(formerly Memorie dell'Istituto Italiano di Idrobiologia)

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CONTENTS
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Dear Reader
The journal Memorie dell'Istituto Italiano di Idrobiologia has been one of the most important historical publication in the field of Limnology. Published since 1942 in one yearly issue, the 57 volumes contain more than 700 papers on fundamental research in limnology and hydrobiology, contributing to the development of freshwater sciences.
While it has been open to important papers from external contributors, the journal has primarily collected the most important results of the research activity of the Istituto Italiano di Idrobiologia, one of the historic Institutes of Limnology in Europe. Monographs, Proceedings of International Symposia and Congresses have also been given due attention.
We have now decided to refresh the journal by making the style more dynamic and by changing the editorial policy to include external contributions of peer-reviewed original papers.
The title of the new edition of the journal will be Journal of Limnology. Each year volume will comprise two issues published every six months, with occasional publication of supplementary issues (Monographs, Proceedings, etc.). The paper edition will be supported by the Web edition, with the full text available on-line at the address: http://www.iii.to.cnr.it.
I hope that scientists will consider our journal a good opportunity to disseminate the results of their research to the more than 600 Public Libraries, Universities and Scientific Institutions all over the world to which the paper edition is mailed. The Web will maximize the diffusion of the journal to the whole scientific world.

Dr. R. de Bernardi
Editor in Chief

Deep water warming in lakes: an indicator of climatic change.
W. Ambrosetti and L. Barbanti.

The deep waters of Lake Maggiore, Italy (z_max =370 m) contain a sort of "climatic memory", represented by variations in the caloric content, which has been located in time and space, and its energetic value calculated on the basis of the depth of the winter mixing, the heat exchange in deep waters and the K_z-N² correlation's during the summer stratification. This analysis, relating to the period 1963-1998, demonstrates that the caloric content in the hypolimnion at the end of the limnological year, i.e. when deep waters are formed, depends strictly on winter meteorological parameters (e.g., wind run, air temperature and solar radiation), as well as on the quantity of heat that can reach the deep layers before and after the onset of thermal stratification. The amount of heat that can reach the deep hypolimnion is around 2-4% of the total heat budget of the lake; while its percentage is small, this quantity of heat can assume considerable importance, and lead to a progressive increase in the heat content of deep waters. The same percentage has been found in the other deep Italian lakes in the sudalpine region, i.e. lakes Como, Iseo, Orta and Garda (z_max between 143 and 410 m). All the lakes also have the same temporal evolution of the climatic memory: both the increasing overall trend and the variations (both positive and negative) occur simultaneously. The pattern we observed is quite similar to that found in historical series of deep water temperatures in lakes in Central Italy (L. Bolsena), North of the Alps, equatorial Africa (L. Victoria) as well as in the Mediterranean below a depth of 2000 m and in the Greenland Sea at depths between 200 and 2000 m. Such a world-wide distribution of the phenomenon suggests that it may be regarded as part of an ongoing process of global climate change.
Key words: deep lakes, vertical mixing, long-term trend of heat content, climatic memory

The trophic evolution of Lake Iseo as related to its holomixis.
L. Garibaldi, V. Mezzanotte, M.C. Brizzio, M. Rogora and R. Mosello.

Lake Iseo is undergoing a progressive deterioration in water quality, mainly due to eutrophication. This paper, based on measurements performed on the main tributaries and the outflow, considers the phosphorus, nitrogen and silica budget of the lake, related to its trophic evolution. The chief conclusion of the study is that the significant reduction in P load over the last few years following the construction of treatment plants is not enough to reverse the trend to eutrophication and the progressive deterioration in water quality. An attempt to use the OECD statistical approach to quantify the relationships between P load and the trophic condition of the lake yielded poor results, because the characteristics of the lake did not fulfil the hypothesis of the model. The reduced tendency to a complete overturn of the water is one of the most important aspects, leading to hypolimnion anoxia and P release from the sediments.
Key words: Iseo, water chemistry, trophic level, nutrient budget

Distribution of native and exotic branchiobdellidans (Annelida: Clitellata) on their respective crayfish hosts in northern Italy, with the first record of native Branchiobdella species on an exotic North American crayfish.
S.R. Gelder, G.B. Delmastro and J.N. Rayburn.

Branchiobdellidans were found on crayfish at the 18 sites examined in northern Italy. At 15 of the sites the native crayfish, Austropotamobius pallipes, was collected and all supported specimens of Branchiobdella italica. A few specimens of B. parasita were found at only two of these sites and no specimens of B. hexodonta were observed during the survey. The North American Louisiana red swamp crayfish, Procambarus (Scapulicambarus) clarkii has been found in three additional sites around Carmagnola, south of Torino. The exotic Cambarincola mesochoreus was recovered from red crayfish at two sites. However, no specimens of C. mesochoreus were found at the third site, but specimens B. italica and B. parasita were identified. This is the first record of native Branchiobdella spp. on an exotic and wild crayfish in northern Italy.
Key words: Branchiobdellidans, Clitellata, Annelida, exotic crayfish, distribution, northern Italy

Population dynamics and production of crustacean zooplankton in two mountain lakes in the Italian Alps (Lake Paione Superiore and Lake Malghette).
M. Manca, P. Comoli and V. Lencioni.

We estimated production of three populations of zooplanktonic crustacea from two lakes in the Italian Alps sampled within the EC-Project MOLAR (ENV-CT95-0007) "Measuring and modelling the dynamic response of remote mountain lake ecosystems to environmental change" (Lake Paione Superiore) and the EC- Guideline 78/659 "On the amelioration and the protection of freshwaters suitable for fish life" (Lake Malghette). The former is a fishless lake, while the latter, in addition to the native Phoxynus phoxynus, hosts populations of Salmo trutta fario and Oncorhynchus mykiss. The Allen curve method was used to estimate production of Acanthodiaptomus denticornis and Daphnia longispina in Lake Malghette and of Cyclops abyssorum tatricus in Lake Paione Superiore. Values of daily production of the three populations are similar to those found by other Authors in oligotrophic and high mountain environments.
Key words: zooplankton, secondary production, acidification, high mountain lakes

An unusual type of Daphnia head shields from plankton and sediments of Himalayan lakes.
M. Manca, P. Comoli and F.G. Margaritora.

The paper reports the finding of a particular type of Daphnia (Ctenodaphnia, Dybowski & Grochowski 1895) head shields, found in the plankton and in the sediments of two lakes in the Khumbu Region (Nepalese Himalayas). The capsules, all of the smallest size, were characterised by the presence of a hole in the medio-dorsal region, behind the posterior median groove. The location of the hole suggests a link with the nuchal organ, which we found in the neonates of the species. The finding is important for taxonomy and provides information for discussing the current hypothesis on the biogeography of the Ctenodaphnia subgenus. Being well preserved in sediment cores, the capsules are useful tools for reconstructing past phases of active reproduction which occurred also in the past.
Key words: Daphnia, head-shields, Himalayan lakes, morphology

Performances of analytical methods for freshwater analysis assessed through intercomparison exercises. II. Major ions.
A.Marchetto, R. Mosello, G.A. Tartari, M. Bianchi, H. Geiss, G. Serrini, G. Serrini-Lanza and H. Muntau.

From 1991 to 1997, nine intercomparison exercises on rainwater and freshwater samples were held in the framework of an activity connecting laboratories participating in different projects (Environmental studies in the Mediterranean basin, Italian network for the study of wet deposition, Acidification of mountain lakes, Limnological studies of Lake Léman, Monitoring of atmospheric deposition in forests). The number of participants increased from 72 in 1991 to 177 in 1997. Among the measured variables (pH, conductivity, alkalinity, major ions and nutrients), Ca, Mg, Na, K, ammonium, sulphate, chloride and nitrate are considered in this paper. The methods commonly used by the laboratories involved in the exercises are compared for precision, and an estimate of the performance of each method at different concentrations is made. The results show that some of the participating laboratories use analytical methods which are not reliable for the concentration values occurring in freshwater and/or in rainwater. However, outliers are also produced by laboratories using well-performing methods, indicating the importance of introducing analytical control procedures.
Key words: intercomparison, freshwater, rainwater

Pluriannual evolution of the hydrochemistry of two Alpine lakes (Lake Paione Inferiore and   Lake Paione Superiore, Ossola Valley) in relation to atmospheric loads.
R. Mosello, A. Marchetto, A. Boggero, M.C. Brizzio, G.A. Tartari and M. Rogora.

Lakes Paione Inferiore and Paione Superiore (LPI, LPS) are extremely sensitive to acidification, so they are useful as indicators in studying changes in atmospheric pollutant fluxes on waterbodies. Regular trends observed in the last 3-4 years cannot merely be a consequence of seasonal or interannual variations. Increasing pH and alkalinity are mainly driven by a decrease in acidic inputs from the atmosphere, which have been halved over the last 10 years. This trend of atmospheric deposition chemistry has emerged in several sampling stations in the subalpine and Alpine area, in the watershed of Lake Maggiore. The decrease in deposition acidity is mainly related to a decrease in sulphate, while nitrate and ammonium deposition is still high, resulting in high nitrate concentrations in lake waters.
Key words: mountain lakes, Alps, acidification, chemistry, trend

A sampler-incubator for studying zooplankton grazing and phytoplankton production.
O. Ravera and S. Scotto.

A new instrument to quantify the zooplankton grazing on phytoplankton and the phytoplankton growth is described. This instrument consists of two cylindrical chambers and a mechanical system for shutting, by a messenger both the chambers simultaneously. With this instrument the grazing may be evaluated by counting the algal cells at the beginning and the end of the experiment as well as by the radioisotope technique. This instrument combines the advantages of others used to the same aim by other authors; e.g. Gliwicz (1968), Haney (1971), Hart & Christmas (1984). As an example, the results of two experiments are reported.
Keywords: grazing chamber, phytoplankton growth, zooplankton grazing, filtering rate, feeding rate, electivity coefficient

Considerations on the biochemical composition of some freshwater zooplankton species.
N. Riccardi and M. Mangoni.

The mean elemental (C, H, N) and biochemical composition (lipids, carbohydrates and proteins) of some abundant crustacean zooplankton species of Italian insubric lakes has been estimated by the analysis of samples collected at different seasons from various environments (Lake Maggiore, Lake Varese, Lake Comabbio, Lake Monate). From each sample an adequate number of specimens of each abundant species was sorted and analyzed by a CHN elemental analyzer. The percentage of lipids, carbohydrates and proteins and the calorific content were calculated from the elemental composition according to Gnaiger & Bitterlich (1984). Inter- and intraspecific variability of biochemical composition was quite high, while elemental composition and calorific content were less variable. An estimate of the mean elemental and biochemical composition of each species was obtained by pooling the data. These mean values have been used to estimate the pools of elements and compounds in the crustacean zooplankton of Lake Comabbio to provide an example of the importance of a multiple approach in zooplankton studies.
Keywords: zooplankton, elemental composition, proximate biochemical composition, calorific content

Studies on the zooplankton of the deep subalpine Lake Garda.
N. Salmaso and L. Naselli-Flores

The specific composition and seasonal dynamics of the zooplankton of Lake Garda have been studied through monthly surveys in two annual cycles (December 1994-November 1995 and January-December 1997). The assemblage is largely dominated by Copipodiaptomus steueri, a typical calanoid presently identified in deep (Garda, Iseo) and shallow lakes of NE Italy and in the hinterland of the central Adriatic region (Dalmatia and Marche Region). Cladocerans and the smaller rotifers represent a significant component of the zooplankton from spring to autumn. A re-examination of the results obtained in previous studies does not seem to demonstrate substantial shifts in the composition of the dominant species. The only documentable and consolidated differences are constituted by the disappearance of Sida crystallina since the '50s and the appearance of new rotifers since the '70s and the '80s. It is stressed that, owing to the fragmentation of the available studies and the use of different methodologies, the qualitative and quantitative modifications of the zooplankton should be interpreted with special caution, requiring further in-depth and continuous monitoring for the meaning they could have as possible signs of modifications of the trophic structure of the lake. However, the results of the comparison of the different investigations do not seem to contrast with the substantial stability of the oligo-mesotrophic character of the lake formally documented since the '70s.
Key words: zooplankton, seasonal dynamics, Lake Garda, deep lakes