Human-induced river runoff overlapping natural climate variability over the last 150 years: Palynological evidence (Bay of Brest, NW France)

Abstract

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 Highlights ► High resolution palynological study (last 150 yrs) in a French estuary ► Increased runoff on mainland since 1945 ► Increased eutrophication indices since 1980 ► Changes in phytoplankton communities and increased toxic algal blooms ► Connection between AMO modes and fossil pollen records (tree pollination rates)

Striking forest revival at the end of the Roman Period in north-western Europe

“…During the 1.7-1.4 ka AP event, humid conditions are supported by increases in riparian pollen percentages as Alnus and Salix (Fig. 2I), the latter being recently discussed as powerful fluvial discharge tracers in the studied area 11,21,24,28 . Temporary increases of total pollen fluxes during the climatic interval from 1700-to 1200 years BP also testify to the increase in fluvial terrigenous inputs, which further enhanced the transport of pollen grains (Fig.…”

Neolithic sheep birth distribution: Results from Nova Nadezhda (sixth millennium BC, Bulgaria) and a reassessment of European data with a new modern reference set including upper and lower molars

“…The region is subjected to an oceanic climate characterized by mild temperatures (around 7.5°C on average in winter and 15°C in summer) and high regular rainfalls. Monthly precipitation amounts exceed 100 mm from October to February and are the lowest in July (Lambert et al, 2018). Data related to the oxygen isotopic composition of precipitation (δ 18 O P ) at the nearest Global Network of Isotopes in Precipitation (GNIP) station at Brest Plouzane is available for the years 1998-2002.…”

Dinoflagellate fossils: Geological and biological applications

“…Large fluctuations in cyst production are then frequently observed and amplified by the abrupt increase of one or two regionally specific taxa (for example Spiniferites spp. : in New Bedford Harbour, NE USA, Pospelova et al, 2002, as well as in the Bay of Brest, NW France, Lambert et al, 2018). Pospelova et al (2002) (Lambert et al, 2018 in NW France, Figure 8;Garcia-Moreiras et al, 2018 in NW Spain).…”

“…It is a widely distributed, coastal species that has been associated with paralytic toxins like saxitoxins (Taylor et al, 1995;Landsberg, 2002;Aylagas et al, 2014). Thanks to a multi-proxy approach combining paleogenetic and palynological (dinocysts and pollen grains) carried out at a very high (annual) resolution (Lambert et al, 2018), increasing abundances of A. minutum since the 1980s has been associated to a warming climate trend superimposed on changes in Bay of Brest agriculture practices in watersheds (Figure 8). Changes in the structure of the dinocyst community observed in the Bay of Brest (i.e.…”

“…Comparison between selected palynological data from the I_09 core taken in a highly anthropized estuary (Bay of Brest, Brittany;Lambert et al, 2018), data from ancient DNA measurements(Klouch et al, 2016), and instrumental data from in situ measurements in watersheds. From left to right: XRF Ti/Ca ratio from the study core, measured precipitations and atmospheric temperatures (Rennes meteorological station; infoclimat.fr), measured nitrate fluxes in the Elorn river (SAGE Elorn), dinocyst percentages (Lingulodinium machaerophorum, Spiniferites bentorii, cysts of Pentapharsodinium dalei and Spiniferites membranaceus) and detection curve of Alexandrium minutum species in the same core (fromKlouch et al, 2016).…”