0 100 200 300 400 0 10 20 30 0 400 800 1,200 1900 1950 2000 Year Volume d’hypoxie (106m 3) Lake Bourget Lake Annecy Lake Geneva Hypoxia 1800 1850 1750 1700
Achieving high time-and- space resolution on 3 GLEON’s sites
JENNY Jean-Philippe1, FRANCUS Pierre1, NORMANDEAU Alexandre2, PERGA Marie-Elodie3
Lakes Geneva, Annecy and Bourget (France) are three deep, peri-alpine lakes which anthopization has been increasing from the beginning of the 20th century. As a consequence, they all underwent eutrophication by the end of the 1970s. Although P abatment measures have been successful, monitored data indicate hypoxia ([O ]< 2mg L-1) remains in bottom waters, causing habitat reduc-tion of benthic macro-organisms and fishes.
This research contributed to the reconstruction of hypoxia dynamics worldwide and to the identification of the actual causes of lake hypoxia, compensating the lack of monitoring data by using paleo-approaches at high time-and space resolution
km
0 2 4 0 5 10 15km 0 1 2 3 4km
Lake Annecy Lake Geneva Lake Bourget
10 missions : 51 cores 5 missions : 45 cores 4 missions : 36 cores
Deepest points 145 m
21 cores Deepest points317 m
28 cores
Deepest points 66 m
23 cores
Fig. 2 Annual dating of core samples based on varves counting Bourget Holocene 1935 1940 1950 1960 1970 1975 1980 2010 Volume: 44×10 150×10 250×10 306×10 200×10 30×10 230×10 251×10 0×106m3 6m3 6m3 6m3 6m3 6m3 6m3 6m3 6m3 Fig. 6 Spatio-temporal extension of lake oxygen depletion in Lake Bourget as recons-tructed from 4D varves preservation (Jenny et al. L&O ; 2013)
Fig. 7 Temporal evolution of lake oxygen depletion in the 3 lakes indicates an unpressedent ecological shift in the mid- 20th cen-tury (Jenny et al. GBC ; accepted)
Identifying GLEON’s sites recording annually-resolved sediments over the last centuries
Fig. 3 Location of the 122 cores collected to recons-truct annual changes in hypoxia volumes
In marine environments, instrumental analysis showed that the number of hypoxic coastal sites has increased exponentially since the 1950th. However, long-term moni-toring in continental zones remains challenging, preven-ting global reconstructions of hypoxia’s dynamic so far.
Paleolimnological investigations can further extend in time
the long term instrumental records of lake oxygen regime,
compensating for the lack of past monitoring datasets, and
address the complexity of the oxygen dynamics in
conti-nental waters.
Global climate change and widespread occurrence of eutrophication have increased the intensity, duration and extent of natural hypoxia, threatening aquatic ecosystem functions, services and biodi-versity. When annual oxygen concentrations fall below a critical point (combining time and inten-sity) macro-benthic life disappears, therefore stopping bioturbation and related sediment mixing. This threshold is well recorded through the formation of laminated sediments in general and «varves» when they specifically track annual deposition.
1 INRS, Centre Eau Terre Environnement, Université du Québec, Canada ; 2 Département de géographie, Université Laval, Québec, Canada ; 3 INRA, Université de Savoie, UMR CARRTEL, Thonon les Bains, France * All references can be found on www.inra.fr/iper_retro
Conclusion
This analysis shows that continental anthropogenic hypoxia has been spreading worldwide since the turn of the 1900th. Therefore, spreading of terrestrial hypoxia has preceded by 50 years that in marine environ-ments, which indicates how reactive and vulnerable are smaller sites, closer to anthropic sources. However, the persistence of hypoxia, well after the implementation of remediation programs in industrialized countries, shows that both marine and terrestrial systems are not res-ponding so far to nutrient emissions reduction.
Statistical population analysis suggests that only anthropogenic factors affected the recent development of hypoxia (Fig. 5). Overall, lakes from urban and agricul-tural regions or with higher human population density (>80 hab.km-2) and higher GDP, are more likely to develop recent hypoxia (Fig. 5, p<0.005).
Global dynamic of hypoxia in continental freshwaters reconstructed from annually-resolved sediments
Location of sites with annually-resolved sediments
Fig. 1 Localisation of the 365 sites recording natural and recent hypoxia in continental waters
Here, we compiled published records of laminated and varved sediments from 1,246 lakes dis-tributed on the six continents in order 1) to depict the dynamics of global hypoxia for freshwa-ters over the last 300 years and 2) to locate high potential archives that recorded past environ-mental conditions with annual resolution, and those related to GLEON’s sites.
Fig. 4 Temporal changes in terrestrial and marine hypoxia over the last 300 years (Jenny et al. submitted)
Fig. 5. Box plot describing proportion of anthropogenic, climatic and land use characteristics in the watershed of hypoxic and oxygenated sites. RH: Recent hypoxic lakes; NH: Naturally hypoxic lakes; OL: oxygenated lakes (Jenny et al. submitted)
RH NH OL RH NH OL RH NH OL RH NH OL RH NH OL RH NH OL
Jenny, J.-P. et al. A spatiotemporal investigation of varved sediments highlights the dynamics of hypolimnetic hypoxia in a large hard-water lake over the last 150 years. Limnol. Oceanogr. 1395–1408 (2013).
Contact: jphilippe.jenny@gmail.com
GLEON varved lakes
GDP 1700-1950
(1,000 bilion dollars)
