Human-induced river runoff overlapping natural climate variability over the last 150 years: Palynological evidence (Bay of Brest, NW France)
|Author(s)||Lambert Clement1, 2, Penaud Aurelie1, Vidal Muriel1, Klouch Khadidja3, Gregoire Gwendoline1, 4, Ehrhold Axel4, Eynaud Frederique5, Schmidt Sabine5, Ragueneau Olivier2, Siano Raffaele3|
|Affiliation(s)||1 : UBO, IUEM, LGO, UMR CNRS 6538, F-29280 Plouzane, France.
2 : UBO, IUEM, Lab Sci Environm MARin LEMAR, UMR CNRS 6539, F-29280 Plouzane, France.
3 : DYNECO PELAGOS, Ctr Brest, IFREMER, F-29280 Plouzane, France.
4 : IFREMER, Ctr Brest, Geosci Marines, F-29280 Plouzane, France.
5 : Univ Bordeaux, EPOC UMR5805, F-33615 Pessac, France.
|Source||Global And Planetary Change (0921-8181) (Elsevier Science Bv), 2018-01 , Vol. 160 , P. 109-122|
|WOS© Times Cited||5|
|Keyword(s)||Pollen grains, Dinoflagellate cysts, Estuarine dynamics, River runoff, Climate variability|
For the first time a very high resolution palynological study (mean resolution of 1 to 5 years) was carried out over the last 150 years in a French estuarine environment (Bay of Brest; NW France), allowing direct comparison between the evolution of landscapes, surface water, and human practices on Bay of Brest watersheds, through continental (especially pollen grains) and marine (phytoplanktonic microalgae: cysts of dinoflagellates or dinocysts) microfossils. Thanks to the small size of the watersheds and the close proximity of the depositional environment to the mainland, the Bay of Brest represents an ideal case study for palynological investigations. Palynological data were then compared to published palaeo-genetic analyses conducted on the same core and to various available instrumental data, allowing us to better characterize past environmental variability since the second half of the 19th century in Western Brittany. We provide evidence of some clues of recent eutrophication and/or pollution that affected phytoplankton communities and which appears linked with increased runoff (higher precipitations, higher percentages of riparian forest pollen, decline of salt marsh-type indicators, and higher values of the XRF Ti/Ca signal), mainly explained by the evolution of agricultural practices since 1945 superimposed on the warming climate trend. We assume that the significant relay observed between dinocyst taxa: Lingulodinium machaerophorum and Spiniferites bentorii around 1965 then followed by Spiniferites membranaceus after 1985, attests to a strong and recent eutrophication of Bay of Brest surface waters induced by high river runoff combined with abnormally elevated air temperatures, especially obvious in the data from 1990. The structure of the dinocyst community has thus been deeply altered, accompanied by an unprecedented increase of Alexandrium minutum toxic form at the same period, as confirmed by the genetic quantification. Despite this recent major anthropogenic forcing, the fossil pollen sequence also records natural climate variability. We highlight, for the first time, a possible connection between climate (AMO modes) and fossil pollen records (especially tree pollination rates) in coastal sediments using tree percentage fluctuations as an indirect proxy for past sea surface and atmospheric temperatures.