Marco Cantonati

Ecological effects of water-level fluctuations in lakes: an urgent issue

Water-level fluctuations (WLF) affect the ecological processes and patterns of lakes in several ways. Aquatic habitats and feeding or breeding grounds are gained or lost, as light, climate and wave impacts change to mention only a few phenomena. While the phenomenon of WLF has been studied in many rivers (better known as flood pulse, Junk & Wantzen, 2004) and environmental flows (Coops et al., 2003), so far it has not received due attention in lakes (Coops et al., 2003). A potential reason for why WLF in lakes have been overlooked for such a long time may be that central Europe and North America—the cradles of limnological research—are situated in landscapes which have been shaped by man for long time. Most central European lakes have been regulated since the middle ages; therefore, naturally fluctuating lakes such as Lake Constance (Fig. 1) are rare today.

Springs of the Alps – Sensitive Ecosystems to Environmental Change: From Biodiversity Assessments to Long-term Studies

The main aim of the present review is to synthesise the present level of knowledge on the hydrobiology of springs in the Alps. Springs are usually small, but complex and taxa rich. They have a mosaic structure, a high degree of individuality and an azonal character, due to the peculiar physicochemical stability. Springs are ecotones linking an aquifer to the uppermost section of a surface running water system. Due to adaptations, of which stenothermy is the most frequent, representatives of many groups of plants and animals have in springs their exclusive (crenobionts) or favourite (crenophiles) habitat. This leads to a peculiar longitudinal distribution of organisms. In spite of this complexity, springs (in particular high elevation springs of the Alps) have received much less attention than other types of inland waters. They are endangered habitats, being menaced by a series of direct impacts (primarily water abstractions) and indirect impacts. The classical Steinmann–Thienemann ecomorphological types are still used with differences in the physical and chemical characteristics and in the biota. In the Alps rheocrene springs (where current velocity is one of the most relevant factors and the importance of the fringing semi-aquatic habitats is reduced) are the most frequent. However, in nature, most springs are transition types among the three traditional ones and these can now be assessed thanks to procedures considering mainly the substrate particle size. The physicochemistry of springs is characterised by limited seasonal fluctuations and determined by the characteristics of the aquifer and by indirect (especially airborne contaminants) and direct impacts. Many groups of organisms are well represented and include indicators of the trophic and acid–base status, of hydrogeology, hydrological stability and biological integrity. The groups with the highest proportions of specialised taxa are mosses, water mites, dipterans, hydrobioid snails and caddisflies. Attempts are being made to add to the traditional spring types hydrochemistry and vegetation and to the regional faunistic types, diatom-based types, to combine different approaches and to consider also functional ones, such as the distinction between POM and mosses’ springs. Given the high proportion of spring-specific taxa, we propose to use the zoobenthos, especially water mites, for an evaluation of the biological integrity of springs, whilst autotrophs, and in particular diatoms, which are the most widespread and taxa-rich group, can provide excellent indicators of eutrophication, acidification and ionic strength. This would require the calibration of the most promising diatom-based procedures to the spring environment and improvement of the knowledge on the taxonomy and distribution of key zoobenthos groups in the Alps. Other areas where further research is required are the definition of integrated spring-types, the potential for recovery of autotrophs in restored springs, functional aspects, the applicability of paleolimnological techniques to springs and the link between impacts on the drainage basin and spring environmental quality. Only once the importance of springs can be understood and effective conservation measures are undertaken, will it be possible to use these peculiar environments characterised by stability and by integration in time and space of direct and indirect impacts as ideal study sites for long-term ecological research.

Effects of water-level fluctuations on lakes: an annotated bibliography

Water-level fluctuations (WLF) in lakes and rivers, especially their extent, frequency and duration, are dominant forces controlling the functioning of these ecosystems. In particular, WLF play an important role in the lake’s littoral and aquatic–terrestrial interface processes. WLF may take place on different spatial and temporal scales under natural conditions but water levels have been artificially modified in regulated lakes, and their impacts are expected to be enhanced within the actual global change scenarios with forthcoming management problems. This article presents an overview on the literature published on this important topic since 1991 by using journals indexed in the ISI Web of Knowledge. The overall objective was to examine temporal and spatial trends in publications on WLF, the specific aspect of WLF concerned and their main effects. Throughout the article we have used case studies to illustrate different effects of WLF on the variety of lake habitats and indicators that have been studied. Overall, the number of papers published on the subject since the 1990s has risen steadily, when less than ten papers were published each year; until 2006 and peaking in 2005. The greatest number of papers on WLF has been carried out in Europe and North America (c. 73%). These data also showed that the effects of WLF have not been studied equally for different groups of organisms. There is a greater interest on macrophytes, which are the most studied group of organisms (18.4% of the papers). Nearly 7% of the papers deal with algae, and zooplankton and invertebrates account for a 7% followed by fish. WLF effects on ecosystems are very complex, and the biological effects in lakes are greatest in shallow water and littoral areas, where even small changes in water levels can result in the conversion of large areas of a standing-water environment in air exposed habitats. Finally, these data might serve to highlight knowledge gaps still existing on this topic and, in particular, some of the approaches that can potentially contribute to solve several of these lacunae are explored.

Benthic Algae in High Altitude Streams of the Alps – a Neglected Component of the Aquatic Biota

This is a review on benthic algae from streams situated above the tree line in the Alps. It integrates published and unpublished data from alpine streams in Austria and in the Trento Province (Northern Italy). The main focus is on the structural and taxonomic composition of benthic algae including macro- and micro-algae and their contribution to the epilithic biofilm and the stream food-web. The environmental conditions relevant to algae within the two major stream types, the glacial (glacier-fed) and non-glacial krenal (spring-fed) stream are discussed. The paper considers both the maximum possible structural complexity of transverse algae zonation in cascading alpine/subalpine stream segments, and the effects of glacial water on species richness in the Central Alps in Austria. Autecological data are given for 46 common diatoms from 42 sites in the Central Eastern and Southern Central Alps and for 30 algae in addition to diatoms for 22 streams in the Central Eastern Alps. Since data on responses of benthic algae to the harsh conditions in high altitude Alpine streams are very scarce, results from literature and the author’s experiences from these and other mountain stream types are synthesized to formulate major objectives for future research in benthic high altitude Alpine stream ecology.

The role of environmental variables in structuring epiphytic and epilithic diatom assemblages in springs and streams of the Dolomiti Bellunesi National Park (south-eastern Alps)

Diatom assemblages were investigated in 16 springs and five streams of the Dolomiti Bellunesi National Park (south-eastern Alps, Italy), by collecting individual samples of the main substrata (stones, bryophytes, surface sediment), and measuring an extensive number of morphological, physical, and chemical variables. An empirical grouping of these aquatic ecosystems, based mainly on morphological features and physical parameters, mostly agreed with groupings produced by non-metric multidimensional scaling. Significant differences among the diatom assemblages were found among the groups identified; however, these differences were more significant in the epilithic than in the epiphytic assemblages. Epiphytic and epilithic diatoms differed significantly according to: a) the identity of several species, b) the relative abundance of some other species, c) the Bray-Curtis distance between the assemblages, and d) taxa richness and diversity index. However, on studying species composition by canonical analysis, with reference to environmental variables (here called macrohabitat) on the one hand, and the type of substrata (stones and mosses) on the other, it was shown that the macrohabitat was responsible for more than 40 % of the variability among diatom assemblages, whereas the microhabitat (i.e. different substrata) accounted for only 3 %. Multivariate analyses (CCA) showed that the environmental factors most influential in structuring diatom assemblages were substrate particle size (influenced in turn by current velocity), permanence of flow, shade, nitrates, and total phosphorus. Characteristic groups of species were identified in the different morphological and hydrological types of springs and streams. Stream sites were often affected to a certain extent by organic pollution, even within the protected area, and consequently colonized by cosmopolitan, eutraphentic or tolerant taxa. In springs, in addition to common and widely distributed species, many poorly known, rare, or Red List species were found, in particular among the less abundant taxa. Our study therefore highlights the important role of springs as valuable habitats for the conservation of aquatic biodiversity.

Environmental controls of epilithic diatom depth-distribution in an oligotrophic lake characterized by marked water-level fluctuations.

The depth-distribution of epilithic diatoms in a carbonate meromictic lake (south-eastern Alps), characterized by marked water-level fluctuations, was investigated. Fixed stations were placed along a depth-profile at intervals of 2–3 m and sampled throughout the year using scuba diving. Diatom analysis included quantification of living cells (biovolume calculations) and the estimation of fucoxanthin concentration, which, coupled with the analysis of digested material, permitted taxonomic determination at specific or subspecific level. Multivariate analyses indicated the following variables to be significant: water-level fluctuations, photosynthetically active radiation, silica and nitrates. Diatoms had distinct depth-distributions. Quantification of densities, biovolumes and Chl a, and physiological (senescence index, fucoxanthin), functional (photosynthetic efficiency), and community structure (diversity) parameters allowed the definition of three depth-distribution zones (shallow, mid-depth and deep). The different diatom communities in these zones were confirmed by ANOSIM (analysis of similarity). The shallow, mid-depth, and deep zone were characterized by disturbance due to water-level fluctuations, high stability with favourable growth conditions and severe light limitation, respectively. Community composition and diversity depth-distribution features remained relatively stable throughout the year, while marked changes in benthic diatom biovolumes were modulated by competition for light with phytoplankton. Fucoxanthin was strongly correlated with epilithic diatom biovolumes. The results indicate that depth-distribution patterns should be considered when performing lake diatom biodiversity inventories and integrity evaluations because water-level fluctuations are likely to increase in many lakes due to increased withdrawal of water and climate change in the future.

Lake benthic algae: toward an understanding of their ecology

Abstract: Lake phytoplankton studies outnumber studies on lake periphyton by an order of magnitude, and most periphyton research has been done in streams. Most benthic algal taxa found in lakes also can be found in lotic systems, but assemblages and taxa differ in a number of ways. The ecological characteristics of some lake benthic algae reflect habitat coupling. Littoral zones (benthic areas above the light compensation depth) are areas of high productivity and biodiversity. The proportion of benthic and planktonic primary production (autotrophic structure) is a key ecosystem property. The distribution of lake benthic algae is markedly influenced by the depth gradient and substratum, and assemblages change with depth from epilithic and epiphytic rheobionts to epipsammic and epipelic limnobionts. At shallow depths, periphyton must cope with the effects of high radiation, water-level fluctuations, wave action (e.g., desiccation, ultraviolet radiation exposure, shear stress), and seasonal shifts in temperature. This situation selects for widely distributed rheophilic species. In contrast, the deeper littoral zone is stable and hosts a distinct subset of lentic periphyton. However, species experience low light intensity, which becomes increasingly severe with increasing depth, and are often threatened by eutrophication-driven increased shading by phytoplankton. Besides change across depths and substrata, adaptations to disturbance levels, competition, microdistribution of phenological stages, and physiomorphological regulation generate multiple and complex spatial patterns at different scales. Lake shores are the focus of human activities with significant consequences for periphyton. In this review, we introduce a series of 15 papers on the topic and suggest directions for future research. Overall, this special series illustrates that, despite the many important ecosystem services provided by lake benthic algae, the topic is understudied.

Diatoms in springs of the Alps: spring types, environmental determinants, and substratum

Abstract.  Spring habitats are highly diverse and have a mosaic microhabitat structure. They are endangered by diffuse exploitation as drinking-water resources, an impact likely to increase with climate change. Diatoms were sampled from stones and bryophytes in 110 nonthermal, near-natural springs in the southeastern Alps (Trentino) and 16 carbonate springs in the Dolomiti Bellunesi National Park. Fuzzy clustering identified 6 assemblages in the main types of springs. Carbonate rheocrenes hosted Achnanthidium spp., Gomphonema elegantissimum, and Nitzschia fonticola. Several of these species are rheophilic. This group was divided into 4 subassemblages defined by decreasing A. lineare and increasing A. pyrenaicum with increasing flow. Carbonate rheocrenes with shading or moderate NO3− enrichment contained a majority of sciaphilic and NO3-tolerant taxa: Cocconeis taxa, Amphora spp., Caloneis fontinalis, Reimeria spp., and Eunotia arcubus. Well-buffered siliceous rheocrenes supported Diatoma spp., Eunotia minor, Encyonema minutum, Navicula exilis, and Planothidium lanceolatum. Many of these species are heliophilic and rheophilic. Carbonate rheocrenes with lower conductivities or seasonal desiccation contained typical taxa of unstable environments: Diadesmis spp., Planothidium frequentissimum, Meridion circulare, and Achnanthidium dolomiticum. Carbonate rock-face seepages and some tufa springs supported xerotolerant diatom species with a preference for higher conductivities: Encyonopsis spp., Delicata spp., Gomphonema lateripunctatum, Denticula spp., and Cymbopleura spp. Siliceous seepages and pool springs, some very-low alkalinity rheocrenes, and 1 iron spring were characterized by acidophilous mire taxa, such as Eunotia spp., Frustulia crassinervia, and Tabellaria flocculosa, and very-low-alkalinity indicators, such as Psammothidium acidoclinatum. The other iron springs harbored species-poor assemblages with low numbers of cells. Many diatom species showed a significant preference for stones or bryophytes. Epibryon had higher richness and diversity than epilithon, and mean diversity did not differ among the most frequent bryophyte species. Bryophytes are quick and easy to sample and are proposed as the target substratum for diatom-based assessments of springs.

Environmental classification of springs of the Italian Alps and its consistency across multiple taxonomic groups

Abstract.  The traditional and widely used ecomorphological spring classification—pool springs (limnocrenes), seepages (helocrenes), and flowing springs (rheocrenes)—is based mainly on the flow regime at the spring mouth. This clear distinction is based purely on environmental conditions, but how and to what extent these spring typologies are reflected by biological assemblages consisting of different taxonomic groups remains largely untested. Classification of habitats typically is based on one or few taxonomic groups. However, groups are likely to differ in their response to the environment, so different, equally valid classifications might result with different groups. We evaluated the responses and their congruence of a wide range of taxonomic groups to different spring types. Eighty-six springs in the Italian Alps were first classified based on environmental factors only. The consistency of this classification was tested using diatoms, bryophytes, vascular plants, nematodes, mollusks, oligochaetes, water mites, copepods, ostracods, chironomids, stoneflies, and caddisflies. When only environmental variables were used, 7 spring types were distinguished: limnocrenes and helocrenes, low- and high-altitude rheocrenes on carbonate rocks, rheocrenes on siliceous rocks, rheocrenes with high discharge, and hygropetric rheocrenes. This classification was reflected by most taxonomic groups, and many species were characteristic for ≥1 spring type. However, the predictive power of the environment for determining species distribution was generally low, a result suggesting that other factors may play an important role in structuring spring assemblages. Concordance among taxonomic groups was found for 2 macrogroups of organisms: autotrophs (diatoms, bryophytes, and vascular plants) and heterotrophs. This result shows that achieving a general classification of springs relevant across all taxonomic groups would be difficult.

Achnanthidium minutissimum (Bacillariophyta) valve deformities as indicators of metal enrichment in diverse widely-distributed freshwater habitats

In the presence of different environmental stressors, diatoms can produce frustules presenting different types of deformities. Metals and trace elements are among the most common causes of these teratological forms. Metal enrichment in water bodies can be attributed to the geological setting of the area or to pollution. The widespread benthic diatom Achnanthidium minutissimum (ADMI) is one of the most metal-tolerant species. In the present study, ADMI teratologies were defined from samples taken from eight very diverse, widely-distributed inland-water habitats: streams affected by active and abandoned mining areas, a metal-contaminated stream, a spring in an old chalcopyrite mine, a mineral-water fountain, and a sediment core taken from a lake affected by metal contamination in the past. Deformed frustules of ADMI were characterised mainly by one (sometimes two) more or less bent off ending, conferring to the specimens a cymbelloid outline (cymbelliclinum-like teratology, CLT). Marked teratologies were distinguished from slight deformities. Hydrochemical analyses, including metals and trace elements, were carried out and enrichment factors (EF) relative to average crustal composition were calculated. To improve our knowledge on the potential of different metals and trace elements to trigger the occurrence of ADMI CLT, we carefully selected 15 springs out of 110 (CRENODAT dataset) where both ADMI and above-average metal or metalloid concentrations occurred, and re-analysed these samples. The results from the eight widely-distributed core sites as well as from the 15 selected CRENODAT springs led to the hypothesis that two metals (copper and zinc) and a metalloid (antimony) were the most likely triggers of ADMI CLT formation. From a quantitative point of view, it is worth noting that the lowest concentrations triggering ADMI CLT can be fairly low, particularly in the case of copper contamination. The antimony-rich site was characterised by a marked-teratology variant where both ends of ADMI were bent off.