Deep Chlorophyll Maxima in the Global Ocean: Occurrences, Drivers and Characteristics.

Stratified oceanic systems are characterized by the presence of a so-called Deep Chlorophyll a Maximum (DCM) not detectable by ocean color satellites. A DCM can either be a phytoplankton (carbon) biomass maximum (Deep Biomass Maximum, DBM), or the consequence of photoacclimation processes (Deep photoAcclimation Maximum, DAM) resulting in the increase of chlorophyll a per phytoplankton carbon. Even though these DCM (further qualified as either DBMs or DAMs) have long been studied, no global-scale assessment has yet been undertaken and large knowledge gaps still remain in relation to the environmental drivers responsible for their formation and maintenance. In order to investigate their spatial and temporal variability in the open ocean, we use a global data set acquired by more than 500 Biogeochemical-Argo floats given that DCMs can be detected from the comparative vertical distribution of chlorophyll a concentrations and particulate backscattering coefficients. Our findings show that the seasonal dynamics of the DCMs are clearly region-dependent. High-latitude environments are characterized by a low occurrence of intense DBMs, restricted to summer. Meanwhile, oligotrophic regions host permanent DAMs, occasionally replaced by DBMs in summer, while subequatorial waters are characterized by permanent DBMs benefiting from favorable conditions in terms of both light and nutrients. Overall, the appearance and depth of DCMs are primarily driven by light attenuation in the upper layer. Our present assessment of DCM occurrence and of environmental conditions prevailing in their development lay the basis for a better understanding and quantification of their role in carbon budgets (primary production and export).

Determination of the fluorescence quantum yield by oceanic phytoplankton in their natural habitat.

Sun-stimulated chlorophyll a fluorescence has been measured in situ, within the upward and downward light fields, in oceanic waters with chlorophyll concentrations of 0.04-3 mg m(-3). We combined these signals with phytoplankton absorption spectra to derive the fluorescence quantum yield, phi (number of photons emitted by fluorescence/number of absorbed photons). phi was derived separately from hyperspectral (upward and downward) irradiance measurements (with a LI-COR Instruments spectroradiometer) and from nadir radiance near 683 nm (with a Biospherical Instruments profiler). The contribution of inelastic Raman scattering to the signal in the red band was assessed and subtracted. Raman-corrected phi values derived from the two instruments compared well. Vertical phi profiles were strongly structured, with maximal (5-6%) values at depth, whereas phi was approximately =1% in near-surface waters (measurements made approximately at solar noon). These near-surface values are needed for interpretation of remotely sensed fluorescence signals. This optical study shows that the fluorescence yield of algae in their natural environment can be accurately derived in a nonintrusive way with available instrumentation and adequate protocols.

Diffuse reflectance of oceanic waters. III. Implication of bidirectionality for the remote-sensing problem.

The upwelling radiance field beneath the ocean surface and the emerging radiance field are not generally isotropic. Their bidirectional structure depends on the illumination conditions (the Sun's position in particular) and on the optical properties of the water body. In oceanic case 1 waters, these properties can be related, for each wavelength λ, to the chlorophyll (Chl) concentration. We aim to quantify systematically the variations of spectral radiances that emerge from an ocean with varying Chl when we change the geometric conditions, namely, the zenith-Sun angle, the viewing angle, and the azimuth difference between the solar and observational vertical planes. The consequences of these important variations on the interpretation of marine signals, as detected by a satelliteborne ocean color sensor, are analyzed. In particular, the derivation of radiometric quantities, such as R (λ), the spectral reflectance, or [ L(w)(λ)](N), the normalized water-leaving radiance that is free from directional effects, is examined, as well as the retrieval of Chl. We propose a practical method that is based on the use of precomputed lookup tables to provide values of the f/Q ratio in all the necessary conditions[ f relates (R(λ) to the backscattering and absorption coefficients, whereas Q is the ratio of upwelling irradiance to any upwelling radiance]. The f/Q ratio, besides being dependent on the geometric configuration (the three angles mentioned above), also varies with λ and with the bio-optical state, conveniently depicted by Chl. Because Chl is one of the entries for the lookup table, it has to be derived at the beginning of the process, before the radiometric quantities R(λ) or [L(W)(λ)](N) can be produced. The determination of Chl can be made through an iterative process, computationally fast, using the information at two wavelengths. In this attempt to remove the bidirectional effect, the commonly accepted view relative to the data-processing strategy is somewhat modified, i.e., reversed, as the Chl index becomes a prerequisite parameter that must be identified prior to the derivation of the fundamental radiometric quantities at all wavelengths.

Diffuse reflectance of oceanic waters. II Bidirectional aspects.

For visible wavelengths and for most of the oceanic waters, the albedo for single scattering (?) is not high enough to generate within the upper layers of the ocean a completely diffuse regime, so that the upwelling radiances below the surface, as well as the water-leaving radiances, generally do not form an isotropic radiant field. The nonisotropic character and the resulting bidirectional reflectance are conveniently expressed by the Q factor, which relates a given upwelling radiance L(u) (θ',φ) to the upwelling irradiance E(u) (θ' is the nadir angle, φ is the azimuth angle, and Q = E(u)/L(u)); in addition the Q function is also dependent on the Sun's position. Another factor, denoted f, controls the magnitude of the global reflectance, R (= E(u) /E(d), where E(d) is the downwelling irradiance below the surface); f relates R to the backscattering and absorption coefficients of the water body (b(b) and a, respectively), according to R = f(b(b)/a). This f factor is also Sun angle dependent. By operating an azimuth-dependent Monte Carlo code, both these quantities, as well as their ratio (f/Q) have been studied as a function of the water optical characteristics, namely ? and η; η is the ratio of the molecular scattering to the total (molecular + particles) scattering. Realistic cases (including oceanic waters, with varying chlorophyll concentrations; several wavelengths involved in the remote sensing of ocean color and variable atmospheric turbidity) have been considered. Emphasis has been put on the geometrical conditions that would be typical of a satellite-based ocean color sensor, to derive and interpret the possible variations of the signal emerging from various oceanic waters, when seen from space under various angles and solar illumination conditions.

Comparison of numerical models for computing underwater light fields.

Seven models for computing underwater radiances and irradiances by numerical solution of the radiative transfer equation are compared. The models are applied to the solution of several problems drawn from optical oceanography. The problems include highly absorbing and highly scattering waters, scattering by molecules and by particulates, stratified water, atmospheric effects, surface-wave effects, bottom effects, and Raman scattering. The models provide consistent output, with errors (resulting from Monte Carlo statistical fluctuations) in computed irradiances that are seldom larger, and are usually smaller, than the experimental errors made in measuring irradiances when using current oceanographic instrumentation. Computed radiances display somewhat larger errors.

Diffuse reflectance of oceanic waters: its dependence on Sun angle as influenced by the molecular scattering contribution.

A spectral model of the inherent optical properties (absorption and scattering coefficients a and b, respectively) of oceanic case 1 waters with varying chlorophyll concentrations C is operated. It provides the initial conditions for Monte Carlo simulations aimed at examining the diffuse reflectance directly beneath the surface R and its variations with the solar zenith angle zeta. In most oceanic waters, molecular scattering is not negligible, and molecular backscattering may largely exceed backscattering. The variable contributions (depending on C and wavelength) of water molecules and particles in the scattering process result in considerable variations in the shape of the volume-scattering function. R(zeta) is sensitive to this shape. From the simulations, R (which increases as zeta increases) appears to be linearly related to cos zeta, with a slope that is strongly dependent on eta(b), the ratio of molecular backscattering to particle backscattering. The value of the single-scattering albedo (?= b/a + b) has a negligible influence on the R(zeta) function provided that ? < 0.8, a condition that is always fulfilled when dealing with oceanic case 1 waters. Practical formulas for R(zeta) are proposed. They include the influence of the diffuse sky radiation. The history of each photon and the number of collisions it experiences before exiting have been recorded. These histories and also a probabilistic approach allow the variations of R with cos zeta, eta(b), and ? to be understood.

[New flavonoid-glycosides from Crataegus monogyna and Crataegus pentagyna]. / Neue flavonoid-glykoside aus Crataegus monogyna und Crataegus pentagyna.

From the leaves and flowers of Crataegus monogyna and C. Pentagyna six new flavonoid-C- and O-glycosides respectively have been isolated and identified as 2''-O-rhamnosyl-orientin, 2''-O-rhamnosyl-isoorientin, 2''-O-rhamnosyl-isovitexin, rutin, spiraeosid, 8-methoxy-kämpferol and 8-methoxy-kämpferol-3-O-glucoside. The structure of O-rhamnosyl-vitexin and O-acetyl-O-rhamnosyl-vitexin isolated previously, have been elucidated unambigiously mainly by NMR- and MS-spectroscopy.