(formerly Memorie dell'Istituto Italiano di Idrobiologia)

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Preface

This issue of the Journal of Limnology reports the results of the work of the Pelagic Food Web Research Group. This is one of the many groups formed as part of the MOLAR project (MOuntain LAke Research), which was initiated in 1996 with the aim of measuring and modelling the dynamic response of remote mountain lake ecosystems to environmental change (No. ENV4-CT95-0007) of the Environmental and Climate Programme of the European Commission. A number of persons have participated over a period of three years and finally contributed to this volume. Participants come from a number of European institutions with the focus points in Ceské Budejovice and Prague (Czech Rep.), Innsbruck (Austria), Pallanza (Italy), Barcelona and Granada (Spain).
The aim of this volume was to evaluate obtained results on microbial assemblage, phytoplankton and zooplankton structure and biomass of mountain lakes in order to give a complex picture of these remote ecosystems to the reader. Besides the description of the taxa encountered, their seasonal cycles and activities are reported. The aim of the Guest Editors was to produce the volume as soon as possible to supply the scientific community with fresh data on remote mountain lakes. This was why data, which are still being processed, as well as some data to be published elsewhere, are not included. We believe this volume is important because the papers, while presenting very recent data, are not limited to reports on results only, but already include discussions and comparison with other lakes. The first two papers concern the methodological protocol used for the assessment of pelagic food webs (Straškrabová et al.) and the chemical characteristics of the MOLAR lakes (MOLAR Water Chemistry Group) and will serve as a reference for the other papers. The following papers are in geographical order of the studied lakes from the North to the South.
It must be noticed that the authors benefited a lot from many theoretical discussions within the group and with the other MOLAR partners during the Workshops organised by the Pelagic Food Web Group (Ceské Budejovice 1996, Pallanza 1998) and during the MOLAR meetings (Prague 1996, Barcelona 1997, Bled 1998, Arcachon 1999). The authors are also indebted to all colleagues involved in sampling, sending and elaborating samples, as well as to all other groups in the MOLAR project, whose data were available for comparison and evaluation. Last but not least, our thanks are due to the coordinators of the MOLAR project Bente Wathne and Simon Patrick.

The Guest Editors

Investigations on pelagic food webs in mountain lakes - aims and methods
V. Straškrabová, C. Callieri, P. Carrillo, L. Cruz-Pizarro, J. Fott, P. Hartman, M. Macek, J.M. Medina-Sánchez, J. Nedoma and K. Šimek.

The MOLAR Project: atmospheric deposition and lake water chemistry
The MOLAR Water Chemistry Group.

In the framework of the Mountain Lake Research (MOLAR) project, the water chemistry of 23 lakes covering Europe from the Svalbard Islands to the South of Spain were studied. The lakes are located above or beyond the timber line in the most important mountain areas in Europe. Atmospheric deposition was sampled at 11 sites, representative of different lake districts. 24 institutions took part in the activity. The comparability of the analytical results performed in 18 laboratories was assured by strict sampling and analysis protocols and by a detailed programme of Analytical Quality Assessment/Control. The paper summarises the main morphometric and hydrological characteristics of the lakes and their watersheds and discusses the results of the atmospheric deposition and lake water chemistry. An overview on the main processes controlling the composition and ionic ratio of deposition and lake chemistry is given.

Phytoplankton in three Tatra mountain lakes of different acidification status
J. Fott, M. Blažo, E. Stuchlík and O. Strunecký.

Seasonality and depth distribution of phytoplankton were studied in three alpine lakes of the High Tatra mountains: Nižné Terianske Pleso, Dlugi Staw and Starolesnianske Pleso. The lakes differ in size, depth, nutrient status (phosphorus) and acidification status (pH, alkalinity, calcium). The highest biomass was found in the small, shallow, phosphorus - rich and acidified lake Starolesnianske. An extremely low phytoplankton biomass was found in the medium sized Dlugi Staw, which is very poor in phosphorus and which is a transition lake between the acidified and bicarbonate ones. The Lake Nižné Terianske is a standard deep, oligotrophic high mountain lake by most of its characters, phytoplankton included. Most of phytoplankton of the three lakes are flagellates. In the non-acidified Lake Nižné Terianske the taxa contributing most to the biomass are Chrysophyceae (Chromulina and other) and dinoflagellates (Gymnodinium uberrimum, Woloszynskia). In the acidified Lake Starolesnianske the most important groups are green flagellates (Chlamydomonas) and dinoflagellates (Peridinium inconspicuum, Woloszynskia ordinata, Katodinium). The non-motile component (Chroococcus, Oocystis, centric diatoms) is sometimes important in the extremely poor phytoplankton of Dlugi Staw. Moreover, resting stages (mainly stomatocysts) often prevail over active phytoplankton in this lake. Seasonal variation in the amount of chlorophyll per unit biovolume was observed in the lakes Nižné Terianske and Starolesnianske, brought about apparently by changes of the underwater light climate.

Abundance, biomass and size-structure of the microbial assemblage in the high mountain lake Gossenköllesee (Tyrol, Austria) during the ice-free period
A. Wille, B. Sonntag, B. Sattler and R. Psenner.

The abundance, biomass and morphology of the microbial components (picocyanobacteria, heterotrophic bacteria, heterotrophic and autotrophic nanoflagellates, ciliates) of the pelagic food web of Gossenköllesee were investigated over two summer periods. The density of bacteria remained relatively stable not only over time but also in vertical profiles (2.5 - 5.5´ 10⁵ cells ml^-1). Bacterial biomass ranged between 35 to 63 mgC m^-2 (5.4 – 15.3 µgC l^-1). Small rod shaped bacteria with mean cell volumes of ~ 0.05 µm³ dominated numerically but filamentous forms (longer than 10 µm), mainly found in the upper water layers, amounting to more than 65% of the total bacterial biomass, increased the mean cell volume up to 0.27 µm³ (SD=0.88). Bacterial biomass represented between 48 and 86% of the total microbial biomass (40 - 90 mgC m^-2), however at 8 m depth the biomass of heterotrophic flagellates (HNF) reached up to 26 mgC m^-3 (2,852 cells ml^-1). From 0-4 m depth small spherical HNF species with cell volumes <10 µm³ were dominant, whereas in deeper water layers large flagellates with cell volumes >50 µm³ dominated throughout the sampling period. Ciliate abundance was low in the upper part of the water column. Only Askenasia chlorelligera and Urotricha cf pelagica occurred with numbers of up to 1,500 cells l^-1. At 8 m depth, Balanion planctonicum was the dominant species throughout the study period reaching numbers of up to 16,000 cells l^-1. Ciliate abundance was significantly correlated with chlorophyll-a concentrations (r_s=0.55, p <0.01) confirming the algivory of the dominant species. No significant correlation was found between the parameters of chlorophyll-a, bacteria and HNF but the abundance of ciliates was negatively correlated with the length of bacteria (r_s= -0.41, p <0.05).

Biomass and species structure of the phytoplankton of an high mountain lake (Lake Paione Superiore, Central Alps, Italy)
A. Pugnetti and R. Bettinetti.

In the framework of the EU MOLAR Project on "Measuring and modelling the dynamic response of remote mountain lake ecosystems to environmental change" a three whole-year study (1996-1998) on the composition and dynamics of phytoplankton community of the high mountain lake, acid sensitive Lago Paione Superiore (LPS) was carried out. The data were analyzed and compared with those gathered during the years 1991-1993. The phytoplankton was made up by nanoplanktonic unicellular algae, the only exception being the colonial Dinobryon sertularia. Just four species, belonging to Chrysophyceae (Chromulina sp., Dinobryon sertularia and Mallomonas alveolata) and to Dinophyceae (Gymnodinium sp.) were important as biomass and density, and they were always present throughout the year. The prevalence of potentially mixotrophic species suggests an adaptive strategy to the low environmental concentrations of inorganic carbon and phosphorus. The seasonal variations of the total biomass were similar to those observed in the previous years. The total number of species has increased; this could be related with the recent increase of the pH and of the alkalinity.

Studies on zooplankton of Lago Paione Superiore
M. Manca and P. Comoli.

We report here the results of a three year study on the zooplankton of Lago Paione Superiore, an acid sensitive lake above the tree line in the Italian Alps. The research was carried out within MOLAR, an EC-founded Project on "Measuring and Modeling the dynamic response of remote mountain lakes ecosystems to environmental change". This study comes after a series of investigations on the effects of acidification, in which we documented the changes occurred with decreasing water pHs, by comparing the recent situation with that in the literature of the 40s, and reconstructed the beginning of anthropogenic disturbance through an analysis of the past cladocera assemblages archived in the lake sediments. A characteristic pattern in seasonal periodicity is a transition from a community dominated by small zooplankton (August) to a community where the large particle-feeder Daphnia longispina dominates. This is a typical pattern observed in fishless, copepod-cladocera lakes. Regardless from which food is able to exploit, Daphnia population of Lago Paione Superiore is composed by well-fed organisms, visually rich in lipids, able to produce more than one generation/year of parthenogenetic females at density levels which are rather high in an oligotrophic high mountain lake.

Organic carbon and microbial food web assemblages in an oligotrophic alpine lake.
C. Callieri and R. Bertoni.

Picoplankton, both autotrophic (APP) and heterotrophic (HPP), ciliates and heterotrophic nanoflagellates (HNF) were counted and their biovolume measured monthly over a 3 year period in Lake Paione Superiore (LPS), a high mountain lake in the Italian alpine region. Analyses of organic carbon, particulate and dissolved, were performed at the same time. APP were negligible and picocyanobacteria almost absent. HPP showed seasonal variations, with low numbers in winter/spring and maxima of nearly 10⁶ cell ml^-1 in August/September, corresponding to 60µg C l^-1. Free-living, non- pigmented flagellates showed a density range from 10⁴ l^-1 to 10⁶ l^-1 with a prevalence of cells <3 µm. Their carbon ranged between 0.1–9 µg C l^-1. Ciliate numbers ranged from 0.02 to 11 10³ l^-1. For much of the year different species of Urotricha were found. Conversely, Strombidium appeared during the ice-free period and Halteria grandinella under the ice, indicating a strict dependence on temperature. Carbon in the microbial loop of LPS (near the bottom) was mainly confined to bacteria (73%), with 20% in HNF and only 7% in ciliates. Total organic carbon (TOC) concentration, measured after the removal of net plankton, ranged from 0.26 to 1.77 mgC l^-1 with a prevalence of the dissolved form (87% av.). The average particulate organic carbon (POC) concentration was 0.24 mgC l^-1. All the components of the microbial loop showed a decline under the ice-cover. Bacterial carbon concentration was three times lower under the ice than in the ice-free season (7.9– 24.4 µgC l^-1, respectively); protozoa carbon too declined under the ice-cover (3.1–5.8 µgC l^-1 for HNF and 0.4–1.7µgC l^-1 for ciliates in the ice-cover and ice-free periods, respectively). The drop in the microbial-loop carbon occurring in late summer may be related to the presence of a Daphnia population peak. At that moment, the structure of the microbial loop is transformed by a top-down control of Daphnia.

Carbon partitioning in the food web of a high mountain lake: from bacteria to zooplankton.
C. Callieri, A. Pugnetti and M. Manca.

The organisms of the microbial loop in Lake Paione Superiore (LPS), a high mountain lake in the Italian Alpine region, were studied together with phytoplankton and zooplankton for three successive years. The biomass of bacteria, HNF (heterotrophic nanoflagellates), ciliates and phytoplankton, as mean carbon concentration in the three years, was 30 and 37 µg C l^-1 near the surface (SUR) and the bottom (BOT) respectively. Under the ice-cover the mean biomass carbon decreased especially at the BOT, whereas at SUR the decrease was less evident due to the maintenance of higher phytoplankton biomass (mixotrophic flagellates). In LPS ~50% of the carbon was confined in bacteria, 20% in protozoa and 30% in phytoplankton. The ratio Autotrophs/Heterotrophs was lower than 1 (mean: 0,97 at SUR and 0,58 at BOT) thus indicating a system with a predominance of the heterotrophs. This might be the result of light inhibition of algal growth coupled to a production of dissolved carbon, utilized by bacteria. During late summer the peak of Daphnia longispina, the main component of the zooplankton of LPS, increased the carbon content in the lake to a total of 158 and 300 µg C l^-1 in 1997 and 1998 respectively. At the late summer peaks, zooplankton represented from 78 to 89% of the total carbon of the pelagic communities. Furthermore, the presence of Daphnia could be responsible for a decrease in the biomass carbon of a variety of organisms (algae, protozoa and bacteria). It may be possible that this is an instance of zooplankton grazing on algae, protozoa and also bacteria, as Daphnia has very broad niches and may eat pico-, nanoplankton and small ciliates. In the oligotrophic LPS, a diet which also includes protozoa could give Daphnia a further chance of survival, as ciliates are an important source of fatty acids and sterols.

Seasonal dynamics and phytoplankton diversity in high mountain lakes (Jöri lakes, Swiss Alps)
B. Hinder, M. Gabathuler, B. Steiner, K. Hanselmann and H. R. Preisig.

The development of the phytoplankton in two years with very distinct weather situations was compared. In 1996, the ice on Jöri Lake III melted in mid June, summer stratification persisted during two months, and ice began to build up again in mid October. In 1997, the ice melted only at the end of July, which strongly influenced the development of the phytoplankton. Stratification persisted during two months and the lake froze up towards the end of October. The average chlorophyll-a concentrations were lower in 1996 than in 1997, which reflects the rather high temperatures and stable weather conditions in 1997 after the late melting of the ice. These observations lead us to suggest that the duration of the ice-free season is less decisive for biomass production than the weather conditions during this period. However, the date and duration of melting of the lake ice strongly influence the development of algal species that are typically observed in early season.

Microbial food web in an oligotrophic high mountain lake (Jöri Lake III, Switzerland)
B. Hinder, I. Baur, K. Hanselmann and F. Schanz.

Jöri Lake III (2512 m a.s.l., z_m = 22 m, A = 57.81 ´ 10³ m², V = 601.1 ´ 10³ m³) is situated in the Vereina region in the eastern part of the Swiss Alps. We studied microbial grazing on bacteria and bacterial productivity during the ice-free period. The lake normally gets thermally stratified for two months between July and September. In 1996, chlorophyll-a concentrations varied from 0.5 to 2.0 µg l^-1 with maximum values just below the thermocline (6 m depth), in 1997, they were between 0.6 and 5.0 µg l^-1 with maximum values at 10 m depth – several meters below the thermocline. Bacterial densities varied between 0.7 and 1.7 ´ 10⁶ ml^-1 with maxima in the thermocline, one to two meters above the chlorophyll maximum. The areal bacterial biomass (volume beneath 1 m² to a depth of 8 m) was 10 µg C l^-1 which remained more or less constant for the periods investigated. In 1997, bacterial growth rate and production rates were determined using [³H]-thymidine incorporation. The rates were as low as 0.002 to 0.006 h^-1 and 0.01 to 0.03 µg C l^-1 h^-1, respectively. We found a carbon ratio of bacteria, phytoplankton, and autotrophic picoplancton (APP) of 1.5:1.1:1 which shows a rather high abundance of bacteria and autotrophic picoplankton (APP) compared to larger phytoplankton. Bacterial growth followed a temperature dependence similar to the one observed for bacteria from Lake Zürich, a prealpine and mesotrophic lake which was studied for comparison. Microbial food web in Jöri Lake III was not top down controlled during the periods of our study and mixotrophic algae like Dinobryon cylindricum var. alpinum and autotrophic nanoflagellates (ANF) were the dominant bacterial grazers observed.

Seasonal dynamics of plankton in a mountain lake in the southern Alps (Laghetto Inferiore, Switzerland).
M. Simona, A. Barbieri, M. Veronesi, S. Malusardi and V. Straškrabová.

The pelagic populations in Laghetto Inferiore (southern Swiss Alps) were studied between 1996 and 1998 during the ice-free period. Investigations revealed a considerable number of phytoplankton species but few zooplankton species. The vertical distribution of the main species of algae shows a marked vertical stratification, with higher biomass concentrations in the deep zone. Along this profile, species in the same taxonomic class tend to be distributed at different depths, according to their specific light requirements. During the study period no clear seasonal succession of populations was observed, also because of the influence of precipitation on the biological cycles. On the whole there was a good correspondence between the biomass values of phytoplankton and those of chlorophyll-a, both in integral samples of the whole water column and in the vertical profile.

Limnological survey in eight high mountain lakes located in Lago Maggiore watershed (Switzerland)
A. Barbieri, M. Veronesi, M. Simona, S. Malusardi and V. Straškrabová.

Eight high mountain lakes located above 2000 m a.s.l. in the watershed of Lago Maggiore, with alkalinity between 10 and 100 µeq l^-1, were studied during summer 1997. The survey revealed that some of the lakes had a saline density gradient which might hinder the mixing of water and restrict the oxygenation in hypolimnion. Following acidification, aluminium was present in some lakes in the form of Al(OH)₂⁺, with pH around 5.9, and as Al(OH)₄^-, with pH around 6.6. The negative correlation observed between pH and nitrate concentrations seems to be linked to vegetal activity. In fact, acidity in soil and water, combined with the presence of aluminium, can partially inhibit the metabolism of plants and algae, and reduce the assimilation of NO₃^-. Major biological differences emerged among the lakes, both at the level of plankton composition and biomass, and in seasonal dynamics. Dinophyceans and chlorophyceans predominated quantitatively in the lakes with low pH and alkalinity values, whereas diatoms were present in the lakes with higher values of these parameters. The phytoplankton biodiversity index showed a considerable variation from lake to lake. Higher alkalinity and salinity was not coupled to a higher biocenosis diversity and one or two species tend to dominate the others.

Microbial plankton assemblages, composition and biomass, during two ice-free periods in a deep high mountain lake (Estany Redó, Pyrenees)
M. Felip, F. Bartumeus, S. Halac and J. Catalan.

Microbial plankton composition and biomass were monitored for two ice-free periods in a deep oligotrophic high-mountain lake (Redó, Pyrenees). Phytoplankton dominated microbial biomass, while the relationship between total water-column-integrated autotrophic and heterotrophic biomass ranged from 1.5 to 6.5 (an average of 4.4). Heterotrophic biomass was dominated by bacteria (an average of 47 %), but heterotrophic nanoflagellates and, to a lesser degree, ciliates occasionally constituted a sizeable proportion. In general, the microbial biomass ratios were 10:2:2:1 for PHY:BAC:HNF:CIL. About one hundred eukaryotic species were found, although most of them in low abundance and frequency. Phytoplankton biomass was dominated by flagellated chrysophytes and dinoflagellates (an average of 40 and 32% respectively); occasionally cryptophytes (in deep layers) and chlorococcal chlorophytes (during the autumn mixing period) were also significant. In the two years sampled, the maximum phytoplankton diversity was observed during the autumn mixing period. Heterotrophic flagellate biomass was dominated by chrysophytes (78% on average), but sporadically a non-identified species reached high abundances. Oligotrichs, (an average of 43% of total ciliate biomass) dominated the ciliate community, still other groups (gymnostomatida and prostomatida) were also significant. Bacteria biomass was largely homogeneous throughout the two periods, but size segregation was observed especially when the lake was stratified, with larger bacteria appearing in the upper layers. The highest planktonic microbial biomass occurred during the mixing periods, mainly during spring. But no clear relationships were found between the temporal distribution of bacteria, phytoplankton, heterotrophic flagellate and ciliate biomass.

The relative importance of the planktonic food web in the carbon cycle of an oligotrophic mountain lake in a poorly vegetated catchment (Redó, Pyrenees)
L. Camarero, M. Felip, M. Ventura, F. Bartumeus and J. Catalan.

The biological activity of the planktonic community of lake Redó, expressed in terms of carbon fluxes, was measured and compared to the changes in DIC, DOC and POC in the water column. Planktonic photosynthesis ranged between c. 0.01 - 0.3 µg C m^-2 h^-1. Release of EOC phytoplankton was highly variable, between 5 and 80% of total fixation. Bacterial uptake of EOC ranged between 1-20% of total fixation. Bacterial activities were, in absolute numbers, very low: 0.005±0.003 µg C m^-2 h^-1, in contrast with the higher grazing rates on bacteria of 0.036±0.021 µg C m^-2 h^-1. Respiration and diffusion of CO₂ to the atmosphere seem to be the main processes controlling DIC concentration. DOC and POC concentrations were highly correlated, and their fluxes presented large fluctuations. These changes in DOC are larger than those due to the processes we have measured. Other processes that might affect DOC include diffusion from sediments, inputs from the catchment, uptake by mixotrophic algae and zooplankton, bacterial respiration, UV photoxidation, and flocculation. Lake Redó seems to act in general terms as an heterotrophic system: respiration is higher than photosynthesis, and the budget is balanced by the import of DOC and, to a lesser extent, POC. Most of the carbon seems to be ultimately released to atmosphere, since little is accumulated in sediments. The estimates of diffusive fluxes agreed with this hypothesis. At this stage, the comparison of biogeochemical budgets with biological activity measurements only serves as a rough approximation of the main pathways in the C cycling in the lake, and to point the issues that need further research in order to calculate the C budget in the lake with accuracy.

Structure changes in a planktonic food web: biotic and abiotic controls
J.M. Medina-Sánchez, M. Villar-Argaiz, P. Sánchez-Castillo, L. Cruz-Pizarro and P. Carrillo.

We examined the response of the plankton community to strong changes in the volume of oligotrophic Lake La Caldera through the ice-free period of 1995-1997. As consequence of the changes in the precipitation regime, the lake volume diminished to 2 m depth during the extremely dry year of 1995. The increase in phosphorus availability for this year caused the diversification of the planktonic community, and the autotrophic:heterotrophic (A/H) ratio was below 1. In spite of the sporadic dust loads associated to atmospheric events, the quick recovery of the lake volumen in 1996 led to a severe phosphorus limitation. Phytoplankton community was dominated by Chrysophyceae (>90% of abundance) and large zooplankters were very scarce (<1 ind l^-1). As a consequence, A/H ratio was over 1 and ciliates developed. Finally, towards the end of 1997, the zooplankton recovery led to a decrease of both A/H ratio and ciliates, and the lake returned to the conditions before drought. A decreasing tendency in both bacterial and phytoplankton densities, possibly as a consequence of top-down controls, was observed through the period of 1995-1997.