Rosario Mosello

Trophic development of the deep lakes south of the Alps: a comparative analysis

This paper reports the results of long-term research carried out in the deep southern subalpine lakes (DSL: Maggiore, Lugano, Como, Iseo and Garda). The measurements performed since the 1970s at the time of the maximum overturn documented an increase of water temperature around 0.012-0.028 °C yc -1 , closely resembling the warming rate found in other lakes in Europe and North America. The DSL showed an increase of total phosphorus after the 1960s and a complete recovery after the 1990s of Lake Maggiore, which reached mean values of TP close to those of the pristine conditions (<10μgPl -1 Present TP concentrations in the other lakes are above 100 μg P l -1 (Lugano), 30-40 μg P l -1 (Como), 50-60 μg P l -1 (Iseo) and c. 20 μg P l -1 (Garda), with only Lake Garda showing a clear tendency to increasing P-concentrations in recent years. The higher trophic status of lakes Lugano and Iseo has favoured hypolimnetic anoxia due to meromixis since the 1960s and the 1990s, respectively, from which there was a partial recovery in 2005. Monthly samplings carried out in the last decades showed a strong coupling between long-term variation in algal nutrients and phytoplankton. On a shorter temporal scale, a significant relationship between the extent of water mixing during the spring months and the replenishment of nutrients and algal biomass in the growing season has also been verified in lakes Iseo, Garda and Lugano.

Phytoplankton as an Indicator of the Water Quality of the Deep Lakes South of the Alps

This paper offers a synoptic account of studies on the phytoplankton communities in the deep southern subalpine lakes (DSL) Garda, Iseo, Como, Lugano and Maggiore. The main cause of the degradation of the water quality in the DSL is eutrophication. The euphotic layers of these lakes are trophically different, ranging from the oligo-mesotrophy of lakes Maggiore and Garda to the meso-eutrophy of lakes Iseo and Lugano. The trophic status as estimated by using total phosphorus and chlorophyll a has provided consistent results in agreement with the models proposed by OECD (1982. Eutrophication of Waters. Monitoring, Assessment and Control, OECD, Paris). Though related with chlorophyll a and TP, the Secchi disk depths have significantly underestimated the trophic status of the DSL. Two trophic indices using the algal orders (PTIorders) and species (PTIspecies) were drawn up on the basis of the distribution of phytoplankton along a trophic gradient defined by the application of multivariate methods; the scores emerging from these indices were used to make a definitive ecological classification of water bodies on a scale from 1 to 5, in accordance with the Water Framework Directive. A third index (PTIOE) was computed as the ratio between the annual mean values of the cumulative biovolumes of two groups of algal orders with opposite trophic characteristics. The three PTI indices were highly correlated, providing a consistent classification of the water bodies. The indices proposed in this work were specifically adopted for use in the DSL. However, the criteria for their implementation constitute a robust and impartial tool for assessing similar indices in other lake typologies and for evaluating the degree of specificity of the trophic indicator values assigned to the single phytoplankton orders and species.

The Effect of Climate Warming on the Hydrochemistry of Alpine Lakes

The hydrochemistry of mountain lakes is highly conditioned by the chemicalcomposition of atmospheric deposition and by climate characteristics. Consequently these ecosystems have proved to be sensitive to long-term changes in both factors. Climate warming seems to be particularly pronounced in the Alpine region. A reduction of snow cover in space and time, due to less precipitation and higher temperatures, means a greater exposure of rocks and soils in the watersheds, which enhances weathering processes. In this paper we aim to evaluate the possible effect of these processes on long-term changes in the chemistry of alpine lakes. Recent climate changes affecting the study area were investigated through a data series referring to temperature, precipitation, snow depth and duration at some stations in the Ossola Valley. Chemical data of 35 lakes located in the Ossola and Sesia Valleys (Central Alps) were used. Lakes were sampled both in the late summer of 2000 and 2001 in the framework of two European Projects and the results compared with previous data (1984–1987). Two lakes (Boden Superiore and Inferiore, 2343 and 2334 m a.s.l., respectively), located in the northern part of the study area, have been sampled more or less continuously since the late 70s, enabling us to evaluate the trends of the main chemical variables. For lakes lying in catchments with highly soluble rocks, a comparison between the two data sets shows an increase of solute contents in the last few years. This result could be attributed to increased weathering rates due to climate warming.

Factors affecting water chemistry of alpine lakes

During a four-year study (1988–1991), 413 lakes in the Central Alps (Italy, Switzerland and Austria) were investigated to quantify their acidification. The ionic content of the lakes was generally low: 68% of them had alkalinity values of less than 200 µeq 1−1 and were regarded as sensitive to acidification. Moreover, 36% of the lakes showed alkalinity values of less than 50 µeq 1−1. Redundancy Analysis was used to relate the hydrochemistry of 187 lakes to their catchment characteristics. Calcite weathering was the main factor influencing lake chemistry. The same analysis, applied to a subset of 101 lakes lying in watersheds exclusively composed of silicic rocks, showed that lake chemistry was influenced by silicate weathering and nitrogen uptake. These processes were found to be mainly related to lake altitude and the fraction of the watershed not covered by vegetation, i.e. controlled by temperature. The importance of these relations to explain the pH shift produced by climatic variation is also discussed.

Bulk and wet atmospheric deposition chemistry at pallanza (N. Italy)

The chemistry of wet only and bulk depositions collected weekly at Pallanza from May, 1984 to December, 1986 is compared and discussed in relation to the composition of dust fall. The comparison shows a very good agreement between pH values (volume weighted mean values of 4.35 and 4.36, respectively, for wet only and bulk samples) and differences between 6 and 12% for ammonium, sulphate, nitrate, and Na. For Ca, Mg, and K the difference range is 30 to 50%. The amount of precipitation over Pallanza (mean value 1951–1985,1709 mm), together with the solute concentrations in atmospheric deposition, mean that there is high bulk deposition of acidifing substances. The values found during the study period were 76, 93, 143, and 81 meq m2 yr−1 for H+, ammonium, sulphate, and nitrate, respectively.

Chemistry and trophic evolution of Lake Lugano in relation to nutrient budget

Chemical studies performed in the last 50 years on the northern and southern basins of Lago di Lugano are reviewed and discussed in relation to the phosphorus and nitrogen loads from the drainage area.Both basins became eutrophic in the fifties as a consequence of the dense population and industrial activity in the drainage area; the effect of these was aggravated by the meromictic condition of the layer below 100 m in the northern basin.The reduction of phosphorus loads in progress since 1976 has resulted in a significant improvement in the chemical and biological condition of the southern basin and the upper 100 m of water in the northern basin. On the other hand, the balance between dissolved oxygen and reducing substances in the northern basin is shifting toward a decrease of the net oxygen concentration, partly as a consequence of the unusually dry conditions of the last three years.A reduction in phosphorus load down to 18 and 22 t a−1 in the northern and southern basins respectively is still necessary in order to reduce the lake phosphorus concentration to 30–35 µg O l−1.

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.

Precision and linearity of inorganic analyses by ion chromatography

Abstract The repeatability of the measurements of peak areas for calibration solutions and the precision of anion and cation determinations (3–600 μM) in freshwater are discussed on the basis of 2 years of measurements on calibration solutions and stabilized internal standards. Anion measurements show higher repeatability of the measurements of peak areas for calibration solutions (R.S.D. 2–5%) and precision (R.S.D. 2–8%) than those of cations (R.S.D. 2–10% and 2–15%, respectively). Results for the calibration technique show that multi-point (6–8 concentrations), quadratic or cubic regressions permit a correct quantification over a wide range (1.5–2 orders of magnitude) of concentrations. Thanks to the repeatability of the measurements of peak areas for calibration solutions, only two calibrations, at the beginning and end of a batch of 20–30 samples, are adequate. These conditions give better results than calibrations performed with 2–3 points and repeated every 8–10 samples.

Precision of ion chromatographic analyses compared with that of other analytical techniques through intercomparison exercises

Abstract Three intercomparison exercises on simulated rainwater were held in 1991–1993 involving 72–98 laboratories in Europe and South America. Ion chromatography was used for the determination of anions (chloride, nitrate and sulphate) by 59–72% of the participating laboratories and for the determination of cations (Na, K, Mg and Ca) by 14–22% of them. The concentration of the single ions ranged between 5 and 150 μmol l−1. The results were used to evaluate the precision of the method, and showed that it was comparable to that of spectrophotometric methods. For ammonium ion, ion chromatography was used by only 4–14% of the laboratories and the results depended on the calibration technique adopted. A general improvement in precision was observed in the course of the exercises.

Response of planktonic communities to calcium hydroxide addition in a hardwater eutrophic lake: results from a mesocosm experiment

The main focus of this study was to investigate the effects of lime treatment with Ca(OH)2 on a hypereutrophic hardwater lake (Lake Alserio, Northern Italy), focusing on its impact on plankton communities. We performed a mesocosm experiment using two large enclosures, one for treatment and one control, in the lake. The addition of Ca(OH)2 proved to be effective in reducing phosphorus concentration, but the effect was limited in time. Redissolution of the precipitating calcium carbonate as it came into contact with the deep water determined a phosphorus increase about 2 weeks after the liming. The liming had a strong impact on the plankton assemblage, as was demonstrated by the sharp decrease in phytoplankton and zooplankton biomass and density, and 1 month after the treatment we observed a different taxonomic composition inside the enclosure from that of the lake. The result of the experiment showed that liming with Ca(OH)2 can be regarded as a suitable treatment for accelerating recovery of the lake. Multiple treatments will be necessary to improve the trophic level over a longer period.