Utilisation des Rapid Light Curves (RLC) et des Steady State Light Curves (SSLC) pour la caractérisation in situ de l'activité photosynthétique du phytoplancton

L'utilisation des rapid light curves (RLC) et des steady state light curves (SSLC), pour la caractérisation in situ de l'activité photosynthétique du phytoplancton, à différentes échelles de temps, a été comparée. La possibilité d'utiliser les paramètres photosynthétiques des RLC pour prédire ceux de SSLC a été évaluée. Les résultats montrent que les RLC sont un bon outil pour la caractérisation de l'activité photosynthétique à différentes échelles temporelle

Rapid light curves (RLC) or non-sequential steady-state light curves (N-SSLC): which fluorescence-based light response curve methodology robustly characterizes phytoplankton photosynthetic activity and acclimation status?

This study is the first in situ work comparing rapid light curves (RLC) and non-sequential steady-state light curves (N-SSLC) in their efficiency to characterize phytoplankton photosynthetic activity and acclimation status. Measurements were carried out at two time scales (daily and annual) using the Pulse Amplitude Modulated fluorometry on samples taken in the coastal waters of a macrotidal ecosystem (the Strait of Dover, eastern English Channel). RLC and N-SSLC were compared under a wide range of environmental conditions and phytoplankton composition in order to define the best methodology to accurately capture short and long-term adjustments in the functioning of the photosynthetic apparatus. The relationships between the photosynthetic parameters extracted from RLC and N-SSLC were also studied to evaluate the possibility to use RLC to predict N-SSLC photosynthetic parameters and thus obtaining the acclimation status at steady state. At daily scale, the maximum electron transport rate and light saturation coefficient resulting from RLC (respectively, ETRm_RLC and Ek_RLC) were found to follow more closely short-term environmental light variations than ETRm and Ek resulting from N-SSLC (ETRm_N-SSLC and Ek_N-SSLC) did. RLC were thus able to detect rapid changes in photosynthetic activity that would have been overlooked with N-SSLC measurements. At annual scale, few differences were found between RLC and N-SSLC. Variations of ETRm and α derived from RLC and N-SSLC were very similar but absolute values were lower for RLC measurements. Because, at daily scale, RLC better capture the short-term changes in photosynthetic activity than N-SSLC do, using RLC to predict N-SSLC photosynthetic parameters and getting information about steady-state acclimation status is not possible at this time scale. However, this can be done at seasonal scale

Pilot Assessment of Primary Productivity OSPAR Convention The Convention for the Protection of the Marine Environment of the North-East Atlantic (the "OSPAR Convention") was opened for signature at the Ministerial Meeting of the former Oslo and Paris Commissions in Paris on

International audiencePrimary production sustains the functioning of marine food webs. Over the long-term (1997-2019) primaryproduction was stable in the Greater North Sea, Celtic Seas and Bay of Biscay and Iberian Coast. Significantdecreases occurred 2015-2019, likely driven by reduced nutrient availability and climate change, which maydisturb higher trophic levels

Pilot Assessment of Primary Productivity OSPAR Convention The Convention for the Protection of the Marine Environment of the North-East Atlantic (the "OSPAR Convention") was opened for signature at the Ministerial Meeting of the former Oslo and Paris Commissions in Paris on

International audiencePrimary production sustains the functioning of marine food webs. Over the long-term (1997-2019) primaryproduction was stable in the Greater North Sea, Celtic Seas and Bay of Biscay and Iberian Coast. Significantdecreases occurred 2015-2019, likely driven by reduced nutrient availability and climate change, which maydisturb higher trophic levels

Underwater light climate and wavelength dependence of microalgae photosynthetic parameters in a temperate sea

International audienceStudying how natural phytoplankton adjust their photosynthetic properties to the quantity and quality of underwater light ( i.e . light climate) is essential to understand primary production. A wavelength-dependent photoacclimation strategy was assessed using a multi-color pulse-amplitude-modulation chlorophyll fluorometer for phytoplankton samples collected in the spring at 19 locations across the English Channel. The functional absorption cross section of photosystem II, photosynthetic electron transport (PET λ ) parameters and non-photochemical quenching were analyzed using an original approach with a sequence of three statistical analyses. Linear mixed-effects models using wavelength as a longitudinal variable were first applied to distinguish the fixed effect of the population from the random effect of individuals. Population and individual trends of wavelength-dependent PET λ parameters were consistent with photosynthesis and photoacclimation theories. The natural phytoplankton communities studied were in a photoprotective state for blue wavelengths (440 and 480 nm), but not for other wavelengths (green (540 nm), amber (590 nm) and light red (625 nm)). Population-detrended PET λ values were then used in multivariate analyses (partial triadic analysis and redundancy analysis) to study ecological implications of PET λ dynamics among water masses. Two wavelength ratios based on the microalgae saturation parameter E k (in relative and absolute units), related to the hydrodynamic regime and underwater light climate, clearly confirmed the physiological state of microalgae. They also illustrate more accurately that natural phytoplankton communities can implement photoacclimation processes that are influenced by in situ light quality during the daylight cycle in temporarily and weakly stratified water. Ecological implications and consequences of PET λ are discussed in the context of turbulent coastal ecosystems

Pilot Assessment of Primary Productivity OSPAR Convention The Convention for the Protection of the Marine Environment of the North-East Atlantic (the "OSPAR Convention") was opened for signature at the Ministerial Meeting of the former Oslo and Paris Commissions in Paris on

International audiencePrimary production sustains the functioning of marine food webs. Over the long-term (1997-2019) primaryproduction was stable in the Greater North Sea, Celtic Seas and Bay of Biscay and Iberian Coast. Significantdecreases occurred 2015-2019, likely driven by reduced nutrient availability and climate change, which maydisturb higher trophic levels

Spectral fluorometric characterization of Haptophyte dynamics using the FluoroProbe: an application in the eastern English Channel for monitoring Phaeocystis globosa

International audienceIn this study, we examined the possibility of using the FluoroProbe for monitoring the dynamics of the Haptophyte Phaecystis globosa in the coastal waters of the eastern English Channel. The FluoroProbe was recalibrated by recording a new fingerprint for P. globosa and the use of this new fingerprint was tested through a series of laboratory and in situ experiments. The annual dynamics of P. globosa estimated using the FluoroProbe and by flow cytometry were similar. A strong relationship was found between the FluoroProbe estimates of P globosa. biomass expressed in terms of chlorophyll a equivalent per litre (eq. μg L 1) and flow cytometric cell counts (r = 0.889, P < 0.001, n = 121). The FluoroProbe can be used to detect the flagellated cells as well as the colonial cells of P. globosa but not to distinguish these two cell types in mixed assemblages. The use of the new fingerprint recorded for P globosa improved the detection of Isochrysis sp. This suggests the possibility of using the FluoroProbe to monitor Haptophytes other than P globosa by calibrating the device with species representative of the region of interest. However, it is important to note that the detection of P. globosa at the species level was possible in the eastern English Channel because it was the only Haptophyte species present with a biomass sufficient to be detected by the FluoroProbe. In areas where several Haptophyte species are simultaneously present, their discrimination will be impossible and in such situations the FluoroProbe can be used to monitor the dynamics of the combined Haptophyte group

Phytoplankton photosynthetic activity dynamics in a temperate macrotidal ecosystem (the Strait of Dover, eastern English Channel): Time scales of variability and environmental control

International audienceThe temporal variability in phytoplankton photosynthetic activity was studied in the coastal waters of the Strait of Dover and related to environmental conditions. Phytoplankton photosynthetic activity data collected from October 2008 and August 2010 at different time scales (hourly to interannual) using a nested sampling design are presented. Photosynthetic parameters including the maximum quantum yield (Fv/Fm), maximal light utilization efficiency (α), maximum electron transport rate (ETRm) and light saturation coefficient (Ek) were measured using Pulse Amplitude Modulation (PAM) fluorometry. Information on the taxonomic composition of phytoplankton assemblages and physicochemical parameters were also collected. Marked changes in photosynthetic parameters were observed at the different time scales investigated. The variability of photosynthetic parameters at sub-seasonal scale (hourly to monthly) could be of the same order of magnitude as at seasonal scale. At short time scale (hour to week), there was no consistent pattern of diel periodicity in photosynthetic parameters. There was a decline of ETRm and α during the day as often as there was a peak at the maximum of irradiance. Light conditions were the main influencing factor on photosynthetic parameters at these time scales. The optimal use of incident irradiance seemed focused on between-day rather than within-day variability. At longer time scale (seasonal to interannual), close interplays between shifts in community taxonomic composition and changes in the physicochemical characteristics of the environment controlled the variability in photosynthetic parameters. Whatever the time scale and period of year considered, variability in photosynthetic parameters was ;Ek-independent;. In well-mixed macrotidal ecosystems, such as the Strait of Dover, a nested sampling design allowing to characterize the short- (intraday and daily) as well as long-term variability (within months, monthly, seasonal, annual and interannual) in photosynthetic parameters is required to get a suitable picture of phytoplankton photosynthesis and avoid potential problems of aliasing

Short-term variability and control of phytoplankton photosynthetic activity in a macrotidal ecosystem (the Strait of Dover, eastern English Channel).

International audienceShort-term changes in phytoplankton photosynthetic activity were studied during different periods of the years 2009 and 2010 in the coastal waters of a macrotidal ecosystem (the Strait of Dover, eastern English Channel). During each sampling period, samples were taken every 1.45 h., from sunrise to sunset, during at least 5 days distributed along a complete spring-neap tide cycle. The photosynthetic parameters were obtained by measuring rapid light curves using pulse amplitude modulated fluorometry and were related to environmental conditions and phytoplankton taxonomic composition. The maximum quantum yield (Fv/Fm) showed clear light-dependent changes and could vary from physiological maxima (0.68-0.60) to values close to 0.30 during the course of 1 day, suggesting the operation of photoprotective mechanisms. The maximum electron transport rate (ETRm) and maximal light utilization efficiency (a) were generally positively correlated and showed large diel variability. These parameters fluctuated significantly from hour to hour within each day and the intraday pattern of variation changed significantly among days of each sampling period. Stepwise multiple linear regressions analyses indicated that light fluctuations explained a part of this variability but a great part of variability stayed unexplained. Fv/Fm, ETRm and a were not only dependent on the light conditions of the sampling day but also on those of the previous days.A time lag of 3 days in the effect of light on ETRm and a variation was highlighted. At these time scales, changes in phytoplankton community structure seemed to have a low importance in the variability in photosynthetic parameters. The photoacclimation index Ek showed a lower variability and was generally different from the incident irradiance, indicating a limited acclimation capacity with a poor optimization of light harvesting during the day. However, in well-mixed systems such as the Strait of Dover, the short-term photoacclimation is disrupted by the high level of variability in environmental conditions. Also, the variability observed in the present study can be associated with a particular kind of photosynthetic response: the ''Ek-independent'' variability. The physiological basis of this photosynthetic response is largely unresolved and further researches on this subject are still required to better explain the dynamics of phytoplankton activity in the Strait of Dover

On biogeochemistry and water quality of river canals in Northern France subject to daily sediment resuspension due to intense boating activities.

International audienceIn Northern France, channelized rivers facilitate greatly the waterway transport that should still increase in the coming years to replace as much as possible the road traffic, considered as a heavier source of pollution. These rivers are now subjected to the good potential status objectives required by the Water Framework Directive. The impact of the recurrent resuspension by the current boat traffic of polluted sediments (due to strong historical pollution) on the water quality is the main concern of this work. Our study reveals that the navigation seems to play a limited role on the enrichment of the water columns by dissolved metals and phosphorus, as well as on the oxygenation of surface waters. Conversely, the cycling of the phytoplankton over the year appears to partly control the physico-chemical and chemical evolutions of the overlying water