COASTAL AND OCEANIC OPTICAL PROPERTIES

A LIGHT ABSORPTION BUDGET FOR SOUTH PACIFIC WATERS

BRICAUD, Annick¹; BABIN, Marcel¹; CLAUSTRE, Hervé¹; RAS, Joséphine¹; TIECHE, Fanny<sup>1

1</sup>Laboratoire d'Océanographie de Villefranche BP 08, Villefranche-sur-Mer, --, 06238, France

The relative contributions of phytoplankton, non-algal particles and colored dissolved organic matter (CDOM) to total light absorption, and their variability, are essential information for bio-optical and biogeochemical models. They remain, however, poorly documented in the open ocean, particularly in clear waters because of the difficulty in measuring very low absorption coefficients. The BIOSOPE cruise (October-December 2004), from Marquesas Islands to Concepcion (Chile), investigated a large range of oceanic regimes from hyper-oligotrophic waters in the sub-tropical gyre, to eutrophic waters in the upwelling area off Chile. The spectral absorption coefficients of phytoplankton and non-algal particles were determined using the classical filter technique, while the CDOM absorption coefficients were measured using a spectrophotometer equipped with a 2-m capillary waveguide (UltraPath, WPI Inc.). Over the whole transect, the absorption coefficients of both dissolved and particulate components covered approximately two orders of magnitude, and in the gyre, they were among the lowest ever reported for open ocean waters. In the oligotrophic and mesotrophic waters, absorption coefficients of phytoplankton were notably lower than those measured in other oceanic areas with similar chlorophyll contents, indicating some deviation from the standard chlorophyll-absorption relationships. The contribution of absorption by non-algal particles to total particulate absorption showed large variations, not only along the vertical but also horizontally, ranging in the surface layer from 10-15% (at 440 nm) in the upwelling area to around 50% in the clearest waters. The slopes of absorption spectra of non-algal particles revealed structured spatial variability. The contribution of CDOM to total absorption showed also structured variations both along the vertical and along the transect, ranging within the surface layer from 25-40% (at 440 nm) in the upwelling area to more than 70% in the gyre. Finally, some features on absorption spectra of phytoplankton and CDOM were observed and will be discussed.

PARTICULATE BACKSCATTERING AND BEAM-ATTENUATION COEFFICIENTS IN THE EQUATORIAL PACIFIC

Dall'Olmo, Giorgio¹; Westberry, Toby K.¹; Behrenfeld, Michael J.¹; Boss, Emmanuel²; Slade, Wayne H.<sup>2

1</sup>Oregon State University 2082 Cordley Hall, Corvallis, OR, OR, 97331, United States; ²University of Maine, Orono, ME, 04469, United States

Light scattering properties of the open ocean have been suggested as a superior index of phytoplankton biomass than chlorophyll-a concentration (chla), because they are insensitive to physiological forcing, such as light and nutrients, that alter the intracellular concentration of pigments. The drawback of particulate scattering is that it is not unique to phytoplankton. Nevertheless, field studies have demonstrated that variations in the beam-attenuation coeffcient (cp) closely track variations in phytoplankton abundance. The relationship between cp and the particulate backscattering coeffcient (bbp), the property retrievable from space, has not been fully evaluated, largely due to a lack of open-ocean field observations. Here, we present extensive data on inherent optical properties from the surface Equatorial Pacific and demonstrate a remarkable coherence in bbp and cp. Coincident measurements of particle size distribution (PSD) indicate that this covariance may be due to the conserved nature of the PSD.

ARE THE NORTHERN BALTIC SEA WATERS EVEN BLACKER ?

BERTHON, Jean-François¹; Zibordi, Giuseppe¹; Canuti, Elisabetta¹; Kaitala, Seppo²; Seppälä, Jukka²; Ylöstalo, Pasi<sup>2

1</sup>Joint Research Centre Via Fermi, Ispra, --, 21027, Italy; ²Finnish Institute of Marine Research, Helsinki, Helsinki, 00561, Finland

The Baltic Sea is well known for the strong light absorption by coloured dissolved organic matter (“CDOM”) in the blue part of the visible spectrum, which leads to an important decrease of the surface reflectance from the green to the blue. This classical definition of the optical properties of the Baltic Sea mainly refers to the well sampled southern Baltic Proper. Typically, in those waters the amount of CDOM is also associated with a relevant amount of particles and a positive correlation between them can be observed. Based on a unique bio-optical data set recently acquired, this study aims at presenting very peculiar optical properties observed in the northern Baltic Sea (from the Baltic Proper to the “almost fresh” waters of the northern Gulf of Bothnia, including the Gulf of Finland and Gulf of Riga). In particular, variations of the b/a ratio at 412nm from >1.5 in the Baltic Proper to less than 0.3 in the northern Gulf of Bothnia are reported. The latter value corresponds to relatively high absorption by CDOM (>1 m^-1) inversely correlated with the concentration of particles and scattering (TSM≈0.5 gm^-3, bp(412)<0.5 and bbp(442)<0.007 m^-1). This allows for a very good representation of the beam attenuation spectral dependency through a decreasing exponential fit. Surface irradiance reflectances at 412nm of these “black waters” are close to 0.001 and the reflectance ratio R(555)/R(412) reaches the value of 9 (against 3 in the Baltic Proper).

These peculiar optical properties are discussed with reference to a large and consistent bio-optical data set previously collected in the southern Baltic Proper (including Gdansk Bay and Pomeranian Bay). The study suggests that the northern Baltic Sea (in particular the northern Gulf of Bothnia) could constitute a separated bio-optical province for remote sensing applications. Further investigations should help confirming such a statement.

ABSORPTION PROPERTIES OF PARTICULATE MATTER IN THE WESTERN MEDITERRANEAN SEA

Vellucci, Vincenzo¹; Ribera d'Alcalà, Maurizio²; Antoine, David<sup>1

1</sup>CNRS-LOV LOV, Villefranche sur mer, --, 06230, France; ²Stazione Zoologica Anton Dohrn, Naples, NA, 80121, Italy

Particulate matter is an optically significant constituent of marine waters playing a major role in absorbing environmental light in case I waters and modifying the radiant field. Compared to the global ocean, the Mediterranean Sea has been shown to be characterized by an anomalous blue to green reflectance ratio (B/G) for a given chlorophyll (Chl) concentration. In this region few in situ observations have been reported for the absorption properties of particulate matter, and its role in modulating the B/G has not yet been thoroughly assessed.

Here the characteristics of the absorption spectra of particulate matter in the Western Mediterranean Sea are described. Data were collected during seven cruises between 2001 and 2004. Absorption spectra were measured, in the visible domain, with filter pad technique and are representative of the oligo- and meso-trophic regimes generally encountered in the basin (measured Chl concentration spanning between 0.05 and 6.00 µg/l) and cover all seasons.

Spectra are divided into their algal and non-algal components, and regional models based on Chl content are derived to fit the observations. The obtained regional models are compared to the most representative homologous models for the global ocean. Differences between the proposed regional and the global models for absorption and specific absorption coefficients are pointed out and discussed. The absorption coefficients for the Mediterranean are introduced into a simple reflectance model to evaluate the impact of the algal and non-algal absorption components on the B/G. Finally the relationship between the underwater radiant field and the mean charachteristics of absorption spectra of phytoplankton is investigated.