CONTENTS
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Phytoplankton association patterns in the deep southern subalpine lakes (Part 1)

1

Freshwater autotrophic picoplankton: a review
Cristiana CALLIERI and John G. STOCKNER
 

Autotrophic picoplankton (APP) are distributed worldwide and are ubiquitous in all types of lakes of varying trophic state. APP are major players in carbon production in all aquatic ecosystems, including extreme environments such as cold ice-covered and/or warm tropical lakes and thermal springs. They often form the base of complex microbial food webs, becoming prey for a multitude of protozoan and micro-invertebrate grazers, that effectively channel APP carbon to higher trophic levels including fish. In this review we examine the existing literature on freshwater autotrophic picoplankton, setting recent findings and current ecological issues within an historic framework, and include a description of the occurrence and distribution of both single-cell and colonial APP (picocyanobacteria) in different types of lakes. In this review we place considerable emphasis on methodology and ecology, including sampling, counting, preservation, molecular techniques, measurement of photosynthesis, and include extensive comment on their im­portant role in microbial food webs. The model outlined by Stockner of an increase of APP abundance and biomass and a decrease of its relative importance with the increase of phosphorus concentration in lakes has been widely accepted, and only recently con­firmed in marine and freshwater ecosystems. Nevertheless the relationship which drives the APP presence and importance in lakes of differing trophic status appears with considerable variation so we must conclude that the success of APP in oligotrophic lakes worldwide is not a certainty but highly probable.
 

2

Evolution of sediment composition of the coastal Lake San Puoto (Latium, Italy) in the last two centuries
Francesca ALVISI and Enrico DINELLI
 

The sediment composition of the coastal Lake San Puoto (Central Italy) was studied to obtain information on its recent environ­mental evolution (last 200 years). The explorative fieldwork was performed within the framework of the EU-funded PALICLAS Proj­ect. Sediment and mass accumulation rates, calculated from ²¹⁰Pb analysis, show mean values of 0.27 cm y^-1 and 0.11 g cm^-2 y^-1 re­spectively, with increasing values from 10 cm to the top (i.e. from ca 1960 AD to the present). The evolution of the recent sedimenta­tion of Lake San Puoto is characterized by the interplay of three main components: detrital (either siliciclastic and/or carbonatic), organic-rich and authigenic phases. The first reflects the input from coastal dunes and rocky outcrops; the other two indicate varia­tions in lake productivity. Sedimentological, geochemical, and mineralogical analyses of two cores collected near the center of the lake were used to define five lithological units: i) a laminated unit representing the last eutrophication period, starting ca 1925 AD; ii) a few turbidite layers, probably due to a lowering of the lake level around 1920 AD; iii) a productive phase of the lake, which oc­curred around the end of the XIX and the beginning of the XX century; iv) a coarse siliciclastic sediment interval, possibly deposited following the digging of the channel connecting Lake San Puoto to Lake Lungo; and v) an older unit representing the natural al­lochthonous detritic sedimentation deriving from the present dune deposits surrounding the lake shores. Metal fluxes were calculated for recent times by means of ²¹⁰Pb sedimentation rates. Zn, Cu and Pb record considerably higher fluxes above 10 cm depth (i.e. 1960 AD), as confirmed by Al-normalized metal depth profiles. Episodes of eutrophication occurred between 1890 and 1920 and between 1925 and the present.
 

3

Present trophic level of Lake Alserio (Northern Italy) and prospect for its recovery
Michela ROGORA, Letizia GARIBALDI, Giuseppe MORABITO, Silvia ARISCI and Rosario MOSELLO
 

Lake Alserio is located in the subalpine area of Northern Italy, in the Brianza region near Lake Como. In November 1999 an EU funded project was started with the aim of devising and implementing interventions leading to the recovery of the lake and the sur­rounding area, which is characterised by a high degree of biodiversity as regards species and natural habitats. Since the seventies, Lake Alserio has been affected by eutrophication, which has resulted in high phosphorus (P) concentrations (annual average in 1999: 80-100 µg P l^-1). The EU project envisaged liming the lake with Ca(OH)₂ to reduce P concentrations in the water. This paper presents and discusses the results of chemical analyses performed on the lake and its tributaries in the first year of the study (No­vember 1999-November 2000). The efficiency of liming with Ca(OH)₂ to reduce P concentrations in laboratory and enclosure ex­periments is also discussed. The present condition of Lake Alserio is between eutrophic and hypereutrophic. Anoxic conditions char­acterise the hypolimnic waters for a prolonged period (May - October) during summer stratification, with a consequent high P re­lease from the sediments (summer P concentrations in the deep water are close to 1 mg P l^-1). Inflowing waters are characterised by high contents of ammonium and phosphorus, showing the importance of domestic sewage in the lake catchment; external load is still too high to permit the water to reach mesotrophic condition, the ultimate goal of recovery. Results of the liming experiments suggest that the treatment, if applied to Lake Alserio, could accelerate its recovery, although the specific procedures to be employed are still to be defined. However, before these or any other remedial operations can be undertaken, the external P load must be reduced by completing the diversion of sewage from the lake.
 

4

Morphological variation and life history changes of a Daphnia hyalina population exposed to Chaoborus flavicans larvae predation (L. Candia, Northern Italy).
Nicoletta RICCARDI, Gianluigi GIUSSANI and Laurence LAGORIO
 

The response of Daphnia populations to invertebrate predators involves morphological changes and  can lead to a trade-off be­tween growth- and reproduction-related traits. The effects of Chaoborus flavicans larvae on the Daphnia hyalina population in Lake Candia were investigated by comparing the morphometric characteristics and life history traits of the preys during one year with low predator density and one year with a ten fold higher predator density. The increase in Chaoborus density seems to have induced morphological changes in Daphnia hyalina which enabled the modified morph to coexist with the predator. The predator impact did not result in any apparent shift in body size pattern of the prey population, while an increase in average clutch size and a tendency towards a decrease in size at first reproduction suggests the possibility of a predator-mediated shift in life history traits.
 

5
Impact of late glacial climate variations on stratification and trophic state of the meromictic lake Längsee (Austria): validation of a conceptual model by multi proxy studies
Roland SCHMIDT, Roland PSENNER, Jens MÜLLER, Peter INDINGER and Christian KAMENIK
 

Selected pigments, diatoms and diatom-inferred phosphorus (Di-TP) concentrations of a late glacial sediment core section of the meromictic Längsee, Austria, were compared with tephra- and varve-dated pollen stratigraphic and geochemical results. A concep­tual model was adopted for Längsee and evaluated using multi proxy data. During the unforested late Pleniglacial, a holomictic lake stage with low primary productivity prevailed. Subsequent to the Lateglacial Betula expansion, at about 14,300 cal. y BP, okenone and isorenieratene, pigments from purple and green sulphur bacteria, indicate the onset of anoxic conditions in the hypolimnion. The formation of laminae coincides with this anoxic, meromictic period with high, though fluctuating, amounts of okenone that persisted throughout the Lateglacial interstadial. The occurrence of unlaminated sediment sections of allochthonous origin, and concurrent low concentrations of okenone, were related to cool and wet climate fluctuations during this period, probably coupled with a com­plete mixing of the water column. Two of these oscillations of the Lateglacial interstadial have been correlated tentatively with the Aegelsee and Gerzensee oscillations in the Alps. The latter climate fluctuation divides a period of enhanced anoxia and primary pro­ductivity, correlated with the Alleröd chronozone. Continental climate conditions were assumed to be the main driving forces for meromictic stability during Alleröd times. In addition, calcite dissolution due to severe hypolimnetic anoxia, appear to have sup­ported meromictic stability. Increased pigment concentrations, which are in contrast to low diatom-inferred total phosphorus (Di-TP), indicate the formation of a productive metalimnion during this period, probably due to a clear-water phase (low catchment ero­sion), increased temperatures, and a steep gradient between the phosphorus enriched hypolimnion and the oligotrophic epilimnion. Meltwater impacts from an extended snow-cover and a summer temperature decline, together with climate instability, are assumed to be the main reasons for the lowering of the anoxic level during the following, climatically heterogeneous, Younger Dryas. Meromic­tic stability was re-established with the termination of the Younger Dryas. The sequence of key pigments, Di-TP, calcite, siliciclas­tics, and organic carbon, seem to be in accordance with the assumptions of the conceptual model.
 

6
Seasonal variation in mortality of brown trout (Salmo trutta) in an acidic aluminium-rich lake
Espen LYDERSEN, Nina W.A. RUKKE, Jannike G.B. JENSEN, Birgitte M. KJELSBERG, Bente TORNSJØ, Rolf D. VOGT, L. Asbjørn VØLLESTAD and Antonio B.S. POLÉO
 

We have studied the seasonal variation in aluminium toxicity in caged brown trout (Salmo trutta), during one year (October 1994 to September 1995) in a small acidic aluminium rich lake, Lake Nepptjern (ANC -22.6 µeq l^-1, pH 5.2, total Al 400 µg l^-1). Trout from two and three different year classes were exposed each month to the lake water for 48 h. Fish were placed in keepnets located in the middle of the lake, at 2 m depth. Fish mortality and water physico-chemistry were monitored during the exposures. The con­centration of inorganic monomeric aluminium in the water was approximately 300 µg l^-1 in average, and the water was acutely toxic to the fish. The observed mortality varied throughout the year, and was highest during spring and summer. During spring, the small fish were more sensitive to the toxic water than larger fish, while the opposite was the case during summer. Water temperature and fish length could explain most of the seasonal variation in mortality. Statistical analyses indicated that water acid neutralising ca­pacity (ANC) and the amounts of total organic carbon (TOC) and silicon in the water also could explain some of the variation in mortality. Variation in other physico-chemical parameters, however, such as silicon, TOC and ANC could only explain the variation in mortality to a limited extent. The mechanism for the temperature dependent mortality is discussed, and we suggest that the de­pendence of water O₂-solubility and fish metabolism upon temperature is of importance. The difference in mortality between small and large fish is discussed in terms of the gill area/body weight ratio, and it seems to be that small fish suffer more from diffusive ion loss having a larger relative gill area than larger fish. Large fish, on the other hand, have a lower relative maximum oxygen uptake than small fish and will suffer more under conditions where aluminium is accumulated on the gill surface.
 

7
The effects of ionic strength on the toxicity of aluminium to Atlantic salmon (Salmo salar) under non-steady state chemical conditions
Espen LYDERSEN, Sigurd ØXNEVAD, Kjartan ØSTBYE, Ronny A. ANDERSEN, Frode BJERKELY,
Leif Asbjørn VØLLESTAD  and Antonio B.S. POLÉO
 

We have tested the influence of water ionic strength on the toxicity of aluminium in fish by comparing the mortality of Atlantic salmon (Salmo salar) parr exposed to Al-rich water with additions of Ca²⁺ or Na⁺. The fish were exposed in parallel to Al-rich water (Al 500 µg l^-1, pH 5.8) under non-steady state conditions, with and without the addition of one of the two base cations. The amount of Na⁺ and Ca²⁺ added to the water was calculated in order to obtain an identical increase in water ionic strength. Fish mortality was dependent on water residence time and whether or not base cations were added to the Al-rich water. In all Al-exposures, the highest mortality was always observed in fish exposed to water with the shortest residence time. Mortality decreased systematically with in­creasing water residence time through the exposure set-up. The addition of a base cation, Ca²⁺ or Na⁺, to the Al-rich water reduced fish mortality significantly compared to the Al-only exposures. Furthermore, increasing ionic strength with Na⁺ reduced mortality to a larger extent than the corresponding increase in ionic strength by the addition of Ca²⁺. The variation in mortality between the various aluminium and base cation treatments is discussed in terms of aluminium chemistry, specific mitigating effects of Ca²⁺ and Na⁺, and the general importance of water ionic strength. This study clearly demonstrates that Ca²⁺ does not play an unique role as an ameliorating cation for Al-toxicity in fish under non-steady state chemical conditions. Thus, ionic strength seems to be important, probably for the interaction between aluminium and the gill surface, reducing the possibility for positively charged aluminium spe­cies to bind to negatively charged sites.

8

Phytoplankton community and limnochemistry of Piburger See (Tyrol, Austria) 28 years after lake restoration
Monica TOLOTTI and Hansjörg THIES
 

Phytoplankton community and limnochemistry of Piburger See, a small soft-water, meromictic lake situated at 913 m a.s.l. in a crystalline area of the Central Eastern Alps of Tyrol (Austria), were investigated 28 years after the beginning of lake restoration. Although long-term data of the lake show a declining trend in total phosphorus concentrations and phytoplankton biovolume, the re­sponse of Piburger See to the restoration measures carried out in 1970 was delayed by about 20 years. At present the lake is ap­proaching its former oligotrophic level. The most evident difference between the past and present phytoplankton species composition of Piburger See is the actual absence of the Cyanophycean Oscillatoria limosa C. A. Agardh, which markedly increased during the first two decades after the lake restoration (1970-1987). The phytoplankton biovolume recorded in 1998 was lower than in the 1970s and 1980s, while seasonal patterns were similar to those recorded before and later on in the lake restoration. The lowest annual phytoplankton biovolume in 1998 occurred in early winter, while the absolute maximum was observed in metalimnetic water layers in late spring. In 1998 the intra-annual patterns of phytoplankton biovolume and chlorophyll-a compare well. Phytoplankton succes­sion started in early 1998 under ice with coccal green algae followed by flagellated Chrysophyceae during spring. The mid-summer phytoplankton community was dominated by centric Bacillariophyceae, which were later replaced by coccal Cyanophyceae. During autumn, Dinophyceae and Chrysophyceae prevailed. Epilimnetic dominance of centric diatoms during mid summer appears to be a new feature, which in 1998 was related to a strong depletion of dissolved silica and nitrate. Long-term water chemistry and phyto­plankton data were checked against local weather data in order to explain the delay in the re-oligotrophication process of Piburger See. However, no clear relationship could be detected between the trends observed in the lake and weather conditions during the past 30 years.
 

9

Preface

Phytoplankton in deep Italian lakes: introductory remarks
Delio RUGGIU
 

10

Ecological patterns of phytoplankton assemblages in Lake Garda: seasonal, spatial and historical features
Nico SALMASO
 

The development of phytoplankton assemblages in the two main basins of the deep (Z_max=350 m) and large (49 km³) subalpine Lake Garda (Northern Italy) was investigated through monthly samplings from 1998 to 2000. The algal development was strongly typified by the alternation, from spring to autumn, of Fragilaria crotonensis and Mougeotia sp. and by the increasing importance of filaments of the complex Planktothrix rubescens/agardhii in summer and autumn. These three “master species” are characterised by higher biovolumes and/or more regular and wide annual development in comparison to the other dominant taxa. The simultaneous application of ordination (NMDS-Non metric Multi Dimensional Scaling) and cluster analysis techniques revealed an ordered and coherent temporal succession of phytoplankton assemblages in the two main basins of the lake. These temporal and spatial regulari­ties arise from the high inertia and resilience against perturbations that characterise the deep and large lakes, and contrast sharply with the less ordered or chaotic and unpredictable seasonal assemblages that may be found in small and shallow lakes. It is stressed that, owing to the use of different methodologies, the modifications of the phytoplankton assemblages in the last fifty years must be interpreted with particular caution. The only demonstrable differences seem to indicate that, with the speeding up of eutrophication processes in the 1960s and 1970s, detectable populations of Planktothrix and irregular increases of Conjugatophyceae (Mougeotia sp., Closterium aciculare) were established. During and after the 1980s, the increase of phosphorus concentrations in the lake and the progression towards more mesotrophic conditions was accompanied by the appearance of further substantial populations of cya­nobacteria (Planktolyngbya limnetica, Anabaena lemmermannii). In contrast with these signs of alteration, Fragilaria crotonensis, Asterionella formosa, Ceratium hirundinella and Dinobryon spp. were among the most abundant taxa from the 1950s to the present time. These modifications are largely consistent with the changes observed during the 1990s in consequence of the different extent of the deep vertical mixing. During complete overturn, with the maximum spring replenishment of nutrients in the euphotic layers, a greater development of Mougeotia sp. and Oscillatoriales was observed. The historical trend and the ecological results allowed elu­cidation of the trophic characteristics of many important species developing in Lake Garda and, in general, in the deep subalpine lakes.
 

11

Phytoplankton assemblages in two sub-basins of Lake Como
Fabio BUZZI
 

This paper reports the results of a two-year study on the chemical characteristics and phytoplankton assemblages in the western (Como) and eastern (Abbadia Lariana) sub-basins of Lake Como. The data were collected from September 1997 to August 1999 in the frame of an investigation financed by the Provincial Administration of Como and Lecco. Over the period of study TP concentra­tions ranged between 6-46 µg P l^-1 at Abbadia, and 5-50 µg P l^-1 at Como. Reactive phosphorus ranged between 6-31 µg P l^-1 at Como, and 6-25 µg P l^-1 at Abbadia. By the end of the study, 85 taxa belonging to six algal groups were identified. Chlorophyta (Chlorophyceae and Conjugatophyceae) (30) were the best represented, followed by Bacillariophyceae (23), Cyanoprokaryota (8), Cryptophyceae (6), Chrysophyceae (6) and Dinophyceae (6). The time variation of chlorophyll a (0.5-9.72 µg l^-1), phytoplankton density (177-64925 cell. ml^-1) and biovolume (21-6665 mm³ m^-3) showed the same trend, with the lowest and maximum values re­corded during winter and from spring to summer, respectively. In order to compare the seasonal succession in the two sub-basins, cluster analysis and non-metric multidimensional scaling ordination were used jointly. Multivariate analysis showed i) a substantial homogeneity in the time seasonality of the dominant phytoplankton assemblages in the two sub-basins and ii) a clear differentiation of summer phytoplankton from the other seasonal phases. From a purely quantitative point of view, there were dissimilarities be­tween the two stations. The western sub-basin showed a greater algal production than Abbadia, especially in spring and summer. Despite the strong reduction of TP since the 1980s, the results of investigations carried out during the 1990s indicated a substantial stability of phytoplankton . It is emphasised that a reduction in biomass and shift in species composition will be possible only by as­suring a severe control of the nutrient loads.
 

12

Recent dynamics (1995-1999) of the phytoplankton assemblages in Lago Maggiore as a basic tool for defining association patterns in the Italian deep lakes
Giuseppe MORABITO, Delio RUGGIU and Pierisa PANZANI
 

The main goal of the study presented here is to identify a repeatable pattern in the seasonal succession of phytoplankton assem­blages in Lago Maggiore. In order to fulfil this objective we analysed the phytoplaktonic succession during a five years period (1995-1999), through the calculation of the Bray-Curtis similarity index applied to biovolume data. A cluster analysis has been then applied to the distance matrix, allowing the identification of sample clusters possessing a similar species composition. The comparison, through the whole period considered, of the phytoplankton assemblages characterising each cluster allowed to recog­nise six seasonal periods (Winter, Early Spring, Late Spring, Early Summer, Late Summer, Autumn), each of them characterised by a peculiar and repeatable species assemblage. Among the most interesting findings we would mention the existence of a Late Spring/Early Summer association, dominated by Planktothrix rubescens and Fragilaria crotonensis, probably peculiar of the deep subalpine lakes, where these species can better take advantage of the physical and chemical environment of the metalimnetic niche. The identification of a pool of dominant and sub-dominant species common to other southern subalpine lakes and the existence of a similar time periodicity in the development and decline of most of them across this lake district seem to be promising in order to give our results a wider application.