Marc Ventura

Seasonal ecosystem variability in remote mountain lakes: implications for detecting climatic signals in sediment records.

Weather variation and climate fluctuations are the main sources of ecosystem variability in remote mountain lakes. Here we describe the main patterns of seasonal variability in the ecosystems of nine lakes in Europe, and discuss the implications for recording climatic features in their sediments. Despite the diversity in latitude and size, the lakes showed a number of common features. They were ice-covered between 5–9 months, and all but one were dimictic. This particular lake was long and shallow, and wind action episodically mixed the water column throughout the ice-free period. All lakes showed characteristic oxygen depletion during the ice-covered-period, which was greater in the most productive lakes. Two types of lakes were distinguished according to the number of production peaks during the ice-free season. Lakes with longer summer stratification tended to have two productive periods: one at the onset of stratification, and the other during the autumn overturn. Lakes with shorter stratification had a single peak during the ice-free period. All lakes presented deep chlorophyll maxima during summer stratification, and subsurface chlorophyll maxima beneath the ice. Phosphorus limitation was common to all lakes, since nitrogen compounds were significantly more abundant than the requirements for the primary production observed. The major chemical components present in the lakes showed a short but extreme dilution during thawing. Certain lake features may favour the recording of particular climatic fluctuations, for instance: lakes with two distinct productive periods, climatic fluctuations in spring or autumn (e.g., through chrysophycean cysts); lakes with higher oxygen consumption, climatic factors affecting the duration of the ice-cover (e.g., through low-oxygen tolerant chironomids); lakes with higher water retention time; changes in atmospheric deposition (e.g., through carbon or pigment burial); lakes with longer stratification, air temperature changes during summer and autumn (e.g., through all epilimnetic species).

Lake Redó ecosystem response to an increasing warming in the Pyrennees during the twentieth century.

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 time-scales. 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-comparison. Finally, our results indicated that, in all cases, there was a significant resilience to high frequency changes and hysteresis despite extreme fluctuations. Although we were dealing with organisms with one or many generations per year, their populations seemed to follow the decadal trends in air temperature.

Tracing Carbon Sources through Aquatic and Terrestrial Food Webs Using Amino Acid Stable Isotope Fingerprinting

Tracing the origin of nutrients is a fundamental goal of food web research but methodological issues associated with current research techniques such as using stable isotope ratios of bulk tissue can lead to confounding results. We investigated whether naturally occurring δ13C patterns among amino acids (δ13CAA) could distinguish between multiple aquatic and terrestrial primary production sources. We found that δ13CAA patterns in contrast to bulk δ13C values distinguished between carbon derived from algae, seagrass, terrestrial plants, bacteria and fungi. Furthermore, we showed for two aquatic producers that their δ13CAA patterns were largely unaffected by different environmental conditions despite substantial shifts in bulk δ13C values. The potential of assessing the major carbon sources at the base of the food web was demonstrated for freshwater, pelagic, and estuarine consumers; consumer δ13C patterns of essential amino acids largely matched those of the dominant primary producers in each system. Since amino acids make up about half of organismal carbon, source diagnostic isotope fingerprints can be used as a new complementary approach to overcome some of the limitations of variable source bulk isotope values commonly encountered in estuarine areas and other complex environments with mixed aquatic and terrestrial inputs.

Reticulate evolution of the Daphnia pulex complex as revealed by nuclear markers

The study of species complexes is of particular interest to understand how evolutionary young species maintain genomic integrity. The Daphnia pulex complex has been intensively studied as it includes species that dominate freshwater environments in the Northern hemisphere and as it is the sole North American complex that shows transitions to obligate parthenogenesis. Past studies using mitochondrial markers have revealed the presence of 10 distinct lineages in the complex. This study is the first to examine genetic relationships among seven species of the complex at nuclear markers (nine microsatellite loci and one protein‐coding gene). Clones belonging to the seven species of the Daphnia pulex complex were characterized at the mitochondrial NADH dehydrogenase (ND5) gene and at the Lactate dehydrogenase (LDH) locus. K‐means, principal coordinate analyses and phylogenetic network analyses on the microsatellite data all separated European D. pulicaria, D. tenebrosa, North American D. pulex, D. pulicaria and their hybrids into distinct clusters. The hybrid cluster was composed of diploid and polyploid hybrids with D. pulex mitochondria and some clones with D. pulicaria mitochondria. By contrast, the phylogeny of the D. pulex complex using Rab4 was not well resolved but still showed clusters consisting mostly of D. pulex alleles and others of D. pulicaria alleles. Incomplete lineage sorting and hybridization may obscure genetic relationships at this locus. This study shows that hybridization and introgression have played an important role in the evolution of this complex.

TRENDS IN THE WATER CHEMISTRY OF HIGH ALTITUDE LAKES IN EUROPE

Here we present the chemical trends of seven high altitude lakes, analysed within the AL:PE and MOLAR Projects of the EU (1999) and selected on the basis of the availability of complete and reliable data for the period 1984–1999. The lakes are representative of the Scandinavian Alps, the Cairngorm Mountains in Scotland, the Alps and the Pyrenees. Significant trends were identified for some indicators of acidification, for instance pH and alkalinity, but not all lakes reacted similarly to decreasing depositions of sulphate and base cations. Differences in lake response are discussed in relation to recent variations of atmospheric deposition chemistry and associated changes in climatic conditions. Beside individual variations of the studied lakes, depending, among other things, on altitude and morphology, catchment characteristics and climate trends play a major role for the reaction of high altitude lakes on changes in atmospheric depositions.

Local and regional founder effects in lake zooplankton persist after thousands of years despite high dispersal potential

We reconstructed the genetic structure of a planktonic crustacean Daphnia longispina living in high mountain lakes and ponds in the Pyrenees to investigate whether it was shaped by persistent founder effects originating shortly after the last glacial maximum or by ongoing dispersal and effective migration (gene flow). We found that the genetic structure can largely be explained by a single colonization event following gradual deglaciation of the Pyrenees ~10 000–15 000 years ago. Nuclear genetic diversity declined steeply from southeast to northwest, suggestive of serial colonization of available habitats with advancing deglaciation. The spatial genetic structure suggests that founder effects were major determinants of the present‐day diversity, both at the catchment level and at the level of individual water bodies, further supporting extremely low effective migration rates. This study reveals a prime example of a founder effect that is both long lasting and maintained at small spatial scales. Our data suggest a process of isolation by colonization as a result of strong priority effects and monopolization. We found evidence for the spread of haplotypes with Pyrenean ancestry across the Palaearctic over distances up to 5500 km, although the local genetic structure after colonization was hardly influenced by contemporary dispersal. Finally, our data also suggest that mitochondrial mutation rates in the studied populations were seven times higher than typically assumed. Overall, we show that founder effects can persist for centuries even at small spatial scales at which the potential for dispersal is high.

Distribution of polycyclic aromatic hydrocarbons in the food web of a high mountain lake, Pyrenees, Catalonia, Spain

We investigated the contents of polycyclic aromatic hydrocarbons (PAHs) in the food web organisms included in the diet of brown trout from a remote mountain lake. The preferential habitat and trophic level of the component species have been assessed from the signature of stable isotopes (delta13C and delta15N). Subsequently, the patterns of accumulation and transformation of these hydrocarbons in the food chain have been elucidated. Most of the food web organisms exhibit PAH distributions largely dominated by phenanthrene, which agrees with its predominance in atmospheric deposition, water, and suspended particles. Total PAH levels are higher in the organisms from the littoral habitat than from the deep sediments or the pelagic water column. However, organisms from deep sediments exhibit higher proportions of higher molecular weight PAH than those in other lake areas. Distinct organisms exhibit specific features in their relative PAH composition that point to different capacities for uptake and metabolic degradation. Brown trout show an elevated capacity for metabolic degradation because they have lower PAH concentrations than food and they are enriched strongly in lower molecular weight compounds. The PAH levels in trout highly depend on organisms living in the littoral areas. Fish exposure to PAH, therefore, may vary from lake to lake according to the relative contribution of littoral organisms to their diet.

Diversity in the Reproductive Modes of European Daphnia pulicaria Deviates from the Geographical Parthenogenesis

Background Multiple transitions to obligate parthenogenesis have occurred in the Daphnia pulex complex in North America. These newly formed asexual lineages are differentially distributed being found predominantly at high latitudes. This conforms to the rule of geographical parthenogenesis postulating prevalence of asexuals at high latitudes and altitudes. While the reproductive mode of high-latitude populations is relatively well studied, little is known about the reproduction mode in high altitudes. This study aimed to assess the reproductive mode of Daphnia pulicaria, a species of the D. pulex complex, from high altitude lakes in Europe. Methodology/Principal Findings Variation at eight microsatellite loci revealed that D. pulicaria from the High Tatra Mountains (HTM) had low genotype richness and showed excess of heterozygotes and significant deviations from Hardy-Weinberg expectations, and was thus congruent with reproduction by obligate parthenogenesis. By contrast, populations from the Pyrenees (Pyr) were generally in Hardy-Weinberg equilibrium and had higher genotypic richness, suggesting that they are cyclic parthenogens. Four lakes from lowland areas (LLaP) had populations with an uncertain or mixed breeding mode. All D. pulicaria had mtDNA ND5 haplotypes of the European D. pulicaria lineage. Pyr were distinct from LLaP and HTM at the ND5 gene. By contrast, HTM shared two haplotypes with LLaP and one with Pyr. Principal Coordinate Analysis of the microsatellite data revealed clear genetic differentiation into three groups. HTM isolates were intermediate to Pyr and LLaP, congruent with a hybrid origin. Conclusion/Significance Inferred transitions to obligate parthenogenesis have occurred only in HTM, most likely as a result of hybridizations. In contrast to North American populations, these transitions do not appear to involve meiosis suppressor genes and have not been accompanied by polyploidy. The absence of obligate parthenogenesis in Pyr, an environment highly similar to the HTM, may be due to the lack of opportunities for hybridization.

DEPOSITION AND STORAGE OF SPHEROIDAL CARBONACEOUS FLY-ASH PARTICLES IN EUROPEAN MOUNTAIN LAKE SEDIMENTS AND CATCHMENT SOILS

Spheroidal carbonaceous particles(SCPs) are produced only from high temperaturecombustion of fossil-fuels. In mountain lakesystems, they provide an unambiguous indicator ofatmospheric deposition. In order to comparedepositional fluxes of SCPs between mountainareas experiencing various pollutant regimes,intensive bulk deposition sampling was undertakenat five sites across Europe. Catchment soil coresand lake sediment cores were also taken at eachsite to compare SCP storage over the post-industrial period. Atmospheric, sediment and soilSCP data showed similar patterns. Highestcontamination was found in Scotland, Slovakia andSpain with the Austrian site intermediate and themid-Norwegian site least contaminated. A highproportion of accumulated SCPs were found to bestored in catchment soils at each site.Therefore, a significant increase in soilerosion, possibly as a result of future climatechange, could lead to the input of largequantities of catchment stored SCPs and, byimplication, other atmospherically depositedcontaminants to the lake ecosystem.

Application of static critical load models for acidity to high mountain lakes in Europe

Critical load models for acidityprovide a measure of the sensitivity of surfacewaters to acid deposition, and can be used todetermine critical load exceedance and potentiallong-term harmful effects. Three static models,the Steady-State Water Chemistry model, diatommodel and First-order Acidity Balance model, arehere applied to 11 high mountain lakes in Norway,Scotland, the Alps, the Pyrenees and the Tatras.Between five and seven of the lakes show criticalload exceedance, depending on the model used.Nitrogen as well as sulphur deposition isimportant in causing exceedance. Since soil andvegetation cover are generally sparse, geologyand lake retention time appear to be key factorsin the determination of critical load. Retentionof nitrogen is observed, but it is unclearwhether this occurs within the lake or theterrestrial part of the catchment.