Micheline Bianchi

Carbon flow in the planktonic food web along a gradient of oligotrophy in the Aegean Sea (Mediterranean Sea)

Abstract This work is a study of plankton food web structure and carbon flow in March and September 1997 in the Aegean Sea, area of outflow of Black Sea waters in the Mediterranean Sea. Biomass and production of autotrophs were measured by size fraction as well as bacterial biomass and production; furthermore, we studied heterotrophic nanoflagellates (HNAN), ciliates and mesozooplankton biomass, copepod production and grazing impact on phytoplankton. The obtained low values of nutrients and plankton biomass and production confirmed the oligotrophic character of this region. Despite the fact that there was no significant horizontal variability in the spatial distribution of nutrients throughout the study area, the planktonic biomass and production revealed a gradual decrease from the Northeast Aegean (NEA) towards the South Aegean (SA). In the Northeast Aegean, a large part of the fixed carbon was channelled through the microbial food web towards copepods; in contrast there was a low transfer of energy in the South Aegean where the multivorous food web was developed. Throughout the study area, almost 60–70% of autotrophic biomass and primary production was performed by cells

Nitrification rates, ammonium and nitrate distribution in upper layers of the water column and in sediments of the Indian sector of the Southern Ocean

Abstract During the ANTARES 1/F-JGOFS cruise (April–May 1993) concentrations of inorganic nitrogen, nitrification rates (ammonium and nitrite oxidation) and 14 C carbonate incorporation by nitrifying bacteria were measured at 0, 50 and 100 m in the water column, and at the water-sediment interface between the polar front (52°S) and the subantarctic and subtropical fronts (42°S) in the Indian Ocean Sector of the Antarctic Ocean. The 0–100 m water layer showed a global increase of NH 4 + (from 0.4 to 0.6 μM) and a global decrease of NO 2 − + NO 3 − (from 25 to 5 μM) along the south-north transect. Anomalies were detected for both ammonium and nitrite + nitrate concentrations in the SubTropical Front (STF) and SubAntarctic Front (SAF). Pelagic nitrifying activities (N oxidation) did not demonstrate any latitudinal gradient in the water column. In terms of integrated rates between 0 and 100 m, the nitrification increased by a factor 1.8–2 in the STF-SAF area. The N oxidation processes mostly depended on the substrate availability. For both ammonium and nitrite oxidizers the C fixation was well correlated ( r = 0.68, p = 0.001 and r = 0.98, p = 0.0001, respectively) with N oxidation, in the range of 15 N (ammonium oxidizers) and 20 N (nitrite oxidizers) oxidized for 1 C incorporated. For nitrite oxidizers, C incorporation was principally influenced by temperature ( Q 10 = 1,73). From the calculation of nitrogen fluxes, the nitrifiers would be able to compete with the primary producers for the regenerated ammonium, while the flux of nitrate produced by nitrification could sustain 10 to 100% of the primary producer requirements for this nitrogen source. In the sediment pore waters, the inorganic nitrogen compounds displayed two different latitudinal distributions. The ammonium concentrations were lower (around 4 μM) south of the 48°S than in the northern part (values reaching 10 μM in the first centimetre). The nitrite + nitrate concentrations were close to 40–45 μM at each end of the transect, and increased to 50–55 μM near 45°S. The benthic nitrification, measured from 0 to 5 cm depth in the sediment cores, showed high rates in the upper 3 cm. The rates of ammonium oxidation demonstrated a good correlation ( r = 0.64, p = 0.0001) with the ammonium content of the interstitial water. The nitrite oxidation rates were well correlated ( r = 0.92, p = 0.0001) with the ammonium oxidation rates. The integrated rates (0–5 cm) of ammonium oxidation were of about 35 nmol NO 2 − produced cm −2 day −1 in the south, and increased by a factor 1.3–1.9 after 44°S. Both ammonium and nitrite oxidation rates showed a maximum near 44°S, corresponding to the SAF-STF frontal zone.

Rates of nitrification and carbon uptake in the Rhône River plume (northwestern Mediterranean Sea)

Nitrification rates were measured along a salinity gradient in the Rhône River estuary, using specific inhibitors (allylthiourea and chlorate) coupled with the measurement of change in nitrite concentration and inorganic carbon uptake by nitrifiers. Rates of ammonium and nitrite oxidation were similar up to 15 practical salinity units (from 1 to 2 μmol N oxidized liter-1 day-1). For higher salinities, nitrite and ammonium oxidation rates were 0.14 and 0.23 μmol N oxidized liter-1 day-1, respectively. Ammonium oxidizers assimilated 19−150 × 10−3 μmol C liter-1 day-1, while nitrite oxidizers fixed 4.8−72.6 × 10−3 μmol C liter-1 day-1. The amounts of nitrogen oxidized and C incorporated demonstrated a linear correlation (r2 > 0.99). The ratio of N oxidized to C incorporated ranged between 14.3 to 12.3 for ammonium oxidizers, and between 31.6 and 29 for nitrite oxidizers, the lower values being measured in seawater.

Bacterial biomass, heterotrophic production and utilization of dissolved organic matter photosynthetically produced in the Almeria-Oran front

Abstract Bacterial biomass, bacterial production and the quantitative importance of the heterotrophic assimilation of photosynthetically produced dissolved organic carbon (PDOC) were examined in relation to the hydrodynamical and biological conditions in the Almeria-Oran front area (Alboran Sea, Western Mediterranean). Although, including all data, bacterial abundance correlated with chlorophyll a ( r = 0.49), the bacteria/phytoplankton carbon ratio decreased in the core of the Atlantic jet. Bacterial integrated secondary production ranged from 124 to 199 mg C m −2 d −1 . Bacterial generation times averaged 3.2 d (S.D.= 1.3) in frontal sites above the pycnocline and 25 d (S.D. = 11) under the pycnocline. In the adjacent Mediterranean waters, bacterial generation times displayed homogenous values from the surface to 150 m (mean 2.7;d; S.D. = 1.5). An isolated Atlantic water mass, at the right side of the jet, showed the longest average bacterial generation times (9.5 d). In the chlorophyll maximum layers, percent extracellular release represented 23.5% of total net primary production in the oligotrophic sites and only 6.5% in the core of the Atlantic jet. The contribution of PDOC to bacterial production exhibited large variations (17–100%). Dissimilarities among sites and hydrodynamical structures of the water masses were mostly observed in bacterial generation times and phytoplankton extracellular release.

Bacterial proteolytic activity in sediments of the Subantarctic Indian Ocean sector.

Abstract Organic material entering the oceanic benthic zone can be permanently buried or recycled to CO2 in the sediment. Therefore it is important to know the carbon flux across the sediment-water interface to determine the initial and rate-limiting step in carbon oxidation, a bacterial enzymatic activity. Bacterial density and ectoproteolytic activity, determined using a fluorogenic substrate analog ( l -leucine-7-amino-4-methyl coumarin, Leu-MCA) were investigated in the water column and in sediments during the ANTARES 1 JGOFS cruise in the Indian sector of the Southern Ocean. A strong decrease in ectoproteolytic activity was observed with increasing water depth. Peak activity in surface water was three orders of magnitude less than in surface sediment. Analysis of experimental data revealed, for most sediment bacterial communities, the existence of a biphasic mechanism with different velocities for organic matter degradation and different affinities of enzymatic systems for substrates. To explain this, we hypothesize a strategy of bacterial communities that use the episodic supplies of organic matter reaching the sediment. Microbial ectoproteolytic activities were highest in surface sediment horizons and decreased progressively with depth. As benthic microbial activities reflect the quantity and quality of organic matter reaching the sea floor, high potential ectoproteolytic activities (PEA) measured in the sediment of the Polar Front Zone could indicate a direct and rapid coupling of relatively high surface productivity and deep ocean water by sinking particle fluxes. Lower values of PEA were found in sediment in the northern study area where lateral processes associated with ocean circulation (Antarctic Circumpolar Current) have an important influence on settling particles. The lowest sediment PEA values were measured at 52°S, a region in which low primary production provides a poor supply of organic matter for the sea floor.

Unusual bloom of star-like prosthecate bacteria and filaments as a consequence of grazing pressure

In seawater used for shrimp aquaculture in French Polynesia, the grazing of small bacteria (rods and coccoids) allowed the growth ofAncalomicrobium cells (to more than 2×106 cells ml−1) and large filaments > 10μm in length (5×106 cells ml−1). Their contribution to the increase in total bacterial number after grazing was 27.8 and 9.8%, respectively. These large bacteria are not grazed on by microflagellates, but are available for mesoplankton larvae.

Nitrification rates, nitrite and nitrate distribution in the Almeria-Oran frontal systems (eastern Alboran Sea)

In the western Mediterranean Sea, the Almeria-Oran frontal zone constitutes the eastern boundary of the Alboran Sea. During the “Almofront-1” cruise (April–May 1991) physical, chemical and biological features of the front and surrounding Atlantic and Mediterranean Waters were investigated. In the present study, the nitrite and nitrate concentrations and nitrification rates were measured. The nitracline was close to the surface in the north, close to the Spanish coast and in the south on the left side of the jet. The highest concentration of nitrite (0.25 μM) was located in the incoming Atlantic Waters. The ammonium oxidation rates were from 0 to 16 nmol 1−1 d−1, except in Atlantic Waters in which the rates reached 28–42 nmol 1−1 d−1. For all types of superficial water, i.e. Atlantic, jet core, Mediterranean, the nitrite produced by nitrification could account for the in situ concentrations in less time than the ages of the water masses. In contrast, to reach the ambient concentration, the production of nitrate would need more time than the lifetime of the water masses, except for the Atlantic Waters. In these Atlantic Waters, showing an increased rate of nitrate production, the in situ concentration of nitrate would be reached within its lifetime. The contribution of the nitrate produced by nitrification to the nitrate uptake by primary producers was estimated to range from 7% (Mediterranean Waters) to 40% (Atlantic Waters).

Bacterial productivity and organic matter flux in the Southern Ocean and in the Antarctic Intermediate Water and Mode Water of the Indian Ocean

Abstract Bacterial biomass production in the water column across the Antarctic Circumpolar Current in the Crozet Basin was determined to estimate the flux of organic carbon through bacteria. South of the Subantarctic Front and Subtropical Front (SAF-STF) zone, bacterial growth was greatest at the surface and decreased exponentially in the deep layers. North of the frontal zone, however, growth was high at the surface and decreased exponentially to 400 m, but then was greater in Mode Water at 500 m and in the Antarctic Intermediate Water (AAIW) at 1250 m. This indicates that the water masses sinking in the frontal zone and being advected towards the equator from the Southern Ocean contain labile organic matter. The paradigm that the majorsource of organic carbon in the interior of the oceans is due to vertical sinking of particles may need to be changed to include horizontal advection for long distances and perhaps long time scales, at least in the major oceans south of the equator.

The comparison of two simple protocols designed to initiate and stimulate ammonia oxidation in closed aquaculture systems.

Abstract This study was conducted to evaluate the use of inocula of frozen cells to accelerate the conditioning of new closed seawater systems. The time required to establish nitrification was shortened by about 30% when using frozen bacteria alone or combined with organic matter (food pellets) enrichment of the seawater before inoculating. When only frozen cells were added, the ammonia concentration stayed below the LC50 of tested animals. Two batches which were not inoculated with frozen ammonia-oxidizing bacteria demonstrated the possible contribution of some heterotrophic nitrification.

Nano- and picoplankton growth and production in the Bay of Villefranche sur Mer (N.W. Mediterranean)

Plankton production in the Bay of Villefranche was relatively constant during March and April 1986 but the particle size at which the production occurred was more variable. At the beginning of the study, production was dominated by the larger (ca. 6 μm) flagellates but towards the end it was more or less equally divided between the nano- and picoplankton. There were considerable differences in the estimates of population growth rates, depending on the methods used, but on average the population doubling times were close to 12 hours for autotrophs and 24 hours for heterotrophs. As autotrophs do not grow during the night, each population was therefore doubling once per day. It seemed that each of the nanoor picoplankton populations could adversely affect the growth of the others. This could be either by simple predation or by some form of inhibition. Although nutrient levels in the bay were uniformly low, the addition of nutrients did not always stimulate algal growth. The plankton populations seemed to be both in a state of equilibrium and intense ecological competition.