Reframing Lake Geneva ecological trajectory in a context of multiple but asynchronous pressures

Abstract

There are no doubts long-term observatories provide unique insight on ecosystems trajectories. Can we use earliest data to set restoration goals? We take the example of Lake Geneva, for which descriptions of the ecosystem are available for as soon as the late 19 th and early 20 th century. Forel writes about how the luxuriant growth of plant communities provided important habitat for aquatic animals, as well as trapping nutrients and affecting water currents. It can be hard to believe Forel is referring to the same lake as present-day Lake Geneva; however, without continuous monitoring, this qualitative description can hardly be compared to recent observations. We resorted to paleolimnology to quantify the changes in plankton communities, as a proxy of general ecological changes, over the past 1,500 years. Our results show that from 563 AD (beginning of the record) to the 20 th century, the cladoceran assemblage remained stable, despite important amplitude of climate variability (3°C). Trajectory of LakeGeneva shifted for the first time in 1946. Online dynamic linear models revealed the following transition, in 1958-1961, transition was critical, i.e., the ecosystem changed state. Littoral associated species were totally lost, and the assemblage is now dominated by pelagic species. The shift took place around the beginning of the long-term monitoring program, when local perturbations (eutrophication) were escalading. Our result raises the vexing observation that the historical dataset, one of the longest records in the world, may not provide a baseline for Lake Geneva's condition.

Characterization of lacustrine harmful algal blooms using multiple biomarkers: Historical processes, driving synergy, and ecological shifts

“…The rising variability of primary producers probably reflects persistent increases in external forcings (Dakos et al, 2015;Huang et al, 2022) or the constraints of similar large lakes to establish alternative stable states (Scheffer and van Nes, 2007;Janssen et al, 2014;Bruel et al, 2021). Although a series of measures costing ~US$30 billion have been implemented to address wastewater discharge and improve water quality in Lake Taihu watershed (Qin et al, 2019), recent warming climate and extreme weather continue to exacerbate non-point nutrient loading (Xu et al, 2021) and internal release from sediments (Yin et al, 2022).…”

“…Systematic monitoring programs are typically initiated once environmental problems have emerged and they rarely capture the full spectrum of aquatic ecosystem deterioration (Battarbee et al, 2005;Qin et al, 2019). One key challenge we face, therefore, is to determine the historical baseline conditions and ecological processes (particularly decadal to centennial) of toxic cyanobacterial blooms in freshwater lakes (Huisman et al, 2018;Ryo et al, 2019;Cao et al, 2020), and what might be the underlying mechanisms of multiple interacting drivers through time (Leavitt et al, 2009;Bruel et al, 2021;Huo et al, 2022).…”

“…phytoplankton versus macrophyte dominance), HABs, and toxin production (Waters et al, 2021). Impacts of multiple stressors on freshwater ecosystems, such as climate change, eutrophication, hydrological alterations, and trophic cascades, may (i) complicate the identification of drivers and their interactions that are responsible for key changes (Leavitt et al, 2009;Deng et al, 2014), (ii) increase the risk of catastrophic shifts (e.g., macrophyte extinction, HABs) (Scheffer et al, 2001), and (iii) compromise the potency of restoration measures by changing the baseline conditions (Battarbee et al, 2005;Bruel et al, 2021). In lake ecosystems, temporal variability in primary production can rise due to persistent increases in nutrient influx (e.g., the paradox of enrichment) (Cottingham et al, 2000;Bunting et al, 2016), after which the state shift to prolific cyanobacteria is triggered by relative minor forcing (eg., warming) that push lakes beyond critical thresholds (Dakos et al, 2015).…”

“…In lake ecosystems, temporal variability in primary production can rise due to persistent increases in nutrient influx (e.g., the paradox of enrichment) (Cottingham et al, 2000;Bunting et al, 2016), after which the state shift to prolific cyanobacteria is triggered by relative minor forcing (eg., warming) that push lakes beyond critical thresholds (Dakos et al, 2015). The complexity and interactivity of stressors on ecosystems further hamper our understanding of how reducing local pressures may increase ecosystem resilience to climate change (Monchamp et al, 2018;Bruel et al, 2021;Carpenter et al, 2022). Retrospective examination of past ecological shifts and variability characteristics using long-term records, therefore, may provide critical insights on how nonlinear and abrupt HABs in response to environmental change may improve lake management and mitigation strategies (Bunting et al, 2016;Ryo et al, 2019;Bjorndahl et al, 2022).…”

A Review of Event Deposits in Lake Sediments

“…Finally, other types of analyses can be used to refine age models, such as anthropogenic perturbations of known age, previously dated tephra deposits (tephrochronology), or the identification of historical events. Geomagnetic field secular variations are a complementary tool to establish more robust age-depth relationships, especially when the age model accuracy suffers from large radiocarbon uncertainties [288,295]. Once the quality of palaeomagnetic secular variations is verified, the declination/inclination variations observed in the sedimentary sequence are then correlated with the reference curve to provide additional stratigraphic chronomarkers (e.g., Figure 18C).…”