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See detailProductivity and temperature as drivers of seasonal and spatial variations of dissolved methane in the Southern Bight of the North Sea, leading to a response from eutrophication and heatwaves
Borges, Alberto ULiege; Royer, Colin ULiege; Lapeyra Martin, Jon et al

Conference (2021, May 19)

Dissolved CH4 concentrations in the Belgian coastal zone (BCZ) (North Sea) ranged between 1607 nmol L-1 near-shore and 4 nmol L-1 off-shore during field cruises in 2016, 2017, 2018 and 2019. Spatial ... [more ▼]

Dissolved CH4 concentrations in the Belgian coastal zone (BCZ) (North Sea) ranged between 1607 nmol L-1 near-shore and 4 nmol L-1 off-shore during field cruises in 2016, 2017, 2018 and 2019. Spatial variations of CH4 were related to sediment organic matter (OM) content and gassy sediments. In near-shore stations with fine sand or muddy sediments, the CH4 seasonal cycle followed water temperature, suggesting methanogenesis control by temperature in these OM rich sediments. In off-shore stations with permeable sediments, the CH4 seasonal cycle showed a yearly peak following the Chlorophyll-a spring peak, suggesting that in these OM poor sediments, methanogenesis depended on freshly produced OM delivery. The annual average CH4 emission was 126 mmol m-2 yr-1 in the most near-shore stations (~4 km from the coast) and 28 mmol m-2 yr-1 in the most off-shore stations (~23 km from the coast), 1,260 to 280 times higher than the open ocean average value (0.1 mmol m-2 yr-1). The strong control of CH4 by sediment OM content and by temperature suggests that marine coastal CH4 emissions, in particular in shallow areas, should respond to future eutrophication and warming of climate. This is supported by the comparison of CH4 concentrations at five stations obtained in March 1990 and 2016, showing a decreasing trend consistent with alleviation of eutrophication in the area. This is also supported by the response to the European heatwave of 2018 that led to record-breaking temperatures in many countries across northern and central Europe. Average seawater temperature in July was 2.5°C higher than the mean from 2004 to 2017 for same month in the BCZ. The mean dissolved CH4 concentration in surface waters in July 2018 (338 nmol L-1) was three times higher than in July 2016 (110 nmol L-1), and an extremely high dissolved CH4 concentration in surface waters (1,607 nmol L-1) was observed at one near-shore station. The high dissolved CH4 concentrations in surface waters in the BCZ in July 2018 seemed to be due to a combination of enhancement of methanogenesis and of release of CH4 from gassy sediments, both most likely related to warmer conditions. The emission of CH4 from the BCZ to the atmosphere was higher in 2018 compared to 2016 by 57% in July (599 versus 382 µmol m-2 d-1) and by 37% at annual scale (221 versus 161 µmol m-2 d-1). The European heatwave of 2018 seems to have led to a major increase of CH4 concentrations in surface waters and CH4 emissions to the atmosphere in the BCZ. [less ▲]

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See detailExperimental assessment of light decrease on the biology of Posidonia oceanica
Richir, Jonathan ULiege; Luyckx, Adrien; Champenois, Willy ULiege et al

Conference (2021, May 18)

Seagrasses have a worldwide distribution and grow from the tidal zone to more than 100 m deep. They are considered ecosystem engineers, by building structurally complex meadows. Seagrass meadows are major ... [more ▼]

Seagrasses have a worldwide distribution and grow from the tidal zone to more than 100 m deep. They are considered ecosystem engineers, by building structurally complex meadows. Seagrass meadows are major coastal ecosystems, are highly productive, provide many goods and services and have considerable environmental, financial, and heritage value. Just like any autotroph, seagrass development relies on light availability. Among the many stressors that threaten seagrasses, light deprivation is consequently a major one. Light availability can decrease because of environmental (e.g., river sediment transport) or anthropogenic (e.g., eutrophication, sediment resuspension) factors, an issue expected to worsen in the future. It is in this context that we experimentally assessed the effects of environmentally relevant shading on a keystone seagrass endemic to the Mediterranean Sea, Posidonia oceanica (Linnaeus) Delile, 1813. Screens of different transparency (nominal reduction of 15, 30 and 60% compared to control) were deployed on a healthy meadow at a depth of 15 m in Corsica, France. The experiment took place between April and August 2018, i.e., during the main annual period of productivity of the plant. Seagrass pigment contents (chlorophylls and xanthophylls), photosynthesis (rapid light curves, photosynthesis/irradiance curves and quantum yield), biometry and primary production, carbohydrates (total insoluble and soluble carbohydrates) were measured monthly. Environmental parameters light, temperature and sediment porewater chemistry (methane, nitrous oxide, sulfide, nutrients) were monitored as well. Results showed the adaptability of P. oceanica to light reduction treatments. The seagrass adapted its photosynthetic activity (RLC) and efficiency (effective quantum yield) to cope with light reduction. This improvement resulted from physiological plasticity because neither the pigment contents nor the photosynthesis/irradiance curves differed between light treatments. P. oceanica shoots further maintained their growth and biomass production despite the decrease in light, but at the expense of storing carbohydrates. Finally, the chemistry of sediment porewater, in particular toxic sulfide was not altered. Results of this work underlined the high resistance and resilience of a healthy P. oceanica meadow to five months in situ light deprivation stress. However, because of the measured decrease of storage carbohydrates, seagrass meadow perennity when exposed to longer, recurrent shading is of concern. Carbohydrates and photosynthetic activity and efficiency could further be investigated as early warning indicators in seagrasses facing light reduction stress. [less ▲]

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See detailNet community metabolism of a Posidonia oceanica meadow
Champenois, Willy ULiege; Borges, Alberto ULiege

in Limnology and Oceanography (2021), 66(0), 2126-2140

We report a 12-yr data-set (August 2006 - October 2018) of nearly-continuous estimates (n=3275) of gross primary production (GPP), community respiration (CR), and net community production (NCP) in a ... [more ▼]

We report a 12-yr data-set (August 2006 - October 2018) of nearly-continuous estimates (n=3275) of gross primary production (GPP), community respiration (CR), and net community production (NCP) in a Posidonia oceanica seagrass meadow, computed from O2 measurements on a mooring at 10 m bottom depth in the Bay of Revellata (Corsica). Both NCP and CR were correlated to GPP and followed the leaf biomass seasonal cycle. The meadow was net autotrophic (NCP of 23±8 mol O2 m-2 yr-1, GPP (83±16 mol O2 m-2 yr-1) > -CR (-60±9 mol O2 m-2 yr-1)), in agreement with oxygen over-saturation (104% at annual scale, 101% in winter and 109% in summer). Calcification (CAL) and CaCO3 dissolution (DIS) rates were evaluated from dissolved inorganic carbon measurements in benthic chamber incubations (August 2006-2009). The meadow was found to be a net sink of CaCO3 (DIS>CAL) at an annual rate of 7 mol CaCO3 m-2 yr-1 that matched estimates of CaCO3 deposition on the meadow by sedimentation from the water column. CAL from epiphyte coralline algae was correlated to GPP, but CAL:GPP ratio (0.1) was lower than reported for coralline algae in cultures (0.6) due to the additional contribution of Posidonia to GPP. Both NCP and net DIS contributed to an annual CO2 sink of -30 mol CO2 m-2 yr-1 distinctly stronger than the estimated net air-sea CO2 flux (-1 mol CO2 m-2 yr-1). This suggests that CO2 input by vertical mixing and/or transport by horizontal advection also strongly contribute to the net atmospheric CO2 exchange. [less ▲]

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See detailEtude du métabolisme, à l'échelle de l'écosystème, de l'herbier de Posidonia oceanica (L.) Delille en Baie de Calvi
Champenois, Willy ULiege

Doctoral thesis (2020)

Human population is increasing exponentially with a growing occupation of the coastal zone. It is estimated that by 2050, about 70% of the world population will live and/or work within 100 km of ... [more ▼]

Human population is increasing exponentially with a growing occupation of the coastal zone. It is estimated that by 2050, about 70% of the world population will live and/or work within 100 km of coastlines. This trend has been going on for decades and will further impact coastal ecosystems. Furthermore, climate changes will heavily impact littoral zones and their biotopes (sea-level increase, change of wind regime, seawater temperature increase, …). But the littoral zone is the source of about 40% of the ecosystem goods and services provided by global ecosystems to humankind. It is then important to study the different coastal ecosystems in terms of biogeochemistry to monitor their evolution. Understand the interdependence between these processes and the response of coastal ecosystems to environmental and human stressors will allow to better manage and protect them. In this context and objective, we monitored metabolic fluxes at community scale in a Posidonia oceanica meadow in the Bay of Calvi (Corsica, France) at -10 m depth. This monitoring contributes to the study of the biology and ecology of the posidonia meadow since the 1970s at this site by ULiège from STARESO. This high-frequency monitoring over 12 years (2006-2018) was carried out with a permanent mooring equipped with oxymeters. This allowed us to compute by a mass balance of O2 the gross primary production (GPP), community respiration (CR) and net community production (NCP) over 3275 24h-cycles. GPP, CR and NCP were highly variable whatever the time scale (daily, seasonal, year-to-year). Day-to-day and seasonal variability of metabolic fluxes follows the one of solar irradiance. Solar irradiance, water temperature and chlorophyll a were the main variables responsible for the inter-annual variations of metabolic fluxes. We also observed the response of the meadow to extreme weather conditions that nowadays are extreme events (heat-wave, absence of fall-winter storms, …) that could become the normal situation in future with climate change in the around Mediterranean Sea. Nowadays, GPP peaks in late Spring / early Summer (May-July), when day length is maximal and seawater temperature optimal (17°C to 20°C), the seawater temperature peaking seasonally (~24°C) in late August. This favorable conjunction of light and temperature for posidonia will not be maintained with a 3°C increase of seawater temperature projected for 2100. Climate models predict that storms in autumn and winter will decrease during the next decades. Data collected during years characterizes by absence of fall storms and/or of mild winters with low winds showed a decrease of both GPP and CR. We showed that this decrease was probably related to the absence of the export of litter, which shadows the benthos, blocking the development of sciaphyle flora. Further, a large part of nutrients stored by P. oceanica are absorbed by leaves from the water column. Mild and calm winters favor an early stratification of the water column, re-enforcing its oligotrophic nature. The mooring of oxymeters was an efficient and robust tool to carry out the monitoring of the P. oceanica meadow. We made incubations with benthic chambers during the first three years of the monitoring allowing to make the full budget of inorganic and organic carbon. The P. oceanica meadow is largely autotrophic at annual and community scales as GPP is higher thatn -CR (NCP>0) with 83, -60 and 23 mol de C m-2 yr-1 for GPP, CR and NCP, respectively. NCP was positive for 90% of the 3275 daily cycles. The incubations showed that a dissolution of calcium carbonate occurred in the meadow, at a rate of 7 mol Ca CO3 m-2 yr-1. For seawater pH and total alkalinity standard conditions, this corresponds to an additional sink of 4 mol O2 m-2 yr-1 to be added to 23 moles de CO2 m-2 yr-1 sustained by NCP. The P. oceanica meadow at -10 m in the Bay of Calvi is a net sink of CO2 of 27 moles de CO2 m- 2 yr-1. [less ▲]

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See detailSea ice CO2 dynamics across seasons: impact of processes at the interfaces
Van der Linden, Fanny ULiege; Tison, Jean-Louis; Champenois, Willy ULiege et al

in Journal of Geophysical Research. Oceans (2020)

Winter to summer CO2 dynamics within landfast sea ice in McMurdo Sound (Antarctica) were investigated using bulk ice pCO2 measurements, air-snow-ice CO2 fluxes, dissolved inorganic carbon (DIC), total ... [more ▼]

Winter to summer CO2 dynamics within landfast sea ice in McMurdo Sound (Antarctica) were investigated using bulk ice pCO2 measurements, air-snow-ice CO2 fluxes, dissolved inorganic carbon (DIC), total alkalinity (TA) and ikaite saturation state. Our results suggest depth-dependent biotic and abiotic controls that led us to discriminate the ice column in three layers. At the surface, winter pCO2 supersaturation drove CO2 release to the atmosphere while spring-summer pCO2 undersaturation led to CO2 uptake most of the time. CO2 fluxes showed a diel pattern superimposed upon this seasonal pattern which was potentially assigned to either ice skin freeze-thaw cycles or diel changes in net community production. The pCO2 decrease across the season was driven by physical processes, mainly independent of the autotrophic and heterotrophic phases encountered in the ice interior. Bottom sea ice was characterized by a massive biomass build-up counterintuitively associated with transient heterotrophic activity and nitrate plus nitrite accumulation. This inconsistency is likely related to the formation of a biofilm. This biofilm hosts both autotrophic and heterotrophic activities at the bottom of the ice during spring and may promote calcium carbonate precipitation. [less ▲]

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See detailProductivity and temperature as drivers of seasonal and spatial variations of dissolved methane in the Southern Bight of the North Sea
Borges, Alberto ULiege; Royer, Colin ULiege; Lapeyra Martin, Jon et al

Poster (2020, May 04)

Dissolved CH4 concentrations in the Belgian coastal zone (BCZ) (North Sea) ranged between 1607 nmol L-1 near-shore and 4 nmol L-1 off-shore during field cruises in 2016, 2017, 2018 and 2019. Spatial ... [more ▼]

Dissolved CH4 concentrations in the Belgian coastal zone (BCZ) (North Sea) ranged between 1607 nmol L-1 near-shore and 4 nmol L-1 off-shore during field cruises in 2016, 2017, 2018 and 2019. Spatial variations of CH4 were related to sediment organic matter (OM) content and gassy sediments. In near-shore stations with fine sand or muddy sediments, the CH4 seasonal cycle followed water temperature, suggesting methanogenesis control by temperature in these OM rich sediments. In off-shore stations with permeable sediments, the CH4 seasonal cycle showed a yearly peak following the Chlorophyll-a spring peak, suggesting that in these OM poor sediments, methanogenesis depended on freshly produced OM delivery. The annual average CH4 emission was 126 mmol m-2 yr-1 in the most near-shore stations (~4 km from the coast) and 28 mmol m-2 yr-1 in the most off-shore stations (~23 km from the coast), 1,260 to 280 times higher than the open ocean average value (0.1 mmol m-2 yr-1). The strong control of CH4 by sediment OM content and by temperature suggests that marine coastal CH4 emissions, in particular in shallow areas, should respond to future eutrophication and warming of climate. This is supported by the comparison of CH4 concentrations at five stations obtained in March 1990 and 2016, showing a decreasing trend consistent with alleviation of eutrophication in the area. This is also supported by the response to the European heatwave of 2018 that led to record-breaking temperatures in many countries across northern and central Europe. Average seawater temperature in July was 2.5°C higher than the mean from 2004 to 2017 for same month in the BCZ. The mean dissolved CH4 concentration in surface waters in July 2018 (338 nmol L-1) was three times higher than in July 2016 (110 nmol L-1), and an extremely high dissolved CH4 concentration in surface waters (1,607 nmol L-1) was observed at one near-shore station. The high dissolved CH4 concentrations in surface waters in the BCZ in July 2018 seemed to be due to a combination of enhancement of methanogenesis and of release of CH4 from gassy sediments, both most likely related to warmer conditions. The emission of CH4 from the BCZ to the atmosphere was higher in 2018 compared to 2016 by 57% in July (599 versus 382 µmol m-2 d-1) and by 37% at annual scale (221 versus 161 µmol m-2 d-1). The European heatwave of 2018 seems to have led to a major increase of CH4 concentrations in surface waters and CH4 emissions to the atmosphere in the BCZ. [less ▲]

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See detailA 15-Month Survey of Dimethylsulfoniopropionate and Dimethylsulfoxide Content in Posidonia oceanica
Richir, Jonathan ULiege; Champenois, Willy ULiege; Engels, Guyliann et al

in Frontiers in Ecology and Evolution (2020), 7(510), 1-15

Posidonia oceanica is the only reported seagrass to produce significant amount of dimethylsulfoniopropionate (DMSP). It is also the largest known producer of DMSP among coastal and inter-tidal higher ... [more ▼]

Posidonia oceanica is the only reported seagrass to produce significant amount of dimethylsulfoniopropionate (DMSP). It is also the largest known producer of DMSP among coastal and inter-tidal higher plants. Here, we studied (i) the weekly to seasonal variability and the depth variability of DMSP and its related compound dimethylsulfoxide (DMSO) in P. oceanica leaves of a non-disturbed meadow in Corsica, France, (ii) the weekly to seasonal variability and the depth variability of DMSP to DMSO concentration to assess the potential of the DMSP:DMSO ratio as indicator of stress, and (iii) the relationships between DMSP, DMSO, and the DMSP:DMSO ratio with potential explanatory variables such as light, temperature, photosynthetic activity (effective quantum yield of photosystem II), and leaf size. The overall average concentrations of organosulfured compounds in P. oceanica leaves were 130 ± 39 μmol.g−1fw for DMSP and 4.9 ± 2.1 μmol.g−1fw for DMSO. Concentrations of DMSP and DMSO in P. oceanica were overall distinctly higher and exhibited a wider range of variations than other marine primary producers such as Spartina alterniflora, phytoplankton communities, epilithic Cyanobacteria and macroalgae. Concentrations of both DMSP and DMSO in P. oceanica leaves decreased from a maximum in autumn to a minimum in summer; they changed little with depth. Potential explanatory variables except the leaf size, i.e., the leaf age were little or not related to measured concentrations. To explain the seasonal pattern of decreasing concentrations with leaf aging, we hypothesized two putative protection functions of DMSP in young leaves: antioxidant against reactive oxygen species and predator-deterrent. The similar variation of the two molecule concentrations over time and with depth suggested that DMSO content in P. oceanica leaves results from oxidation of DMSP. The DMSP:DMSO ratio remained constant around a mean value of 29.2 ± 9.0 μmol:μmol for the non-disturbed harvested meadow regardless of the time of the year, the depth or the leaf size. As suggested for the salt march plant S. alterniflora, we hypothesized the DMSP:DMSO ratio could be considered as indicator of stress in seagrasses exposed to environmental or anthropogenic stressors. More research would now be needed to confirm the functions of DMSP and DMSO in seagrasses and how the DMSP:DMSO ratio will vary under various disturbances. [less ▲]

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See detailPosidonia oceanica, a top producer of dimethylsulfoniopropionate and dimethylsulfoxide
Richir, Jonathan ULiege; Champenois, Willy ULiege; Engels, Guyliann et al

in CIESM WORKSHOP MONOGRAPHS (2019, October 10)

We studied the dynamic of dimethylsulfoniopropionate and its derivative dimethylsulfoxide in Posidonia oceanica. The annual average concentrations in leaves were 129 ± 39 μmol.g for DMSP and 5.0 ± 2.1 ... [more ▼]

We studied the dynamic of dimethylsulfoniopropionate and its derivative dimethylsulfoxide in Posidonia oceanica. The annual average concentrations in leaves were 129 ± 39 μmol.g for DMSP and 5.0 ± 2.1 μmol.g for DMSO. DMSP and DMSO concentrations decreased from a maximum in the fall to a minimum in the summer and were mainly correlated to the seagrass leaf size. The similar variation of the two molecule concentrations suggested that DMSO content results from oxidation of DMSP. The DMSP:DMSO ratio, considered as indicator of stress in Spartina alterniflora, remained constant around a mean value of 27.7 μmol:μmol. More research is now needed to investigate the functions of DMSP and DMSO in seagrasses, how the DMSP:DMSO ratio will vary under disturbance and whether it is useful as indicator of stress. [less ▲]

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See detailCO2 transfer in landfast sea ice: impact of processes at the interfaces
Van der Linden, Fanny ULiege; Moreau, Sébastien; Tison, Jean-Louis et al

Poster (2019, August)

Sea ice is a biome actively participating in the regional cycling of CO2 both as a source and a sink at different times of the year depending on ice physics, ice chemistry and ice trophic status ... [more ▼]

Sea ice is a biome actively participating in the regional cycling of CO2 both as a source and a sink at different times of the year depending on ice physics, ice chemistry and ice trophic status (autotrophic vs heterotrophic). The porous sea ice provides a dynamic habitat hosting diverse communities of microorganisms (algae, bacteria, heterotrophic protists, fungi and viruses), particularly concentrated at the bottom of the ice at McMurdo Sound (Antarctica). Bacterial and algal productions affect the CO2 dynamics by releasing or consuming CO2, which in turn impacts concentrations of dissolved inorganic carbon (DIC) and total alkalinity (TA) - key parameters to describe the ocean-sea ice-atmosphere CO2 fluxes. The balance between photosynthesis and respiration of both algae and bacteria, expressed as the net community production (NCP), determines the net trophic status of the ice. NCP relates directly to the biogenic contribution of sea ice to CO2 uptake or release. During the YROSIAE project, which took place at Cape Evans in McMurdo Sound from Nov. 2011 to Dec. 2012, we carried out the first long-term monitoring of pCO2 and CO2 fluxes at sea ice interfaces. The seasonal pattern of air-ice CO2 fluxes was consistent with pCO2 changes, i.e. brine pCO2 over-saturation during late winter (brine concentration of DIC and upward brine expulsion) leading to CO2 degassing, and under-saturation during spring (brine dilution and DIC depletion) leading to atmospheric CO2 uptake. However, diurnal cycles of air-snow-ice CO2 fluxes were superimposed on seasonal changes and appeared to be controlled by the diurnal cycle of basal snow and ice skin temperatures. Though the ice trophic status is likely to affect CO2 fluxes, it appeared that seasonal and diurnal changes at the sea ice surface were decoupled from the succession of autotrophic and heterotrophic phases observed in the ice interior. At the bottom of the ice, a large biomass build-up was associated with high remineralisation and heterotrophic activity. Such condition is likely due to the presence of a biofilm (microbial assemblages embedded in extracellular polymeric substances). The biofilm may further promote CaCO3 precipitation in parallel with an increase of salinity-normalized TA. Such sea ice system, where significant heterotrophic activity is maintained independently of the biomass build-up and which supports CaCO3 precipitation jointly with increasing alkalinity, challenges previous insights. [less ▲]

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See detailInter-annual variations over a decade of primary production of the seagrass Posidonia oceanica
Champenois, Willy ULiege; Borges, Alberto ULiege

in Limnology and Oceanography (2019), 64(1), 32-45

We acquired quasi-continuous measurements of community gross primary production (GPP) by mass balanceof O2 measured on a mooring, from August 2006 to October 2016 over a Posidonia oceanica meadow (10 m ... [more ▼]

We acquired quasi-continuous measurements of community gross primary production (GPP) by mass balanceof O2 measured on a mooring, from August 2006 to October 2016 over a Posidonia oceanica meadow (10 m depth) in the Bay of Revelleta (Corsica). Over the 2006–2016 period, annual GPP averaged 88 molO2 m−2 yr−1 and ranged from 61 to 108 molO2 m−2 yr−1 . The 2 yr with the lowest annual GPP (2007 and 2015) were characterized by a low occurrence of fall–winter storms, probably leading to the accumulation of leaf litter in fall and early winter; we hypothesize this might have led to occultation of benthic macro-algae. Among the other years, the inter-annual variability of GPP was related to changes during the February–August period, as GPP was repeatable among years during the September–January period. For the February–August period, inter-annual variations of GPP were correlated to chlorophyll a (Chl a), solar radiation and water temperature. Computed phytoplankton GPP corresponded to a small fraction of community GPP, so the relation between GPP and Chl a probably reflected inter-annual variations of a common driver that we hypothesize to be nutrient inputs. The correlation of GPP with solar radiation shows that light availability contributed to inter-annual variations of the development of P. oceanica. The positive relation between GPP and temperature was consistent with the fact that the observed temperatures in the Bay of Revelleta were during the study period within the comfort range for the growth of P. oceanica. [less ▲]

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See detailDetermination of dimethylsulfoniopropionate and dimethylsulfoxide in Posidonia oceanica leaf tissue
Champenois, Willy ULiege; Borges, Alberto ULiege

in MethodsX (2019), 6(0), 56-62

In order to investigate the possible use of the dimethylsulfoniopropionate (DMSP) and dimethylsulfoxide (DMSO) ratio as a stress indicator of Posidonia oceanica a method for the determination of these ... [more ▼]

In order to investigate the possible use of the dimethylsulfoniopropionate (DMSP) and dimethylsulfoxide (DMSO) ratio as a stress indicator of Posidonia oceanica a method for the determination of these quantities was developed for this type of material. The method relies on gas chromatography with headspace technique, instead of the purge-and-trap technique commonly used. The method allows the determination of both DMSP and DMSO on the same sample. This method allows to quantify DMSP, DMSO and DMSP:DMSO ratio for calibration curves with a coefficient of variation around 2% and a relative error around 2% and within the ranges natural variability of DMSP and DMSO in P. oceanica leaf tissue. Preliminary tests showed that DMSP in P. oceanica leaf tissue ranged from 20 to 200 mmol g 1 of fresh weight (FW) and 2 to 5 mmol gfw 1 for DMSO. The DMSP:DMSO ratio ranged from 2 to 40. The quantifications were conducted with different mixtures of DMSP and DMSO by measurements of DMSP and DMSO in the same sample of P. oceanica leaf tissue. [less ▲]

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See detailResponse of marine methane dissolved concentrations and emissions in the Southern North Sea to the European 2018 heatwave
Borges, Alberto ULiege; Royer, Colin ULiege; Lapeyra Martin, Jon et al

in Continental Shelf Research (2019), 190(104004), 1-8

During the European heatwave of 2018 that led to record-breaking temperatures in many countries across northern and central Europe, average seawater temperature in July was 2.5 °C higher than the mean ... [more ▼]

During the European heatwave of 2018 that led to record-breaking temperatures in many countries across northern and central Europe, average seawater temperature in July was 2.5 °C higher than the mean from 2004 to 2017 for same month in the Belgian coastal zone (BCZ) (Southern Bight of the North Sea). The mean dissolved CH4 concentration in surface waters in July 2018 (338 nmol L−1) was three times higher than in July 2016 (110 nmol L−1), and an extremely high dissolved CH4 concentration in surface waters (1607 nmol L−1) was observed at one near-shore station. The high dissolved CH4 concentrations in surface waters in the BCZ in July 2018 seemed to be due to a combination of enhancement of methanogenesis and of release of CH4 from gassy sediments, both most likely related to warmer conditions. The emission of CH4 from the BCZ to the atmosphere was higher in 2018 compared to 2016 by 57% in July (599 versus 382 μmol m−2 d−1) and by 37% at annual scale (221 versus 161 μmol m−2 d−1). The European heatwave of 2018 seems to have led to a major increase of CH4 concentrations in surface waters and CH4 emissions to the atmosphere in the BCZ. [less ▲]

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See detailExperimental shading effects on the photosynthetic activity of the seagrass Posidonia oceanica
Richir, Jonathan ULiege; Borges, Alberto ULiege; Champenois, Willy ULiege et al

Scientific conference (2018, October 11)

The Mediterranean seagrass Posidonia oceanica (L.) Delile forms dense coastal meadows from the surface to 40m depth. As benthic primary producer, its growth and development rely on the light intensity and ... [more ▼]

The Mediterranean seagrass Posidonia oceanica (L.) Delile forms dense coastal meadows from the surface to 40m depth. As benthic primary producer, its growth and development rely on the light intensity and quality of Mediterranean oligotrophic coastal waters. A major environmental stressor is the deterioration of the water column transparency resulting from human activities, e.g., eutrophication, sediment resuspension, installation of marine structures. There is a lack of knowledge regarding low to medium light reduction effects on P. oceanica. The aim of this study was to follow its photosynthetic response to such light reduction over the growing period. The experiment was performed in the healthy P. oceanica meadow of the Calvi Bay (Corsica, France), in front of the STARESO research station. Three 9 m2 shading nets were deployed at 15 m depth above the meadow canopy. Light reductions were 15, 30 and 60% compared to the nearby unshaded control meadow. Structures were deployed from April until August 2018. Posidonia oceanica leaves were monitored monthly for photosynthesis with a DIVING-PAM-II fluorometer. We recorded in situ rapid light curves (RLCs) and chlorophyll fluorescence effective and maximum quantum yield of PSII (ΔF/Fm' or Fv/Fm) at dawn or zenith. Photosynthesis-irradiance curves (PI-curves) were recorded on dark adapted samples in the laboratory. Light and temperature were continuously monitored. Water samples were collected for CH4, N2O and H2S measurements in pore-water and nutrient analyses in pore- and canopy-water. RLC parameters, i.e., maximum relative electron transfer rate (rETRm), alpha (α) and irradiance at the onset of light saturation (Ik) responded distinctly to the 4 treatments. rETRm decreased with shading and showed up to one month of time lag for summer maxima between control and under 60 % shading. Regarding α and Ik, they respectively increased or decreased with shading. Spring to summer decreases of ΔF/Fm' in control and under 15% shading were higher than under 30 and 60 % shading. Spring to summer seasonal trends of PI-curve parameters, i.e., maximum production (Pmax), α and Ik did not differ among treatments. A similar trend among treatments was also determined for Fv/Fm, which showed a slight decrease over time. Of all environmental parameters (nutrient data not available yet), only light availability changed according to treatment. Our results on RLCs and ΔF/Fm' suggest that P. oceanica has adapted its photosynthetic activity to light availability: the seagrass shows great photosynthetic plasticity to changing ambient light conditions. Further studies and complementary analyses on seagrass samples (biometry, growth, sugars, C, pigments, DMSP and DMSO) are still necessary to strengthen these conclusions and verify that the low to medium light reduction did not affect the plant photosynthetic apparatus over the five months of experiment. [less ▲]

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See detailEtude de la photosynthèse de Posidonia oceanica par fluorimétrie modulée
Richir, Jonathan ULiege; Abadie, Arnaud ULiege; Borges, Alberto ULiege et al

Scientific conference (2018, July 26)

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See detailAntarctic landfast sea ice: autotrophy vs heterotrophy, sink vs source of CO2
Van der Linden, Fanny ULiege; Moreau, Sébastien; Champenois, Willy ULiege et al

Conference (2018, June 20)

Sea ice is a biome actively participating in the regional cycling of CO2 as both a source and a sink at different times of the year depending on its trophic status (autotrophic vs heterotrophic). In the ... [more ▼]

Sea ice is a biome actively participating in the regional cycling of CO2 as both a source and a sink at different times of the year depending on its trophic status (autotrophic vs heterotrophic). In the frame of the YROSIAE project (Year-Round survey of Ocean-Sea-Ice-Atmosphere Exchanges), carried out at Cape Evans in McMurdo Sound (Antarctica) from Nov. 2011 to Dec. 2012, ice cores, seawater, and brines were collected at regular time intervals. We used dissolved inorganic carbon (DIC) and chlorophyll-a (chl-a) as proxies of net community production and autotrophic biomass, respectively. From spring, very high chl-a concentrations (>2400𝜇𝑔.𝐿!!) were observed at the bottom of the ice. This suggests high primary production. Strikingly, at the same time, nutrients increased significantly indicating strong remineralization at the bottom. In the ice interior, evolution of DIC was marked by a succession of autotrophic and heterotrophic phases. The overall increase of DIC suggests that the ice interior was rather heterotroph. Such sea ice system should expel CO2. Yet, strong under-saturation in CO2 and DIC depletion appeared at the ice surface, suggesting that sea ice should take up CO2 from the atmosphere. On the whole, land fast sea ice in McMurdo Sound appears as a puzzling ecosystem. High primary production and remineralization develop simultaneously at the bottom while the top of the ice is rather heterotrophic but still able to pump CO2 from the atmosphere. [less ▲]

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See detailInter-annual variations over a decade of primary production of the Mediterranean seagrass Posidonia oceanica
Borges, Alberto ULiege; Champenois, Willy ULiege

Conference (2018, May 29)

Posidonia oceanica meadows border nearly of all the coastlines of the Mediterranean Sea, and are present from depths between ~1 m and ~40m. They are among the most productive ecosystems on Earth, host a ... [more ▼]

Posidonia oceanica meadows border nearly of all the coastlines of the Mediterranean Sea, and are present from depths between ~1 m and ~40m. They are among the most productive ecosystems on Earth, host a large biodiversity and provide numerous ecosystem services and goods. These ecosystems are declining in response to several human stressors: eutrophication (waste water and aquaculture), physical destruction (urbanisation of the littoral, anchoring and trawling), and climate change (sea-level rise and warming). We acquired quasi-continuous measurements of community gross primary production (GPP) by mass balance of O2 measured on a mooring, from August 2006 to October 2016 over a Posidonia oceanica meadow (10 m depth) in the Bay of Revelleta (Corsica). Over the 2006-2016 period, annual GPP averaged 88 molO2 m-2 yr-1 and ranged from 61 to 108 molO2 m-2 yr-1. Two years (2007 and 2013) were characterized by the lowest annual GPP, due to accumulation of leaf litter in fall and early winter due to the low occurrence of storms (absence of litter export), leading to occultation of benthic macro-algae. Among the other years, the inter-annual variability of GPP was related to changes during the February-August period, as GPP was repeatable among years during the September-January period. For the February-August period, inter-annual variations of GPP were related to Chlorophyll-a (Chl-a), solar radiation and water temperature. Phytoplankton GPP corresponded to a small fraction of community GPP, so the relation between GPP and Chl-a probably reflected inter-annual variations of nutrient inputs that favored both phytoplanktonic and benthic GPP. The correlation of GPP with solar radiation show that light availability contributes to inter-annual variations of the development of P. oceanica, in line with previous studies that showed the control of light availability on primary production seasonal and depth variations. The positive relation between GPP and temperature was consistent with the fact that the observed temperatures in the Bay of Revelleta were within the comfort range for the growth of P. oceanica. [less ▲]

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See detailPAM fluorometry research in Posidonia oceanica
Richir, Jonathan ULiege; Abadie, Arnaud ULiege; Borges, Alberto ULiege et al

Scientific conference (2018, April 26)

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See detailEtude de la photosynthèse de Posidonia oceanica par fluorimétrie modulée
Richir, Jonathan ULiege; Abadie, Arnaud ULiege; Borges, Alberto ULiege et al

Conference (2018, April 10)

Numerous methods for measuring seagrass productivity and growth exist: evolution of O2 or CO2 (incubation chambers, optodes), biomass, shoot leaf elongation, determination of elementary contents … Another ... [more ▼]

Numerous methods for measuring seagrass productivity and growth exist: evolution of O2 or CO2 (incubation chambers, optodes), biomass, shoot leaf elongation, determination of elementary contents … Another possible method relies on pulse amplitude modulation fluorometry (PAM). This technique allows the determination of the photosynthetic quantum yield (Yield) from fluorescence re-emitted by chlorophyll a before and after the application of a saturating light pulse. The Yield determined along a gradient of irradiance draws a light curve (RLC, Rapid Light Curve) similar to photosynthesis-irradiance curves. Since spring 2015 several measurements of Posidonia oceanica photosynthetic activity have been performed at STARESO using diving-PAM fluorometers, for multiple related purposes. The results of these works show that: (i) the absorbance of light by P. oceanica leaves is lower than the average value of terrestrial plants, (ii) the Yield remains constant, being influenced neither by season nor by depth and only the strong light intensities at shallow depths cause its decrease (photoinhibition), (iii) RLCs highlight the high photochemical plasticity of the plant to environmental conditions, (iv) the maximum electron transfer rate modelled from RLCs seems to be a good indicator of the average elongation of shoot leaves and hence of shoot growth and (v) photosynthesis as a biomarker responds to short-term Cu exposures at environmentally relevant levels. Posidonia oceanica photosynthetic activity, which will further be monitored during an in situ shading experiment, is studied in parallel with the development of a new generic biomarker of stress, the ratio of concentrations of organosulfured coumpounds in the plant (dimethylsulfonioproprionate, DMSP and dimethylsulfoxide, DMSO). In conclusion, whether the scientific issue is ecophysiological, environmental, ecotoxicological, PAM fluorometry is a technical approach to consider. [less ▲]

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