Sylvie Mathot

Calculating carbon biomass of Phaeocystis sp. from microscopic observations

Conversion factors for calculating carbon biomass ofPhaeocystis sp. colonies and free-living cells were determined from microscopic observations and chemical analysis conducted on cultured and naturalPhaeocystis sp. populations originating from the Southern Bight of the North Sea in 1986 and 1987. They allow calculation, in terms of carbon biomass, of the different forms ofPhaeocystis sp. that succeed each other when the population is growing, on the basis of microscopic observations. The latter include enumerations of free-living cells (flagellated and non-motile) and colonies, as well as colonial biovolume measurement. Specific application to natural populations from Dutch coastal waters during spring 1986 shows that more than 90% ofPhaeocystis sp. carbon biomass is under colonial form, most of it exceeding the grazing characteristics of current zooplankton at this period of the year. Detailed analysis of seasonal changes shows in addition that the size of the colonies greatly increases during the course ofPhaeocystis sp. flowering, reaching sizes as high as 1 mm diameter at the top of the bloom when nutrients are depleted. Physiologically this corresponds to an enhanced synthesis of mucilaginous substances, with the decrease of available nutrients leading to an increasing contribution of the matrix to the total colonial carbon during the course of the bloom. Carbon content ofPhaeocystis sp. colonies therefore greatly varies with their size, ranging from 0.3 to 1430 ngC colony−1.

Factors controlling phytoplankton ice-edge blooms in the marginal ice-zone of the northwestern Weddell Sea during sea ice retreat 1988 : field observations and mathematical modelling

The factors controlling phytoplankton bloom development in the marginal ice zone of the northwestern Weddell Sea were investigated during the EPOS (Leg 2) expedition (1988). Measurements were made of physical and chemical processes and biological activities associated with the process of ice-melting and their controlling variables particularly light limitation mediated by vertical stability and ice-cover, trace metal deficiency and grazing pressure. The combined observations and process studies show that the initiation of the phytoplankton bloom, dominated by nanoplanktonic species, was determined by the physical processes operating in the marginal ice zone at the time of ice melting. The additional effects of grazing pressure by protozoa and deep mixing appeared responsible for a rather moderate phytoplankton biomass (4 mg Chla m−3) with a relatively narrow geographical extent (100–150 km). The rôle of trace constituents, in particular iron, was minor. The importance of each factor during the seasonal development of the ice-edge phytoplankton bloom was studied through modelling of reasonable scenarios of meteorological and biological forcing, making use of a one-dimensional coupled physicalbiological model. The analysis of simulations clearly shows that wind mixing events — their duration, strength and frequency — determines both the distance from the iceedge of the sea ice associated phytoplankton bloom and the occurrence in the ice-free area of secondary phytoplankton blooms during the summer period. The magnitude and extent of the ice-edge bloom is determined by the combined action of meteorological conditions and grazing pressure. In the absence of grazers, a maximum ice-edge bloom of 7.5 mg Chla m−3 is predicted under averaged wind conditions of 8 m s−1. Extreme constant wind scenarios (4–14 m s−1) combined with realistic grazing pressure predict maximum ice-edge phytoplankton concentrations varying from 11.5 to 2 mg Chla m−3. Persistent violent wind conditions (≥ 14 m s−1) are shown to prevent blooms from developing even during the brightest period of the year.

Biochemical fractionation of primary production by phytoplankton in Belgian coastal waters during short- and long-term incubations with 14C-bicarbonate. I: Mixed diatom population

Short- and long-term time course studies of radiocarbon accumulation in the intracellular end-products of photosynthesis (proteins, polysaccharides, lipids, small metabolites) and extracellular monomers and polymers were conducted at natural light intensity during a 24-h period in Belgian coastal waters dominated by large diatoms species in September, 1983. It is shown that carbon losses observed during the long-term incubation are due to the catabolism of reserve products (polysaccharides and lipids), which occurs both during the light and dark periods and provides carbon and energy for pursuing protein synthesis during the dark. Catabolism rates, as calculated by means of a simple mathematical model, indicate reduced rates of lipid catabolism (1–2% h-1, respectively for the light and dark periods), although polysaccharide catabolism proceeds at much higher rates, namely 20% h-1 during the light and 8% h-1 during the dark period. Assuming that protein synthesis proceeds at a constant rate during the 24-h period and that β 1–3 glucan constitutes the main storage product of this diatom population, it is shown that at least 65% of the gross primary production is catabolized by the cells. From this, only 16% are mobilized for dark protein synthesis. The remaining is respired, especially during the light period.

Size-fractionated primary production in the open Southern Ocean in austral spring

Size-fractionated primary production was measured by carbon-14 uptake incubations on three transects between 47°S and 59°30′S along 6°W in October/November 1992. Open Antarctic Circumpolar Current and ice-covered Weddell Gyre water showed comparable low productivity (∼0.3 gCm−2 day−1) and size distribution. Picoplankton (<2 μm) was the dominant size fraction, contributing approximately half to the total water column production. The significance of larger (>20 μm) phytoplankton was only minor. Productivity in the Polar Front Zone north of 50°S, with higher water column stability, was up to 10 times higher with microplankton (>20 μm) being predominant. No ice-edge bloom occurred over the 2 months study period; this is explained by non-favourable hydrographic conditions for blooming and the lack of melt-water lenses upon ice retreat. Picoplankton tended to make higher contributions at lower water column stability, and microplankton to make higher contributions at higher stability. Mixing, together with light climate, are discussed as the driving forces for Antarctic primary production and for its size structure.

Biochemical fractionation of primary production by phytoplankton in Belgian coastal waters during short- and long-term incubations with 14C-bicarbonate - II. Phaeocystis poucheti colonial population

Short- and long-term, light-dark, time-course studies of radiocarbon accumulation in the major intracellular end-products of photosynthesis (proteins, polysaccharides, lipids, small metabolites) and extracellular monomers and polymers were conducted at natural light intensity in Belgian coastal waters and in the English Channel on Phaeocystis poucheti colonies growing under depleted and non-limited, inorganic nitrogen concentration. Evidence is given that the exopolymeric substances which compose the colony envelope, massively secreted during the photoperiod, are used during the dark, together with the intracellular reserve products, to cover the carbon and energetic needs of the colonies either for the maintenance or for pursuing protein synthesis, according to the external inorganic nitrogen level.

Modelling ice-edge phytoplankton bloom in the Scotia-Weddell sea sector of the Southern Ocean during spring 1988

Abstract Determination of the biomass of phytoplankton and their associated micrograzers along the 49°W meridian in the Scotia-Weddell Sea sector of the Southern Ocean was carried out at several times during the austral spring 1988–1989. The data reveal a peak of algal biomass moving southward and located at a constant distance of about half a degree of latitude from the ice edge. Microzooplankton biomass closely follows that of phytoplankton. Photosynthesis, growth, excretion and respiration were determined from experiments combining tracer technology and biochemical fractionation. The kinetics of the processes and their control by available light and temperature has been established. On the basis of these data, a mathematical model was developed for calculating daily integrated growth of phytoplankton from the knowledge of light regime, temperature and vertical structure of the water column. Coupling of this model with a one-dimensional hydrodynamical model, calculating the depth of the wind mixed layer from meteorological data and ice cover, allowed simulation of the dynamics of phytoplankton development during the ice-retreat period in this sector of the Scotia Sea-Weddell Sea area.

Modelling carbon cycling through phytoplankton and microbes in the Scotia-Weddell Sea area during sea ice retreat

An ecological model to calculate phytoplankton development and microbial loop dynamics in the marginal ice zone of the antarctic ecosystem has been established on the basis of physical and biological (phyto- and bacterioplankton biomass and activity and counting of two classes of heterotrophic nanoplankton) measurements carried out in the marginal ice zone of the Scotia-Weddell Sea sector of the Southern Ocean during sea ice retreat 1988 (EPOS 1 and 2 expeditions). Application of this model at latitudes where sea ice retreat occurs and in adjacent open sea and permanently ice-covered areas demonstrated that the marginal ice zone is a region of enhanced primary and bacterioplankton production. Combining the results of the phyto- and bacterioplankton models allowed the quantitative estimate of the carbon fluxes through the lower level of the planktonic food web of the Weddell Sea marginal ice zone during the sea ice retreat period. The resulting carbon budget revealed the quantitative importance of microbial and micrograzing processes in the pathways of net primary production, 71% of this latter being assimilated in the microbial food web. However, total net microbial food web secondary production contributed 28% of‘marginal ice zone produced’ food resources available to krill and other Zooplankton.

Gross and net primary production in the Scotia-Weddell Sea sector of the Southern Ocean during spring 1988"

SummaryDaily rates of gross and net primary production were calculated in the Scotia-Weddell Sea sector of the Southern Ocean during spring 1988 (EPOS, Leg 2) on the basis of kinetic experiments, which combine radiotracer technology and classic biochemical procedures, and by taking into account the light regime, the physical structure of the water column, the vertical distribution of chlorophyll a, and the protozoan grazing pressure. From these calculations, three distinct sub-areas were identified: the Closed Pack Ice Zone (CPIZ), characterized by the lowest average gross primary production (0.36 gC · m−2 · day−1); the Marginal Ice Zone (MIZ) with a maximum mean value of 1.76 gC · m−2 · day−1; and the Open Ocean Zone off the ice edge (OOZ) with an intermediate mean value of 0.87 gC · m−2 · day−1. Net primary production fluctuated nearly in the same proportions, averaging 0.55, 0.2 and 1.13 gC · m−2 · day−1 in the OOZ, CPIZ and MIZ respectively, representing 53% of the total photo-assimilated carbon under heavy ice cover (CPIZ) and 64% in the two other areas. Available light, strongly dependent on the ice cover, was shown to control the level of primary production in the sea ice associated sub-areas, whilst protozoa grazing on phytoplankton determined the moderate primary production level characteristic of the “well illuminated” OOZ area.

Carbon fluxes to Antarctic top predators

The role of birds, seals and whales in the overall biological carbon fluxes of the Southern Ocean has been estimated based on census counts of top predator individuals in the region. Using standard routines for conversion to food consumption and respiration rates we demonstrate that at most 0.3-0.6% of primary production in the Southern Ocean is exhaled, even if primary production by ice-algae is ignored. Food requirements of top predators indicate that photosynthetic production in the ice biota likely is substantial, deserving future attention and research. The results of these field observations deviate strongly from much higher top-predator respiration of 2-22.5% of primary production, as recently suggested from theorethical models. The findings illustrate that the Antarctic food web is more complex than hitherto acknowledged.

Interactions in the microbial community of the marginal ice zone of the northwestern Weddell Sea through size distribution analysis

SummaryEnumeration and identification of planktonic microorganisms (phytoplankton, bacteria, protozoa) were carried out for 16 stations sampled in the marginal ice zone of the northwestern Weddell Sea during sea-ice retreat in 1988 (EPOS Leg 2). From these data, carbon biomass distribution among various classes, chosen according to size and trophic mode, has been determined. This analysis reveals the general dominance of nano-phytoplankton (74 %), mainly Cryptomonas sp.. In two stations only, significant microphytoplanktonic biomass occurred. Bacterioplankton biomass was 16 % of the phytoplanktonic biomass. Protozooplankton appeared as a significant group whose biomass represented an average of 23 % of the total microbial biomass. Maximum phytoplankton and protozooplankton biomass was reached at about 100–150 km north of the receding ice edge whilst bacteria did not show marked spatial variations. From these results, indirect evidence for close relationships between protozoa and bacteria, as well as protozoa and autotrophs, is given. The size range of autotrophic prey and predators overlaps (equivalent spherical diameter range = 6 to 11 μm). This size overlapping increases the complexity of the trophic organization of the microbial community. Our results thus support the idea of a flux of energy not always oriented towards an increasing particle size range. Potential ingestion rate, calculated from a mean clearance rate in the literature, indicated that protozooplankton might ingest as high as 48 % of the daily phytoplankton production in the marginal ice zone.