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Percentage of diatoms in microphytoplankton in 1981 (light grey) (Quéguiner 1982), 1993 to 1994 (dark grey) (Del Amo et al. 1997b) and in 2001 to 2002 (black) (this study)
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This study presents (1) the first annual time-series recorded for any marine system of the weekly variability in rates of production (P) and dissolution (D) of biosilica (BSiO2) and (2) the first evidence of the end of the year-round dominance of diatoms in the Bay of Brest, a well-mixed anthropogenically nitrate-enriched macrotidal ecosystem, typi...
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... biosilica production was only active from spring to mid-summer 2001 (Fig. 5a), suggesting that diatoms might not be dominant throughout the year (Fig. 4). Compared to 1981Compared to -1982Compared to and 1993Compared to -1994, our study confirms that, from spring to the be- ginning of summer, diatoms remain predomi- nant in the microphytoplankton (Fig. 6). This indicates that, in spite of increasing riverine deliveries of nitrate to the Bay of Brest over the last 20 yr, especially during winter, the recycling of Si(OH) 4 (either at the sediment- water interface [Ragueneau et al. 2002], and/or in the water column [this study, Fig. 5b]) remains sufficient to account for the Si requirement ...
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... study also shows that, in contrast to 1981 to 1982 and 1993 to 1994, the non-siliceous species (dinoflagel- lates) became predominant over diatoms from mid- summer to fall 2001 (Fig. 6). This change might have taken place progressively over the last decade, as a summer decrease in diatom dominance was already evidenced in 1993. What triggered this drastic mid- summer change in microphytoplankton dominance? It is beyond the scope of our paper to discuss the details of the inter-annual variability of the environmental ...
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A massive diatom-bloom is observed annually in the surface waters of the naturally Fe-fertilized Kerguelen Plateau (Southern Ocean). In this study, silicon isotopic signatures (δ30Si) of silicic acid (DSi) and suspended biogenic silica (BSi) were investigated in the whole water column with an unprecedented spatial resolution in this region, during...
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... To regenerate their damaged biomass, sponges must consume not only food to supply their energetic and growth needs but also dissolved silicon to build their siliceous skeletons as they grow. Unexpectedly, the highest silicon consumption occurred in the time of the year when dissolved silicon in seawater was minimal in the Bay of Brest (April-June; < 1.5 µM Si), when diatoms bloom and deplete dissolved silicon availability 50 . Furthermore, the estimated sponge silicon consumption from mid-April to mid-June during regeneration (0.4 ± 0.5 g Si m −2 ; Table 1) could not be fulfilled even if the concentration of dissolved silicon in seawater had been 100-150 µM Si, the concentration at which H. perlevis reaches its maximum silicon consumption rate 37,51 . ...
Diatoms play a key role in the marine silica cycle, but recent studies have shown that sponges can also have an important effect on this dynamic. They accumulate large stocks of biogenic silica within their bodies over long periods, which are thought to vary little on an intra-annual scale. The observation of an abrupt decline in sponge biomass in parallel with large increases in abundance of a spongivorous nudibranch (Doris verrucosa) led us to conduct a year-long study on the effect of nudibranch predation on the silicon budget of a sponge (Hymeniacidon perlevis) population. After 5 months of predation, the abundance of sponge individuals did not change but their biomass decreased by 95%, of which 48% was explained by nudibranch predation. About 97% of sponge spicules ingested by nudibranchs while feeding was excreted, most of them unbroken, implying a high rate of sponge silica deposition in the surrounding sediments. After predation, sponges partially recovered their biomass stocks within 7 months. This involved a rapid growth rate and large assimilation of dissolved silicon. Surprisingly, the highest rates of silicon absorption occurred when dissolved silicon concentration in seawater was minimal (< 1.5 µM). These findings suggest that the annual sponge predation-recovery cycle triggers unprecedented intra-annual changes in sponge silicon stocks and boosts the cycling of this nutrient. They also highlight the need for intra-annual data collection to understand the dynamics and resilience of sponge ecosystem functioning.
... Due to the macrotidal regime, associated with a strong vertical mixing, the high nitrate concentrations do not generate important green tides (Le . Strong decreases in the Si : N and Si : P ratios did not exhibit dramatic phytoplankton community shifts from diatoms to non-siliceous species in spring (Del Amo et al., 1997) because of the high Si recycling (Ragueneau et al., 2002;Beucher et al., 2004). ...
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.
... The amount of sponge silica accumulated in only the surface (upper centimeter) sediment layer of the Bay of Brest (1775 AE 162 Â 10 3 kg Si) falls in the same order of magnitude than that accumulated in the living sponge fauna of the Bay Ragueneau et al. (2005), stock of biogenic silica in planktonic diatoms has been calculated from Beucher et al. (2004), and reservoir of diatom silica in surface sediments are from Song and Ragueneau (2007). Asterisks refer to fluxes derived indirectly. ...
... The biogeochemical cycling of Si in the Bay of Brest through planktonic diatoms is well studied (Delmas and Tréguer, 1983;Del Amo et al. 1997;Chauvaud et al. 2000;Beucher et al. 2004) and summarized in Ragueneau et al. (2005). This budget was based on diatom Si flux rates and did not consider the sponge Si flux rates, nor the biogenic silica stocks of sponges and diatoms in the living communities and sediments. ...
... To compare the contribution of diatoms and sponges to the Si budget of the Bay, the standing stocks and Si reservoirs in the form of diatom silica were determined from the literature. According to it, the standing stock of diatoms in the water column of the Bay ranges from 0.12 to 1.98 μmol Si L À1 over the yr (Beucher et al. 2004). Unlike sponges, diatom cells have an ephemeral life of only days. ...
In coastal systems, planktonic and benthic silicifiers compete for the pool of dissolved silicon, a nutrient required to make their skeletons. The contribution of planktonic diatoms to the silicon cycle in coastal systems is often well characterized, while that of benthic silicifiers such as sponges has rarely been quantified. Herein, silicon fluxes and stocks are quantified for the sponge fauna in the benthic communities of the Bay of Brest (France). A total of 45 siliceous sponge species living in the Bay account for a silicon standing stock of 1215 tons, while that of diatoms is only 27 tons. The silicon reservoir accumulated as sponge skeletons in the surface sediments of the Bay rises to 1775 tons, while that of diatom skeletons is only 248 tons. These comparatively large stocks of sponge silicon were estimated to cycle two orders of magnitude slower than the diatom stocks. Sponge silicon stocks need years to decades to be renewed, while diatom turnover lasts only days. Although the sponge monitoring over the last 6 yr indicates no major changes of the sponge stocks, our results do not allow us to conclude if the silicon sponge budget of the Bay is at steady state, and potential scenarios are discussed. The findings buttress the idea that sponges and diatoms play contrasting roles in the marine silicon cycle. The budgets of these two major silicon users need to be integrated and their connections revealed, if we aim to improve our understanding of the silicon cycling in coastal ecosystems.
... Freshwater inputs are essentially from the Aulne river (catchment area 1875 km 2 , mean river flow 26 m 3 s -1 ), and also from two smaller rivers, the Elorn (catchment area 385 km 2 , mean river flow 6 m 3 s -1 ) and the Mignonne (catchment area 111 km 2 , mean river flow 1.5 m 3 s -1 ). Because of the macrotidal regime, the high nitrate concentrations do not generate important green tides (Le and the strong decreases in the Si:N and Si:P ratios did not 95 exhibit dramatic phytoplankton community shifts from diatoms to non-siliceous species in spring (Del Amo et al., 1997) according to the high Si recycling (Ragueneau et al., 2002;Beucher et al, 2004). ...
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.
... The NAO produces large changes in surface air temperatures, storm-track position, and precipitation over the North Atlantic and is strongly related to changes in sea surface temperature (SST) at the Brest and Roscoff sites (Tréguer et al., 2014 and reference therein). Freshwater inputs release important nutrient stocks into the aquatic environment (Meybeck et al., 2006;Dürr et al., 2011;Tréguer and De La Rocha, 2013), fueling phytoplankton blooms (e.g., Del Amo et al., 1997;Beucher et al., 2004). In these ecosystems, nutrient inputs maintain sustained phytoplankton biomass during the summer. ...
Weekly and bi-monthly carbonate system parameters and ancillary data were collected from 2008 to 2020 in three coastal ecosystems of the southern Western English Channel (sWEC) (SOMLIT-pier and SOMLIT-offshore) and Bay of Brest (SOMLIT-Brest) located in the North East Atlantic Ocean. The main drivers of seasonal and interannual partial pressure of CO2 (pCO2) and dissolved inorganic carbon (DIC) variabilities were the net ecosystem production (NEP) and thermodynamics. Differences were observed between stations, with a higher biological influence on pCO2 and DIC in the near-shore ecosystems, driven by both benthic and pelagic communities. The impact of riverine inputs on DIC dynamics was more pronounced at SOMLIT-Brest (7%) than at SOMLIT-pier (3%) and SOMLIT-offshore (<1%). These three ecosystems acted as a weak source of CO2 to the atmosphere of 0.18 ± 0.10, 0.11 ± 0.12, and 0.39 ± 0.08 mol m–2 year–1, respectively. Interannually, air-sea CO2 fluxes (FCO2) variability was low at SOMLIT-offshore and SOMLIT-pier, whereas SOMLIT-Brest occasionally switched to weak annual sinks of atmospheric CO2, driven by enhanced spring NEP compared to annual means. Over the 2008–2018 period, monthly total alkalinity (TA) and DIC anomalies were characterized by significant positive trends (p-values < 0.001), from 0.49 ± 0.20 to 2.21 ± 0.39 μmol kg⁻¹ year⁻¹ for TA, and from 1.93 ± 0.28 to 2.98 ± 0.39 μmol kg–1 year–1 for DIC. These trends were associated with significant increases of calculated seawater pCO2, ranging from +2.95 ± 1.04 to 3.52 ± 0.47 μatm year–1, and strong reductions of calculated pHin situ, with a mean pHin situ decrease of 0.0028 year–1. This ocean acidification (OA) was driven by atmospheric CO2 forcing (57–66%), Sea surface temperature (SST) increase (31–37%), and changes in salinity (2–5%). Additional pHin situ data extended these observed trends to the 2008–2020 period and indicated an acceleration of OA, reflected by a mean pHin situ decrease of 0.0046 year–1 in the sWEC for that period. Further observations over the 1998–2020 period revealed that the climatic indices North Atlantic Oscillation (NAO) and Atlantic Multidecadal Variability (AMV) were linked to trends of SST, with cooling during 1998–2010 and warming during 2010–2020, which might have impacted OA trends at our coastal stations. These results suggested large temporal variability of OA in coastal ecosystems of the sWEC and underlined the necessity to maintain high-resolution and long-term observations of carbonate parameters in coastal ecosystems.
... La dynamique phytoplanctonique de la Rade de Brest a fait l'objet de nombreuses études depuis les années 80 [Quéguiner et Tréguer 1984 ;Ragueneau, De Bias Varela et al. 1994 ;Del Amo et al. 1997 ;Beucher et al. 2004 ;Ragueneau, Chauvaud, Moriceau et al. 2005 ;Claquin et al. 2010]. Elle suit un schéma classique des zones tempérées avec une efflorescence printanière principale (démarrant autour du mois avril) suivie de plus petites efflorescences jusqu'au mois d'octobre. ...
Les systèmes automatisés de mesures à haute fréquence (HF) déployés dans des écosystèmes contrastés sont censés permettre une meilleure compréhension de la dynamique de l'environnement (et du phytoplancton) en réponse aux pressions d'origine naturelle et anthropiques, ainsi que les effets directs et indirects des proliférations du phytoplancton nuisible pouvant conduire à des dysfonctionnements des écosystèmes. La compréhension de cette dynamique est également importante afin de proposer des indicateurs conformes aux objectifs des directives européennes et des conventions des mers régionales. Alors que les données basses fréquences (BF) continuent de livrer leurs secrets, la complexité des données HF (bases volumineuses, convexes, avec des données manquantes,...) rend leur exploitation difficile. Dans ce contexte, cette thèse a pour objectif de développer un système numérique Open Source basé sur plusieurs méthodes du Machine Learning. Ce système doit permettre (i) de définir des schémas de fonctionnement des efflorescences multi-sources et multi-échelles à partir de données multivariées, (ii) de disposer d'un système de prédiction et d'alerte, (iii) de pouvoir adapter en temps (quasi) réel les stratégies d'échantillonnage pour les besoins de l'Observation, de la Surveillance et de la Recherche. La mise en évidence d'états environnementaux favorables ou pas aux efflorescences algales permet de hiérarchiser les facteurs de contrôle et d'identifier des schémas de fonctionnements. Ainsi, cette thèse a été conduite en différentes phases. Tout d'abord, une étude exploratoire des différentes variables, sources et échelles de données et des sites d'études a permis de définit les spécificités de chaque environnement et d'appliquer une stratégie d'étude multicritères. Ensuite, une nouvelle méthode de classification adaptée aux problématiques écologiques et HF est développée : la Classification Spectrale Multi-Niveaux. Cette approche appliquée aux données de la station MAREL-Carnot (IR ILICO, SNO COAST-HF) a permis la description (au niveau biogéochimique et taxonomique) d'événements récurrents mais aussi extrêmes, dont la période peut être infra-hebdomadaire ou même horaire. En plus de la caractérisation par état, cette approche multicritères identifie des schémas relationnels (états pressions et réponses). Ainsi, elle met en évidence des schémas de succession des facteurs d'influences, et des communautés. Enfin, une comparaison des réponses du modèle de classification à d'autres bases de données (MAREL-Iroise et MesuRho) est présentée. Pour aller plus loin, la méthodologie proposée est étendue à la prédiction d'événements météorologiques à partir de réanalyses (prévisions ERA5) et à un cas d'étude spatialisé (campagne océanographique CGFS). Elle démontre des classes cohérentes avec les expertises et s'ouvre ainsi vers une variété d'applications.
... Many rivers and estuaries discharge freshwater into the sWEC, with enhanced river influx during winter due to intense precipitation (Tréguer et al., 2014). These freshwater inputs release nutrient stocks into the marine environment (Meybeck et al., 2006;Dürr et al., 2011;Tréguer and De La Rocha, 2013) fueling spring phytoplankton blooms (e.g., Del Amo et al., 1997;Beucher et al., 2004) and maintaining substantial primary productivity in summer, when light is the limiting factor, not nutrients (Wafar et al., 1983). ...
From 2015 to 2019 we installed high-frequency (HF) sea surface temperature (SST), salinity, fluorescence, dissolved oxygen (DO) and partial pressure of CO 2 (pCO 2) sensors on a cardinal buoy of opportunity (ASTAN) at a coastal site in the southern Western English Channel (sWEC) highly influenced by tidal cycles. The sensors were calibrated against bimonthly discrete measurements performed at two long-term time series stations near the buoy, thus providing a robust multi-annual HF dataset. The tidal transport of a previously unidentified coastal water mass and an offshore water mass strongly impacted the daily and seasonal variability of pCO 2 and pH. The maximum tidal variability associated to spring tides (>7 m) during phytoplankton blooms represented up to 40% of the pCO 2 annual signal at ASTAN. At the same time, the daily variability of 0.12 pH units associated to this tidal transport was 6 times larger than the annual acidification trend observed in the area. A frequency/time analysis of the HF signal revealed the presence of a day/night cycle in the tidal signal. The diel biological cycle accounted for 9% of the annual pCO 2 amplitude during spring phytoplankton blooms. The duration and intensity of the biologically productive periods, characterized by large inter-annual variability, were the main drivers of pCO 2 dynamics. HF monitoring enabled us to accurately constrain, for the first-time, annual estimates of air-sea CO 2 exchanges in the nearshore tidally-influenced waters of the sWEC, which were a weak source to the atmosphere at 0.51 mol CO 2 m −2 yr −1. This estimate, combined with previous studies, provided a full latitudinal representation of the WEC (from 48 • 75 N to 50 • 25 N) over multiple years for air-sea CO 2 fluxes in contrasted coastal ecosystems. The latitudinal comparison showed a clear gradient from a weak source of CO 2 in the tidal mixing region toward sinks of CO 2 in the stratified region with a seasonal thermal front separating these hydrographical provinces. In view of the fact that several continental shelf regions have been reported to have switched from sources to sinks of CO 2 in the last century, weak CO 2 sources in such tidal mixing areas could potentially become sinks of atmospheric CO 2 in coming decades.
... The control chamber showed no detectable change in DSi concentration during its incubation ( Table 1), indicating that diatoms were scarce or non-active during the period of incubation. This result was expected because diatom abundance is minimal during November in the bay [39,40]. The individuals of T. citrina consumed DSi at an average rate of 0.046 ± 0.018 μmol Si h -1 mL -1 over the incubation period (Table 1). ...
Sponges consume dissolved silicon (DSi) to build their skeletons. Few studies have attempted to quantify DSi utilization by these organisms and all available determinations come from laboratory measurements. Here we measured DSi consumption rates of the sponge Tethya citrina in its natural habitat, conducting 24h incubations in benthic chambers. Sponges consumed DSi at an average rate of 0.046 ± 0.018 μmol h-1 mL-1 when DSi availability in its habitat was 8.3 ± 1.8 μM. Such DSi consumption rates significantly matched the values predicted by a kinetic model elsewhere developed previously for this species through laboratory incubations. These results support the use of laboratory incubations as a suitable approach to learn about DSi consumption. During the field incubations, utilization of other dissolved inorganic nutrients by this low-microbial-abundance (LMA) sponge was also measured. The sponges were net sources of ammonium (-0.043 ± 0.031 μmol h-1 mL-1), nitrate (-0.063 ± 0.031 μmol h-1 mL-1), nitrite (-0.007 ± 0.003 μmol h-1 mL-1), and phosphate (-0.004 ± 0.005 μmol h-1 mL-1), in agreement with the general pattern in other LMA species. The detected effluxes were among the lowest reported for sponges, which agreed with the low respiration rates characterizing this species (0.35 ± 0.11 μmol-O 2 h-1 mL-1). Despite relatively low flux, the dense population of T. citrina modifies the availability of dissolved inorganic nutrients in the demersal water of its habitat, contributing up to 14% of nitrate and nitrite stocks. Through these effects, the bottom layer contacting the benthic communities where siliceous LMA sponges abound can be partially depleted in DSi, but can benefit from inputs of N and P dissolved inorganic nutrients that are critical to primary producers.
... The mean concentration of dissolved silica (DSi) was 3.33 mgl -1 (0.3-10.3 mgl -1 ) in pre monsoon seasons. The spatio-temporal variations of DSi in coastal water is influenced by several factors, including the proportional physical mixing of seawater with fresh water, adsorption of reactive silicate into sedimentary particles, chemical interaction with clay minerals (Gouda and Panigrahy, 1992), co-precipitation with humic compounds and iron and biological removal by phytoplankton, especially by diatoms and silico flagellates (Beucher et al., 2004;Sahoo et al., 2014). The DSi was found to be highest in the northern sector due to maximum river discharge in this region (Figure 9.10). ...
... The mean concentration of dissolved silica (DSi) was 3.33 mgl -1 (0.3-10.3 mgl -1 ) in pre monsoon seasons. The spatio-temporal variations of DSi in coastal water is influenced by several factors, including the proportional physical mixing of seawater with fresh water, adsorption of reactive silicate into sedimentary particles, chemical interaction with clay minerals (Gouda and Panigrahy, 1992), co-precipitation with humic compounds and iron and biological removal by phytoplankton, especially by diatoms and silico flagellates (Beucher et al., 2004;Sahoo et al., 2014). The DSi was found to be highest in the northern sector due to maximum river discharge in this region (Figure 9.10). ...
