hidden to observation. Recovery included some rare species only observed in the slush layers of the seasonal snow and ice cover. 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 species level according to the type of nitrogen supply, and resting stage development depending on nutrient conditions. Response similarities between autotrophic and heterotrophic organisms indicate a network of interactions that may reinforce coexistence. 4. Synthesis. The results indicate that many planktonic protist species in oligotrophic waters can show stable long-term non-equilibrium coexistence by alternately recovering from very low densities when episodic nutrient enrichments ¿ of varying P and N amounts and composition ¿ occur

Homeostasis and non-linear shift in the stoichiometry of P-limited planktonic communities

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 ( NH+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 to Redfield proportions in P‐repleted conditions suggest a complex regulation of P scarcity in planktonic communities that goes beyond immediate acclimation growth responses and might include alternative physiological and biogeochemical states

The Arctic in the twenty-first century: changing biogeochemical linkages across a paraglacial landscape of Greenland

The Kangerlussuaq area of southwest Greenland encompasses diverse ecological, geomorphic, and climate gradients that function over a range of spatial and temporal scales. Ecosystems range from the microbial communities on the ice sheet and moisture-stressed terrestrial vegetation (and their associated herbivores) to freshwater and oligosaline lakes. These ecosystems are linked by a dynamic glacio-fluvial-aeolian geomorphic system that transports water, geological material, organic carbon and nutrients from the glacier surface to adjacent terrestrial and aquatic systems. This paraglacial system is now subject to substantial change because of rapid regional warming since 2000. Here, we describe changes in the eco- and geomorphic systems at a range of timescales and explore rapid future change in the links that integrate these systems. We highlight the importance of cross-system subsidies at the landscape scale and, importantly, how these might change in the near future as the Arctic is expected to continue to warm

Using paleolimnology to find restoration solutions: the case of Lake Muzzano, Switzerland

Lake Muzzano (45°59′50″N 8°55′41″E, 337 m a.s.l.) is a hyper-eutrophied lake located in the Tessin region of Switzerland. Almost every year, algal blooms (Microcystis) cover the lake with a thickness of 1–2 cm. These blooms associated with periods of anoxia in summer have led to fish kills in 1967 and 1994. In the hope of avoiding these blooms, a bypass bringing water away from the lake has been established in 1999. This solution was not adequate as blooms kept reoccurring. Sediment removal was then proposed by the Tessin Canton as a possible remediation technique and The L.A.K.E.S Institute had a mandate in 2010 to study the lake (present and past state) to determine the reasons creating anoxia and algal blooms. The present state of the lake shows that anoxia is still occurring when the algal bloom covers the lake's surface. Subfossil diatom and chironomid analyses show that the baseline conditions were those found before 1922 AD when the lake was oligotrophic and supported a diversified community of chironomids suggesting good oxygenation. After 1922 AD, circulation to the lake was cut out and nutrients accumulated in the lake leading to anoxia and the establishment of Microcystis. Heavy metal analysis in the sediment shows that the concentration is above the national recommendation and thus sediment should not be removed or should be stored with hazardous material. Based on the present status of the lake and paleolimnological results, two solutions are proposed: to further decrease the nutrients coming in the lake (possibly using filtrating plants) followed by flushing to increase lake water circulation. Physical capping of the sediment to avoid exchange of heavy metals and phosphorus release at the water/sediment interface could also be envisaged once the two prime solutions are in place.Peer reviewedPeer Reviewe

Effects of episodic nutrients enrichments on P-limited planktonic communities: Lake Redon ENEX 2013 experiment

[Methods] Analytical methods are described in the associated publication: Giménez-Grau, P., Felip, M., Zufiaurre, A., Pla-Rabès, S., Camarero, L. & Catalan, J. (2020) Homeostasis and non-linear changes in the stoichiometry of P limited planktonic communities. Ecosphere,11:e03249 [Usage Notes] Initial reference conditions in the lake water column were sampled during three dates. They are reported separately, but they should be amalgamated if used as initial experimental conditions. There are a few missing values across the biogeochemical data. Data from the enclosure water column and the sediment traps are reported in separated files.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.Ministerio de Ciencia e Innovación, Award: CGL2010–19737. Ministerio de Ciencia e Innovación, Award: CGL2016–80124-C2-1-P. Generalitat de Catalunya, Award: 2017 SGR 910. Ministerio de Ciencia e Innovación, Award: FPI BES-2014-070196. Ministerio de Educación, Cultura y Deporte, Award: FPU AP2010-3596.Peer reviewe

Historical variations in lead fluxes in the Pyrenees (NE Spain) from a dated lake sediment core

Variations in Pb concentration in lake sediments reflect changes in the fluxes of this element in the past. A sediment core from a lake in the Pyrenees (Lake Redó at 2,240 m a.s.l., NE Spain) was studied, with the aim of reconstructing past environmental and climatic conditions in the lake and its catchment area. The core was dated using both 210Pb and 14C. A surface peak of Pb concentration, which was about 10 times higher than the background level, was found. This peak is attributed to mining activities since the beginning of this century. Although Pb pollution due to the combustion of gasoline is expected to be present, no evidence can be deduced from Pb isotope ratios of sediment due to the masking effect of Pb from mines. A second peak appeared in a deeper layer, with a maximum Pb concentration of about 17 times higher than the background level. The origin of this peak is unclear. All evidence suggests that the ancient peak is due to mining operations in the central Pyrenees area, which were not documented historically.Peer reviewe

Depositional and geochemical dynamics of Mediterranean Watershed-Lake Systems (WLS) during the Anthropocene: disentangling human and climate forcings in the Iberian Peninsula during the last millennium

In the context of sustainable socio-economic growth, water availability, soil conservation and land degradation are key factors in the Mediterranean regions of the Iberian Peninsula. Our knowledge of Mediterranean Watershed-Lake Systems (WLS) dynamics in the context of recent Global Warming and the Great Acceleration is hindered by the absence of integrated studies that include varied geographic contexts and long term time series. In the Mediterranean regions of the Iberian Peninsula disentangling climate and anthropic factors is more complex due to the long history of human impact. The forecasted intensification of the hydrological cycle (flood intensity and frequency) associated to global warming will likely lead to higher sediment mobilization and sediment delivery to the lakes, increase in carbon fluxes and bioproductivity and also in metal and other pollutant mobilization from the watersheds The MEDLANT project applies a multidisciplinary approach to understand environmental, paleohydrological and climate dynamics in WLS during the Anthropocene based on high resolution lake records for the last millennium. We use a transect of lake paleorecords from NE Iberian Peninsula to test these hypotheses by comparing recent changes with those occurred during other warmer periods - as the end of the Little Ice Age and during the Medieval Climate Anomaly - and also during the main phases of human impact (Roman, Medieval, late 19th- Early 20th century). Available data suggest that synergetic effects between climate and humans have intensified erosion, heavy metal mobilization and C storage in Mediterranean WLS. However, increase in extreme events caused by climate change and reforestation due to rural exodus have had opposite impacts in sediment delivery in recent times. Dynamic models for Mediterranean WLS will include their response during climate and anthropogenic disturbances and the complex synergetic effects obtained from paleolimnological records.Peer Reviewe

The Arctic in the Twenty-First Century: Changing Biogeochemical Linkages across a Paraglacial Landscape of Greenland.

The Kangerlussuaq area of southwest Greenland encompasses diverse ecological, geomorphic, and climate gradients that function over a range of spatial and temporal scales. Ecosystems range from the microbial communities on the ice sheet and moisture-stressed terrestrial vegetation (and their associated herbivores) to freshwater and oligosaline lakes. These ecosystems are linked by a dynamic glacio-fluvial-aeolian geomorphic system that transports water, geological material, organic carbon and nutrients from the glacier surface to adjacent terrestrial and aquatic systems. This paraglacial system is now subject to substantial change because of rapid regional warming since 2000. Here, we describe changes in the eco- and geomorphic systems at a range of timescales and explore rapid future change in the links that integrate these systems. We highlight the importance of cross-system subsidies at the landscape scale and, importantly, how these might change in the near future as the Arctic is expected to continue to warm