David Antoine

Oceanic primary production: 2. Estimation at global scale from satellite (Coastal Zone Color Scanner) chlorophyll

A fast method has been proposed [Antoine and Morel, this issue] to compute the oceanic primary production from the upper ocean chlorophyll-like pigment concentration, as it can be routinely detected by a spaceborne ocean color sensor. This method is applied here to the monthly global maps of the photosynthetic pigments that were derived from the coastal zone color scanner (CZCS) data archive [Feldman et al., 1989]. The photosynthetically active radiation (PAR) field is computed from the astronomical constant and by using an atmospheric model, thereafter combined with averaged cloud information, derived from the International Satellite Cloud Climatology Project (ISCCP). The aim is to assess the seasonal evolution, as well as the spatial distribution of the photosynthetic carbon fixation within the world ocean and for a “climatological year”, to the extent that both the chlorophyll information and the cloud coverage statistics actually are averages obtained over several years. The computed global annual production actually ranges between 36.5 and 45.6 Gt C yr−1 according to the assumption which is made (0.8 or 1) about the ratio of active-to-total pigments (recall that chlorophyll and pheopigments are not radiometrically resolved by CZCS). The relative contributions to the global productivity of the various oceans and zonal belts are examined. By considering the hypotheses needed in such computations, the nature of the data used as inputs, and the results of the sensitivity studies, the global numbers have to be cautiously considered. Improving the reliability of the primary production estimates implies (1) new global data sets allowing a higher temporal resolution and a better coverage, (2) progress in the knowledge of physiological responses of phytoplankton and therefore refinements of the time and space dependent parameterizations of these responses.

Bidirectional reflectance of oceanic waters: accounting for Raman emission and varying particle scattering phase function

The bidirectionality of the upward radiance field in oceanic case 1 waters has been reinvestigated by incorporation of revised parameterizations of inherent optical properties as a function of the chlorophyll concentration (Chl), considering Raman scattering and making the particle phase function shape (beta(rho)) continuously varying along with the Chl. Internal consistency is thus reached, as the decrease in backscattering probability (for increasing Chl) translates into a correlative change in beta(rho). The single particle phase function (previously used) precluded a realistic assessment of bidirectionality for waters with Chl > 1 mg m(-3). This limitation is now removed. For low Chl, Raman emissions significantly affect the radiance field. For moderate Chl (0.1-1 mg m(-3)), new and previous bidirectional parameters remain close. The ocean reflectance anisotropy has implications in ocean color remote-sensing problems, in derivation of coherent water-leaving radiances, in associated calibration-validation activities, and in the merging of data obtained under various geometrical configurations.

Heating Rate within the Upper Ocean in Relation to its Bio–optical State

Abstract Solar radiation absorption and local heating within the upper layers of the open ocean are strongly influenced by the abundance of phytoplankton as depicted by the chlorophyll concentration. According to whether this concentration is high or low, the heat deposition occurs within a layer that may vary in thickness from low than 10 m to more than 100 m. A simple parameterization, accounting for this dependence, is developed. It allows the vertical profiles of heating rate to be predicted from the phytoplanktonic pigment concentration, as it can (and will) be remotely detected from space, by using ocean color sensors. This computationally efficient parameterization has been validated in reference to the results of a full spectral model. In the simplified computation, the solar spectrum is partitioned into two domains, below and above the wavelength 0.75 µm. For the infrared waveband, not influenced by biological materials the irradiance profile is described by a single exponential function. For the...

Oceanic primary production: 1. Adaptation of a spectral light‐photosynthesis model in view of application to satellite chlorophyll observations

A global equation, designed to estimate the column-integrated oceanic primary production realized by a given phytoplankton biomass under various environmental conditions, is used to develop a practical method to assess the primary production (P) from the chlorophyll concentration as provided by satellite imagery. This basic equation combines three terms, namely the photosynthetically available radiation impinging at the sea surface, PAR(0+), the column-integrated chlorophyll content, tot, and the cross section for photosynthesis per unit of chlorophyll, Ψ*. Global monitoring of incident irradiance and near-surface algal biomass is now achievable from space, and thus the next step toward a monitoring of oceanic primary production would be to dispose in parallel of a “climatological field” of the Ψ* quantity. Actually, Ψ* depends on the two other terms of the equation (PAR(0+) and tot,), and, in addition, on temperature (also detectable from satellite). Therefore such a “climatological field” is variable and complex and it can be conveniently replaced by lookup tables allowing easy interpolation. The entries are date, latitude, cloudiness, temperature, and remotely sensed chlorophyll concentration. This upper layer concentration is extended downward owing to previous results of a statistical analysis of the chlorophyll vertical distribution; accordingly, two parallel tables, corresponding to well-mixed or stratified upper layers with uniform or non uniform chlorophyll vertical profiles, respectively, are constructed. These tables are produced by systematically using a previously published spectral light-photosynthesis model. For such extensive computations, the model necessarily relies on, and is operated with, a standard set of ecological and physiological parameters. Therefore sensitivity analyses have been carried out in view of assessing the impact on Ψ*, and on the resulting production of deviations in these parameters or parameterizations, vis-a-vis the standard values or formulations which were adopted when building the tables. The effects of the biomass vertical structure, of possible light and temperature adaptation, and of the presence of degraded pigments are among the sensitivity studies which have been performed. The method as proposed can accomodate any improvement and complexity in parameterization to the extent that additional computation time is faced only when generating the lookup tables, not when using them in conjunction with satellite data.

Climate-driven basin-scale decadal oscillations of oceanic phytoplankton.

Untangling the Web Chlorophyll-containing phytoplankton is at the core of the marine food web. Martinez et al. (p. 1253) combined satellite data about upper ocean chlorophyll and sea surface temperatures to demonstrate a clear connection between phytoplankton and sea surface temperatures on a multidecadal time scale. Basin-scale ocean dynamic processes such as the Pacific Decadal Oscillation and the Atlantic Multidecadal Oscillation connect the physical, climate-related variability to changes in phytoplankton distribution and amount. Thus, improving the reliability of forecasts of large-scale ocean dynamics may help to improve predictions of changes in ocean community ecology. Satellite data show that upper ocean chlorophyll and sea surface temperatures are connected on a multidecadal time scale. Phytoplankton—the microalgae that populate the upper lit layers of the ocean—fuel the oceanic food web and affect oceanic and atmospheric carbon dioxide levels through photosynthetic carbon fixation. Here, we show that multidecadal changes in global phytoplankton abundances are related to basin-scale oscillations of the physical ocean, specifically the Pacific Decadal Oscillation and the Atlantic Multidecadal Oscillation. This relationship is revealed in ~20 years of satellite observations of chlorophyll and sea surface temperature. Interaction between the main pycnocline and the upper ocean seasonal mixed layer is one mechanism behind this correlation. Our findings provide a context for the interpretation of contemporary changes in global phytoplankton and should improve predictions of their future evolution with climate change.

Algal pigment distribution and primary production in the eastern Mediterranean as derived from coastal zone color scanner observations

About 300 coastal zone color scanner (CZCS) scenes, gathered over the eastern Mediterranean basin mostly during the years 1979–1981, have been processed from level 1 by using improved pixel-by-pixel procedures for the atmospheric correction and pigment retrieval. The seasonal evolution of the upper ocean pigment concentration is described and analyzed within the whole basin and its subbasins. From the chlorophyll concentration in the top layer, and by using statistical relationships, the depth-integrated pigment content is estimated and used in conjunction with a light-photosynthesis model to estimate the carbon fixation. The model relies on a set of physiological parameters, selected after the validation of the light-photosynthesis model and not on locally measured parameters. Additional information needed in the modeling are the photosynthetically available radiation (computed from astronomic and atmospheric parameters, combined with a cloud climatology), sea temperature and mixed-layer depth (taken from Levitus (1982)). Actually, the model is used to generate look-up tables in such a way that all possible situations (concerning available radiation, chlorophyll concentration, and temperature) are covered. The appropriate situation associated with any pixel is selected from these tables to generate primary production maps. Despite a relatively good spatial coverage, studying the interannual variability of the pigment distribution and primary production appeared to be impossible. Therefore 12 “climatological” monthly chlorophyll maps have been produced by merging the data corresponding to several years. The carbon fixation rates in each of the subbasins have been computed on a monthly basis, and annual mean values derived thereafter. The primary production values are compared with sparse field determinations. They are also compared with those previously derived for the Western basin, also by using CZCS data (Morel and Andre, 1991). When put together, these companion works provide a kind of record of the trophic status of the entire Mediterranean Sea in the early 1980s. Ocean color sensors to be launched next, like SeaWIFS, will allow the seasonal and interannual variabilities in the late 1990s to be addressed.

A multiple scattering algorithm for atmospheric correction of remotely sensed ocean colour (MERIS instrument): Principle and implementation for atmospheres carrying various aerosols including absorbing ones

A multiple scattering algorithm for atmospheric correction of satellite ocean colour observations is described. This algorithm, precisely designed for the MERIS instrument, globally assesses the combined contributions of aerosols and molecules to the multiple scattering regime. The approach was introduced in a previous work, where it was shown that, for a given aerosol, multiple scattering effects can be assessed through the relationship between the aerosol optical thickness and the relative increase in the path radiance that results from the progressive introduction of this aerosol within an aerosol-free atmosphere. Based on considerations about the accuracy to which the water-leaving radiances should be retrieved, the need to account for multiple scattering is argued. The principle of the algorithm is then presented, and tests and sensitivity studies (especially as regards aerosol type and vertical distribution) are performed to assess its performance in terms of errors on the retrieved water-leaving re...

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.

Seasonal variability of the mixed layer depth in the Mediterranean Sea as derived from in situ profiles

A new 0.5° resolution Mediterranean climatology of the mixed layer depth based on individual profiles of temperature and salinity has been constructed. The criterion selected is a threshold value of temperature from a near-surface value at 10 m depth, mainly derived by a method applied on the global (de Boyer Montegut et al., 2004 dBM04). With respect to dBM04, the main differences reside in the absence of spatial interpolation of the final fields and in the improved spatial resolution. These changes to the method are necessary to reproduce the Mediterranean mixed layers behavior. In the derived climatological maps, the most relevant features of the basin surface circulation are reproduced, as well as the areas prone of the deep water formation are clearly identified. Finally, the role of density in the definition of the mixed layers differing behaviors between the oriental and the occidental regions of the basin is presented.