Monica Tolotti

Steady state phytoplankton in a deep pre-alpine lake: species and pigments of epilimnetic versus metalimnetic assemblages

The vertical distribution pattern of algal species, chlorophylls and specific carotenoids present in the dimictic pre-alpine Ammersee (Bavaria, Germany) are given for the year 2001. A detailed taxonomic list of the phytoplankton species is recorded, along with light micrographs and detailed descriptions of the flagellates. A deep chorophyll maximum, mainly built by Planktothrix rubescens, was common in this deep mesotrophic lake. The three most dominant species among 83 identified taxa alternated seasonally and reached significant biovolumes in both the epi- and the metalimnion (Planktothrix rubescens > Ceratium hirundinella > unicellular centric diatoms > Asterionella formosa > Fragilaria spp. > Anabaena lemmermannii > Phacotus lenticularis and less frequent dominant was Rhodomonas minuta). We define a steady state phytoplankton assemblage in Ammersee as a stable community in terms of species composition and standing crop. The stability of species composition was measured by Bray-Curtis similarity between monthly samples and indicate the change of individual biovolumes of species from month to month. The stability of standing crop was evaluated by the net change of total biovolume for the same time intervals. Focussing on steady state phytoplankton assemblages we compared three spatially heterogeneous environments for vertical niche separation within the top 12 m: the euphotic epilimnion (2 and 5 m), the euphotic metalimnion (7 m) and the metalimnion below the euphotic zone with dim-light less than 1% (10 and 12 m). The definition of a steady state assemblage on both ends hold true only for metalimnetic layers at dim-light levels below 1% in Ammersee. At this metalimnetic layer more than 80% similarity in phytoplankton composition between successive monthly samples was reached, associated with almost zero net changes of total biovolume only. The greater the contribution of the three most dominant taxa to biovolume, the higher were the Bray-Curtis similarities at metalimnetic depths below the euphotic zone. Zooplankton biomass had very little effect on species assemblages in the metalimnion, while parameters related to stratification (Schmidt stability) as well as those of trophy (TP, Chl) correlated with species changes. The similarity values between successive monthly samples from all the euphotic layers never reached more than 60% and were usually significantly lower, even if biovolume net changes were around zero. Both the high fluctuations of the ratio of photosynthetic versus photo-protective carotenoids (PSC:PPC) and the statistical significance of correlations between the change of species and environmental-biotic parameters separate the euphotic layer of the top 7 m as a homogenous community from deeper strata. At all sampling depths within this euphotic zone the increase of sunshine duration was associated with an increase of the carotenoid ratio PSC:PPC, but no relationship was found for the deeper layers. The change of species in the euphotic layer was not significantly related to thermal stability, TP or the dominance structure of phytoplankton, but linked with the zooplankton biomass and therefore seemed to be top-down controlled. From our observations, we can conclude that only during stratification and only in the metalimnion below the euphotic zone steady state assemblages can be expected in the deep mesotrophic Ammersee.

Weather-driven ecology of planktonic diatoms in Lake Tovel (Trentino, Italy)

The seasonal development of planktonic diatoms in Lake Tovel has been studied from 2002 to 2004 within the SALTO research project. The objective was to investigate the role of weather conditions and lake hydrology in regulating the development of Cyclotella and Fragilaria functional species groups. The different responses of the two species to the environmental variability and their reciprocal relationships were considered. Cyclotella showed a weak relationship with the selected environmental variables and its seasonal development was difficult to interpret, also in relation to its opportunistic behaviour. On the contrary, Fragilaria complex showed a prompt response to environmental variability and its seasonal development appeared to be mainly regulated by hydrological conditions, through nitrate nitrogen and, secondarily, by silica concentrations. Water temperature and thermal stability affected only Fragilaria species, while different nutrient requirements allowed Cyclotella species to grow under conditions of limited resources for Fragilaria. Since Lake Tovel has scarce internal nutrient reserves and its hydrology is strongly affected by precipitation both in winter and summer, diatom phenology seems to be indirectly regulated by weather conditions.

Structure of the diatom community of the River Adige (North-Eastern Italy) along a hydrological gradient

Physical constrains such as water discharge, suspended solids and turbidity act as dominant factors in driving the planktonic diatom assemblages of the River Adige (North-Eastern Italy). Two sampling stations, characterised by different hydromorphological features (Cortina all’Adige and Boara Pisani, with torrential and more potamal characteristics, respectively) were sampled fortnightly following an integrated approach encompassing physical, chemical and biological measurements and aiming at identifying the dominant factors controlling the temporal development of the community. A morpho-functional approach was used to classify the diatom assemblages where Morpho-Functional Diatom Groups (MFDG) were defined for diatom genera, according to their morphology, habitat selection and modality of adhesion to river substrate. In the two sampling points, algal growth was never limited by nutrients or zooplankton. The irregular development of MFDG was determined by the stochastic hydrological events and changes in variables related to water discharge (suspended solids and light attenuation). Tychoplanktonic, benthic and drifted taxa (such as Diatoma spp., Encyonema spp., Navicula spp. and Nitzschia spp.) were dominant in the torrential station (Cortina all’Adige), while the contribution of euplanktonic unicellular centric taxa (such as Cyclotella spp., and Stephanodiscus spp.) was higher in the potamal station (Boara Pisani).

Phytoplankton and Zooplankton Associations in a Set of Alpine High Altitude Lakes: Geographic Distribution and Ecology

Species composition and interactions, biomass dominance, geographic distribution and driving variables were investigated for two key elements of the pelagic food web of Alpine lakes, the phytoplankton and the zooplankton, based on a single sampling campaign during summer 2000. Altogether, 70 lakes were surveyed, 49 of which located in three different lake districts of the west and eastern Italian Alps and 21 in the central Austrian Alps (within the uppermost Danube catchment). In addition to the analysis of environmental variables affecting distribution and species structure of the two planktonic compartments, a brief review of the main research lines and hypotheses adopted in the past for the study of phytoplankton and zooplankton in high Alpine lakes is given. The lakes, investigated partly within the European project EMERGE (EVK1-CT-1999-00032) and partly within a regional project in the eastern Alps, comprise a wide range of morphological, chemical and trophic conditions. The phytoplankton communities were found to be diverse and mostly dominated by flagellates (chrysophytes, cryptophytes and dinoflagellates), and only to a lesser extent by non-motile green algae, desmids and centric diatoms. The zooplankton communities were mainly dominated by Alpine cladocerans and copepod species, while rotifers were abundant within one group of Italian lakes (sampled in early summer). The multivariate statistical analyses (CCA) showed that catchment features (i.e. percentage of vegetation cover and geochemical composition) and nitrate concentration are essential drivers for the phytoplankton, whereas for zooplankton also trophic status of the lakes and phytoplankton structure are important. The combined variance analysis of the lake clusters outlined by the multivariate analyses on phytoplankton and zooplankton data, respectively, allowed the identification of four principal lake types (three located on siliceous and one on carbonaceous bedrock), each one characterised by a certain combination of habitat features, which in their turn influence trophic state, and phytoplankton and zooplankton species composition and functionality.

Temperature modulated effects of nutrients on phytoplankton changes in a mountain lake

Piburger See, a dimictic mountain lake in Austria, experienced moderate cultural eutrophication in the 1950s. Lake restoration led to a re-oligotrophication in the 1990s with a decrease in seasonal phytoplankton biovolume until the late 1990s, but a reversed trend from the early 2000s onwards. We hypothesize that recent changes in phytoplankton biomass and functional structure are triggered by changes in lake nitrogen and silica concentrations, and we expect climate-related factors to modulate the trophic status of Piburger See. Phytoplankton data were analyzed by non-metric multidimensional scaling (NMDS) applied on biovolume of morpho-functional groups, combined with correlation analyses of environmental variables. Since the 2000s, short-term changes in phytoplankton of Piburger See were explained by varying concentrations and ratios of nitrogen and silica, while the inter-annual variability in phytoplankton species composition was rather attributed to superimposed rising water temperature and lake thermal stability. Our results underline the co-dominant role of phosphorus and nitrogen as phytoplankton drivers in lakes that experience periods of nitrogen limitation. The combined impact of nutrients and climate on phytoplankton development can thus mimic short-term increases in the trophic level of less productive lakes.

Flagellate algae (Chrysophyceae, Dinophyceae, Cryptophyceae) in 48 high mountain lakes of the Northern and Southern slope of the Eastern Alps: biodiversity, taxa distribution and their driving variables

Survey samplings on late summer phytoplankton communities were carried out on 48 high mountain lakes located on the Austrian and Italian slopes of the Eastern Alps. The lakes of North Tyrol (A) and South Tyrol (I) were sampled in 2000 as part of the EU project EMERGE (EVK1-CT-1999- 00032). The lakes of Trentino (I) were investigated in other research projects during 1996 and 1997 (Adamello mountain range) and 2000 (catchment of the River Avisio), respectively. The objectives of this paper are: (1) to study taxonomy and biodiversity of Chrysophyceae, Dinophyceae and Cryptophyceae in high altitude lakes of the Eastern Alps; (2) to identify functional flagellate groups characterising lakes with similar habitat properties, (3) to identify the environmental variables driving abundance and distribution of the three selected algal groups, thus contributing to the selection of sensitive bioindicator taxa. The lakes investigated show rather wide morphological, chemical and trophic state gradients. Flagellate algae account for a median relative abundance (R.A.) of 68%. Chrysophyceae are the most important group in terms of biodiversity and R.A.. Special flagellate associations could be related to lake features, like catchment geology, mineralization level and nutrient concentrations. However, the distribution of flagellate algae did not allow a complete geographical separation of the lakes studied in the different districts. Multivariate canonical analyses indicate that the distribution of Chrysophyceae is mainly driven by NO3-N concentration and thermal conditions, while Dinophyceae are driven by a combination of alkalinity, altitude, thermal condition and, less importantly, nutrient concentration. Physical properties of the lakes, such as thermal condition and lake depth, represent the principal driving variables for Cryptophyceae. The responses to the different environmental variables suggest that the three flagellate groups analysed might be used as indicators for environmental changes in high mountain lakes of the Eastern Alps.

Ecological and taxonomic observations on the flagellate algae characterising four years of enclosure experiments in Lake Tovel (Southern Alps)

Lake Tovel (Italian Alps, Brenta Dolomites, 1178 m a.s.l.), with a dimictic mixolimnion, is a meromictic and oligotrophic mountain lake, once notorious for the summer reddening of its waters due to dinoflagellate blooms that ceased suddenly in 1964. Since 1997 the Museo Tridentino di Scienze Naturali has been carrying out in situ experiments in order to understand the factors regulating the blooms. Experiments were carried out in different types of enclosures to test the reaction of phytoplankton to phosphorus enrichment and light reduction, with special reference to Glenodinium sanguineum Marchesoni, the dinoflagellate responsible for the reddening. The aim of the present contribution is to study the ecology of other flagellate algae typical of Lake Tovel, by analysing the data from the enclosure experiments. Particular attention is given to competition for resources, the comparison with G. sanguineum and some interesting taxonomic observations. The four flagellate taxa selected for this investigation were Gymnodinium uberrimum (Allman) Kofoid & Swezy, Dinobryon cf. sociale var. americana (Brunnthaler) Bachmann, Campylomonas sp. and Tetraselmis sp. Number of individuals and biovolume calculated for each taxon during the experiments were statistically related to NO3-N and TP concentrations, water temperature, pH and oxygen saturation. A prompt and positive response to P-enrichment was found in all experiments for both G. sanguineum and G. uberrimum. However, the first taxon showed a pronounced development only when nutrients were coupled with high light intensities, while the latter appeared to be a better competitor in low light intensities and low water temperature. The behaviour of Tetraselmis sp. was similar to that of G. sanguineum, while the development pattern of Campylomonas sp. was similar to that of G. uberrimum. D. sociale did not show a clear relationship either to nutrient enrichment or weather conditions.

First human impacts and responses of aquatic systems: A review of palaeolimnological records from around the world:

Lake sediments constitute natural archives of past environmental changes. Historically, research has focused mainly on generating regional climate records, but records of human impacts caused by land use and exploitation of freshwater resources are now attracting scientific and management interests. Long-term environmental records are useful to establish ecosystem reference conditions, enabling comparisons with current environments and potentially allowing future trajectories to be more tightly constrained. Here we review the timing and onset of human disturbance in and around inland water ecosystems as revealed through sedimentary archives from around the world. Palaeolimnology provides access to a wealth of information reflecting early human activities and their corresponding aquatic ecological shifts. First human impacts on aquatic systems and their watersheds are highly variable in time and space. Landscape disturbance often constitutes the first anthropogenic signal in palaeolimnological records. While the effects of humans at the landscape level are relatively easily demonstrated, the earliest signals of human-induced changes in the structure and functioning of aquatic ecosystems need very careful investigation using multiple proxies. Additional studies will improve our understanding of linkages between human settlements, their exploitation of land and water resources, and the downstream effects on continental waters.

Steady state of phytoplankton and implications for climatic changes in a deep pre-alpine lake: epilimnetic versus metalimnetic assemblages

The thermodynamic term ‘steady state’ originally coined by studies on chemical reaction kinetics is also used to define growth in continuous culture systems. It is also used to describe the persistence of biota and relatively stable conditions as a whole in an ecosystem such as the rain forest or parts of the ocean (SOMMER 1985, DOKULIL & TEUBNER 2003, NASELLI-FLORES et al. 2003). According to SOMMER et al. (1993) three criteria must be met to consider a phytoplankton community to be in steady state: (1) no more than three species contribute >80% to the total biomass, (2) the persistence of these species for more than 1–2 weeks and (3) during this period the total biomass does not significantly change. In the present study, we searched for time periods and depth layers in which phytoplankton steady states are most likely. We defined a steady-state phytoplankton assemblage as a stable community in terms of species composition (Bray-Curtis similarity) and standing crop (net change of total biovolume). We compared three spatially heterogeneous environments for vertical niche separation within the top 12 m in the dimictic pre-alpine Ammersee (Bavaria, Germany): the euphotic epilimnion, the euphotic metalimnion, and the metalimnion below the euphotic zone with light < 1% of the surface. We evaluated the applicability of the definition of steady state assemblages given above and discuss results with respect to environmental parameters and climatic change.

Comparison between natural and impacted Alpine lakes six years after hydropower exploitation has ceased

Abstract Many lakes in mountain regions have been used for hydropower generation since the 1950s. It has been estimated that as many as 79% of the rivers in the Alps have been affected by the presence of hydropower plants. In this context, the shutting down of hydropower plants on a group of Alpine lakes represented a good opportunity to study the ecological impact on them. We selected nine lakes that had been affected and nine that had not, and analysed the differences in environment, littoral diatoms and zoobenthos, phytoplankton, zooplankton, and fish. Results showed that benthic biota -diatoms and zoobenthos- were the most affected by water-level drawdown during winter months. Even six years after the end of hydroelectric operations, diatom species richness and diversity were lower in impacted lakes. Assemblage structure was different for both diatoms and zoobenthos. Phytoplankton and zooplankton were similar in impacted and unaffected lakes in terms of both species richness (and diversity) and assemblage structure. The degree of impact on fish was unclear because illegal stocking of lakes with allochthonous fish species had taken place. This study showed that compared to limnetic biota, littoral communities were the most affected by the decrease in water volume every winter. Six years after the end of hydroelectric operations, diatoms, and to lesser extent zoobenthos, were still different compared to those in natural (unaffected) lakes. Planktic communities seem to be either more resistant to the disturbances, or else able to recover more quickly to their former condition.