Effects of long term nutrient and climate variability on subfossil Cladocera in a deep, subalpine lake (Lake Garda, northern Italy)

Abstract

Albeit subfossil Cladocera remains are considered as a reliable proxy for tracking historical lake development, they have been scarcely studied in large subalpine lakes south of the Alps. In this study, subfossil Cladocera remains from Lake Garda in northern Italy were analyzed to track the lake's environmental changes since the Middle Ages. One core was retrieved from the largest sub-basin of Lake Garda (Brenzone, 350 m deep) and two cores were retrieved from the profundal and littoral zone of the smaller lake sub-basin (Bardolino, 80 and 40 m deep, respectively). The species distribution of Cladocera remains in recent sediment layers was similar to that observed in contemporary water samples. The deepest sections of the three cores were characterized by species sensitive to water temperature, which suggest a key role of major climatic events in driving the lake response during the late Holocene. The most evident change in Cladocera assemblages in the studied cores was observed during the 1960s, when planktonic taxa such as Daphnia spp. and Bosmina spp. became dominant at the expense of littoral taxa. Despite the highly coherent general pattern of subfossil Cladocera, the cores showed a set of minor differences, which were attributed to different morphological and hydrological features of the two basins forming Lake Garda. Multivariate analysis revealed a clear relation of Cladocera to climate variability during periods of low lake nutrient levels (i.e. before the 1960s). This provides additional information on past ecological responses of Lake Garda, as contrast previous data from subfossil diatom and pigment analyses did not fully capture effects of climate change during the same period. Since the 1960s shifts in assemblage composition of Cladocera and diatoms were highly coherent, in response to nutrient increase and the following, less pronounced, decrease in nutrient concentrations. During the last five decades, the response of the Cladocera to climate variability appeared to be masked by nutrient change. This work points up the potential of the multi-proxy approach for disentangling the multifaceted biological

Cladocera paleocommunity to disentangle the impact of anthropogenic and climatic stressors on a deep subalpine lake ecosystem (Lake Iseo, Italy)

“…Planktonic crustaceans, particularly the cladocerans, are highly sensitive to environmental changes and represent good sentinels to anthropic and climatic impacts (Berthon et al 2014). The Cladocera remains detected in Lake Iseo sediment reflected the species composition and density of modern pelagic Cladocera assemblages (Leoni 2017;Leoni et al 2018a), and this confirms the potential of these biological proxies for paleoecological reconstructions, as previously reported in several studies on different lake types (Milan et al 2017;Pociecha et al 2020). A relation between the principal pelagic species has been pointed out, highlighting that the density of predators, as B. longimanus and L. kindti, is related to the density of their principal prey, Bosminidae and D. longispina.…”

“…In this phase, species such as B. longirostris, B. longispina and D. longispina became dominant. In particular, B. longirostris is considered as a typical indicator of eutrophication (Zawiska et al 2014;Milan et al 2017). The increase of pelagic taxa in times of high nutrient availability was an expected result following the bottom-up mechanisms reported in the literature and verified in other lakes (McQueen et al 1989).…”

“…Indeed, in Lake Iseo it was possible to observe a negative correlation between the inorganic sedimentation rate and some vulnerable pelagic species as B. coregoni and L. kindti. Moreover, the littoral species C. sphaericus and the high adaptable B. longirostris (Milan et al 2017) increased in phases characterized by high sedimentation rate. This relation can be due to a major material transportation from the littoral zone to the centre of the lake, caused by high hydrological variability in the catchment area.…”

Sedimentary Record of Cladoceran Functionality under Eutrophication and Re-Oligotrophication in Lake Maggiore, Northern Italy

“…Typically, Bythotrephes remains (caudal spines and sometimes resting eggs) are well recovered from sediments and their accumulation has been used to estimate presence and abundance [28][29][30][31]. Bythotrephes remains have been found from sediments in large and deep subalpine lakes, such as Lake Garda in northern Italy [32], in similar type of geo-limnological settings than Lake Maggiore. The use of a larger (76-µm) than the commonly used~50-µm sieve [19] for processing sediment samples for fossil cladoceran analysis in the current study is not a reasonable explanation for the lack of Bythotrephes remains, since the previous studies have used even a 250-µm aperture sieve [28][29][30].…”

Forty years of my adventures with Cladocera remains: retrospection on my scientific work

“…My PhD students not only conduct basic analyses on Cladocera remains, but have developed a number of methodological modifications, and expanded application of Cladocera analysis for sediment studies across time and space. Monika Niska is an expert in the analysis of Eemian (Milan et al 2017) have analysed Cladocera remains to explore the genesis and ontogeny of water bodies, including oxbow lakes, peat bogs, and both shallow and deep lakes of different trophic status. In addition to Cladocera analysis, these researchers conducted chemical analyses and other biological analyses, to reconstruct past conditions.…”