Suchergebnisse

8 páginas, 1 figura. ; Aim Recent papers have used large palaeolimnological datasets to reveal the biodiversity patterns of aquatic microorganisms. However, scant attention has been paid to the influence of time on these patterns. Where lake surficial sediment samples are used as integrals of diversity, the time interval of each sample varies according to differences in sediment accumulation rates. This paper aims to test the reliability of using lake surface sediments to measure and to compare microbial diversity when the potential influences of the species–time relationships are taken into account. Location Alpine lakes in Europe. Methods We analysed microorganism (siliceous microalgae) assemblages in three European Alpine lakes using short sediment cores (210Pb-dated) and annual sediment trap samples from 12 UK lakes. The same number of individuals was pooled for each sample 500 times to avoid sampling effort effects and to standardize species diversity estimation. The influence of time on the diversity score was assessed by simulating an increase of time span for surface sediment samples by cumulatively adding in successive sediment core samples (from the most recent to the oldest). We used species richness (S) and the exponential of the bias-corrected Shannon entropy index (exp(Hb-c)) to estimate diversity. Results Increasing the time interval represented by a surficial sediment sample did not affect the diversity results. The estimation of diversity was similar for cumulative and non-cumulative samples. Diversity estimation was only altered in lakes experiencing high community turnover due to strong environmental forcing during the time period spanned by the cumulative sample. Main conclusions The use of surface lake sediments is suitable for estimating the average site diversity of free-living microorganisms. Diversity is integrated in a single sample and species assemblage composition is derived from microbial communities living in distinct lake microhabitats. Species remains, accumulated in a single sample over several years of environmental variability, represent a diversity integral that captures a spatio-temporal component equivalent to the γ-diversity measure. ; This research was supported by an I3P grant (2007–2010) from the Spanish government to Sergi Pla-Rabes ; Peer reviewed

Este artículo contiene 18 páginas, 9 figuras. ; 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 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 research was funded by research grants of the Spanish Government NitroPir (CGL2010–19737) and Transfer (CGL2016–80124-C2-1-P) and the Catalan Government GECA (2017 SGR 910). P.G-G (FPU AP2010-3596) and A.Z. (FPI BES-2014-070196) acknowledge their respective predoctoral scholarships ; Peer reviewed

Barton Peninsula is an ice-free area located in the southwest corner of King George Island (South Shetland Islands, Antarctica). Following the Last Glacial Maximum, several geomorphological features developed in newly exposed ice-free terrain and their distribution provide insights about past environmental evolution of the area. Three moraine systems are indicative of three main glacial phases within the long-term glacial retreat, which also favoured the development of numerous lakes. Five of these lakes were cored to understand in greater detail the pattern of deglaciation through the study of lacustrine records. Radiocarbon dates from basal lacustrine sediments enabled the reconstruction of the chronology of Holocene glacial retreat. Tephra layers present in lake sediments provided additional independent age constraints on environmental changes based on geochemical and geochronological correlation with Deception Island-derived tephra. Shrinking of the Collins Glacier exposed the southern coastal fringe of Barton Peninsula at 8 cal ky BP. After a period of relative stability during the mid-Holocene, the ice cap started retreating northwards after 3.7 cal ky BP, confining some glaciers within valleys as shown by moraine systems. Lake sediments confirm a period of relative glacial stability during the last 2.4 cal ky BP. © Cambridge University Press 2019. ; The authors are grateful to the Portuguese Polar Program (PROPOLAR), the Korea Polar Research Institute (KOPRI) and the Chilean Antarctic Institute (INACH) for providing logistic support for the fieldwork. Marc Oliva is supported by the Ramón y Cajal Program (RYC-2015-17597) and the Research Group ANTALP (Antarctic, Arctic, Alpine Environments; 2017-SGR-1102) funded by the Government of Catalonia through the AGAUR agency. The work complements the research topics examined in the PALEOGREEN (CTM2017-87976-P) and CRONOANTAR (CTM2016-77878-P) projects of the Ministry of Economy, Industry and Competitiveness, Spain, and the projects Analysis of nunataks of the Antarctic Peninsula as multiproxy data sources on environmental change and climate dynamics – NUNANTAR (02/SAICT/2017 – 32002) and Holocene environments in Barton Peninsula (South Shetland Islands, Antarctica) funded by the Fundação para a Ciência e a Tecnologia, Portugal. ; Peer reviewed