Fabio Lepori

Warming trends of perialpine lakes from homogenised time series of historical satellite and in-situ data

The availability of more than thirty years of historical satellite data is a valuable source which could be used as an alternative to the sparse in-situ data. We developed a new homogenised time series of daily day time Lake Surface Water Temperature (LSWT) over the last thirty years (1986-2015) at a spatial resolution of 1km from thirteen polar orbiting satellites. The new homogenisation procedure implemented in this study corrects for the different acquisition times of the satellites standardizing the derived LSWT to 12:00 UTC. In this study, we developed new time series of LSWT for five large lakes in Italy and evaluated the product with in-situ data from the respective lakes. Furthermore, we estimated the long-term annual and summer trends, the temporal coherence of mean LSWT between the lakes, and studied the intra-annual variations and long-term trends from the newly developed LSWT time series. We found a regional warming trend at a rate of 0.017°Cyr-1 annually and 0.032°Cyr-1 during summer. Mean annual and summer LSWT temporal patterns in these lakes were found to be highly coherent. Amidst the reported rapid warming of lakes globally, it is important to understand the long-term variations of surface temperature at a regional scale. This study contributes a new method to derive long-term accurate LSWT for lakes with sparse in-situ data thereby facilitating understanding of regional level changes in lakes surface temperature.

Thirty Years of Chemical Changes in Alpine Acid-Sensitive Lakes in the Alps

The subalpine and alpine areas in North-Western Italy and Southern Switzerland (Canton Ticino) receive high deposition of atmospheric pollutants transported from emission sources in the Po Valley. Long-term studies on high-altitude lakes in these areas indicate widespread recovery from acidification, even though most of them are still substantially affected, especially by deposition of nitrogen compounds. We analysed long-term trends of the major chemical compounds in a sample (n = 41) of high-altitude lakes, both at the site and regional levels, with the aim to assess the response of water chemistry to changes in atmospheric deposition and climate. These lakes have been studied since the early 1980s in the context of research programmes on acidification and atmospheric pollution. The significant decrease of sulfate and acidity in atmospheric deposition led to acidification recovery in the majority of the lakes. However, some lakes are still acidic or show a high sensitivity to acidification. This sensitivity is particularly evident at the snowmelt, when alkalinity is still fully depleted in some lakes. At present, nitrate is the dominant acidifying agent in the studied lakes, due to the high input of nitrogen compounds from atmospheric deposition. Our study also demonstrated that climatic factors interact with atmospheric deposition affecting the long-term changes in lake water.

Climatic effects on vertical mixing and deep-water oxygen content in the subalpine lakes in Italy

Deep lakes south of the Alps (DSL: Maggiore, Lugano, Como, Iseo and Garda) are characterised by varying trophic states and dissolved oxygen (DO) concentrations. Some of these lakes experience anoxic conditions in deep waters. We hypothesised that the increase in temperature and water-column stability observed in these lakes during recent decades influenced the deep-water DO concentration. In particular, we tested whether the thermal regime of the lakes and the depth of mixing affect oxygen replenishment during winter–spring turnover. To this aim, we analysed long-term trends and seasonal variability of oxygen levels in the DSL during 1992–2016. We included in our analysis the effects of environmental variables, such as winter air temperature and atmospheric modes of variability. Our results showed a recent decrease in the deep-water oxygen content in lakes Maggiore, Como and Garda and an increase of the extent of anoxic conditions in lakes Lugano and Iseo. Our results suggest that, beside cultural eutrophication, rising environmental pressures, such as global warming, can influence the future trends of the oxygen levels and ecological states of deep lakes.

Prokaryoplankton and phytoplankton community compositions in five large deep perialpine lakes

Phytoplankton (PCC) and prokaryoplankton community compositions were studied in five perialpine lakes (Garda, Maggiore, Como, Iseo, and Lugano) of different trophic conditions and mixing regimes, using inverted microscopy and CARD-FISH technique. The aim of this research was to assess, for the first time in these lakes, the relationships among PCC, bacteria, and archaea, and the effects of environmental drivers. We observed a clear difference of PCC compositions in spring and summer. All the lakes showed Bacillariophyta dominating in spring, whereas in summer, there were remarkable differences of PCC. Prokaryoplankton composition showed more pronounced differences in the vertical profile than those between spring and summer. The oligomictic lakes showed a uniform vertical gradient of prokaryotes in spring, while in the meromictic lakes, their abundances were incremented with depth. In summer, the prokaryotic community changed, and niche differentiation occurred in almost all lakes. In conclusion, our study showed a general pattern, common to all the lakes, of a first appearance of the large-sized “opportunistic” bacteria in spring, followed by ultramicrobacteria, less vulnerable to predation in summer. Significant correlations between a few PCC and bacterial groups were found, thus elucidating that functional interactions can be the key to understand plankton successions.

Improving estimates of primary production in lakes: a test and a case study from a peri-alpine lake (Lake Lugano)

Abstract Although primary production (i.e., the carbon fixed by autotrophs through photosynthesis) is assessed extensively by limnologists, its measurement presents methodological challenges. The objectives of this study were to (1) address some of these challenges by proposing an improvement to current mathematical models used to calculate cumulative (e.g., seasonal or annual) production rates from instantaneous (e.g., hourly) production values, and (2) explore the response of primary production to 3 decades (1983–2014) of nutrient management in Lake Lugano (Switzerland and Italy) as a case study. Objective 1 was prompted by difficulties in estimating the photosynthetic parameters Pbmax, the maximum photosynthetic potential, and Iopt, the optimum light intensity, in days without photoinhibition. We developed a model extension to simulate these parameters from irradiance and total phosphorus (TP). To this end, we adapted equations previously used in marine studies and calibrated and tested them against values of primary production measured using the carbon-14 (14C) light-and-dark bottle method during 1983–2014. A Nash-Sutcliffe efficiency of 0.53 and a ratio between the root mean square error and the standard deviation of the observed values (RSR) of 0.68 indicated a satisfactory performance of our model extension. Therefore, we suggest that our modelling approach presents a step toward a more precise quantitative estimate of primary production. Concerning objective 2, in Lake Lugano, annual rates of primary production (measured as C) declined by 21.26 mg m−2 d−1 from 1983 to 2014. The environmental drivers included TP in spring, summer, and autumn, and water temperature in winter. These results suggest that primary production in Lake Lugano was limited mainly by TP availability and therefore seemed to be responding to nutrient management.

Redox-dependent niche differentiation provides evidence for multiple bacterial sources of glycerol tetraether lipids in lakes

Significance Reliable prediction of future climate conditions requires a thorough understanding of climate variability throughout Earth’s history. Microbial molecular fossils, such as bacterial membrane-spanning tetraether lipids [branched glycerol dialkyl glycerol tetraethers (brGDGTs)], have proven to be particularly useful for the assessment of past climatic conditions, because they occur ubiquitously in the environment and show compositional changes related to temperature. However, the identity and ecology of brGDGT-producing bacteria is largely unknown, and a mechanistic basis for brGDGT-based paleoclimate reconstruction is still lacking. Here, we present insights into the ecological parameters that affect brGDGT synthesis in lakes, demonstrating that eutrophic lakes with oxygen-deprived bottom waters are the preferred sites for brGDGT-based reconstructions of continental climate. Terrestrial paleoclimate archives such as lake sediments are essential for our understanding of the continental climate system and for the modeling of future climate scenarios. However, quantitative proxies for the determination of paleotemperatures are sparse. The relative abundances of certain bacterial lipids, i.e., branched glycerol dialkyl glycerol tetraethers (brGDGTs), respond to changes in environmental temperature, and thus have great potential for climate reconstruction. Their application to lake deposits, however, is hampered by the lack of fundamental knowledge on the ecology of brGDGT-producing microbes in lakes. Here, we show that brGDGTs are synthesized by multiple groups of bacteria thriving under contrasting redox regimes in a deep meromictic Swiss lake (Lake Lugano). This niche partitioning is evidenced by highly distinct brGDGT inventories in oxic vs. anoxic water masses, and corresponding vertical patterns in bacterial 16S rRNA gene abundances, implying that sedimentary brGDGT records are affected by temperature-independent changes in the community composition of their microbial producers. Furthermore, the stable carbon isotope composition (δ13C) of brGDGTs in Lake Lugano and 34 other (peri-)Alpine lakes attests to the widespread heterotrophic incorporation of 13C-depleted, methane-derived biomass at the redox transition zone of mesotrophic to eutrophic lake systems. The brGDGTs produced under such hypoxic/methanotrophic conditions reflect near-bottom water temperatures, and are characterized by comparatively low δ13C values. Depending on climate zone and water depth, lake sediment archives predominated by deeper water/low-13C brGDGTs may provide more reliable records of climate variability than those where brGDGTs derive from terrestrial and/or aquatic sources with distinct temperature imprints.

Are swiss alpine lakes recovering from acidification

What are the effects on acid-sensitive ecosystems? Drawing from long-term monitoring data, we evaluated trends in the chemistry of 20 high-elevation lakes in Canton Ticino, Switzerland, an Alpine region that was severely impacted by acid deposition in the past. We report on the trends of the main acid anions (sulfate and nitrate), alkalinity, and pH in the lakes from 1986 to 2011, and compare these trends to changes in regional atmospheric deposition during the same period.