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

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

A long-term case study of a large sub-Alpine lake

Availability of remotely sensed multi-spectral images since the 1980’s, which cover three decades of voluminous data could help researchers to study the changing dynamics of bio-physical characteristics of land and water. In this study, we introduce a new methodology to develop homogenised Lake Surface Water Temperature (LSWT) from multiple polar orbiting satellites. Precisely, we developed homogenised 1 km daily LSWT maps covering the last 30 years (1986 to 2015) combining data from 13 satellites. We used a split-window technique to derive LSWT from brightness temperatures and a modified diurnal temperature cycle model to homogenise data which were acquired between 8:00 to 17:00 UTC. Gaps in the temporal LSWT data due to the presence of clouds were filled by applying Harmonic ANalysis of Time Series (HANTS). The satellite derived LSWT maps were validated based on long-term monthly in-situ bulk temperature measurements in Lake Garda, the largest lake in Italy. We found the satellite derived homogenised LSWT being significantly correlated to in-situ data. The new LSWT time series showed a significant annual rate of increase of 0.020 °C yr−1 (*P < 0.05), and of 0.036 °C yr−1 (***P < 0.001) during summer

Phytoplankton configuration in six deep lakes in the peri-Alpine region: are the key drivers related to eutrophication and climate?

The aim of this study was to draw a general picture of the phytoplankton community in peri-Alpine lakes, including for the first time a broad data set of six deep peri-Alpine lakes, belonging to the same geographical region. The objective was to define the main key drivers that influence the phytoplankton community composition in this particular vulnerable region, for which the impacts of climate change have been demonstrated to be stronger than on a global average. The phytoplankton was investigated with a particular focus on cyanobacteria and using a classification approach based on morpho-functional groups. We hypothesized that phytoplankton in peri-Alpine lakes is mainly driven by nutrient loads as well as by water temperatures, variables that are strongly influenced by climate change and eutrophication. Though different phytoplankton configurations among lakes were partly due to their geographical (altitude) position, assemblages were mostly linked to temperature and nutrients. Furthermore, the results confirmed the significant role of the spring fertilization on the seasonal phytoplankton development. Cyanobacteria were related to the increasing annual average of air and water temperature gradient and therefore might become more important under future warming scenario. Air temperatures have a significant impact on water temperature in the uppermost meters of the water column, with a stronger influence on warmer lake

A new deal for Advances in Oceanography and Limnology (AIOL Journal)

With this first double issue of 2015, Advances In Oceanography and Limnology (AIOL Journal) is facing an important regime shift. Published in house until 2007 by the Italian Association of Oceanography and Limnology (AIOL, www.aiol.info) as the Proceedings of the AIOL National Congress, it has been transformed starting from 2011 in a regular scientific journal with the aim of offering a new publication outlet for oceanographers and limnologists...

First record of quagga mussel, Dreissena bugensis Andrusov, 1897, in Italy: morphological and genetic evidence in Lake Garda

Here, we report the first record of quagga mussel, Dreissena bugensis Andrusov, 1897, in Italy. This mollusc is native of the Dnieper River drainage of Ukraine and is one of the most aggressive invaders in freshwater ecosystems. The spread of quagga mussels throughout Europe and Northern America begun during the 1940s and 1989, respectively. Individuals of quagga mussel were identified in February and March 2022 in two stations located in the shallow (Bardolino) and deeper basins (Castelletto di Brenzone) of Lake Garda. Samples, collected with an Eckman grab, allowed identifying individuals of quagga mussel attached on macrophytes, stones and other mollusc shells. The identification of the individuals was carried out both through the analysis of morphological characters and genetic and phylogenetic analyses using the mitochondrial COI gene. The discovery of quagga mussel is the last of a long series of reports of non-indigenous species introduced into Lake Garda. Lacking direct connection by rivers or canals with other waterbodies colonized by D. bugensis at the northern side of the Alps, a more probable cause of introduction of this new species into Lake Garda can be due to unintentional overland transport through recreational boats and fishing gear. With this new discovery, Lake Garda has confirmed its pivotal role as a southern Alpine corridor for the introduction of non-indigenous species previously established at the northern border of the Alps. Considering its high colonization rate and compared with other invasion patterns observed in Europe and North America, it can be assumed that D. bugensis will soon establish itself with dominant populations throughout Lake Garda. At the same time, it can also be assumed that it will soon spread to other Italian water bodie

Cyanotoxins in Lake Garda? The critical evaluation of chemodiversity

Anthropic pressure and climate changes are the major stressors in lakes. Both factors play an important role in the development of cyanobacteria with the capability of producing toxic metabolites (cyanotoxins). Cyanotoxins have negative effects on aquatic flora and fauna, and can be cause of serious intoxications episodes in animals and humans. Some cyanotoxins moreover can undergo bioaccumulation. Lake Garda hosts some potentially toxic cyanobacteria (Planktothrix rubescens and Dolichospermum lemmermannii) which are known to produce microcystins (hepatotoxins) and anatoxins (neurotoxins). The European Community funded the EULAKES project, which supported a monitoring campaign in 2010-2012 aimed at assessing the presence of cyanotoxins in Lake Garda. In that period the chemical diversity and the abundance of cyanotoxins were investigated by advanced analytical techniques (LC-MS) with a monthly frequency. The survey showed that some cyanotoxins (in particular microcystin RRdm) were present in the lake water with concentrations, however, which did not represent a sanitary risk. The most abundant toxins were microcystins, which five variants have been identified, and anatoxin-a. The highest content of toxins has been registered in spring-summer periods. The data collected, in combination with biological and environmental parameters, provide useful indications for a rational management of possible toxic bloom

Potential toxicity and distribution patterns of Dolichospermum lemmermannii (Cyanobacteria) in European lakes

Dolichospermum lemmermannii (Nostocales) is distributed in temperate and boreal regions. Despite this species is typical of cold environments, many populations were shown to be able to form huge water blooms in summer stratified conditions. Indeed, this species is characterized by high variability to temperature adaptation and some strains show high temperature optima (i.e. between 19°C and 26°C). In the large and deep lakes south of the Alps D.lemmermannii developed only recently. Extended surface bloom occurred firstly in Lake Garda (1990/91) and progressively in lakes Iseo (second half of the 1990s), Maggiore (2005), and Como (2006). The recent spread of this species in Southern Europe emphasizes its ecological heterogeneity. Since all these evidences suggest the existence of different ecotypes adapted to different European climatic regions, a wide research on populations isolated from several European waterbodies was carried out. Several strains of D.lemmermannii were studied applying a multidisciplinary approach, which includes taxonomical, genetic and metabolomic analyses. A phylogenetic study on the 16S rRNA and housekeeping genes (e.g. rpoB,) was integrated by the assessment of the toxic potential, evaluating the presence of cyanotoxins (i.e. microcystins, nodularins, anatoxins, cylindrospermopsins) and cyanotoxins encoding genes. Further studies will allow gaining insight about the phylogeography of this fast spreading species at a continental level, along climatic and trophic gradients

Protocollo per il campionamento di fitoplancton in ambiente lacustre

No abstract availabl

A synoptic study of phytoplankton in the deep lakes south of the Alps (lakes Garda, Iseo, Como, Lugano and Maggiore)

This paper presents a synoptic account of the most important results emerging from studies on the phytoplankton communities in the deep southern subalpine lakes Garda, Iseo, Como, Lugano and Maggiore (DSL) in the second half of the 1990s. At present, the trophogenic 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 research confirmed the existence of a common pool of species developing in the DSL, as already suggested by early studies conducted on a seasonal basis from the end of the 1970s to the first half of the 1980s. However, multivariate analyses (Correspondence Analysis, CA, and a subsequent application of Non Metric Multidimensional Scaling) demonstrated that the species in this common pool were developing differently or exclusively along a geographic and a trophic gradient. The major differences in the geographic distribution were found between the easternmost lakes (Garda and Iseo) and those farthest to the West (Lugano and, partly, Maggiore), with intermediate characteristics in Lake Como. These differences were due mainly to changes in the dominance relationships and only secondarily to compositional changes. The detection of the ultimate causes of these differences should take into account other factors not considered in the paper (i.e. the specific analysis of the food webs, local climatic conditions, hydrology and seasonal input of nutrients). Despite the observed differences, common patterns in the sequence of seasonal assemblages in the DSL could be recognised and defined. The second gradient in the species distribution identified by CA was strongly correlated with the principal trophic descriptors (algal biomass and total phosphorus); this meant that the phytoplankton taxa could be ranked along a trophic spectrum, from oligotrophy to eutrophy. A brief examination of the main differences which have historically arisen with the progress of eutrophication in the DSL showed that the species identified in this study as being indicative of more eutrophic conditions were increasing in importance in some of the lakes

The extent and variability of storm-induced temperature changes in lakes measured with long-term and high-frequency data

The intensity and frequency of storms are projected to increase in many regions of the world because of climate change. Storms can alter environmental conditions in many ecosystems. In lakes and reservoirs, storms can reduce epilimnetic temperatures from wind-induced mixing with colder hypolimnetic waters, direct precipitation to the lake's surface, and watershed runoff. We analyzed 18 long-term and high-frequency lake datasets from 11 countries to assess the magnitude of wind- vs. rainstorm-induced changes in epilimnetic temperature. We found small day-to-day epilimnetic temperature decreases in response to strong wind and heavy rain during stratified conditions. Day-to-day epilimnetic temperature decreased, on average, by 0.28°C during the strongest windstorms (storm mean daily wind speed among lakes: 6.7 ± 2.7 m s−1, 1 SD) and by 0.15°C after the heaviest rainstorms (storm mean daily rainfall: 21.3 ± 9.0 mm). The largest decreases in epilimnetic temperature were observed ≥2 d after sustained strong wind or heavy rain (top 5th percentile of wind and rain events for each lake) in shallow and medium-depth lakes. The smallest decreases occurred in deep lakes. Epilimnetic temperature change from windstorms, but not rainstorms, was negatively correlated with maximum lake depth. However, even the largest storm-induced mean epilimnetic temperature decreases were typically <2°C. Day-to-day temperature change, in the absence of storms, often exceeded storm-induced temperature changes. Because storm-induced temperature changes to lake surface waters were minimal, changes in other limnological variables (e.g., nutrient concentrations or light) from storms may have larger impacts on biological communities than temperature changes