Annick Bricaud

Variability in the chlorophyll‐specific absorption coefficients of natural phytoplankton: Analysis and parameterization

Variability in the chlorophyll (chl) a-specific absorption coefficients of living phytoplankton aph*(λ) was analyzed using a data set including 815 spectra determined with the wet filter technique in different regions of the world ocean (covering the chlorophyll concentration range 0.02–25 mg m−3). The aph* values were observed to decrease rather regularly from oligotrophic to eutrophic waters, spanning over more than 1 order of magnitude (0.18 to 0.01 m2 mg−1) at the blue absorption maximum. The observed covariation between aph*(λ) and the field chl a concentration (chl) can be explained considering (1) the level or pigment packaging and (2) the contribution of accessory pigments to absorption. Empirical relationships between aph*(λ) and 〈chl〉 were derived by least squares fitting to power functions. These relationships can be used to produce aph* spectra as a function of 〈chl〉. Such a simple parameterization, if confirmed with further data, can be used, e.g., for refining estimates of the carbon fixation rate at global or regional scales, such as those obtained by combining satellite pigment concentration maps with primary production models based on physiological parameters, among which aph* is an important one.

Theoretical results concerning light absorption in a discrete medium, and application to specific absorption of phytoplankton

The radiation absorption within a medium supposedly non-absorbing in itself but containing an absorbing substance as particles is examined theoretically. Absorption by such particles can be treated independently of (Mie) scattering under an assumption concerning the complex refractive index that is reasonable in case of algal cells (index close to that of water and weakly absorbing substance). The effect of discreteness on absorption properties is ruled by a function of the dimensionless parameter that combines through their product the size (d) and the absorption coefficient of the cell material (acm). The limiting value of this function (d → 0) describes the case of a true solution of the same material, whereas its variations with d and acm imply that the specific absorption coefficient, for a given substance, is variable in magnitude and in spectral behaviour. Consequently, Beers law, which rests on the existence of a constant specific coefficient, generally cannot apply when canopy changes intervene in an algal population. Various cases of such changes are studied along with their consequences on absorptive properties. The theoretical conclusions are exemplified by some experimental results concerning algal cultures. The absorption properties of the cell material for each species can be extrapolated from the actual absorption spectra of intact cells. Problems that originate from the non-constancy of the specific absorption coefficient (spectral values or mean value) are examined in view of two kinds of applications: the photosynthetic (quantum) yield evaluation and the algal biomass assessment by remote sensing, for which the constancy is generally postulated.

Variations of light absorption by suspended particles with chlorophyll a concentration in oceanic (case 1) waters: Analysis and implications for bio-optical models

Spectral absorption coefficients of total particulate matter ap(λ) were determined using the in vitro filter technique. The present analysis deals with a set of 1166 spectra, determined in various oceanic (case 1) waters, with field chl a concentrations (〈chl〉) spanning 3 orders of magnitude (0.02–25 mg m−3). As previously shown [Bricaud et al., 1995] for the absorption coefficients of living phytoplankton ao(λ), the ap(λ) coefficients also increase nonlinearly with 〈chl〉. The relationships (power laws) that link ap(λ) and ao(λ) to 〈chl〉 show striking similarities. Despite large fluctuations, the relative contribution of nonalgal particles to total absorption oscillates around an average value of 25–30% throughout the 〈chl〉 range. The spectral dependence of absorption by these nonalgal particles follows an exponential increase toward short wavelengths, with a weakly variable slope (0.011±0.0025 nm−1). The empirical relationships linking ap(λ) to (〈chl〉) can be used in bio-optical models. This parameterization based on in vitro measurements leads to a good agreement with a former modeling of the diffuse attenuation coefficient based on in situ measurements. This agreement is worth noting as independent methods and data sets are compared. It is stressed that for a given (〈chl〉), the ap(λ) coefficients show large residual variability around the regression lines (for instance, by a factor of 3 at 440 nm). The consequences of such a variability, when predicting or interpreting the diffuse reflectance of the ocean, are examined, according to whether or not these variations in ap are associated with concomitant variations in particle scattering. In most situations the deviations in ap actually are not compensated by those in particle scattering, so that the amplitude of reflectance is affected by these variations.

Modeling the inherent optical properties of the ocean based on the detailed composition of the planktonic community

We describe an approach to modeling the oceans inherent optical properties (IOPs) that permits extensive analyses of IOPs as the detailed composition of suspended particulate matter is varied in a controlled manner. Example simulations of the IOP model, which includes 18 planktonic components covering a size range from submicrometer viruses and heterotrophic bacteria to microplanktonic species of 30-mum cell diameter, are discussed. Input data to the model include the spectral optical cross sections on a per particle basis and the particle-number concentration for each individual component. This approach represents a significant departure from traditional IOP and bio-optical models in which the composition of seawater is described in terms of a few components only or chlorophyll concentration alone. The simulations illustrate how the separation and understanding of the effects of various types of particle present within a water body can be achieved. In an example simulation representing an oligotrophic water body with a chlorophyll a concentration of 0.18 mg m(-3), the planktonic microorganisms altogether are the dominant particulate component in the process of light absorption, but their relative contribution to light scattering is smaller than that of nonliving particles. A series of simulations of water bodies with the same chlorophyll a concentration but dominated by different phytoplankton species shows that composition of the planktonic community is an important source of optical variability in the ocean.

Nitrogen- and irradiance-dependent variations of the maximum quantum yield of carbon fixation in eutrophic, mesotrophic and oligotrophic marine systems

Abstract Natural variability of the maximum quantum yield of carbon fixation ( φ C max ), as determined from the initial slope of the photosynthesis-irradiance curve and from light absorption measurements, was studied at three sites in the northeast tropical Atlantic representing typical eutrophic, mesotrophic and oligotrophic regimes. At the eutrophic and mesotrophic sites, where the mixed layer extended deeper than the euphotic layer, all photosynthetic parameters were nearly constant with depth, and φ C max averaged between 0.05 and 0.03 molC (mol quanta absorbed) −1 , respectively. At the oligotrophic site, a deep chlorophyll maximum (DCM) existed and φ C max varied from ca 0.005 in the upper nutrient-depleted mixed layer to 0.063 below the DCM in stratified waters. firstly, φ C max was found roughly to covary with nitrate concentration between sites and with depth at the oligotrophic site, and secondly, it was found to decrease with increasing relative concentrations of non-photosynthetic pigments. The extent of φ C max variations directly related to nitrate concentration was inferred from variations in the fraction of functional PS2 reaction centers ( f ), measured using fast repetition rate fluorometry. Covariations between f and nitrate concentration indicate that the latter factor may be responsible for a 2-fold variation in φ C max . Moreover, partitioning light absorption between photosynthetic and non-photosynthetic pigments suggests that the variable contribution of the non-photosynthetic absorption may explain a 3-fold variation in φ C max , as indicated by variations in the effective absorption cross-section of photosystem 2 ( σ PS2 ). Results confirm the role of nitrate in φ C max variation, and emphasize those of light and vertical mixing.

Light attenuation and scattering by phytoplanktonic cells: a theoretical modeling.

A theoretical modeling is proposed to predict the efficiency factors for attenuation, total scattering, and backscattering for spherical and homogeneous phytoplanktonic cells in suspension. The input parameters of this modeling are the actual size distribution, the spectral values of absorption by the living cells, and an adjustable value of the real part of the refractive index. The variations in these parameters lead to very diverse spectral behavior of the efficiency factors. Theoretical predictions are compared to experimental results for some species to evaluate the reliability of the model for algal cells of various indices and morphologies.

Light backscattering efficiency and related properties of some phytoplankters

By using a set-up that combines an integrating sphere with a spectroradiometer LI-1800 UW, the backscattering properties of nine different phytoplankters grown in culture have been determined experimentally for the wavelengths domain ν = 400 up to 850 nm. Simultaneously, the absorption and attenuation properties, as well as the size distribution function, have been measured. This set of measurements allowed the spectral values of refractive index, and subsequently the volume scattering functions (VSF) of the cells, to be derived, by operating a scattering model previously developed for spherical and homogeneous cells. The backscattering properties, measured within a restricted angular domain (approximately between 132 and 174°), have been compared to theoretical predictions. Although there appear some discrepancies between experimental and predicted values (probably due to experimental errors as well as deviations of actual cells from computational hypotheses), the overall agreement is good; in particular the observed interspecific variations of backscattering values, as well as the backscattering spectral variation typical of each species, are well accounted for by theory. Using the computed VSF, the measured backscattering properties can be converted (assuming spherical and homogeneous cells) into efficiency factors for backscattering (Qbb). Thhe spectral behavior of Qbb appears to be radically different from that for total scattering Qb. For small cells, Q (λ) is practically constant over the spectrum, whereas Qb(λ) varies approximately according to a power law (λ−2). As the cell size increases, Qbb conversely, becomes increasingly featured, whilst Qb becomes spectrally flat. The chlorophyll-specific backscattering coefficients (bb∗ appear highly variable and span nearly two orders of magnitude. The chlorophyll-specific absorption and scattering coefficients, a∗ and b∗, are mainly ruled by the interspecific variations in cellssize (D) and intracellular pigment concentration (Ci) (actually by the variations of the product DCi). Though bb∗ is involved in the modelling of the diffuse reflectance of waters, the impact of its actual variation is greatly limited because typical bb∗ values, even at their maximum (10−3 m2 mg−1), are very low. This result confirms that living algae have a negligible influence on the backscattering process by oceanic waters; other particles (bacteria, detritus, etc.) associated with algae are mainly responsible for this process.

Algal biomass and sea surface temperature in the Mediterranean Basin: Intercomparison of data from various satellite sensors, and implications for primary production estimates

Abstract The Mediterranean Basin, subject both to climate changes and to increasing anthropogenic inputs, is an appropriate test site for observing the evolution of algal biomass and primary production on a long-term basis. With this aim, it is first necessary to study the consistency of the various sets of satellite data as provided by the space agencies, and to compare them to in situ available data. Pixel-by-pixel comparisons of the Level 3 chlorophyll products derived from the ocean color and temperature scanner (OCTS; Version 4, August 1999), polarization and directionality of earth reflectances (POLDER; reprocessing no. 2, July 2000), and the sea-viewing wide field-of-view sensor (SeaWiFS; reprocessing no. 3, May 2000) reveal a reasonably good agreement. Discrepancies, however, appear particularly in oligotrophic areas: weekly (or 10-day) means for OCTS and POLDER (which were operating simultaneously) differ in these areas by 30–70% on average, and OCTS and SeaWiFS weekly means, at 1-year distance, reveal differences by up to a factor of 2. Comparisons with measurements at sea, performed during various cruises, show that all these sensors tend to overestimate chlorophyll concentrations in oligotrophic waters. A “regional algorithm” is proposed to reduce this bias. The impact of using the various datasets for chlorophyll concentration, and for seawater temperature (Reynolds sea surface temperature [SST] analyses, Levitus climatological profiles) for primary production computations is shown. Because they are simultaneous to ocean color data, Reynolds analyses appear to be the most appropriate inputs to such computations. They have, however, to be combined with climatological vertical profiles of seawater temperature, so as to provide representative values for the productive layer.

Is desert dust making oligotrophic waters greener

In situ optical measurements provide evidence that oligotrophic waters of the Mediterranean Sea have a greener color than would result from their phytoplankton content alone. This anomaly, detectable in low chlorophyll waters, remains unnoticed in the chlorophyll-rich waters of the nearby waters of the Moroccan upwelling zone. It is due to enhanced absorption in the blue and enhanced backscattering in the green parts of the visible spectrum likely resulting from the presence of submicron Saharan dust in suspension within the upper layer. This result implies that regional estimations of carbon fixation from ocean color images might be significantly overestimated, not only in the Mediterranean Sea, but also in other oligotrophic areas of the Northern hemisphere, subjected to desert dust deposition.

Satellite‐derived phytoplankton pigment structures in the Portuguese upwelling area

A series of 25 Nimbus 7 coastal zone color scanner images obtained from July 1981 to September 1983 in Portuguese coastal waters was analyzed in order to study the space-time distribution of phytoplankton pigment. In the images from winter and spring, the phytoplankton distributions showed no significant spatial patterns. During the upwelling season, from late June to October, several recurrent patterns were observed. Filaments of high pigment concentration extended zonally 200 km off the west coast of Portugal, north of Lisbon. Pigment-rich plumes were observed south of capes along the west coast, south of Lisbon, and parallel to the south coast of Portugal. The location of the filaments and plumes coincided with topographic features such as submarine ridges. South of Lisbon the location of plumes was also related to coastal morphology. The temporal variability of the pigment patterns was compared with the wind-induced offshore Ekman transport calculated from measurements made at two meteorological stations, located at Lisbon and Faro, on the south coast. A significant relationship was found: well-developed phytoplankton structures were generally related to moderate or intense offshore transport, whereas the absence of plumes corresponded to either weak offshore transport or coastal convergence. For each of the three coastal areas in this study, scatter plots showed linear relationships between digital counts of phytoplankton concentration and sea surface temperature. The spatial variation of the temperature range decreased in time after the onset of an upwelling event. One cause of this variation could be the north-south change in temperature of the upwelled Eastern North Atlantic Central Water along the Portuguese coast.