Meridional and vertical distributions of several biogeochemical parameters were studied along a section in the southeastern Atlantic and the Southern Ocean south of South Africa during the austral ...summer 2008 of the International Polar Year to characterize the biogeochemical provinces and to assess the seasonal net diatom production. Based on analyses of macro-nutrients, ammonium (NH4), chlorophyll a, (Chl a), phaeopigments, biogenic silica (BSi), particulate inorganic carbon (PIC), and particulate organic carbon and nitrogen (POC and PON, respectively), four biogeochemical domains were distinguished along the section: the subtropical Atlantic, the confluence zone of the subtropical and subantarctic domains, the Polar Frontal Zone (PFZ) in the Antarctic Circumpolar Current (ACC), and the north-eastern branch of the Weddell Gyre. The subtropical region displayed extremely low nutrient concentrations featuring oligotrophic conditions, and sub-surface maxima of Chl a and phaeopigments never exceeded 0.5 µg L−1 and 0.25 µg L−1, respectively. The anticyclonic and cyclonic eddies crossed in the Cape Basin were characterized by a deepening and a rise, respectively, of the nutrients isoclines. The confluence zone of the subtropical domain and the northern side of the ACC within the subantarctic domain displayed remnant nitrate and phosphate levels, whereas silicate concentrations kept to extremely low levels. In this area, Chl a level of 0.4–0.5 µg L−1 distributed homogenously within the mixed layer, and POC and PON accumulated to values up to 10 µM and 1.5 µM, respectively, indicative of biomass accumulation along the confluence zone during the late productive period. In the ACC domain, the Polar Frontal Zone was marked by a post-bloom of diatoms that extended beyond the Polar Front (PF) during this late summer condition, as primarily evidenced by the massive depletion of silicic acid in the surface waters. The accumulation of NH4 to values up to 1.25 µM at 100 m depth centred on the PF and the accumulation of BSi up to 0.5 µM in the surface waters of the central part of the PFZ also featured a late stage of the seasonal diatom bloom. The silica daily net production rate based on the seasonal depletion of silicic acid was estimated to be 11.9 ± 6.5 mmol m−2 d−1 in the domain of the vast diatom post-bloom, agreeing well with the previously recorded values in this province. The Weddell Gyre occasionally displayed relative surface depletion of silicic acid, suggesting a late stage of a relatively minor diatom bloom possibly driven by iceberg drifting releases of iron. In this domain the estimated range of silica daily net production rate (e.g. 21.1 ± 8.8 mmol m−2 d−1) is consistent with previous studies, but was not significantly higher than that in the Polar Front region.
The Kerguelen Plateau is characterized by a naturally Fe-fertilized phytoplankton bloom that extends more than 1000km downstream in the Antarctic Circumpolar Current. During the KEOPS2 study, in ...austral spring, we measured particulate nitrogen (PN), biogenic silica (BSi) and particulate iron (PFe) export fluxes in order to investigate how the natural fertilization impacts the stoichiometry and the magnitude of export fluxes and therefore the efficiency of the biological carbon pump. At 9 stations, we estimated elemental export fluxes based on element concentration to 234Th activity ratios for particulate material collected with in-situ pumps and 234Th export fluxes (Planchon et al., 2015). This study revealed that the natural Fe-fertilization increased export fluxes but to variable degrees. Export fluxes for the bloom impacted area were compared with those of a high-nutrient, low-chlorophyll (HNLC), low-productive reference site located to the south-west of Kerguelen and which had the lowest BSi and PFe export fluxes (2.55mmolBSim−2d−1 and 1.92µmol PFem−2d−1) and amongst the lowest PN export flux (0.73mmolPNm−2d−1). The impact of the Fe fertilization was the greatest within a meander of the polar front (PF), to the east of Kerguelen, with fluxes reaching 1.26mmolPNm−2d−1; 20.4mmol BSim−2d−1 and 22.4µmolPFem−2d−1. A highly productive site above the Kerguelen Plateau, on the contrary, was less impacted by the fertilization with export fluxes reaching 0.72mmolPNm−2d−1; 4.50mmol BSim−2d−1 and 21.4µmolPFem−2d−1. Our results suggest that ecosystem features (i.e. type of diatom community) could play an important role in setting the magnitude of export fluxes of these elements. Indeed, for the PF meander, the moderate productivity was sustained by the presence of large and strongly silicified diatom species while at the higher productivity sites, smaller and slightly silicified diatoms dominated. Interestingly, our results suggest that PFe export fluxes can be driven by the lithogenic pool of particles, especially over the Plateau where such inputs from the sediments are important. Finally, for the Plateau and the PF meander, the comparison between PFe export and the particulate PFe stock integrated over the mixed layer depth revealed an efficient PFe export out of the mixed layer at these sites. Export efficiencies (i.e. the ratio between export and uptake) exhibit a very efficient silica pump especially at the HNLC reference station where heavily silicified diatoms were present. On the contrary, the increase with depth of the C:N ratio and the low nitrogen export efficiencies support the idea of a strong remineralization and nitrification activity.
•Natural iron fertilization induces an increase of PN, BSi and PFe export fluxes.•Ecosystem plays a key role on the magnitude, stoichiometry and efficiency of fluxes.•Particulate iron export fluxes are mainly driven by the lithogenic pool of particles.
From 2008 to 2014, the MAREL-Iroise buoy, located in the Bay of Brest, collected high-frequency measurements of partial pressure of CO2 (pCO2) and ancillary hydrographic parameters, in conjunction ...with a comprehensive sampling regime of two additional carbonate system variables total alkalinity (AT), and dissolved inorganic carbon (DIC). Biological processes drive variations in AT and DIC throughout the year, except in winter, when primary production is negligible and large freshwater inputs occur. Annually, the Bay of Brest generally behaves as a source of CO2 to the atmosphere (0.14±0.20molCm−2yr−1), showing inter-annual variability significantly linked to annual net community production (NCP). The presence of a large community of benthic filter feeders leads to high levels of particulate organic matter (POM) and opal deposition during the spring diatom bloom. Over the following few months, benthic POM remineralisation reduces the spring CO2 deficit relative to the atmosphere, and remineralisation of biogenic silica supplies further late spring primary production. The result is an inverse spring NCP – air-sea CO2 flux relationship, whereby greater NCP in early spring results in lower fluxes of CO2 into the Bay in late spring. This recycling mechanism, or silicic acid pump, also links the spring and summer NCP values, which are both determined by the peak wintertime nutrient concentrations. The carbonate system is further affected by the benthic community in winter, when CaCO3 dissolution is evident from notable deviations in the ΔAT:ΔDIC ratio. This study highlights the necessity of individual study of coastal, temperate ecosystems and contributes to a better understanding of what determines coastal areas as sinks or sources of CO2 to the atmosphere.
•The Bay of Brest generally behaves as a source of CO2 to the atmosphere.•Inter-annual variability in air-sea CO2 exchange is linked to net community production.•Springtime net community production is determined by the winter, river dissolved silica supply.•Total alkalinity and dissolved inorganic carbon variability is driven by biology.
High-frequency pCO2 and ancillary data were recorded for seven years during the first deployment of a CARbon Interface OCean Atmosphere (CARIOCA) sensor in the surface waters of a temperate coastal ...ecosystem, the Bay of Brest, which is impacted by both coastal (via estuaries) and oceanic (North Atlantic via the Iroise Sea) water inputs. The CARIOCA sensor proved to be an excellent tool to constrain the high pCO2 variability in such dynamic coastal ecosystem. Biological processes (e.g. pelagic photosynthesis/respiration) were the main drivers of the seasonal and diurnal pCO2 dynamics throughout seven years of observations. Autotrophic processes were responsible for abrupt pCO2 drawdown of 100 to 200 mu atm in spring. During the spring bloom, diurnal variations were driven by diel biological cycle. The average daily drawdown due to autotrophy (observed during highest daily PAR) was equivalent to 10 to 60% of the total pCO2 drawdown observed every year during the spring season. From late summer to fall, heterotrophic processes increased pCO2 in the surface water of the Bay back to the pre-bloom level. The average daily increase due to heterotrophy (observed during lowest daily PAR) corresponded to 10 to 70% of the total pCO2 increase observed every year during the late summer to fall period. Air-sea CO2 fluxes estimates based on hourly, daily and monthly calculations showed that careful consideration of the diurnal variability was needed to accurately estimate air-sea CO2 fluxes in the Bay of Brest. Sampling only during daytime or night-time would induce 8 to 36% error on monthly air-sea CO2 fluxes. This would in turn reverse the direction of the fluxes at annual level for the Bay. The annual emissions of CO2 from the surface waters of the Bay to the atmosphere showed relatively low inter-annual variations with an average of +0.7+/-0.4molCm-2yr super(-1) computed for the study period. Further, air-sea CO2 fluxes computed for the adjacent inner-estuaries and Iroise Sea for an annual cycle were +17+/-3molCm-2yr super(-1) and -0.2+/-0.2molCm-2yr super(-1), respectively. The spatial gradient showed a clear pattern from strong source to sink of CO2, from the inner-estuaries to the open oceanic waters of the North Atlantic. We suggest that semi-enclosed Bays act as buffers for sea to air emissions of CO2 from inner estuaries to adjacent costal seas.
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.
Coastal marine ecosystems, which play a crucial role in the biogeochemical and ecological functioning of the Earth, are highly sensitive to the combined effects of climate and human activities. ...Because of their location, coastal ecosystems are directly influenced by human activities, but it remains challenging to assess the spatial and temporal scales at which climate influences coastal ecosystems. We monitored 12 sampling stations, distributed in 8 ecosystems in France, over 2 decades for physico-biogeochemical parameters (temperature, salinity, concentrations of dissolved oxygen, nutrients and particulate material). The study encompasses a large diversity of temperate coastal ecosystems with respect to e.g. geomorphology, trophic status, tidal regime, river influence and turbidity. Time-series analysis coupled with standardised 3-mode principal component analyses, partial triadic analyses and correlations were used to assess bi-decadal variability and ecosystem trajectories, and to identify large-scale, regional and local drivers. Our results highlighted 2 abrupt changes in 2001 and 2005. The bi-decadal changes were related to changes in large-scale and regional climate, detected through proxies of temperature and atmospheric circulation, as well as through river discharge. Ecosystem trajectories tended to move towards an increase in temperature and salinity, and/or a decrease in chlorophyll
a
, nutrients and particulate matter. However, the magnitude of change, the year-to-year variability and the sensitivity to the 2001 and 2005 changes varied among the ecosystems. This study highlights the need for establishing long-term time series and combining data sets as well as undertaking multi-ecosystem and local studies to better understand the long-term variability of coastal ecosystems and its associated drivers.
Since the beginning of the industrial revolution, atmospheric carbon dioxide (CO2) concentrations have risen steadily and have induced a decrease of the averaged surface ocean pH by 0.1 units, ...corresponding to an increase in ocean acidity of about 30 %. In addition to ocean warming, ocean acidification poses a tremendous challenge to some marine organisms, especially calcifiers. The need for long-term oceanic observations of pH and temperature is a key element to assess the vulnerability of marine communities and ecosystems to these pressures. Nearshore productive environments, where a large majority of shellfish farming activities are conducted, are known to present pH levels as well as amplitudes of daily and seasonal variations that are much larger than those observed in the open ocean. Yet, to date, there are very few coastal observation sites where these parameters are measured simultaneously and at high frequency.To bridge this gap, an observation network was initiated in 2021 in the framework of the CocoriCO2 project. Six sites were selected along the French Atlantic and Mediterranean coastlines based on their importance in terms of shellfish production and the presence of high- and low-frequency monitoring activities. At each site, autonomous pH sensors were deployed, both inside and outside shellfish production areas, next to high-frequency CTD (conductivity–temperature–depth) probes operated through two operating monitoring networks. pH sensors were set to an acquisition rate of 15 min, and discrete seawater samples were collected biweekly in order to control the quality of pH data (laboratory spectrophotometric measurements) as well as to measure total alkalinity and dissolved inorganic carbon concentrations for full characterization of the carbonate system. While this network has been up and running for more than 2 years, the acquired dataset has already revealed important differences in terms of pH variations between monitored sites related to the influence of diverse processes (freshwater inputs, tides, temperature, biological processes). Data are available at 10.17882/96982 (Petton et al., 2023a).
High-frequency pCO
2 and ancillary data were recorded for seven years during the first deployment of a CARbon Interface OCean Atmosphere (CARIOCA) sensor in the surface waters of a temperate coastal ...ecosystem, the Bay of Brest, which is impacted by both coastal (via estuaries) and oceanic (North Atlantic via the Iroise Sea) water inputs. The CARIOCA sensor proved to be an excellent tool to constrain the high pCO
2 variability in such dynamic coastal ecosystem. Biological processes (e.g. pelagic photosynthesis/respiration) were the main drivers of the seasonal and diurnal pCO
2 dynamics throughout seven years of observations. Autotrophic processes were responsible for abrupt pCO
2 drawdown of 100 to 200
μatm in spring. During the spring bloom, diurnal variations were driven by diel biological cycle. The average daily drawdown due to autotrophy (observed during highest daily PAR) was equivalent to 10 to 60% of the total pCO
2 drawdown observed every year during the spring season. From late summer to fall, heterotrophic processes increased pCO
2 in the surface water of the Bay back to the pre-bloom level. The average daily increase due to heterotrophy (observed during lowest daily PAR) corresponded to 10 to 70% of the total pCO
2 increase observed every year during the late summer to fall period. Air–sea CO
2 fluxes estimates based on hourly, daily and monthly calculations showed that careful consideration of the diurnal variability was needed to accurately estimate air–sea CO
2 fluxes in the Bay of Brest. Sampling only during daytime or night-time would induce 8 to 36% error on monthly air–sea CO
2 fluxes. This would in turn reverse the direction of the fluxes at annual level for the Bay. The annual emissions of CO
2 from the surface waters of the Bay to the atmosphere showed relatively low inter-annual variations with an average of +
0.7
±
0.4
mol
C
m
−2
yr
−
1
computed for the study period. Further, air–sea CO
2 fluxes computed for the adjacent inner-estuaries and Iroise Sea for an annual cycle were +
17
±
3
mol
C
m
−2
yr
−
1
and −
0.2
±
0.2
mol
C
m
−2
yr
−
1
, respectively. The spatial gradient showed a clear pattern from strong source to sink of CO
2, from the inner-estuaries to the open oceanic waters of the North Atlantic. We suggest that semi-enclosed Bays act as buffers for sea to air emissions of CO
2 from inner estuaries to adjacent costal seas.
► We assessed pCO
2 dynamics from diurnal to inter-annual level in the Bay of Brest. ► Pelagic biological processes controlled pCO
2 dynamics from spring to fall. ► Integrating pCO
2 diurnal variability is imperative for air–sea CO
2 fluxes estimates. ► Bays act as buffers for sea to air CO
2 emissions from estuaries to coastal seas.
Meridional and vertical distributions of several biogeochemical parameters were studied along a section in the southeastern Atlantic and the Southern Ocean south of South Africa during the austral ...summer 2008 of the International Polar Year to characterize the biogeochemical provinces and to assess the seasonal net diatom production. Based on analyses of macro-nutrients, ammonium (NH4), chlorophyll a, (chl a) phaeopigments, biogenic silica (BSi), particulate inorganic carbon (PIC), and particulate organic carbon and nitrogen (POC and PON, respectively) four biogeochemical domains were distinguished along the section: the subtropical Atlantic, the confluence zone of the subtropical and subantarctic domains, the Polar Frontal Zone (PFZ) in the Antarctic Circumpolar Current (ACC) and the north-eastern branch of the Weddell Gyre. The subtropical region displayed extremely low nutrient concentrations featuring oligotrophic conditions, and sub-surface maxima of chl a and phaeopigments never exceeded, 0.5 μg l−1 and 0.25 μg l−1 respectively. The anticyclonic and cyclonic eddies crossed in the Cape Basin were characterized by a deepening and a rise, respectively, of the nutrients isolines. Mesoscale eddies can bring episodic pulse of nutrients into the photic zone. The confluence zone of the subtropical domain and the northern side of the ACC within the subantarctic domain displayed remnant nitrate and phosphate levels, whereas silicate concentrations kept to extremely low levels. In this area chl a level of 0.4–0.5 μg l−1 distributed homogenously within the mixed layer, and POC and PON accumulated to values up to 10 μM and 1.5 μM, respectively; still indicative of biomass accumulation along the confluence zone during the late productive period. In the ACC domain, the Polar Frontal Zone was marked by a postbloom of diatoms that extended beyond the Polar Front (PF) during this late summer condition, as primarily evidenced by the massive depletion of silicic acid in the surface waters. The accumulation of NH4 to values up to 1.25 μM at 100 m depth centred on the PF and the accumulation of BSi up to 0.5 μM in the surface waters of the central part of the PFZ also featured a late stage of the seasonal diatom bloom. Similar southward displacement of the silicic acid depletion beyond the PF has been previously observed throughout the productive period, associated with the development and extension of the seasonal bloom of diatoms. The silica daily net production rate based on the seasonal depletion of silicic acid was estimated to be 11.9 ± 6.5 mmol m−2 d−1 in the domain of the vast diatom post-bloom, agreeing well with the previously recorded values in this province. The Weddell Gyre occasionally displayed relative surface depletion of silicic acid suggesting a late stage of a relatively minor diatom bloom, possibly driven by iceberg drifting releases of iron. An accumulation of BSi up to 0.5 μM was recorded in the top 350 m of the southern branch of the ACC and in the Weddell Gyre which may be seen as the presence of heavily silicified diatoms due to lack of iron in this HNLC area. In this domain the estimated range of silica daily net production rate (e.g. 21.1 ± 8.8 mmol m−2 d−1) is consistent with previous studies, but was not significantly higher than that in the Polar Front region.
The GEOTRACES Intermediate Data Product 2014 (IDP2014) is the first publicly available data product of the international GEOTRACES programme, and contains data measured and quality controlled before ...the end of 2013. It consists of two parts: (1) a compilation of digital data for more than 200 trace elements and isotopes (TEIs) as well as classical hydrographic parameters, and (2) the eGEOTRACES Electronic Atlas providing a strongly inter-linked on-line atlas including more than 300 section plots and 90 animated 3D scenes. The IDP2014 covers the Atlantic, Arctic, and Indian oceans, exhibiting highest data density in the Atlantic. The TEI data in the IDP2014 are quality controlled by careful assessment of intercalibration results and multi-laboratory data comparisons at cross-over stations. The digital data are provided in several formats, including ASCII spreadsheet, Excel spreadsheet, netCDF, and Ocean Data View collection. In addition to the actual data values the IDP2014 also contains data quality flags and 1-σ data error values where available. Quality flags and error values are useful for data filtering. Metadata about data originators, analytical methods and original publications related to the data are linked to the data in an easily accessible way. The eGEOTRACES Electronic Atlas is the visual representation of the IDP2014 data providing section plots and a new kind of animated 3D scenes. The basin-wide 3D scenes allow for viewing of data from many cruises at the same time, thereby providing quick overviews of large-scale tracer distributions. In addition, the 3D scenes provide geographical and bathymetric context that is crucial for the interpretation and assessment of observed tracer plumes, as well as for making inferences about controlling processes.
•GEOTRACES releases its first integrated and quality controlled Intermediate Data Product 2014 (IDP2014).•The IDP2014 digital data are available at http://www.bodc.ac.uk/geotraces/data/idp2014/ in 4 different formats.•The eGEOTRACES Electronic Atlas at http://egeotraces.org/ provides 329 section plots and 90 animated 3D tracer scenes.•The new 3D scenes provide geographical and bathymetric context crucial for tracer assessment and interpretation.
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