Distributions of phytoplankton carbohydrate, protein and lipid in the world oceans from satellite ocean colour

Energy value of phytoplankton regulates the growth of higher trophic species, affecting the tropic balance and sustainability of marine food webs. Therefore, developing our capability to estimate and monitor, on a global scale, the concentrations of macromolecules that determine phytoplankton energy value, would be invaluable. Reported here are the first estimates of carbohydrate, protein, lipid, and overall energy value of phytoplankton in the world oceans, using ocean-colour data from satellites. The estimates are based on a novel bio-optical method that utilises satellite-derived bio-optical fingerprints of living phytoplankton combined with allometric relationships between phytoplankton cells and cellular macromolecular contents. The annually-averaged phytoplankton energy value, per cubic meter of sub-surface ocean, varied from less than 0.1 kJ in subtropical gyres, to 0.5–1.0 kJ in parts of the equatorial, northern and southern latitudes, and rising to more than 10 kJ in certain coastal and optically complex waters. The annually-averaged global stocks of carbohydrate, protein and lipid were 0.044, 0.17 and 0.108 gigatonnes, respectively, with monthly stocks highest in September and lowest in June, over 1997-2013. The fractional contributions of phytoplankton size classes e.g., picoplankton, nanoplankton and microplankton to surface concentrations and global stocks of macromolecules varied considerably across marine biomes classified as Longhurst provinces. Among these provinces, the highest annually-averaged surface concentrations of carbohydrate, protein, and lipid were in North-East Atlantic Coastal Shelves, whereas, the lowest concentration of carbohydrate or lipid were in North Atlantic Tropical Gyral, and that of protein was in North Pacific Subtropical Gyre West. The regional accuracy of the estimates and their sensitivity to satellite inputs are quantified from the bio-optical model, which show promise for possible operational monitoring of phytoplankton energy value from satellite ocean colour. Adequate in situ measurements of macromolecules and improved retrievals of inherent optical properties from high-resolution satellite images, would be required to validate these estimates at local sites, and to further improve their accuracy in the world oceans

Planktonic ciliates in the Mediterranean Sea: longitudinal trends

International audienceWe analysed samples taken through the euphotic zone from 18 stations between the Ligurian Sea (6 degrees E) and the Levantin Basin (32 degrees E) from 24 May to 25 June 1996. Both ciliate and chlorophyll concentrations ranged over a factor of about 7, but ciliate concentrations (0.4-2.8 mg C m(3)) varied irregularly compared to a longitudinal decline, west to east, in chlorophyll concentration (0.07-0.48 mg m(3)). The lower chlorophyll concentrations (0.1 mg m(2)) of the eastern basin stations corresponded with a relatively high stock of ciliates (0.5 mg C m(2)). Large mixotrophic ciliates were more abundant, in both absolute and relative terms, in the eastern Mediterranean stations with less chlorophyll. The species diversity of tintinnid ciliates appeared higher in the central and eastern basins compared to the west. Our results suggest a shift from the western to eastern Mediterranean in the planktonic food towards a microbially dominated system. (C) 1999 Elsevier Science Ltd. All rights reserved

Phytoplankton pigment variations during the transition from spring bloom to oligotrophy in the northwestern Mediterranean sea

Phytoplankton taxonomic pigments and primary production were measured at the JGOFS-France time-series station DYFAMED in the northwestern Mediterranean Sea during May 1995 to investigate changes in phytoplankton composition and the biogeochemical implications (DYNAPROC experiment). The study period covered the transitional situation from late spring bloom to pre-oligotrophic. The late spring bloom situation, occurring at the beginning of the study, revealed high chlorophyll a concentrations (maximum 3 mg m(-3) at 30 m) and high primary production (maximum 497 mg C m(-2) 14 h(-1)). At the end of the experiment, the trophic regime shifted towards pre-oligotrophic and was characterized by lower chlorophyll a concentrations (<1 mg m(-3)), although primary production still remained high (659 mg C m-2 14 h(-1)). At termination of the spring bloom, the phytoplankton community was composed of chromophyte nanoflagellates (38 +/- 4%), diatoms (39 +/- 2%), cryptophytes (12 +/- 1%) and cyanobacteria (8 +/- 1%). During the transition to the prl-oligotrophic period, the contribution of small cells increased (e.g. cyanobacteria 18 +/- 2%, green flagellates 5 +/- 1%), Vertical profiles of pigments revealed a partition of the phytoplankton groups: cyanobacteria were most abundant in the surface layer, nanoflagellates containing 19'-HF + 19'BF at the depth of chlorophyll maximum, whereas diatoms were located below the chlorophyll maximum. At termination of the spring bloom, a wind event induced vertical transport of nutrients into the euphotic layer. Phytoplankton groups responded differently to the event: initially, diatom concentrations increased (for 24 h) then rapidly decreased. In contrast, all others groupsdecreased just after the event. The long-term effect was a decrease of biomass of dominant groups (diatoms and chromophyte nanoflagellates), which accelerated the community succession and hence contributed to the oligotrophic transition. (C) 2000 Elsevier Science Ltd. All rights reserved

Diel vertical distribution of planktonic ciliates within the surface layer of the NW Mediterranean (May 1995)

The composition and vertical distribution of planktonic ciliates within the surface layer was monitored over four diel cycles in May 95, during the JGOFS-France DYNAPROC cruise in the Ligurian Sea (NW Mediterranean). Ciliates were placed into size and trophic categories: micro- and nano-heterotrophic ciliates, mixotrophic ciliates, tintinnids and the autotrophic Mesodinium rubrum. Mixotrophic ciliates (micro and nano) represented an average of 46% of oligotrich abundance and 39% of oligotrich biomass; nano-ciliates (hetero and mixotrophic) were abundant, representing about 60 and 17% of oligotrich abundance and biomass, respectively. Tintinnid ciliates were a minor part of heterotrophic ciliates. The estimated contribution of mixotrophs to chlorophyll a concentration was modest, never exceeding 9% in discrete samples. Vertical profiles of ciliates showed that chlorophyll-containing ciliates (mixotrophs and autotrophs) were mainly concentrated and remained at the chlorophyll a maximum depth. In contrast, among heterotrophic ciliates, a portion of the population appeared to migrate from 20-30 m depth during the day to the surface at night or in the early morning. Correlation analyses of ciliate groups and phytoplankton pigments showed a strong relationship between nano-ciliates and zeaxanthin, and between chlorophyll-containing ciliates and chlorophyll a, as well as other pigments that were maximal at the chlorophyll a maximum depth. Total surface layer concentrations showed minima of ciliates during nightime/early morning hours. (C) 2000 Elsevier Science Ltd. All rights reserved

Ultraphytoplankton basin-scale distribution in the eastern Mediterranean Sea in winter : link to hydrodynamism and nutrients

International audienceThe basin-scale distribution of ultraphytoplankton (<10 µm) was determined in the upper 200 m of the eastern Mediterranean Sea during the winter season. Four clusters were resolved by flow cytometry on the basis of their optical properties and identified as Synechococcus, Prochloro-coccus, pico-(<3 µm) and nanoeukaryotes (3-10 µm). Syne-chococcus was the most abundant population (maximum abundance of about 37 000 cells cm −3) and contributed up to 67.7% to the overall ultraphytoplanktonic carbon biomass, whereas the contribution of Prochlorococcus never exceeded 6.5%. The maximum integrated carbon biomass was 1763, 453, 58 and 571 mg C m −2 for nanoeukaryotes, picoeukary-otes, Prochlorococcus and Synechococcus respectively. Water mass properties were analyzed on the basis of temperature and salinity distributions in order to account for the general circulation and locate the main hydrodynamic structures (fronts, gyres, transition between western and eastern basins). The effect of the main hydrodynamic structures and nutrients on the ultraphytoplankton distribution was investigated. No positive correlation between nutrients and phytoplankton could be established when considering large scales. However, below 50 m depth, nutrient ratios between particular stations were correlated to corresponding density ratios. In contrast, significant relationships were found be-Correspondence to: M. Denis ([email protected]) tween Synechococcus abundance and density, resulting from the impact of a gyre in southern Adriatic basin and a ther-mohaline front in the Ionian basin. A significant relationship was also found between picoeukaryotes and salinity in the comparison of western and eastern Mediterranean Sea

Carbon biomass, and gross growth rates as estimated from C-14 pigment labelling, during photoacclimation in Prochlorococcus CCMP 1378

International audienceThe C-14 labelling of chlorophylls and carotenoids is increasingly used to evaluate phytoplanktonic biomass and growth rates in oceanic systems. Rigorous testing of the technique in the laboratory, however, is necessary prior to its application in the field. A Mediterranean clone of Prochlorococcus, a photosynthetic prokaryote which is an important component of the autotrophic biomass in oligotrophic environments, was subjected to shifts in light intensity. Particulate organic carbon (POC) was monitored by CHN analysis, pigments by HPLC and Prochlorococcus and heterotrophic bacteria concentrations by flow cytometry. Using a combination of HPLC and on-line radioactivity detection, C-14 labelling kinetics of divinyl-chlorophyll a (Dv-chl a) and zeaxanthin were followed. Prochlorococcus changed its Dv-chl a content markedly in response to change in light intensity, but not its zeaxanthin content, which remained nearly constant around 1.07 fg cell(-1) regardless of the irradiance. Pigment synthesis rates were correctly estimated from their C-14 incorporation rates whatever the Light level. From POC measurements and cell concentrations, the Prochlorococcus carbon content was estimated to be 49 fg C cell(-1). Moreover, under both constant and shifted (high to low and vice versa) Light conditions, Prochlorococcus growth rate (as computed from variations in cell. density) was much better estimated from zeaxanthin than from Dv-chl a labelling rates

Diel variations of the DMSP-to-chlorophyll a ratio in Northwestern Mediterranean surface waters

International audienc

Heterotrophic bacteria show weak competition for nitrogen in Mediterranean coastal waters (Thau Lagoon) in autumn

The importance of heterotrophic bacteria relative to phytoplankton in the uptake of ammonium and nitrate was studied in Mediterranean coastal waters (Thau Lagoon) during autumn, when the Mediterranean Sea received the greatest allochthonous nutrient loads. Specific inhibitors and size-fractionation methods were used in combination with isotopic N-15 tracers. NO3 (-) and NH4 (+) uptake was dominated by phytoplankton (60 % on average) during the study period, which included a flood event. Despite lower biomass specific NH4 (+) and NO3 uptake rates, free-living heterotrophic bacteria contributed significantly (> 30 %) to total microbial NH4 (+) and NO3 (-) uptake rates in low chlorophyll waters. Under these conditions, heterotrophic bacteria may be responsible for more than 50 % of primary production, using very little freshly produced phytoplankton exudates. In low chlorophyll coastal waters as reported during the present 3-month study, the heterotrophic bacteria seemed to depend to a greater extent on allochthonous N and C substrates than on autochthonous substrates derived from phytoplankton

Impact of oysters as top predators on microbial food web dynamics: a modelling approach with parameter optimisation. Running page head: Modelling oyster impact on MFW

International audienceAquaculture is becoming a relevant and productive source of seafood, and production is expected to double in the near future. However, bivalve activities can significantly impact coastal ecosystem functioning. To study the direct and indirect impacts of oysters on the microbial food web, a 0D biogeochemical modelling approach was adopted. The model was adjusted by parameter optimisation, assimilating data from several mesocosm observations of concentrations of nitrate, phosphate, silicate, dissolved organic carbon, chlorophyll, and bacterial biomass. The optimisation method provided a set of optimal parameters to fit the experimental observations of ‘control’ (i.e. natural water without oysters) and ‘oyster’ (i.e. natural water with oysters) mesocosms. The modelling results showed good accordance with the experimental observations, suggesting that the oysters directly reduced phytoplankton community biomass, thus constraining the ecosystem to a more heterotrophic state. Oysters also reduced competition between bacteria and phytoplankton for nutrient uptake, favouring higher bacterial biomass than in the control experiment. Additionally, the presence of oysters strongly increased large micro-zooplankton biomass (50-200 µm; mainly ciliates and large flagellates). This was a consequence of bacterivory by small zooplankton (5-50 µm; mostly flagellates and small ciliates), providing a trophic link between bacteria and larger zooplankton. In conclusion, parameter optimisation showed good capacity to manage experimental data in order to build a more realistic model. Such models, in connection with future developments in aquaculture and global change scenarios, could be a promising tool for exploited ecosystem management and testing different environmental scenarios