Alain Herbland

Size structure of phytoplankton biomass in the equatorial Atlantic Ocean

The size structure of chlorophyll a (Chl a) and phaeopigments (<20, <10, <3, <2, and <1 μm) has been studied along three transects (4°, 23°, and 35°W) covering the entire equatorial Atlantic Ocean, with special attention to the small sizes (<3 μm). Everywhere in the studied area, even in the equatorial upwelling, the bulk of Chl a is within organisms which pass through a 3 μm Nuclepore filter. The vertical distribution of the <1 μm Chl a is closely related to the depth of the nitracline. In the nitrate-depleted mixed layer, the <1 μm Chl a always dominates, and represents 71% of the total Chl a on the average. At the top of the nitracline, the <1 μm Chl a concentration is maximum but represents only 50% of the total Chl a. In the nitrate-rich waters, whatever the depth, the percentage of <1 μm Chl a values are nearly zero. When integrated over the whole euphotic layer, photic zone, the < μm Chl a values are nearly zero. When integrated over the whole euphotic layer, the <1 μm Chl a represents about 25% of the total Chl a when nitrate is present at surface; this percentage reaches 60% when the top of the nitracline deepens to 100 m depth. Preliminary measurements of photosynthetic activity (light gradient and time course experiments) indicate that the <1 μm fraction contains actively photosynthetic organisms but also organisms which are able to fix CO2 in the darkness in a significant proportion. The roles of sinking, nutrients, and light in the vertical distribution of picoplankton are discussed. Our results indicate that the size distribution of Chl a (and especially the relationship between abundance of picoplankton and nitrate distribution) is the same throughout the whole equatorial Atlantic belt, from Brazil to the Gulf of Guinea. From an ecological point of view, the open equatorial Atlantic, in spite of large variations in hydrological structure, can be considered as a unique ecosystem. Probably the same is true for the entire open tropical Atlantic zone.

Spatial and temporal extension of eutrophication associated with shrimp farm wastewater discharges in the New Caledonia lagoon.

Shrimp farming in New Caledonia typically uses a flow-through system with water exchange rates as a tool to maintain optimum hydrological and biological parameters for the crop. Moreover, the effluent shows hydrobiological characteristics (minerals, phytoplankton biomass and organic matter) significantly higher than that of the receiving environment. Separate surveys were carried out in a bay (CH Bay) with a medium-size intensive farm (30 ha) (PO) and in a mangrove-lined creek (TE Creek) near a larger semi-intensive farm (133 ha) (SO). Net loads of nitrogen exported from the semi-intensive farm and the intensive farm amounted to 0.68 and 1.36 kg ha(-1)day(-1), respectively. At CH Bay, discharge effects were spatially limited and clearly restricted to periods of effluent release. The high residence time at site TE favoured the installation of a feedback system in which organic matter was not exported. Mineralization of organic matter led to the release of nutrients, which in turn, caused in an increased eutrophication of this ecosystem. The study of the pico- and nanophytoplankton assemblages showed (i) a shift in composition from picophytoplankton to nanophytoplankton from offshore towards the coast and (ii) a shift within the picophytoplankton with the disappearance of Prochlorococcus and the increase of picoeucaryotes towards the shoreline. These community changes may partially be related to a nitrogen enrichment of the environment by shrimp farm discharges. Thus, in view of the recent addition of the New Caledonian lagoon to the UNESCO World Heritage list, the data presented here could be a first approach to quantify farm discharges and evaluate their impact on the lagoon.

Satellite and in situ observations of a late winter phytoplankton bloom, in the northern Bay of Biscay

A phytoplankton bloom was observed in late winter 2000, on the continental shelf offshore of southern Brittany, in northwestern Bay of Biscay. This bloom appeared initially along the 120-m isobath, in stratified and clear waters, at the interface between the oceanic water and the plumes of southern Brittany rivers (mainly the Loire and Vilaine). The development of the bloom was triggered by favourable meteorological conditions, characterised by solar irradiance reaching the maximum level expected for that period of the year. Outside of the bloom area, the phytoplankton photosynthesis was irradiance limited: inshore, because of the stronger attenuation of the light; offshore, because of the weak stratification. The hydrological conditions at the onset of the bloom were observed in the field, during the oceanographic cruise MODYCOT. However, without SeaWiFS, the only observations related to this major event in the primary production would have been those of the coastal phytoplankton network (REPHY (REseau PHYtoplankton)).

Dynamique du phytoplancton et matière organique particulaire dans la zone euphotique de l'Atlantique Equatorial

At two fixed stations in the Equatorial Atlantic Ocean (0°–4° W), the physical, chemical and biological properties of the euphotic layer were determined for 14 d (Station A: 5–18 February, 1979) and 13 d (Station B: 20 October–7 November, 1979), respectively. The stability of the water column allowed comparison of 3 different “systems”: (i) a well-illuminated and nitrate-depleted mixed layer; (ii) a chlorophyll maximum layer (chl amax) in the thermocline which is poorly illuminated (6.3% of surface irradiance); (iii) a well-illuminated but nitrate-rich (>0.9 μg-at l-1) mixed layer. In each layer the particulate organic carbon (COP), nitrogen (NOP) and phosphorus (POP) contents were measured and compared with the phytoplankton biomass. In the chlorophyll maximum layer, the phytoplankton biomass contributed significantly to the total particulate organic matter (between 55 and 75%). In the nitrate-depleted mixed layer, the results varied according to whether the regression technique [COP=f(chl a)] was used, or the chl a synthesis during the incubation of the samples. With the former technique, the phytoplankton carbon (Cp) content appeared minimal, because the y intercept, computed using all the data of the water column, was probably overestimated for this layer. POP would be more associated with living protoplasm than with carbon and nitrogen in the three layers. In the chlorophyll a maximum layer it constitutes a valuable detritus-free biomass measurement, since 80% of the POP consist of phytoplankton phosphorus. The assimilation numbers (NA=μg C μg chl a-1 h-1) were high in all three layers, but the highest values were recorded in the nitrate-depleted mixed layer (NA=15 μg C μg chl a-1 h-1). In the chlorophyll maximum layer, light would be a limiting factor during incubation: between 1025 and 8.1024 quanta m-2 d-1 NA and light are positively correlated independant of nitrate concentration. The growth rates of phytoplankton (μ) were estimated and compared to the maximum expected growth rate. Our main conclusion was that despite very low biomass and nutrient content, the mixed layer was in a highly dynamic state, as evidenced by high rates of phytoplankton growth and short nutrient turnover times (1 d or less for PO-P4 in the mixed layer versus 3 d in the thermocline). The presence of nitrate in the water column allows the development of a higher phytoplankton biomass but does not increase growth rate.

The soluble fluorescence in the open sea: Distribution and ecological significance in the equatorial Atlantic Ocean

Abstract The red fluorescence of filtered sea water has been measured on 216 samples in the 0–150 m layer of the equatorial Atlantic Ocean. Soluble fluorescence is maximum where chlorophyll a and in vivo fluorescence are maximum, but the percentage of soluble fluorescence, (soluble fluorescence/ in vivo fluorescence) × 100, is minimum at these levels; in recently upwelled waters of the equatorial divergence, the percentage of soluble fluorescence is equal to 10 in the 0–20 m layer and regularly increases to 60 or more at 100–150 m; in the nitrate depleted mixed layer of a convergence it averages 30, decreases to 15 in the thermocline maximum of chlorophyll a , and again reaches 60 in deep waters. A significant positive correlation has been found between the percentage of soluble fluorescence and the amount of phaeophytin, and soluble fluorescence in the open sea is thought to be the result of the degradation and release of chloroplastic products by aged or grazed phytoplankton populations. Low values ( 30) are evidence of unfavourable growth conditions (e.g., limiting nutrients or darkness) or high grazing pressure. The simultaneous measurement of in vivo fluorescence and soluble fluorescence is a method of obtaining valuable information rapidly on the physiological state of the phytoplankton population in the water column.

The deep phaeopigments maximum in the ocean : reality or illusion ?

The vertical distribution of phaeopigments in the ocean is traditionally interpreted as the result of the balance between two processes: (1) Grazing by herbivores (= production) and (2) photooxidation by excess of light (= degradation). If this interpretation is still correct for coastal and surface waters (e.g. spring bloom of phytoplankton and upwelled waters) there is today some evidence that the permanent (or seasonal) deep phaeopigment maximum layer in the bottom of the stratified euphotic zone (tropical and subtropical oceans) is largely overestimated. The artifact would be due to the presence of chlorophyll b which interferes with phaeopigments in the widely and routinely used acid fluorometric method for determination of Chla . The high concentration of Chlb relative to Chla at depth (and only at depth) suggests the existence, and probably the dominance, of shade adapted green algae (eukaryotes) in the deep chlorophyll maximum layer of the stratified euphotic zones. Recent direct observations and counting of cells support this hypothesis. The example in this paper shows that although they are irreplaceable, universal and simple techniques must be used with due considerations of their properties and limits when they are applied in structures where the conditions are expected to be very different from those where the method has been developed.