Invasions and re-emergences: an analysis of the success of Bythotrephes in Lago Maggiore (Italy)

Bythotrephes was first seen in North America in 1982 (single individual, Lake Ontario) and noticed elsewhere in the Laurentian Great Lakes in abundance in 1985. Starting from 1987 it sharply increased in the open-water zooplankton of Lago Maggiore, reaching values never recorded in the past 40 years. Despite being native, the species\u27 impact on Lago Maggiore was somewhat comparable to that observed in invaded North American lakes. The re-emergence led to an overall increase in invertebrate predation which became permanent five years after Bythotrephes\u27 establishment, causing direct and indirect effects on the food web. Changes included a sharp decrease in the large filter-feeder Daphnia, whose mortality rate increased, and consequently an increase in phytoplankton cell density; an increase in abundance and size of colonial rotifers with a decline in Leptodora abundance, and an overall decrease in zooplankton biomass were also observed. The increase of Bythotrephes in Lago Maggiore, however, was related to a shift in population phenology, which resulted in seasonal growth starting earlier and lasting longer. Daphnia phenology changed only later, suggesting that the impact was consequent to a temporary decoupling between Bythotrephes and its preferred prey. The shift in density and phenology became permanent. Over the long-term, however, potential competitors and prey were able to recover, reaching levels of abundance comparable to those recorded before the Bythotrephes\u27 re-emergence. Such a response is likely attributable to behavioural adaptation mechanisms resulting in temporal and spatial displacement of potential competitors, predators and prey

In memory of Oscar Ravera

Oscar Ravera was one of the most lively and active protagonists in the field of ecology. A man of great culture, Oscar had a genuine interest in knowledge, an interest fed by his insatiable childlike curiosity. Arriving young, at Pallanza, he found fertile ground to develop his passion for aquatic ecology, a passion that led both to important scientific achievements, and to very large-scale initiatives..

Not only Cladocera: what we can learn from RRE analysis in deep lake sediment cores

Since early palaeolimnological studies, Cladocera have been largely investigated in lakes of different typologies and from a large variety of sites. Analyses of their subfossil remains provide evidence for changes in trophodynamics, habitat and water level fluctuations and of impact of drivers such as climate change on lake ecosystem functioning. Rotifers are an important component of lacustrine food webs. They are responsible for the largest part of zooplankton diversity. Because of short developmental times and intrinsic rate of increase, they promptly respond to different impacts, such as changes in trophy, pollution and recovery as well as climatedriven changes. Rotifers have been overlooked in palaeolimnological studies because they do not leave subfossil remains. They do produce, however, resting eggs of a variety of morphotypes (MTs), which preserve well in sediments. We report here results of a study in which we applied RRE MTs (Rotifer Resting Eggs MorphoTypes) analyses to a sediment core of Lake Orta on which cladocera and diatoms were analysed along with lake chronic, heavy metal pollution and acidification as well as recovery. We found that RRE abundance increased during pollution. RRE MTs differed substantially before vs. during pollution and along with the different recovery phases. The restored RRE community differed substantially from the pre-pollution one. Unexpectedly, RREs persisted over the full pollution phase, when copper concentration in the water column was as high as 108 ?g/L (in the late fifties) and when lake pH (value at the winter mixing) was of 3.8 (in the middle eighties). The presence of open egg cases also proves attempts of e.g. Brachionus calyciflorus to establish also during pollution phase. Extending RREMTs analyse to other deep subalpine lakes (i.e. Leman, Annecy and Bourget) encourage to further develop this approach to contribute to understanding impact of local vs. global drivers affecting lake ecosystem functioning

What have we learned about ecological recovery from liming interventions of acid lakes in Canada and Italy?

The idea of launching another special issue of the Journal of limnology on Lake Orta was born in 2014, on the 25th anniversary of its liming intervention, during an International symposium on Lake Orta organized and hosted by the Pallanza Institute (http://www.ise.cnr.it/vb). The conference did not simply celebrate the past. While the liming of Lake Orta was undoubtedly a great national and international success, the speakers at the conference, instead sought to enlarge and deepen knowledge of patterns and mechanisms of lake ecosystem responses to the water quality improvements, or chemical recovery, that accompanied Lake Orta\u27s liming

Mechanisms underlying recovery of zooplankton in Lake Orta after liming

The goal of this study was to improve the understanding of the large-scale mechanisms underlying the recovery of the zooplankton of Lake Orta from historical contamination, following reduced input of ammonia and metals and the subsequent 1989/90 liming intervention. The industrial pollution had been severe and long-lasting (1929-1990). Zooplankton biodiversity has improved, but most of the new taxa appearing in our counts are rotifers, while many calanoids and the large cladoceran predators (Bythotrephes and Leptodora) that are common in the nearby Lake Maggiore, were still absent from Lake Orta 17 years after liming. To aid understanding of the large-scale mechanisms controlling changes in annual richness, we assessed the annual persistence (P) of Crustacea and Rotifera taxa as an estimator of whether propagules that survived introduction, as result of the natural recolonization process, also thrived. We found that the rate of introduction of zooplankton colonists and their persistence in the water column of Lake Orta changed from 1971 to 2007. New rotifer taxa appeared in the lake after the mid-1980s, when discharge of toxic substances decreased, but their annual persistence was low (P<0.5) until the turn of the century. The numerical values of rotifer and crustacean persistence in Lake Orta were unexpectedly high in 2001 and 2007 (0.55 and 0.72 for rotifers, 0.85 and 0.86 for crustacean, respectively), much higher than in limed lakes in Sudbury, Canada, and in adjacent Lake Maggiore. We hypothesize this could be related to the lack of Cladoceran predators and zooplanktivorous fish in the pelagic waters of Lake Orta

Invasione di specie aliene: comparsa di Polyphemus pediculus (Linnaeus, 1761; Crustacea Cladocera) nel Lago Maggiore

Polyphemus pediculus (Linnaeus, 1761) is a holoarctic cladoceran species typically inhabiting the littoral zone of lakes. It is a voracious predator, hunting its prey by visual predation, able to deplete the zooplankton population. Its detection for the first time in Lake Maggiore, at a sampling station close to the littoral, in which zooplankton was monitored for 20 years is worth of being discussed. First, because it represents a further increase in predatory Cladocera, after Bythotrephes longimanus (Leydig, 1860) have successfully re-emerged in the open water. Second, because it may be regarded as an increasing invasion trend in a lake traditionally considered as a stable, mature environment, in which invasions should be hardly successful.Polyphemus pediculus (Linnaeus, 1761) ? un cladocero oloartico che tipicamente colonizza la zona litorale dei laghi. ? un predatore vorace, che attua una strategia di predazione visiva, in grado di decimare il popolamento zooplanctonico. La presenza di questo organismo, rilevata per la prima volta in campioni provenienti da una stazione del Lago Maggiore, posta vicino alla zona litorale nella quale lo zooplancton viene monitorato da 20 anni, ? meritevole di discussione. Primo, perch? rappresenta un ulteriore aumento della frazione di Cladoceri predatori, dopo l\u27accresciuta presenza numerica di Bythotrephes longimanus (Leydig, 1860) nelle acque pelagiche. Secondo, perch? pu? essere considerato un aumento della tendenza alle invasioni, in un lago tradizionalmente considerato come un ambiente stabile e maturo, nel quale pertanto la probabilit? di successo delle invasioni dovrebbe essere bassa

Long-term research to understand impact of perturbations on lakes: the example of Lake Maggiore

Perturbations linked to the direct and indirect impacts of human activities during the so-called Anthropocene, affect the structure and functioning of lake ecosystems to varying degrees. To understand the patterns and mechanisms of these anthropogenic effects and the extent to which they may drive irreversible changes in ecosystem services, long-term research is required. Studies on the long-term dynamics of plankton may be particularly useful for large and deep lakes whose overall productivity is dominated by pelagic processes. In the open-waters of such lakes, planktonic organisms link and interact with both abiotic and biotic compartments. Here we will analyze 60 years of data on the plankton of the large, deep, subalpine, Lake Maggiore, tracing changes in the pelagic food web which occurred during different phases of the lake\u27s recent evolution. We will document short- to- medium response times by different trophic levels, from microbes, to primary producers and secondary consumers. We will revisit results of past studies based on contemporary and paleolimnological studies and present new analyses to: i) identify any tipping points of the lake trophic evolution, ii) discern effects of recent climatic change, iii) quantify whether inter-annual variability has changed perhaps in responses to changes in thermal stratification regime and warming. By supplementing structural with functional descriptions of long term changes in phyto- and zooplankton communities, we aim to test competing mechanisms underpinning the decade-scale changes we observed

Il Lago Omodeo: caratteristiche fisiche e chimiche

The chemical and physical data collected in Lago Omodeo (Central Sardinia) during 1976 are presented. Particulary temperature and oxygen distributions show the influences of the deep water withdrawl. The high winter nitrate and orthophosphate concentrations and their seasonal changes, the dissolved oxygen depletion in deep waters are importat indices of high eutrophic conditions