11 research outputs found
Sentinel-2 remote sensing of Zostera noltei-dominated intertidal seagrass meadows
Accurate habitat mapping methods are urgently required for the monitoring, conservation, and management of blue carbon ecosystems and their associated services. This study focuses on exposed intertidal seagrass meadows, which play a major role in the functioning of nearshore ecosystems. Using Sentinel-2 (S2) data, we demonstrate that satellite remote sensing can be used to map seagrass percent cover (SPC) and leaf biomass (SB), and to characterize its seasonal dynamics. In situ radiometric and biological data were acquired from three intertidal meadows of Zostera noltei along the European Atlantic coast in the summers of 2018 and 2019. This information allowed algorithms to estimate SPC and SB from a vegetation index to be developed and assessed. Importantly, a single SPC algorithm could consistently be used to study Z. noltei-dominated meadows at several sites along the European Atlantic coast. To analyze the seagrass seasonal cycle and to select images corresponding to its maximal development, a two-year S2 dataset was acquired for a French study site in Bourgneuf Bay. The po-tential of S2 to characterize the Z. noltei seasonal cycle was demonstrated for exposed intertidal meadows. The SPC map that best represented seagrass growth annual maximum was validated using in situ measurements, resulting in a root mean square difference of 14%. The SPC and SB maps displayed a patchy distribution, influenced by emersion time, mudflat topology, and seagrass growth pattern. The ability of S2 to measure the surface area of different classes of seagrass cover was investigated, and surface metrics based on seagrass areas with SPC >= 50% and SPC >= 80% were computed to estimate the interannual variation in the areal extent of the meadow. Due to the high spatial resolution (pixel size of 10 m), frequent revisit time (<= 5 days), and long-term objective of the S2 mission, S2-derived seagrass time-series are expected to contribute to current coastal ecosystem management, such as the European Water Framework Directive, but to also guide future adaptation plans to face global change in coastal areas. Finally, recommendations for future intertidal seagrass studies are proposed
Satellite-assisted monitoring of water quality to support the implementation of the Water Framework Directive
The EU Water Framework Directive1 (WFD) is an ambitious legislation framework to achieve good ecological and chemical status for all surface waters and good quantitative and chemical status for groundwater by 2027.âA total of 111,062 surface waterbodies are presently reported on under the Directive, 46% of which are actively monitored for ecological status.âOf these waterbodies 80% are rivers, 16% are lakes, and 4% are coastal and transitional waters.âIn the last assessment, 4% (4,442) of waterbodies still had unknown ecological status, while in 23% monitoring did not include in situ water sampling to support ecological status assessment2.âFor individual (mainly biological) assessment criteria the proportion of waterbodies without observation data is much larger; the full scope of monitoring under the WFD is therefore still far from being realised.âAt the same time, 60% of surface waters did not achieve âgoodâ status in the second river basin management plan and waterbodies in Europe are considered to be at high risk of having poor water quality based on combined microbial, physical and physicochemical indicators3
Modélisation de l'écophysiologie de l'hußtre
Un modĂšle Ă©cophysiologique dĂ©terministe de la croissance, de la reproduction et des interactions entre le bivalve et son environnement estuarien a Ă©tĂ© Ă©laborĂ© pour l'huĂźtre japonaise Crassostrea gigas. Pour fonctionner, le modĂšle a besoin de la tempĂ©rature de l'eau, des concentrations en matiĂšre en suspension, matiĂšre organique particulaire, matiĂšre minĂ©rale particulaire, chlorophylle-a, phĂ©opigments, protĂ©ines, lipides et glucides particulaires. Les fonctions de filtration, consommation, ingestion, absorption et respiration ainsi que les efficacitĂ©s de rĂ©tention, sĂ©lection et absorption sont modĂ©lisĂ©es Ă l'aide de variables endogĂšnes (poids sec de l'animal, allocation d'Ă©nergie entre la croissance et la reproduction) et exogĂšnes (tempĂ©rature, quantitĂ© et qualitĂ© de la nourriture). Le modĂšle simule l'Ă©volution temporelle de deux compartiments : somatique et rĂ©serves-gonades. L'Ă©nergie provenant de la nourriture absorbĂ©e est allouĂ©e au compartiment somatique jusqu'Ă un maximum de croissance somatique. Ce maximum dĂ©croĂźt avec l'Ăąge du bivalve et ne dĂ©passe jamais 0,02 g.jâ1.individuâ1. L'Ă©nergie en excĂšs est alors attribuĂ©e au compartiment rĂ©serves-gonades. Cette reprĂ©sentation permet d'identifier les pĂ©riodes de stockage de rĂ©serves et de gamĂ©togĂ©nĂšse ainsi qu'une bonne simulation de l'intensitĂ© des pontes. Le modĂšle tient compte d'une efficacitĂ© de sĂ©lection plus importante pour les microphytes que pour les particules dĂ©tritiques. L'efficacitĂ© d'absorption a Ă©tĂ© reprĂ©sentĂ©e en fonction du pourcentage de matiĂšre organique dans la fraction ingĂ©rĂ©e afin de tenir compte de la dilution de la nourriture par les particules minĂ©rales. L'efficacitĂ© d'absorption des microalgues varie de 20 Ă 60 %, tandis que pour les particules organiques dĂ©tritiques, le modĂšle simule des valeurs nĂ©gatives pour tenir compte des pertes fĂ©cales mĂ©taboliques. Ces formulations permettent de suivre l'Ă©volution temporelle des quantitĂ©s de microphytes et de matiĂšres dĂ©tritiques rejetĂ©es sous la forme de pseudofĂšces et de fĂšces
Temporal changes in community structure of tide pools following the â
The impact of the âErikaâ oil spill on the tidal rock pool community, and
particularly on two species of sea urchin (Paracentrotus lividus and Psammechinus miliaris), was investigated over a
3-year period, at Piriac-sur-Mer (Department of Loire-Atlantique, France,
47°21.6' N; 2°31.7' W). A dramatic increase in the abundance of
two macroalgae Ulva sp. and Grateloupia doryphora occurred following a 100% mortality of sea urchins
observed three weeks after the oil spill. The density of sea urchins and of
other main herbivores, the periwinkle Littorina littorea and the trochid mollusks Gibbula umbilicalis and
Gibbula pennantii, were monitored between January 2000 and March 2003. There was significant
inverse relationship between the overall density of herbivores (sea urchins,
periwinkles and trochid mollusks) and the percent cover of algae in the
tidal pools. The first urchins in the tidal pools were observed two years
after the oil spill and it took three years to reach sea urchin densities
comparable to the reference value of 63Â ind.mâ2 obtained before the oil
spill
Temporal changes in nickel and vanadium concentrations and in condition index and metallothionein levels in three species of molluscs following the â
The petroleum spilt by the tanker âErikaâ contained environmentally high
concentrations of nickel (45Â mgâkgâ1) and vanadium (83Â mgâkgâ1).
Our aim was to show that nickel and vanadium concentrations in marine
organisms could be used as tracers of their exposure to oil deposits along
the coast. Two biomarkers were determined, condition index (CI) and
metallothionein levels.
Samples were collected monthly from January to May 2000 from five sites
along the coast of Vendée and Loire Atlantique: (1) Lérat, (2) La
Govelle, (3)Â Saint Gildas, (4)Â La Bernerie and (5)Â La Fosse. Among benthic
invertebrates, mussels Mytilus edulis (filter-feeders), periwinkles Littorina littorea (grazing-feeders) and
dogwhelks Nucella lapillus (carnivora, bivalve predators) were selected. In addition,
mussels were collected from a control site, Fier d'Ars (RĂ© Island).
The species chosen as bioindicators have responded to the presence of oil in
their environment by accumulating nickel and vanadium. The bioaccumulation
of vanadium occurred early one month after oil spill whereas nickel
bioaccumulation was deferred, probably as a consequence of a lower stability
of vanadylporphyrins compared to nickelporphyrins which are known in
particular for their role in stabilizing emulsions (film at the water/oil
interface). Interspecific differences may be explained by different food
habits: periwinkles grazed contaminated algae; mussels as filter-feeders
retained particles and colloids from the water column; dogwhelks fed on
mussels. Spatio-temporal changes of nickel and vanadium concentrations may
result from (i)Â the arrival of new oil slicks, (ii)Â the action of cleaning
of the coasts contributing to the re-suspension of petroleum. In all of the
three species, few changes of the CI were observed from site to site. CI
variations were linked to sexual ripening in mussels. Mussels originating
from the control site showed MT concentrations significantly lower than
those in specimens from impacted sites. The highest MT concentrations were
observed in January and February, and then a consistent decrease was
registered in March and May. MTÂ concentrations in periwinkles increased very
significantly in March and May. An increase in MTÂ concentrations was also
shown at this period in dogwhelks. Depending on the species, positive
correlations were shown between MT and nickel and/or vanadium
concentrations
Model of Bio-Colonisation on Mooring Lines: Updating Strategy Based on a Static Qualifying Sea State for Floating Wind Turbines
International audienceBio-colonisation affects the ageing of materials and the behaviour of offshore structures. Mooring systems and umbilicals belong to the family of slender bodies which are components sensitive to bio-colonisation because of a change of dynamic behaviour due to shape, roughness and mass modifications. However, this stochastic process in time and space is hard to predict. The purpose is then twofold: first, to provide a stochastic spatial model of the bio-colonisation on a mooring line; second, to show that in some defined environmental conditions, such as low wave height, low wind and current velocities, the monitoring of mooring lines tension can help to assess and reduce uncertainty on this model. Therefore, a comprehensive stochastic modelling based on mussels colonisation was carried out using on-site videotapes, experimental campaigns and expert knowledge. We studied the efficiency of a virtual sensing network using this model and a conditional entropy metric. It is first shown that the spatial model fits well with experimental data, and second that a denser medium accuracy sensor network is to be preferred to a single high accuracy fairlead sensor to reduce the uncertainty on the model parameters. It is then worth updating bio-colonisation on mooring lines during the lifetime of a floating wind turbine
Temperature and seston quantity and quality effects on field reproduction of farmed oysters,
The proliferation of the voluntarily-introduced cupped oyster, Crassostrea gigas, has attained the proportions of species invasion in many intertidal habitats in Europe, presumably resulting from successful reproduction of farmed individuals. It is thus imperative to better understand the reproductive characteristics of farmed oysters, since they are directly under human
control. We quantified the dry tissue mass (DTM), gametosomatic index (GSI),
and reproductive cycle of farmed oysters at two sites in Bourgneuf Bay,
France, in relation to environmental parameters using continuously-recording
probes in 2005 and 2006. The GSI was developed for this study, based on the
actual area occupied by gametes, rather than the area of the gonad
previously used for quantitative histological estimation of reproductive
effort. The two sites, intermediate â (IT) and high-turbidity (HT), differed
markedly in the amount and quality of particulate suspended matter, and also
in fine-scale temperature variations. Oysters at both sites presented two
spawning periods in both 2005 and 2006; Bourgneuf Bay is thus near the
northernmost European limit for a 2-spawning cycle in Crassostrea gigas. Gonad maturation was initiated when spring water temperature reached 8â10 °C, and gamete
atresia occurred when water temperatures transiently dipped to 15â18 °C.
Spawns, which occurred above 18 °C, were timed by fine-scale water
temperature variations. Particulate organic matter quality peaks, coinciding
with gonad maturation, were related to DTM variations before spawning
periods, for the IT oysters in both years, and for the HT oysters in 2006.
The reproductive effort (GSI) of oysters was similar at both sites; however,
the fates of the gametes differed according to site. At the first spawning,
the IT oyster gamete emissions were +1 month delayed, as were peak water
temperatures greater than 18 °C, and more pronounced, compared to the HT
site. Although the second spawning showed high proportions of atretic
oocytes at both sites in both years, the IT oysters evacuated twice as many
gametes as the HT oysters in 2005. The IT conditions therefore appear more
suited to Crassostrea gigas gamete evacuation than the HT conditions
Modélisation de l'affinage de l'hußtre
L'affinage traditionnel des hußtres creuses, pratiqué dans les
claires ostréicoles de la façade Atlantique française, est soumis aux
fluctuations des facteurs environnementaux. Un procédé d'affinage
contrÎlé en terme de matiÚres organique et inorganique particulaires
(MOP et MIP) et de température a été
développé afin de réduire la variabilité de croissance de la
chair des huĂźtres. Cet Ă©levage intensif repose sur la production de la
diatomée Skeletonema costatum, distribuée aux hußtres avec
une concentration moyenne de 4â5 mg de
MOP·Lâ1. Un modĂšle Ă©cophysiologique de
l'huĂźtre Crassostrea gigas, simulant les croissances du soma et des
rĂ©servesâgonades, a Ă©tĂ© appliquĂ© aux conditions d'affinage
contrÎlé dans le but d'analyser les réponses du bivalve. Son
élaboration a nécessité un retour à l'expérimentation. Deux
fonctions d'alimentation ont été étudiées au laboratoire : la
filtration et la production de pseudofÚces, intervenant dans la régulation
de l'ingestion. Les résultats, pour une température de 14 °C, et
pour une gamme de MOP et de MIP variant respectivement de 4 Ă
18 mg·Lâ1 et de 15 Ă
55 mg·Lâ1, montrent que l'ingestion est rĂ©gulĂ©e
par la production de pseudofÚces, la filtration ne présentant pas de
variation significative (moyenne de
2,09 ± 0,11 L·hâ1·gâ1,
avec un taux d'activité de 59%). Cette production de pseudofÚces, qui
engendre une augmentation de la fraction organique ingérée par le
mécanisme de sélection pré-ingestive, permet de compenser la
variabilité des concentrations en MIP. Des simulations ont permis
dâanalyser les effets nĂ©gatifs de ce facteur environnemental sur la
croissance du bivalve. Il apparaßt alors que la ration utilisée en affinage
contrĂŽlĂ© (4â5 mg de MOP.Lâ1) permet une
croissance en chair sÚche malgré des concentrations simulées
atteignant 50 mg·Lâ1 de MIP. Lors de
l'Ă©laboration du modĂšle pour les conditions spĂ©cifiques Ă
l'affinage contrÎlé, la gamétogenÚse est apparue comme un
processus déterminant pour la répartition de l'énergie chez le
bivalve
Temporal and spatial variations in benthic nitrogen cycling in a temperate macro-tidal coastal ecosystem: Observation and modeling
International audienceWe used field observations, laboratory measurements and a reactive transport model (RTM) to investigate temporal and spatial variations in benthic nitrogen (N) cycling in the eutrophic temperate macro-tidal Vilaine Bay (VB), France. A time series of benthic flux measurements and pore-water profiles of dissolved inorganic N (DIN: ammonium, nitrate, nitrite) and dissolved organic N (DON) was conducted at a single station between April and September 2015 (six times). A spatial investigation of the benthic fluxes was performed in July 2016 at this station and three other stations in the VB. All measurements were accompanied by a large panel of physical, chemical and biological descriptors in the water column. In 2015, benthic ammonium fluxes at the monitoring station varied between 75 ÎŒmol m-2 h-1 in spring and were less than 10 ÎŒmol m-2 h-1 in summer. The benthic DON fluxes co-varied with the ammonium fluxes, ranging from 100 ÎŒmol m-2 h-1 in spring to zero in summer. In the summer of 2016, a phytoplankton bloom occurred and as a result the benthic ammonium and DON fluxes reached higher values than in the spring of 2015, accompanied by bottom water hypoxia at one measured station. Benthic nitrate and nitrite fluxes varied between -31 (towards the sediments) and 22 ÎŒmol m-2 h-1 and were explained by the bottom water concentration and nitrification rates. After fitting the existing pore-water profiles, the applied RTM correctly simulated the temporal and spatial variations in the benthic DIN fluxes and predicted that a large part of the deposited organic matter (OM) is remineralized aerobically at the sediment-water interface (SWI). The overall results showed a synthetic pattern of benthic N cycling in the VB, based on the occurrence of diatom blooms as the main source of OM in the sediments. The rapid decomposition of this deposited diatom material at the SWI releases large amounts of ammonium and DON to the water column and rapidly consumes oxygen at the sediment surface. When blooms occur in summer, their decomposition can be followed by hypoxia/anoxia in the bottom water. When blooms are absent, benthic N fluxes are weak and mainly fed by the diffusion from the pore-water. By integrating the present results in a 3D ecological model, it should be possible to more accurately predict the development of bottom water hypoxia in the VB
Temporal and spatial variations in benthic nitrogen cycling in a temperate macro-tidal coastal ecosystem: Observation and modeling
We used field observations, laboratory measurements and a reactive transport model (RTM) to investigate temporal and spatial variations in benthic nitrogen (N) cycling in the eutrophic temperate macro-tidal Vilaine Bay (VB), France. A time series of benthic flux measurements and pore-water profiles of dissolved inorganic N (DIN: ammonium, nitrate, nitrite) and dissolved organic N (DON) was conducted at a single station between April and September 2015 (six times). A spatial investigation of the benthic fluxes was performed in July 2016âŻat this station and three other stations in the VB. All measurements were accompanied by a large panel of physical, chemical and biological descriptors in the water column. In 2015, benthic ammonium fluxes at the monitoring station varied between 75âŻÎŒmolâŻmâ2 hâ1 in spring and were less than 10âŻÎŒmolâŻmâ2 hâ1 in summer. The benthic DON fluxes co-varied with the ammonium fluxes, ranging from 100âŻÎŒmolâŻmâ2 hâ1 in spring to zero in summer. In the summer of 2016, a phytoplankton bloom occurred and as a result the benthic ammonium and DON fluxes reached higher values than in the spring of 2015, accompanied by bottom water hypoxia at one measured station. Benthic nitrate and nitrite fluxes varied between â31 (towards the sediments) and 22âŻÎŒmolâŻmâ2 hâ1 and were explained by the bottom water concentration and nitrification rates. After fitting the existing pore-water profiles, the applied RTM correctly simulated the temporal and spatial variations in the benthic DIN fluxes and predicted that a large part of the deposited organic matter (OM) is remineralized aerobically at the sediment-water interface (SWI). The overall results showed a synthetic pattern of benthic N cycling in the VB, based on the occurrence of diatom blooms as the main source of OM in the sediments. The rapid decomposition of this deposited diatom material at the SWI releases large amounts of ammonium and DON to the water column and rapidly consumes oxygen at the sediment surface. When blooms occur in summer, their decomposition can be followed by hypoxia/anoxia in the bottom water. When blooms are absent, benthic N fluxes are weak and mainly fed by the diffusion from the pore-water. By integrating the present results in a 3D ecological model, it should be possible to more accurately predict the development of bottom water hypoxia in the VB