Fabio Buzzi

Phytoplankton as an Indicator of the Water Quality of the Deep Lakes South of the Alps

This paper offers a synoptic account of studies on the phytoplankton communities in the deep southern subalpine lakes (DSL) Garda, Iseo, Como, Lugano and Maggiore. The main cause of the degradation of the water quality in the DSL is eutrophication. The euphotic layers of these lakes are trophically different, ranging from the oligo-mesotrophy of lakes Maggiore and Garda to the meso-eutrophy of lakes Iseo and Lugano. The trophic status as estimated by using total phosphorus and chlorophyll a has provided consistent results in agreement with the models proposed by OECD (1982. Eutrophication of Waters. Monitoring, Assessment and Control, OECD, Paris). Though related with chlorophyll a and TP, the Secchi disk depths have significantly underestimated the trophic status of the DSL. Two trophic indices using the algal orders (PTIorders) and species (PTIspecies) were drawn up on the basis of the distribution of phytoplankton along a trophic gradient defined by the application of multivariate methods; the scores emerging from these indices were used to make a definitive ecological classification of water bodies on a scale from 1 to 5, in accordance with the Water Framework Directive. A third index (PTIOE) was computed as the ratio between the annual mean values of the cumulative biovolumes of two groups of algal orders with opposite trophic characteristics. The three PTI indices were highly correlated, providing a consistent classification of the water bodies. The indices proposed in this work were specifically adopted for use in the DSL. However, the criteria for their implementation constitute a robust and impartial tool for assessing similar indices in other lake typologies and for evaluating the degree of specificity of the trophic indicator values assigned to the single phytoplankton orders and species.

Effects of nutrient availability and temperature on phytoplankton development: a case study from large lakes south of the Alps

This work investigated the combined effects of nutrient availability and temperature on phytoplankton in large and deep lakes south of the Alps (lakes Garda, Iseo, Como, Lugano and Maggiore). The more eutrophic basins (Lugano and Iseo) showed a higher presence of cyanobacteria, green algae (Chlorophyta and Charophyta) and dinoflagellates (Dinophyta). Besides these two water bodies, high biomasses of cyanobacteria were recorded also in the oligo-mesotrophic Lake Garda. The development of these algal groups during the growing season showed a strong dependence on the surface spring availability of SRP, which, in turn, was related to winter climatic oscillations, deep mixing dynamics, and trophic status. A specific analysis carried out by applying additive mixed modelling, generalized least squares and mixed modelling, allowed investigation of the direct, seasonal effects of water temperature variations and trophic status on different algal groups. The dominant cyanobacteria (Oscillatoriales) showed only a partial relationship with temperature, while Nostocales and Chroococcales, which did not appear to have a close relationship with the trophic status of the lakes, were characterised by abrupt increases during the warmer months. High positive relationships with temperature were found for a few other algal groups (e.g., Chlorophyta, Charophyta and Dinophyta). Overall, the results indicated a positive relationship between the seasonal development of the more abundant and eutrophic-sensitive algal groups and the concurrent effect of trophic status and water temperature. Nevertheless, it was stressed that specific differences could be interpreted taking into account the different autoecological characteristics and susceptibilities of different species and functional groups to other stressing factors favouring losses, including, e.g., vertical sinking and grazing.

Influence of atmospheric modes of variability on the limnological characteristics of large lakes south of the Alps: a new emerging paradigm

Winter air and spring surface water temperatures, spring epilimnetic phosphorus, and hypolimnetic oxygen in the deep lakes south of the Alps (Garda, Iseo, Como, Lugano, and Maggiore) showed a high degree of temporal coherence. The common temporal patterns were originating from the effects caused by winter climate, and from corresponding synchronisms in the interannual variations in the extent of the spring water renewal and replenishment of nutrients. In turn, the sequence of linked causal events was triggered by two atmospheric modes of variability relevant for the Mediterranean region, i.e. the East Atlantic pattern (EA) and the Eastern Mediterranean Pattern (EMP). In contrast, there were no significant relationships of air and water temperatures with the North Atlantic Oscillation. In oligotrophic lakes, which were characterised by weak vertical nutrient gradients, the spring replenishment of P was negligible and difficult to detect (Maggiore), or detectable but of minor importance (Garda and Como) compared to that measured in more enriched lakes (Iseo and Lugano). The applicability of EA and EMP in the study of the impact of climate on aquatic ecosystems will require to be tested by expanding the number, typology, and geographical location of water bodies in the Alpine and Mediterranean regions.

Warming trends of perialpine lakes from homogenised time series of historical satellite and in-situ data

The availability of more than thirty years of historical satellite data is a valuable source which could be used as an alternative to the sparse in-situ data. We developed a new homogenised time series of daily day time Lake Surface Water Temperature (LSWT) over the last thirty years (1986-2015) at a spatial resolution of 1km from thirteen polar orbiting satellites. The new homogenisation procedure implemented in this study corrects for the different acquisition times of the satellites standardizing the derived LSWT to 12:00 UTC. In this study, we developed new time series of LSWT for five large lakes in Italy and evaluated the product with in-situ data from the respective lakes. Furthermore, we estimated the long-term annual and summer trends, the temporal coherence of mean LSWT between the lakes, and studied the intra-annual variations and long-term trends from the newly developed LSWT time series. We found a regional warming trend at a rate of 0.017°Cyr-1 annually and 0.032°Cyr-1 during summer. Mean annual and summer LSWT temporal patterns in these lakes were found to be highly coherent. Amidst the reported rapid warming of lakes globally, it is important to understand the long-term variations of surface temperature at a regional scale. This study contributes a new method to derive long-term accurate LSWT for lakes with sparse in-situ data thereby facilitating understanding of regional level changes in lakes surface temperature.

Climatic effects on vertical mixing and deep-water oxygen content in the subalpine lakes in Italy

Deep lakes south of the Alps (DSL: Maggiore, Lugano, Como, Iseo and Garda) are characterised by varying trophic states and dissolved oxygen (DO) concentrations. Some of these lakes experience anoxic conditions in deep waters. We hypothesised that the increase in temperature and water-column stability observed in these lakes during recent decades influenced the deep-water DO concentration. In particular, we tested whether the thermal regime of the lakes and the depth of mixing affect oxygen replenishment during winter–spring turnover. To this aim, we analysed long-term trends and seasonal variability of oxygen levels in the DSL during 1992–2016. We included in our analysis the effects of environmental variables, such as winter air temperature and atmospheric modes of variability. Our results showed a recent decrease in the deep-water oxygen content in lakes Maggiore, Como and Garda and an increase of the extent of anoxic conditions in lakes Lugano and Iseo. Our results suggest that, beside cultural eutrophication, rising environmental pressures, such as global warming, can influence the future trends of the oxygen levels and ecological states of deep lakes.

Restoring lakes through external phosphorus load reduction: the case of Lake Pusiano (Southern Alps)

Abstract We estimated the external phosphorus load (EPL) between 1960 and 2015 in Lake Pusiano, a mid-sized subalpine lake that reached its maximum trophic state in the mid-1980s. Using historical data we also estimated the internal phosphorus load (IPL) between 1972 and 2015. EPL reached its maximum in the mid-1980s with phosphorus (P) values ~21 t yr−1 compared to the current value close to 6 t yr−1. IPL was one order of magnitude less and ranged between 0.25 t yr−1 (2015) and 3.6 (1985) t yr−1. The strong reduction of the P load determined a marked decrease of both P (from 200 to 23 μg L−1) and chlorophyll a (from 18 to 8 μg L−1) in-lake concentrations. The process of eutrophication and subsequent recovery, however, showed hysteresis between P load and in-lake P concentrations. In recent years, in particular, the P concentrations at winter overturn seem to be independent from the P load, related to modifications in the hydrological management of the lake that favored P flush-out in October–November, when the EPL is maximal. This process led to a marked decrease of the total P concentrations at winter overturn (23 μg L−1), which recently resulted in lower-than-target concentrations (30 μg L−1) established by the Lombardy Region. The lake, nevertheless, suffers from the presence of the toxic cyanobacterium Planktothrix rubescens, and eradicating this species and further improving water quality will likely require an additional abatement of the EPL.

Plankton dynamics across the freshwater, transitional and marine research sites of the LTER-Italy Network: patterns, fluctuations, drivers

A first synoptic and trans-domain overview of plankton dynamics was conducted across the aquatic sites belonging to the Italian Long-Term Ecological Research Network (LTER-Italy). Based on published studies, checked and complemented with unpublished information, we investigated phytoplankton and zooplankton annual dynamics and long-term changes across domains: from the large subalpine lakes to mountain lakes and artificial lakes, from lagoons to marine coastal ecosystems. This study permitted identifying common and unique environmental drivers and ecological functional processes controlling seasonal and long-term temporal course. The most relevant patterns of plankton seasonal succession were revealed, showing that the driving factors were nutrient availability, stratification regime, and freshwater inflow. Phytoplankton and mesozooplankton displayed a wide interannual variability at most sites. Unidirectional or linear long-term trends were rarely detected but all sites were impacted across the years by at least one, but in many case several major stressor(s): nutrient inputs, meteo-climatic variability at the local and regional scale, and direct human activities at specific sites. Different climatic and anthropic forcings frequently co-occurred, whereby the responses of plankton communities were the result of this environmental complexity. Overall, the LTER investigations are providing an unparalleled framework of knowledge to evaluate changes in the aquatic pelagic systems and management options.

Prokaryoplankton and phytoplankton community compositions in five large deep perialpine lakes

Phytoplankton (PCC) and prokaryoplankton community compositions were studied in five perialpine lakes (Garda, Maggiore, Como, Iseo, and Lugano) of different trophic conditions and mixing regimes, using inverted microscopy and CARD-FISH technique. The aim of this research was to assess, for the first time in these lakes, the relationships among PCC, bacteria, and archaea, and the effects of environmental drivers. We observed a clear difference of PCC compositions in spring and summer. All the lakes showed Bacillariophyta dominating in spring, whereas in summer, there were remarkable differences of PCC. Prokaryoplankton composition showed more pronounced differences in the vertical profile than those between spring and summer. The oligomictic lakes showed a uniform vertical gradient of prokaryotes in spring, while in the meromictic lakes, their abundances were incremented with depth. In summer, the prokaryotic community changed, and niche differentiation occurred in almost all lakes. In conclusion, our study showed a general pattern, common to all the lakes, of a first appearance of the large-sized “opportunistic” bacteria in spring, followed by ultramicrobacteria, less vulnerable to predation in summer. Significant correlations between a few PCC and bacterial groups were found, thus elucidating that functional interactions can be the key to understand plankton successions.