"High-altitude lakes · Pearls in the mountain landscape" edited by Fabio Stoch. Texts. Marco Cantonati · Alberto Carton · Luca Lapini · Valeria Lencioni · Bruno ...
I TA L I A N H A B I TAT S
High-altitude lakes
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Italian habitats Italian Ministry of the Environment and Territory Protection / Ministero dell’Ambiente e della Tutela del Territorio Friuli Museum of Natural History / Museo Friulano di Storia Naturale · Comune di Udine
I TA L I A N H A B I TAT S
Scientific coordinators Alessandro Minelli · Sandro Ruffo · Fabio Stoch Editorial committee Aldo Cosentino · Alessandro La Posta · Carlo Morandini · Giuseppe Muscio
"High-altitude lakes · Pearls in the mountain landscape" edited by Fabio Stoch
Texts Marco Cantonati · Alberto Carton · Luca Lapini · Valeria Lencioni · Bruno Maiolini · Sergio Paradisi · Margherita Solari · Fabio Stoch In collaboration with Paola Zarattini English translation Eleba Caladruccio · Alison Garside · Gabriel Walton Illustrations Roberto Zanella
High-altitude lakes
Graphic design Furio Colman
Pearls in the mountain landscape
Photographs Nevio Agostini 67, 81, 92, 101, 139 · Archive Museo Friulano di Storia Naturale (Tomasi) 63, 65/1 · Archive Museo Tridentino di Scienze Naturali (Cadin) 48 · Marco Cantonati 46, 47, 49, 51, 52, 53, 54, 55, 56, 58, 59, 61, 62, 65/2, 96, 130, 131, 132, 133 · Stefano Caresana 26/2 · Alberto Carton 8, 12, 16, 17, 19, 20, 21, 22, 23, 24, 27, 28, 31, 32, 33, 35, 123, 124, 125 · Carlo Càssola 103 · Compagnia Generale Ripreseaeree 30 · Ulderica Da Pozzo 38, 136, 138 · Adalberto D'Andrea 102, 105, 108, 110, 141 · Vitantonio Dell'Orto 6, 10, 14, 44, 60 · Angelo Leandro Dreon 114, 115, 116 · Paolo Fabbro 80 · Tiziano Fiorenza 100, 117 · Luca Lapini 121 · Bruno Maiolini, 82, 84, 85, 87, 91, 97, 98, 99, 134 Michele Mendi 118, 119/1 · Eugenio Miotti 111, 127 · Giuseppe Muscio 106, 129 · Paolo Paolucci 120 · Roberto Parodi 119/2 · Ivo Pecile 11, 41, 79, 89, 90, 128, 135, 143 · Roberto Seppi 13, 26/1 · Fabio Stoch 7, 42, 66, 68, 69, 70, 71, 72, 74, 75, 76, 77, 88, 93, 94, 95, 126, 144, 145 · Augusto Vigna Taglianti 9, 15, 39, 45, 83, 86, 122, 137 · Paola Zarattini 78 · Roberto Zucchini 112, 113, 142
© 2006 Museo Friulano di Storia Naturale, Udine, Italy All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, without the prior permission in writing of the publishers. ISBN 88 88192 28 X Cover photo: Laghetto di Avostanis (Carnic Alps, Friuli Venezia Giulia, photo: Ulderica Da Pozzo)
M I N I S T E R O D E L L’ A M B I E N T E E D E L L A T U T E L A D E L T E R R I T O R I O M U S E O F R I U L A N O D I S T O R I A N AT U R A L E · C O M U N E D I U D I N E
Contents
Italian habitats
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Alberto Carton · Fabio Stoch
Geomorphology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Alberto Carton 1 Caves and karstic phenomena
2 Springs and spring watercourses
3 Woodlands of the Po Plain
4 Sand dunes and beaches
5 Mountain streams
6 The Mediterranean maquis
Flora . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 Marco Cantonati
Invertebrates: zooplankton . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 Fabio Stoch
7 Sea cliffs and rocky coastlines
8 Brackish coastal lakes
9 Mountain peat-bogs
10 Realms of snow and ice
11 Pools, ponds and marshland
12 Arid meadows
Invertebrates: zoobenthos. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 Valeria Lencioni · Bruno Maiolini
Vertebrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101 Luca Lapini · Sergio Paradisi
Conservation and management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123 Marco Cantonati · Luca Lapini · Sergio Paradisi · Fabio Stoch 13 Rocky slopes and screes
14 High-altitude lakes
15 16 Beech forests The pelagic of the domain Apennines
17 Volcanic lakes
18 Mountain conifer forests
Suggestions for teaching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137 Margherita Solari
Select bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147
Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151 19 Seagrass meadows
20 21 Subterranean Rivers and waters riverine woodlands
22 23 Marine bioLagoons, constructions estuaries and deltas
24 Italian habitats
List of species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153
Flora MARCO CANTONATI
■ Algae in free waters Typical high-altitude lakes have crystal-clear water. Sometimes, however, excursionists may notice that the water has greenish hues, due to microscopic algal proliferation. As we shall see below, a series of factors make free waters in mountain lakes extremely difficult to colonise by algae. These lakes are covered by a thick layer of ice for about seven months in Vegetation surrounding the Lago della the year. When snow covers the ice Maddalena (Piedmont) sheet, the extent to which light can penetrate into the water is greatly reduced. The water is cold, and only its upper layer is warmed in summer. Mountain lakes are often in areas where the substrate is composed of rocky walls or debris, the soil is underdeveloped, and human activities are minimal or totally absent. These lakes therefore lack nutrients. When the basin is made up of crystal rock, which is almost insoluble, the lake water lacks dissolved minerals and is therefore likely to be acidified by atmospheric pollutants. Moreover, at high altitudes, UV radiation increases, and water transparency favours its penetration. Some mountain lakes are very shallow (10 m max), and UV radiation can easily reach their bottoms. This means that organisms living in them have nowhere to shelter from the negative effects of these rays. Living conditions in high-altitude lakes are therefore generally difficult, and algae can colonise them only by means of special adaptations. In these standing waters, algal plankton, or phytoplankton, is typically dominated by organisms with flagella, i.e., elongated filiform appendages, the primary organs of motion for these tiny animals. Flagellated algae are generally chrysophytes (or golden-brown algae), dinoflagellates, cryptophytes (those
Lagarot di Lourousa (Maritime Alps, Piedmont)
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producing buds underwater) and chlorophytes (also called green algae). Thanks to their capacity for movement, they are not at the mercy of currents and sediments (as planktonic diatoms are), and may be able to swim weakly to areas of the lake where conditions are more favourable. For instance, at night, when light is insufficient in surface water, these plankters may reach deeper water (vertical migration), where decay of organic matter occurs and nutrients are more abundant. Similarly, in winter they can accumulate under the surface covered with snow and ice, to exploit the dim filtered light. It is now known that several types of high-altitude lacustrine phytoplankton many more than was originally thought - adopt a peculiar strategy, called mixotrophy, to make up for the lack of nutrients. This is an elaborate mode of deriving nourishment from both autotrophic and heterotrophic mechanisms, so that these types of plankters behave sometimes as plants and at others as animals. If nutrients are sufficient, they behave as we suppose they should - that is, as plants: they consume dissolved salts and obtain energy from sunlight (or rather, those components of it that can penetrate at various levels) through photosynthesis. If nutrients are insufficient, the algae turn into predators, and feed on bacteria as well as on other algae. This technique is especially, but not only, adopted by microalgae belonging to the cryptophytic and dinoflagellate groups.
As regards UV radiation, when lakes are deep and algae can move independently, they move to the deepest areas of the lake, where the rays cannot penetrate. When waters are shallow and clear, they activate natural substances that function as UV filters, such as aminoacids similar to mycosporins and a few carotenoids. In addition to flagellate algae, phytoplankton in high-altitude lakes may include other groups, such as diatoms and cyanobacteria. Both prefer nonacid water, and planktonic diatoms (the round, radial ones) actually disappear from acid water. However, high-altitude standing waters often host benthic diatoms (i.e., those adapted to life on the bottom). A few species may even spend stages of their lives in free water (as planktonic algae). Among benthic diatoms, some species are perfectly adapted both to life in waters containing minimal amounts of minerals subjected to sudden, seasonal peaks of acidity, and to those that are permanently acid due to natural or anthropic causes. Cyanobacteria include species specially adapted to environments that have reached high trophic levels due to organic contamination, and are very numerous in mountain lakes affected by animals grazing, large numbers of tourists, etc. Open waters in high-altitude lakes therefore host stable algal communities which are well adapted to the harsh living conditions and the challenges that
The green alga Ankistrodesmus
Cyanobacteria (Tolypothrix) with yellow sheath
The golden-brown alga Dinobryon
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The phytoplankton of the Lago di Tovel and summer algal blooms The Lago di Tovel in the Brenta Dolomites (Trentino) - often mentioned in this section - is well-known for the intense red algal bloom that forms on its surface, especially along its southwestern shore (called Baia Rossa, or red bay) in hot, sunny summer weather. The phytoplankton of the Lago di Tovel are mainly composed of flagellate algae (especially dinoflagellates and chrysophytes), typical inhabitants of high-altitude lakes. Tovel is actually at only 1178 m asl, is surrounded by a forest of silver fir and Norway spruce, and is not a typical mountain lake. However, it is supplied by a large permeable aquifer from several sublacustrine springs in the Baia Rossa. The continuous supply of cold water and the local cool, humid climate lower the temperature of its surface water
Reddening of the Lago di Tovel in1961
even in summer, like that of lakes well above the treeline. In addition to dinoflagellates and chrysophytes, lake phytoplankton include a large group of diatoms (both centric, with radial symmetry, and pinnate, with bilateral symmetry, stickor shuttle-shaped), cryptophytes, several green algae and cyanobacteria (especially those of the genus Synechococcus, typically found in clean water). The large number of planktonic diatoms and cyanobacteria are due to the slightly alkaline (never acid) water, caused by the type of rock (limestone and dolomite). However, this lake phytoplankton would never have become so well-known outside scientific circles if it were not for the particular seasonal behaviour displayed until 1964 by one of their dinoflagellate species. In summers with long periods of nice weather, this species bloomed very profusely, reaching, particularly in the Baia Rossa, considerable density (from more than half a million to tens of millions of cells per litre). The cytoplasm of these cells contains red carotenoids that coloured the water scarlet. An important research project (SAL-TO), financed by the Autonomous Province of Trento, enabled scientists to shed light on many aspects of this phenomenon, including the taxonomic classification of the alga, the causes of the bloom and, consequently, possible reasons why it ended in the mid-1960s. Although these reasons are still being examined, it is likely that the algal blooms were supported by a large quantity of nutrients introduced into the lake, caused by a different use of the territory. In the past, the area had been
Marco Cantonati used for grazing by cattle in way which were unlike pravious ones, and stalls and barns were cleaned in a very different way. Since the late 1930s, when the zoologist Edgardo Baldi carried out research on the Lago di Tovel and its bloom, the dinoflagellate that was held responsible for the phenomenon - at the time named Glenodinium sanguineum by the scientist Marchesoni - was thought to be present in two physiological stages, one green and the other red. The passage from the former to the latter stage occurred through accumulation of carotenoids in its cytoplasm, due to specific environmental stimuli (excessive sunlight and lack of nitrogen - what in fact occurs in algae that give rise to minor, occasional blooms in smaller
bodies of water, like the green alga Haematococcus pluvialis. See the section in “Pools, ponds and marshes” of the Habitat series). Isolation and harvesting of specimens showed that, of the three species composing the morphologically similar group once thought to be responsible for the green and red stages of G. sanguineum, only one is the actual reddening agent (now present in very small quantities), and the harvested form is always red. This recently identified species has been named Tovellia sanguinea, and what were thought to be the green stages of G. sanguineum are now classified as the new species Baldinia anauniensis. Recent analyses have also shown that this species contains aminoacids similar to mycosporin.
Tovellia sanguinea
Campylomonas sp.
Baldinia anauniensis
Fragilaria tenera
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these environments pose season by season. As these conditions are similar m in high-altitude lakes all over the world, and as these algae can spread 5 efficiently thanks to their adaptations, the result is a type of high-altitude lake 10 phytoplankton dominated by cosmopolitan species. 15 As already mentioned, the groups of algae and cyanobacteria living in high20 altitude lakes vary according to the type of rock of the basin itself, and therefore to the level of mineralisation, 25 and that the distribution of these organisms in the water column is not 30 random. Algal groups also vary according to seasonal changes, and Seasonal variation of phytoplanktonic biovolume (mm3/litre) at various depths species distribution may also vary (metres) considerably throughout the year. Due to the complex combination of environmental factors that endow each high-altitude lake with unique characteristics, it is therefore difficult to provide a clear chart of seasonal phytoplanktonic variations. Having said that, however, the following generalisation can be made. In spring-early summer, after winter ice has melted, the dominating algae are flagellate, belonging to the dinoflagellates and cryptophytes. Green algae develop in summer, and planktonic diatoms are abundant in non-acid lakes in autumn and spring (although they may also be numerous in summer, if weather conditions are favourable to their growth). Planktonic cyanobacteria peak in autumn. Large numbers of chrysophytes are found in many high-altitude lakes, independently of the sampling period, especially at average depths. The abundant post-thaw development of acidophilic armoured dinoflagellates, like Peridinium umbonatum, is typical of high-altitude lakes lacking minerals. This is evident in diagrams showing the vertical distribution of phytoplankton biomass and the seasonal variations in numbers of algal groups in poorly mineralised and slightly acidic alpine lakes. In these same lakes, dinoflagellates also dominate in summer, and are later replaced by green algae of the genus Oocystis. In winter, phytoplankton is greatly reduced. As expected in lakes with these chemical characteristics, planktonic diatoms are barely present. 0
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■ Shore and bottom algae
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In addition to vascular plants, bryophytes and lichens that mark the maximum water level of a lake, excursionists’ attention may be caught by sometimes large, filamentous algal masses resembling aquatic vascular plants, which are, in fact, charophytes (stoneworts). Although they generally live on carbonate substrates, their two most important genera are found in alpine lakes: the genus Chara on carbonate and Nitella on silica. These macroscopic algae have complex structures, with whorls of nodes and internodes. When handled, they typically smell of garlic. They usually colonise lake shores, where the water is so shallow that they are often left totally dry, or trapped in snow in winter, although they can also live at certain depths within the lake. Other types of gaudy plants along lake shores are filamentous green algae, which form stringy masses on stones and mosses. Although they are coloured in different hues of green, bright green indicates the development of Zygnemales. The chloroplasts of the most common genera have typical shapes easily recognisable under a microscope: spiralling in Spirogyra, starshaped in Zygnema and rod-like in Mougeotia. Drawdown areas and those just above the usual water level may contain the filamentous green alga
Vertical zonation of cyanobacteria and lichens on a boulder near the shore of a high-altitude lake
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Ulothrix (Ulothrichales). Generally submerged but near the lake shores are Oedogoniales, with the genus Oedogonium, the cells of which have a series of particular “caps” at one end (produced by cellular division and growth of their bilayered cell walls, whose number corresponds to that of cell divisions). Filamentous green algae are quite frequent in high-altitude lakes, although they bloom in precise environmental conditions, i.e., when nutrient supply to the lake increases (cattle grazing nearby, barns in the vicinity, etc.). Another unusual situation that can cause excessive growth of these algae is produced by precipitation of acid pollutants. Lake basins in crystal rock near peaty areas or deposits of sediments and debris often contain large numbers of Desmidiales. Desmids are unicellular green algae (occasionally filamentous) also known as monifiliform algae, due to their interesting shapes which are only visible under a microscope. The most important algal groups colonising the shores and bottoms of mountain lakes are those which are barely visible to the eye: diatoms and blue-green algae (cyanophytes/cyanobacteria). The latter actually form typical bands of various colours (black, red, turquoise, reddish-brown, brown) arranged in a typical pattern starting from the mean hydrometric water level. Instead, diatoms can only be identified as golden-brown covers on the rocks
Filamentous green algae (Spirogyra sp.) and bryophytes in a mountain basin
or films, which can actually be quite thick on aquatic plants (especially on decaying parts). Some cyanobacterial films are particularly gaudy. In high-altitude standing waters, their blackish mucilage is frequently seen a dozen centimetres above the average water level - on the exposed littoral zone especially in the many lakes that undergo frequent variations in water level. This film is known as the “Gloeocapsa line”, from the name of the cyanobacterial genus that is commonly found on the rocks. The blackish hue is due to the violet sunproof pigments (gloeocapsins) Filamentous green alga (Spirogyra sp.) under the microscope which are accumulated by these cyanobacteria to prevent damage caused by UV radiation, so typical of the harsh, exposed habitats they colonise. Algae and cyanobacteria are not only found in shallow water, but all along the littoral zone and at the bottom of lakes, both in areas intermittently sprayed by water, and at considerable depth, if the lake waters are transparent enough. These organisms obtain energy from light thanks to photosynthesis, and therefore require a minimal source of light to colonise bottoms with stable, vital populations. It is interesting to note that cyanobacteria, also called blue-green algae, have a simple cellular structure - like that of bacteria - without a true nucleus separated by membranes, and they are therefore more closely related to non-photosynthetic bacteria than to other algae. However, they use solar energy through photosynthesis, thus delivering oxygen, just as algae and vascular plants do. Very little research has been carried out on cyanobacteria populations living at the bottom of lakes in the European mountain chains, including the Alps and Apennines. On-site research is very difficult, as the distribution of algae in water is only visible to scuba-divers who require special scientific equipment and instruments, which have to be carried to the site. The most common algae at varying levels of a lake are cyanobacteria. They change colours according to their vertical distribution. Species of Gloeocapsa, Calothrix parietina and Chamaesiphon polonicus are generally found in littoral zones sprayed by waves.
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Rocks in shallow water are covered by a sometimes thick, brownish film containing several cyanobacteria, such as Scytonema, Schizothrix, Dichothrix, Tolypothrix and Ammatoidea. As depth increases, other taxa are found, like Tolypothrix, Nostoc and Phormidium. Further down, at considerable depth, on limestone, there are Geitleribactron periphyticum and the rare Chlorogloea purpurea. Microdiatoms are also arranged according to depth, although they are barely visible. Some species only colonise shallow water (e.g., Denticula tenuis, Delicata delicatula); others, even in large numbers, only intermediate levels, like Gomphocymbellopsis ancyli, Brachysira calcicola and Achnanthes trinodis. Still other species can only live in deep water, such as Achnanthes montana, Staurosira pinnata and Staurosira brevistriata, and there are also algae that can live at any depth, showing no preference, like Achnanthidium minutissimum. Diatom species may live at various depths in different lakes, and if one lake has varying characteristics, at various depths even within the same lake. Red algae (rhodophytes) are sometimes found at considerable depth, and several genera of this group are known to be adapted to life in dim habitats. The environmental conditions to which cyanobacteria and algae are exposed
in high-altitude lakes vary greatly with depth. In the upper layer, especially if subject to level variations, drought is likely and these organisms, frequently exposed to the air, must resist UV radiation. As depth increases, the main environmental problem becomes the rapid dimming of available light. Algal development can occur down to a depth where light available for photosynthesis is only 1% of the total light penetrating the lake surface. The area in which algae develop is called the euphotic region; below this is cold, dark water - the aphotic region - in which the organic matter produced in the upper layer of water decomposes. It must also be noted that not all the components of the light spectrum have the same capacity for penetrating water, and are therefore absorbed selectively, partly according to the characteristics of the dissolved substances and suspended particles in water. UV radiation is generally absorbed in the upper few metres of water (although in very clear lakes it may penetrate for a dozen metres), and in oligotrophic lakes (those lacking nutrients), the light spectrum penetrating deeper (for several metres) is predominantly blue and green. As depth increases, temperature decreases and, in the lower levels, seasonal variations fall and stabilise around 4°C throughout the year in the deepest levels (temperature of maximum water density). Temperature decrease with
The cyanobacterium Chlorogloea purpurea. This rare species has only been found at depth, on the rocky beds of two Alpine lakes (Lunzer Untersee in Austria and Lago di Tovel, Brenta Dolomites, Trentino)
The diatom Gomphocymbellopsis ancyli - a species of holarctic distribution - found at depths between 9 and 18 metres in the Lago di Tovel (Brenta Dolomites, Trentino)
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depth is not regular, and there is often a layer - called the thermocline marking the point at which water temperature decreases steeply, up to several degrees per metre. However, small high-altitude lakes that are generally shallow and only occasionally undergo wind turnover during summer storms, seldom have thermoclines and well-developed epi- and hypolimnia. The thermocline separates the upper layers of well-illuminated water (epilimnion), which are warmer and more productive, from the deep, very dim or totally dark, dense water (hypolimnion). Since algae and other autotrophs capable of producing oxygen colonise the upper layer, and organic matter is decomposed in the lower layer by organisms that consume oxygen by means of respiration, the epilimnion of deep lakes is well oxygenated, whereas dissolved oxygen may plummet in the hypolimnion. Conversely, algal nutrients are less abundant in the epilimnion, where they are consumed by algae, cyanobacteria, lichens, bryophytes and vascular plants, and more abundant in the hypolimnion, where they can even be generated by mineralisation of organic matter. In the transitional zone between the poorly oxygenated waters of the hypolimnion and the highly oxygenated ones of the epilimnion, the stones at the bottom of siliceous basins where water contains iron are colonised by
Chamaesiphon polonicus, a cyanobacterium with a thick reddish sheath protecting it from desiccation
large numbers of ferrobacteria (or sheathed bacteria). These betaproteobacteria are typically found in aquatic environments rich in iron, in the interface between anoxic and oxygenated habitats (e.g., in ferruginous springs). As already mentioned, algae and cyanobacteria living on the bottom of highaltitude lakes overcome the harsh environmental conditions by means of a series of adaptations. In the shallow water of the littoral zone, algae and cyanobacteria must protect themselves from intense UV radiation. They therefore look for shelter, repair the damage caused to their cells, and protect themselves from UV radiation with pigmented compounds. The first procedure can only be used by motile species (e.g., by filamentous, creeping algae) and this, together with the second procedure, requires physiologically active cells. The ability to move and repair molecular damage, generally to their DNA, requires cellular metabolism and enzymatic systems responsible for repair to be active. The third procedure has the great advantage that it can be carried out even if the cell is quiescent or metabolism reduced, enabling the organisms to overcome unfavourable conditions. In addition to gloeocapsin, one of the most common sunproof pigments of cyanobacteria is scytonemin, which can absorb UV-A and part of UV-B, i.e., most of the dangerous components of UV radiation. Scytonemin is only produced by some cyanobacterial taxa, which collect it outside their cells, generally within the sheath enclosing some species. Scytonemin turns the sheaths protecting cells yellow, even when their metabolism is almost inactive. The sheaths may also be thick and robust (as in the species Chamaesiphon polonicus, in which they are a bright reddish-brown), thus protecting cells from dehydration in periods when species lacking locomotion are exposed to drought. As already mentioned, many diatom species have powers of locomotion and can therefore seek refuge from desiccation. Other non-motile species produce large amounts of mucilage in which their cells are immersed to prevent dehydration. Diatoms, other algae and cyanobacteria do not only colonise stones, but also surface sediment. This substrate is exclusively inhabited by motile diatoms - especially species of the genera Navicula s.l., Nitzschia and Surirella - which may even carry out daily vertical migrations in the upper sediment layers. These migrations enable them first to exploit daylight and then to burrow into sediment to nutrient-rich microhabitats at night. Locomotion is also essential for sediment-living algae that would otherwise be buried by micro-mudslides and accumulation of debris.
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As depth increases, algae no longer have to protect themselves from radiation - in fact, they need to collect and make the best possible use of the dim light that penetrates through the water. Microalgae can do this effectively by increasing the efficiency and yield of their photosynthetic apparatuses. Other adaptations include changes in the absolute quantity of chlorophyll, accessorial pigments and their relative proportions. Cyanobacteria on a stone Accessorial pigments are compounds typical of photosynthetic processes, the role of which is to collect fractions of light energy in reaction centres within chlorophylls. Generally, microalgae living on the bottom at considerable depth have higher amounts of chlorophyll in their cells. The cell content of the main accessorial pigment also increases. In diatoms it is fucoxanthin, which is golden-brown, and this is why diatoms collected deep in the water are dark brown. In addition, fucoxanthin absorbs light in the green spectral zone, and this is why these algae live at depths at which the light spectrum is blue and green. They are totally different from the algae of shallow water, the chloroplasts of which are greenish-yellow. Cyanobacteria have two main accessorial pigments: one is blue (phycocyanin) and the other red (phycoerythrin). In order to adapt to poorly lit environments at the bottom of lakes, these organisms can vary the absolute quantities of these pigments within their cells and the relative amount of chlorophyll. This adaptive mechanism is known as photochromia. Individuals of the same species may be typically blue-green if they live in shallow water, and pinkish-violet if they live at depth. A similar mechanism can also occur within a community, and species which are always pinkish-violet generally live in deep water. During research work on the algal distribution in the Lago di Tovel, samples taken from the deep, lightless zone (aphotic zone) contained cells of diatoms that were not only healthy, but even motile. The reason is that, in the absence of light, some diatoms become heterotrophs and consume various types of organic compounds. More generally, algae survive long periods in the dark areas of a lake by becoming quiescent and reducing their metabolism to a minimum.
■ Aquatic and riparian plants High-altitude lakes are environments where aquatic and semi-aquatic vascular plants are not expected to be found. However, while hiking to mountain lakes, excursionists will certainly come across sedges, rushes and horsetails along the shores of lakes containing deposits of fine debris. They will marvel at the elegant designs created by the thin, tapering leaves of floating bur-reed (Sparganium angustifolium) drifting on the water surface, and notice the tiny white flowers of the delicate water crowfoot emerging from the surface of the water. High-altitude lakes can therefore host submerged, semi-submerged and shore (hygrophilous peri-lacustrine) plants. These types of vascular plants colonise lakes in which sediment accumulates, giving rise to sandy-silty bottoms with organic debris. This generally occurs near the outlets of lakes with a certain trophic load, or those collapsing - a very slow phenomenon involving all lakes. In these areas, peri-lacustrine and aquatic plants play an important role. At the end of their vegetative cycle, a large quantity of plant material deriving from their development accumulates on the bottom of the lake, favouring its rise.
Riparian sedge meadow surrounding a high-altitude lake
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Cotton-grass (Eriophorum scheuchzeri)
This process produces new substrate for colonisation by peri-lacustrine plants which, in high-altitude lakes, are the true protagonists, as only a limited number of species of truly aquatic plants are present. Botanical hand-books and texts generally quote the Latin names of the plant associations rather than guide species, which are defined according to precise rules. Occasionally, however, in some territories the guide species which give the name to the association may not be very common, although there are plant communities having characteristics which clearly belong Flowerheads of floating bur-reed (Sparganium angustifolium) to a particular association. The description given below does not quote the Latin names of the associations, but lists the plant communities and their dominating species, among which are these most commonly found in high-altitude lakes. Several species of the genus Potamogeton are found in mountain lakes. They are rhizophytes rooted in the bottom of lakes and ponds. The community with reddish pondweed (Potamogeton alpinus) grows in oligotrophic and mesotrophic lakes, on both carbonate and siliceous substrates. It is therefore found in slightly alkaline, poorly mineralised water. The community with longstalked pondweed (Potamogeton praelongus) colonises deep water in oligotrophic lakes with siliceous substrates. It is difficult to find because it grows far from the shore at considerable depths. This is a very rare species and is generally collected during scuba-diving excursions and limnological samplings with grabs. The association with slender-leaved pondweed (Potamogeton filiformis) is the aquatic plant community that reaches the highest altitudes on the Eastern Alps. It grows near the shores of shallow, mesotrophic lakes on carbonate substrates. It includes marestail (Hippuris vulgaris), characeous algae (Chara sp.) and bottle sedge (Carex rostrata). Although marestail usually lives at low altitudes (up to 600 m), it has also been found in high-altitude lakes, probably thanks to transportation of seeds by migrating birds.
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At intermediate altitudes, mountain basins subject to eutrophication on carbonate rocks host, at a certain depth (3 m), the community with perfoliate pondweed (Potamogeton perfoliatus), including the invasive Canadian waterweed (Elodea canadensis). Groups with water starwort (Callitriche), bur-reed (Sparganium) and threadleaved water crowfoot (Ranunculus trichophyllus ssp. eradicatus) dot with their grassy mats shallow lakes in the sub-alpine and alpine zones. Vernal water starwort (Callitriche palustris) grows in high-altitude basins on siliceous substrates and on sand in shallow water (it is typically found in the Lago delle Buse in Trentino). At intermediate altitudes, eutrophic carbonate lakes may host white water-lily (Nymphaea alba). The community with this plant is relatively rare in mountain regions, because it prefers bodies of water that warm up in summer. Communities with common reed (Phragmites australis) are also found at lower altitudes, in eutrophic (nutrient-rich) lakes. On carbonate mountains, the shallow waters of mesotrophic lakes, which easily dry up in summer, may be colonised by broad-leaved pondweed (Potamogeton natans) and water knotgrass (Polygonum amphibium). These lakes are generally found in the upper sub-alpine and lower alpine limits. Their
Lush growth of floating bur-reed (Sparganium angustifolium) along the shores of an alpine lake
environments may undergo severe damage due to eutrophic factors such as cattle drinking. Knotgrass is quite rare on the Alps, especially on the southern versant. The community dominated by broadleaved pondweed and thread-leaved water crowfoot typically grows in the littoral zone of lakes in the upper subalpine/lower alpine limit of siliceous mountains. It lives in shallow water, along shores undergoing summer drought, on sandy-silty substrates with organic debris. Floating bur-reed has inconspicuous greenish flowers, so that the presence of this macrophyte in alpine lakes is only revealed by its thin, Thread-leaved water crowfoot (Ranunculus trichophyllus ssp. eradicatus) ribbonlike leaves, which may exceed one metre in length. Rooted into the bottom, it grows vertically until it reaches the water surface, when it starts developing horizontally, one plant next to the other. Floating bur-reed is not very common, and its populations are scattered over several mountain areas. Other communities are mainly composed of thread-leaved water crowfoot, which colonises the shallow waters of high-altitude oligotrophic lakes on siliceous substrates, sometimes cohabiting with floating bur-reed. Communities exclusively containing thread-leaved water crowfoot may be the initial colonising stage of lakes on siliceous mountains. The substrate on which this plant lives is composed of coarse mineral debris lacking organic matter. The water is neutral or slightly alkaline and moderately mineralised. As organic matter accumulates in sediment, it deprives it of ions, a process which gives rise to water acidification, so that thread-leaved water crowfoot is gradually replaced by floating bur-reed. At intermediate altitudes, siliceous substrates with structured shores covered with vegetation host bogbean (Menyanthes trifoliata), which is found in the Lago delle Buse and Lago delle Malghette in Trentino. The shores of alpine lakes may be dotted with the downy white hairs of common cotton-grass (Eriophorum angustifolium). The community with Eriophorum scheuchzeri is often found in alpine bogs and along lake shores. The distribution of plants on the bottoms and shores of high-altitude lakes is
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not random. Typical distribution starts from free, open water in the middle of the lake and then gradually develops towards the shore: submerged plants and aquatic moss, semi-submerged plants, hygrophilous shore moss, and then swamp plants. When vegetation is well developed, it is arranged in a belt pattern called vegetation series. The area surrounding the open water of oligotrophic lakes and ponds in the Dolomites also frequently hosts communities with bur-reed and water starwort; the shores are inhabited by plant communities, the first one nearest to the water containing bog Tufted bulrush (Trichophorum caespitosum) sedge (Carex limosa), followed by one with harestail cotton-grass (Eriophorum vaginatum) and tufted bulrush (Trichophorum caespitosum). Again, starting from the middle of the lake towards the shore, mesotrophic lakes are colonised by slender-leaved pondweed, followed by broad-leaved pondweed and water knotgrass. At lower altitudes (up to 1000 m), eutrophic lakes may be densely covered with white water-lily, fennel pondweed (Potamogeton pectinatus) and perfoliate pondweed. Besides vascular plants, bryophytes also play an important role, either as very large semi-floating masses in shallow water (especially if combined with peat
Succession of vegetation types on the shores of a high-altitude lake (Lago delle Buse, Trentino); from the open water and moving towards the shore are communities of floating bur-reed and vernal water starwort, two sedge groups and, lastly, one community of cotton-grass and tufted bulrush
moss) or as shore colonisers. These are ideal habitats for the development of the grass frog. Submerged and shore aquatic plants and bryophytes, which are covered with films of epiphytic microalgae, favour colonisation by rich animal and plant communities. Large amounts of lichens cover the rocky shores of mountain lakes. They are generally emergent species, which occasionally live on intermittently sprayed areas. Very little is known of those adapted to aquatic environments, and they are probably only found in shallow water. This is because lichens are the product of symbiosis between fungi and algae, and the latter, also called phycobionts, require sunlight to carry out photosynthesis. The transparency of high-altitude lakes certainly favours them, enabling them to colonise even deep water. There is a considerable difference between lichens found on the shores of carbonate as opposed to siliceous rocks. Other natural variables affecting lichens on lake shores are shade and humidity. These communities are also arranged in typical belts in the lower and upper portions of the intermittently sprayed areas of the lake, where humidity is higher. Seasonal and short-term variations in lichens are far smaller than those affecting algae, because lichen grows very slowly, never exceeding a few millimetres a year.
Floating masses of bryophytes in a high-altitude basin
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High-altitude lakes I TA L I A N H A B I TAT S Ministero dell’Ambiente e della Tutela del Territorio Museo Friulano di Storia Naturale
This book is an emergency message, an SOS that the Habitat series is sending out in defence of some of the most fascinating and breathtakingly beautiful Italian environments.
High-altitude lakes
Unfortunately, mountain lakes are now far from being remote and pristine. Their exploitation by mass tourism, for hydro-electrical purposes and as sources of drinking water, the indiscriminate introduction of fish species, acidification and pollution by rain, increased UV radiation due to ozone reduction in the atmosphere, and the rising temperatures caused by global climatic changes are all factors jeopardising the survival of these ecosystems.
H A B I TAT S
Animal and plant species, which colonised these environments after the retreat of the Quaternary glaciers, found refuge - and were sometimes “trapped” - in these areas as relicts. They therefore had to cope with the harsh environmental conditions of the mountains, such as the cold climate, high UV radiation and lack of nutrients. Only a few species have made the necessary adaptations to survive, and this is why glacial relicts are extremely interesting from the scientific and conservation viewpoints.
I TA L I A N
Whenever we think of lakes high up in the mountains, we envisage the classic small bodies of water set in green conifer woodland or glinting in sunny valleys surrounded by rocks and high peaks. However, reality goes beyond this commonplace. Mountain lakes display a whole series of complex geomorphological and biological characteristics.
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