A spatiotemporal investigation of varved sediments highlights the dynamics of hypolimnetic hypoxia in a large hard‐water lake over the last 150 years

Abstract

The spatiotemporal distribution of biochemical varves spanning the last 150 yr was investigated using 40 cores collected over a depth gradient in a large subalpine lake-Lake Bourget-in the French Alps. Four-dimensional sedimentological, biological, and geochemical analyses show that varve preservation can be used as a reliable proxy to reconstruct annual-to-decadal oscillations of hypoxia in large lakes. The volume of hypoxic waters was calculated by integrating the volume between the lake bottom and the depth of the shallowest varve-bearing core for each year. Although Lake Bourget bottom waters have been oxic over the last 9000 yr, severe hypoxia has occurred only since 1933 6 1. The volume of hypoxic waters showed, thereafter, a succession of pronounced fluctuations, leading to an increase of 8% of the total lake volume in the 1960s, a decline in the 1980s, and a second, ongoing increase since 1990. Whereas the initial onset of persistent hypoxic conditions could be attributed to eutrophication due to nutrient-rich inputs from sewage water and/or diffuse contamination, the later fluctuations were also driven by climatic factors, i.e., flooding, rising air temperatures, and phosphorus-independent changes in primary production. Hence, cumulative effects related to global warming seem to have driven hypolimnetic hypoxic conditions since equilibrium was initially disrupted due to a drastic shift in the trophic state.In both marine and freshwater ecosystems, hypoxia In many marine and freshwater ecosystems, humaninduced eutrophication, as a result of nutrient-mainly N and P-over-enrichment of waters, has caused bottomwater hypoxia (Carpenter 2005;Diaz and Rosenberg 2008). It seems logical, therefore, that nutrient abatement measures would result in a return to oxic conditions in bottom waters. However, recent research has shown that climatic changes can also trigger deoxygenation of deep habitats (Deutsch et al. 2011). Climate warming alters the water temperature and water-mass stability, which can affect bottom oxygen conditions by decreasing dioxygen In Lake Bourget, most symptoms of eutrophication (the increase in N : P ratio, the increase in water column transparency in spring, and the reduction in chlorophyll a concentrations) disappeared within the last 30 yr in response to a drastic drop in dissolved P content in water (Jacquet et al. 2005) following a restoration program that started in the 1980s. Counterintuitively, severe hypoxic conditions (defined here as the prolonged absence of bioavailable O 2 in the hypolimnion, leading to a drastic reduction of benthic fauna) still prevail in bottom waters, precluding a return to pre-industrial ecosystem conditions ). Our objective is to reconstruct the yearly intensity of hypolimnetic hypoxia over the last 150 yr in Lake Bourget and to qualitatively explore potential forcing factors related to nutrient concentration and climate.Because long-term monitoring data are scarce or missing and almost never cover the pre-industrial period, long-term

Urban point sources of nutrients were the leading cause for the historical spread of hypoxia across European lakes

“…S6) that were previously oxygenated over the past millennia shifted to hypolimnic hypoxia between CE 1930 and CE 1950 following a slight P increase (i.e., with enrichments of only ∼8-10 μg TP L −1 ; refs. 35,46,47). This finding illustrates that even a small increase in P availability can stimulate enough primary productivity to trigger hypoxia without generating algal blooms (because blooms were only observed after CE 1950).…”

High-resolution paleolimnology opens new management perspectives for lakes adaptation to climate warming

“…Fine oscillations of hypoxia through complete large lake basins were quantitatively reconstructed by computing the extension of the hypoxic water volumes using multiple sediment cores collected at various depths (40 cores for lake Bourget, 20 for lake Annecy, and 21 for Geneva; Jenny et al, 2013Jenny et al, , 2014a.…”

Effects of nutrients and warming on Planktothrix dynamics and diversity: a palaeolimnological view based on sedimentary DNA and RNA

“…They comprise (i) diatom frustules and large calcite crystals; (ii) small calcite crystals and (iii) organic matter including DNA and detrital material (Giguet‐Covex et al ., ). When laminae were not visible (sediment deposited between 1910 and 1933), the sampling interval was based on the known annual sedimentation rate to maintain an appropriate temporal resolution (Jenny et al ., ). The age model and the methodology used for dating are fully described in Appendix S1.…”

“…The sediment cores analysed were precisely dated both by counting visible annual layers and by using radionuclide‐based chronologies and reported flood events (Jenny et al ., ). Preserved nucleic acids extracted from sediment samples were subjected to phylogenetic analyses targeting cyanobacteria based on the 16S rRNA gene and internal transcribed spacer (ITS).…”