Introduction to the French GEOTRACES North Atlantic transect (GA01): GEOVIDE cruise

© 2018 Author(s). The GEOVIDE cruise, a collaborative project within the framework of the international GEOTRACES programme, was conducted along the French-led section in the North Atlantic Ocean (Section GA01), between 15 May and 30 June 2014. In this special issue (https://www.biogeosciences.net/special-issue900.html), results from GEOVIDE, including physical oceanography and trace element and isotope cyclings, are presented among 18 articles. Here, the scientific context, project objectives, and scientific strategy of GEOVIDE are provided, along with an overview of the main results from the articles published in the special issue

The GEOTRACES Intermediate Data Product 2014

The GEOTRACES Intermediate Data Product 2014 (IDP2014) is the first publicly available data product of the international GEOTRACES programme, and contains data measured and quality controlled before the end of 2013. It consists of two parts: (1) a compilation of digital data for more than 200 trace elements and isotopes (TEIs) as well as classical hydrographic parameters, and (2) the eGEOTRACES Electronic Atlas providing a strongly inter-linked on-line atlas including more than 300 section plots and 90 animated 3D scenes. The IDP2014 covers the Atlantic, Arctic, and Indian oceans, exhibiting highest data density in the Atlantic. The TEI data in the IDP2014 are quality controlled by careful assessment of intercalibration results and multi-laboratory data comparisons at cross-over stations. The digital data are provided in several formats, including ASCII spreadsheet, Excel spreadsheet, netCDF, and Ocean Data View collection. In addition to the actual data values the IDP2014 also contains data quality flags and 1-? data error values where available. Quality flags and error values are useful for data filtering. Metadata about data originators, analytical methods and original publications related to the data are linked to the data in an easily accessible way. The eGEOTRACES Electronic Atlas is the visual representation of the IDP2014 data providing section plots and a new kind of animated 3D scenes. The basin-wide 3D scenes allow for viewing of data from many cruises at the same time, thereby providing quick overviews of large-scale tracer distributions. In addition, the 3D scenes provide geographical and bathymetric context that is crucial for the interpretation and assessment of observed tracer plumes, as well as for making inferences about controlling processes

Interannual variability of the initiation of the phytoplankton growing period in two French coastal ecosystems

International audienceAbstract. Decadal time series of chlorophyll a concentrations sampled at high and low frequencies are explored to study climate-induced impacts on the processes inducing interannual variations in the initiation of the phytoplankton growing period (IPGP) in early spring. We specifically detail the IPGP in two contrasting coastal temperate ecosystems under the influence of rivers highly rich in nutrients: the Bay of Brest and the Bay of Vilaine. In both coastal ecosystems, we observed a large interannual variation in the IPGP influenced by sea temperature, river inputs, light availability (modulated by solar radiation and water turbidity), and turbulent mixing generated by tidal currents, wind stress, and river runoff. We show that the IPGP is delayed by around 30 d in 2019 in comparison with 2010. In situ observations and a one-dimensional vertical model coupling hydrodynamics, biogeochemistry, and sediment dynamics show that the IPGP generally does not depend on one specific environmental factor but on the interaction between several environmental factors. In these two bays, we demonstrate that the IPGP is mainly caused by sea surface temperature and available light conditions, mostly controlled by the turbidity of the system before first blooms. While both bays are hydrodynamically contrasted, the processes that modulate the IPGP are similar. In both bays, the IPGP can be delayed by cold spells and flood events at the end of winter, provided that these extreme events last several days

Interannual variability of the initiation of the phytoplankton growing period in two French coastal ecosystems

International audienceAbstract. Decadal time series of chlorophyll a concentrations sampled at high and low frequencies are explored to study climate-induced impacts on the processes inducing interannual variations in the initiation of the phytoplankton growing period (IPGP) in early spring. We specifically detail the IPGP in two contrasting coastal temperate ecosystems under the influence of rivers highly rich in nutrients: the Bay of Brest and the Bay of Vilaine. In both coastal ecosystems, we observed a large interannual variation in the IPGP influenced by sea temperature, river inputs, light availability (modulated by solar radiation and water turbidity), and turbulent mixing generated by tidal currents, wind stress, and river runoff. We show that the IPGP is delayed by around 30 d in 2019 in comparison with 2010. In situ observations and a one-dimensional vertical model coupling hydrodynamics, biogeochemistry, and sediment dynamics show that the IPGP generally does not depend on one specific environmental factor but on the interaction between several environmental factors. In these two bays, we demonstrate that the IPGP is mainly caused by sea surface temperature and available light conditions, mostly controlled by the turbidity of the system before first blooms. While both bays are hydrodynamically contrasted, the processes that modulate the IPGP are similar. In both bays, the IPGP can be delayed by cold spells and flood events at the end of winter, provided that these extreme events last several days

Interannual variability of the initiation of the phytoplankton growing period in two French coastal ecosystems

Abstract. Decadal time series of chlorophyll-a concentrations sampled at high and low frequencies are explored to study climate-induced changes on the processes inducing interannual variations in the Initiation of the Phytoplankton Growing Period (IPGP) in early spring. In this study, we specifically detail the IPGP in two contrasting French coastal ecosystems: the Bay of Brest and the Bay of Vilaine. A large interannual variability in the IPGP is observed in both ecosystems in connection with variations of environmental drivers (solar radiation, sea temperature, wind direction and intensity, precipitation, river flow, sea level, currents and turbidity). We show that the IPGP is delayed by around 30 days in 2019 in comparison with 2010. The use of a one-dimensional vertical model coupling hydrodynamics, biogeochemistry and sediment dynamics shows that the IPGP is generally dependent on the interaction between several drivers. Interannual changes are therefore not associated with a unique driver (such as increasing sea surface temperature). Extreme events also impact the IPGP. In both bays, IPGP is sensitive to cold spells and flood events. The interannual variability of the IPGP is significant and strongly conditioned, at the local scale, by a combination of several environmental parameters, with a larger sensitivity to sea temperature and light conditions, linked to the turbidity of the system. While both bays are hydrodynamically contrasted, the processes that modulate IPGP are similar. Keywords Phytoplankton biomass, Long-term in situ observations, Coastal ecosystems, Extreme events, Climate change

Variabilité décennale de la biomasse phytoplanctonique dans deux écosystèmes côtiers français aux conditions contrastées

Les environnements côtiers sont fortement impactés par l'influence combinée du changement climatique d’origine naturelle et anthropique. Les impacts des épisodes d’évènements météorologiques extrêmes sur l’océan côtier restent à identifier et quantifier. Nous étudions ici l'impact du changement climatique sur la dynamique de la biomasse phytoplanctonique en analysant les mesures de fluorescence et de chlorophylle-a du phytoplancton à haute (détection des évènements extrêmes) et basse fréquence (détection des évolutions à plus long terme) dans deux écosystèmes eutrophes côtiers contrastés (rade de Brest 2000-2019 et baie de Vilaine 2011-2019). Alors que la fréquence et l'intensité des efflorescences phytoplanctoniques dans la baie de Vilaine sont sous influence directe des flux benthiques et des apports continentaux de nutriments, aucune relation n'est détectée dans la rade de Brest où le temps de résidence des masses d'eau est relativement court, et les limitations en nutriments peu fréquentes. Nos résultats mettent en évidence la forte variabilité interannuelle de la période productive des efflorescences algales - et de leur intensité - entre ces deux écosystèmes aux conditions écologiques contrastées. En se concentrant sur le début de la période productive, l'étude de ces années de floraison atypiques révèle une réponse différente entre ces deux sites d'étude face aux vagues de chaleur, aux crues extrêmes ou encore aux tempêtes. Après avoir relié l’occurrence des évènements de dessalures en rade de Brest à différents régimes de temps (AR and NAOp), les liens entre la réponse hydrodynamique de l’océan côtier, leur impact sur la biomasse phytoplanctonique et les processus à plus grande échelle  sont explorés.

Temperature and Salinity Changes in Coastal Waters of Western Europe: Variability, Trends and Extreme Events

International audienc

Large and local-scale influences on physical and chemical characteristics of coastal waters of Western Europe during winter

There is now a strong scientific consensus that coastal marine systems of Western Europe are highly sensitive to the combined effects of natural climate variability and anthropogenic climate change. However, it still remains challenging to assess the spatial and temporal scales at which climate influence operates. While large-scale hydro-climatic indices, such as the North Atlantic Oscillation (NAO) or the East Atlantic Pattern (EAP) and the weather regimes such as the Atlantic Ridge (AR), are known to be relevant predictors of physical processes, changes in coastal waters can also be related to local hydro-meteorological and geochemical forcing. Here, we study the temporal variability of physical and chemical characteristics of coastal waters located at about 48°N over the period 1998-2013 using (1) sea surface temperature, (2) sea surface salinity and (3) nutrient concentration observations for two coastal sites located at the outlet of the Bay of Brest and off Roscoff, (4) river discharges of the major tributaries close to these two sites and (5) regional and local precipitation data over the region of interest. Focusing on the winter months, we characterize the physical and chemical variability of these coastal waters and document changes in both precipitation and river runoffs. Our study reveals that variability in coastal waters is connected to the large-scale North Atlantic atmospheric circulation but is also partly explained by local river influences. Indeed, while the NAO is strongly related to changes in sea surface temperature at the Brest and Roscoff sites, the EAP and the AR have a major influence on precipitations, which in turn modulate river discharges that impact sea surface salinity at the scale of the two coastal stations

Hydrological extreme event occurrences and impacts linked with climate variations in coastal waters of western Europe

The dynamics of physico-chemical processes in the bay of Brest are studied to better understand the impact of local events on the coastal ocean. We are analyzing long-term in situ data (since 2000), high and low frequencies, from the COAST-HF (buoy Marel Iroise) and SOMLIT sites, located at the entrance to the bay of Brest. This study is divided into two main stages: the detection and characterization of extreme events, followed by a numerical simulation of these events to understand the underlying oceanic processes. The focus is on desalination events during the winter months (December, January, February and March), considering the season with the greatest number of extreme events. We show the relationship between local extreme events and variability at larger scales, considering climate indices such as the North Atlantic Oscillation (NAO). A comparison between the low frequency data of the SOMLIT program and the high frequency data of the Marel buoy is carried out, highlighting the interest of high frequency measurements for the detection of extreme events. A comparison between in situ data and two numerical simulations of different resolutions is also performed over salinity time series. This study also show an interannual variability of extreme events interesting in a context of climate change. We make the link between these extreme desalination and the nitrate level in the bay of Brest. To finally study the relationship between nutrients and chlorophyll in order to look at the correlation that it can exists between extreme events and biology in the coastal environment.

Unraveling Salinity Extreme Events in Coastal Environments: A Winter Focus on the Bay of Brest

Extreme weather events affect coastal marine ecosystems. The increase in intensity and occurrence of such events drive modifications in coastal hydrology and hydrodynamics. Here, focusing on the winter period (from December to March), we investigated multi-decade (2000–2018) changes in the hydrological properties of the Bay of Brest (French Atlantic coast) as an example of the response of a semi-enclosed bay to extreme weather episodes and large-scale atmospheric circulation patterns. The relationships between extreme weather events and severe low salinity conditions (as a proxy for changes in water density) were investigated using high-frequency in situ observations and high-resolution numerical simulations. The identification of intense episodes was based on the timing, duration, and annual occurrence of extreme events. By examining the interannual variability of extreme low salinity events, we detect a patent influence of local and regional weather conditions on atmospheric and oceanic circulation patterns, precipitation, and river runoff. We revealed that low salinity events in Brittany were controlled by large-scale forcings: they prevailed during the positive phase of the North Atlantic Oscillation and periods of low occurrences of the Atlantic Ridge weather regime. The increase in severe storms observed in western France since 2010 has led to a doubling of the occurrence and duration of extreme low salinity events in Brittany