Combined Earth observations reveal the sequence of conditions leading to a large algal bloom in Lake Geneva.

Freshwater algae exhibit complex dynamics, particularly in meso-oligotrophic lakes with sudden and dramatic increases in algal biomass following long periods of low background concentration. While the fundamental prerequisites for algal blooms, namely light and nutrient availability, are well-known, their specific causation involves an intricate chain of conditions. Here we examine a recent massive Uroglena bloom in Lake Geneva (Switzerland/France). We show that a certain sequence of meteorological conditions triggered this specific algal bloom event: heavy rainfall promoting excessive organic matter and nutrients loading, followed by wind-induced coastal upwelling, and a prolonged period of warm, calm weather. The combination of satellite remote sensing, in-situ measurements, ad-hoc biogeochemical analyses, and three-dimensional modeling proved invaluable in unraveling the complex dynamics of algal blooms highlighting the substantial role of littoral-pelagic connectivities in large low-nutrient lakes. These findings underscore the advantages of state-of-the-art multidisciplinary approaches for an improved understanding of dynamic systems as a whole.

Douglas-fir plantations impact stream and groundwater chemistry in western Europe: Insights from three case studies in France and Luxembourg.

In rural areas, nitrate concentrations in surface waters most often originate from the leaching of excess N fertilizer in agricultural lands, whereas forested catchments often have good water quality. However, Douglas-fir plantations may induce nitrogen cycle unbalances which may lead to an excess of nitrate production in the soil. We hypothesize that the excess of production of nitrate in the soil and nitrate leaching to streamwater is greater in catchments planted with Douglas fir. We used paired catchments in both France and Luxembourg with different land covers (Douglas-fir, Spruce, Deciduous, Grassland and clearcut) which were monitored over a 3-5 year period in order to assess the effect of Douglas-fir plantations on the chemical composition of surface water. Nitrate concentration in the soil and groundwater were also monitored. The results show that nitrate concentrations in streams draining Douglas-fir catchments were two to ten times higher than in streams draining other land covers, but were similar to the clearcut catchment. Nitrate concentrations under Douglas-fir in groundwater (up to 50 mg L-1) and in the soil were also higher than under all other land covers. Soil nitrate concentration was related to stream nitrate concentration. This suggests that soil processes, through excessive nitrate production under Douglas-fir, are driving the nitrate concentration in the stream water and our hypothesis of a transfer of a fairly large proportion of this excessive production from the soil to the stream is supported. This study also shows that nitrate concentrations in surface and ground waters in rural areas could also originate from Douglas fir forested catchments. The impact of Douglas-fir is nevertheless reduced downstream through a dilution effect: mixing tree species at the catchment scale could thus be a solution to mitigate the effect of Douglas-fir on nitrate concentration in surface waters.

Long-term influence of early human occupations on current forests of the Guiana Shield.

To decipher the long-term influences of pre-Columbian land occupations on contemporary forest structure, diversity, and functioning in Amazonia, most of the previous research focused on the alluvial plains of the major rivers of the Amazon basin. Terra firme, that is, nonflooded forests, particularly from the Guiana Shield, are yet to be explored. In this study, we aim to give new insights into the subtle traces of pre-Columbian influences on present-day forests given the archaeological context of terra firme forests of the Guiana Shield. Following archaeological prospects on 13 sites in French Guiana, we carried out forest inventories inside and outside archaeological sites and assessed the potential pre-Columbian use of the sampled tree species using an original ethnobotanical database of the Guiana Shield region. Aboveground biomass (320 and 380 T/ha, respectively), basal area (25-30 and 30-35 m2 /ha, respectively), and tree density (550 and 700 stem/ha, respectively) were all significantly lower on anthropized plots (As) than on nonanthropized plots (NAs). Ancient human presence shaped the species composition of the sampled forests with Arecaceae, Burseraceae, and Lauraceae significantly more frequent in As and Annonaceae and Lecythidaceae more frequent in NAs. Although alpha diversity was not different between As and NAs, the presence of pre-Columbian sites enhances significantly the forest beta diversity at the landscape level. Finally, trees with edible fruits are positively associated with pre-Columbian sites, whereas trees used for construction or for their bark are negatively associated with pre-Columbian sites. Half a millennium after their abandonment, former occupied places from the inner Guiana Shield still bear noticeable differences with nonanthropized places. Considering the lack of data concerning archaeology of terra firme Amazonian forests, our results suggest that pre-Columbian influences on the structure (lower current biomass), diversity (higher beta diversity), and composition (linked to the past human tree uses) of current Amazonian forests might be more important than previously thought.

Magnesium Isotope Variations to Trace Liming Input to Terrestrial Ecosystems: A Case Study in the Vosges Mountains.

Liming with Ca and Mg carbonates is commonly used to reduce soil and stream acidity and to improve vegetation growth and nutrition in forests. Ten years ago, dolomite lime was experimentally applied to a forest catchment on granite in the Vosges Mountains (northeast France), which is characterized by acid soils and drained by an acid stream. The average Mg isotope composition of the dolomite lime (-1.75‰) was low compared with that of tree foliage (-0.70‰), granite and deep soil layers (-0.40‰), and stream water (-0.80‰) in the control catchment. After liming, the exchangeable Mg concentrations in surface soil layers, which were initially very low, increased, and the Mg isotope composition decreased (up to -0.60‰). The decrease was smaller in deeper layers but not in proportion to the increase in exchangeable Mg content, suggesting contributions from mineralization of organic matter and/or displacement of exchangeable Mg from surface layers. Before application, Mg concentration in beech and fir leaves was low, and that of 1-yr-old fir needles was lower than that in current needles. Internal Mg translocation within fir needles also resulted in a lower δMg of older needles. Three years after dolomite application, the Mg isotope composition of plant leaves was lower than that in the control catchment; this decrease (up to -1.00‰) was attributed to direct uptake of Mg from dissolving dolomite. Liming doubled the concentration of Mg in the stream, whereas the Mg isotope composition decreased correspondingly from -0.80 to -1.20‰, indicating a fast transfer of dolomite Mg to the stream. Our findings indicate that monitoring of δMg may be a promising tool to study the fate of dolomitic inputs in terrestrial and aquatic ecosystems.