COASTAL OPTICAL MEASUREMENTS I

SPECTRAL VARIATIONS OF PARTICULATE LIGHT SCATTERING IN COASTAL WATERS, FROM THE VISIBLE TO THE NEAR-INFRARED

Doxaran, David Pierre¹; Ruddick, Kevin G.²; McKee, David³; Gentili, Bernard¹; Chami, Malik¹; Babin, Marcel<sup>1

1</sup>CNRS - Laboratoire d'Océanographie de Villefranche Quai de la Darse, Villefranche-sur-Mer, --, 06230, France; ²Royal Belgian Institute of Natural Sciences, Management Unit of the North Sea Mathematical Models, Brussels, Gulledele, 1200, Belgium; ³Department of Physics, University of Strathclyde, Glasgow, Rottenrow, G4 0NG, United Kingdom

In coastal waters, light scattering by suspended particles determines to a large extent the propagation of solar radiation and the magnitude of surface reflectance. Based on theory and simplifying assumptions, the spectral dependence of light scattering by hydrosols is often modeled using a power-law function. Such a model is appropriate for the near-infrared spectral domain but is not valid in the visible part of the spectrum where both mineral and biogenic suspended particles significantly absorb light. The objective of this study is to propose, based on theory and observations, a model that describes the spectral variations in the scattering coefficient of marine particles (b_p), over the visible and near-infrared spectral domains. To be realistic, this model must account for the impact of particle light absorption on their scattering properties.

A simple power-law function closely reproduces the near-infrared b_p spectral variations, with a spectral slope varying in the range [0.1 – 1.4]. In the visible, particulate absorption effects systematically lead to b_p values 5 to 30% lower than values predicted using a power-law function fitted in the near- infrared and extrapolated to 440 nm. The respective influences of the particle size distribution and composition are investigated. Based on theoretical calculations, a new model is found to closely reproduce the actual b_p spectral variations from near-infrared to short visible wavelengths, taking into account particulate absorption effects. This result is confirmed by numerous field measurements carried out in European and Arctic estuarine and coastal waters.

OPTICAL EFFECTS OF TIDAL STIRRING IN A SEASONALLY STRATIFIED SHELF SEA.

Cunningham, Alex¹; McKee, David¹; Brown, Ian¹; Ramage, Leanne¹; Neil, Claire¹; Creanor, Danielle<sup>1

1</sup>University of Strathclyde 107 Rottenrow, Glasgow, --, G40NG, United Kingdom

The Irish and Celtic Seas constitute a semi-enclosed, relatively shallow region (maximum depth 175 m) between Ireland and the UK mainland. In spring and summer, these seas exhibit classical examples of shelf fronts separating regions where the water column is thermally stratified from those where it is tidally mixed. The mixed regions carry relatively low (< 5 mg m^-3) concentrations of suspended minerals throughout the year, and so particle loads are low compared to estuaries and major river plumes. However the fine mineral particles in these seas are efficient sources of scattering and backscattering, and they have a marked effect on the underwater light field and on the water-leaving radiance spectra. One consequence is that patterns of mixing and stratification predicted by hydrodynamic models (using the Simpson-Hunter parameter) are highly correlated with regions of enhanced reflectance and distinctive spectral signatures are observed in ocean colour images. A second consequence is that the depth of penetration of photosynthetically active radiation is greatly reduced, with important implications for primary production modelling.

This talk will review the results of 5 cruises in the Irish and Celtic Seas, during which extensive measurements were made of both inherent optical properties and in situ radiometric profiles. Specific inherent optical properties for the main optically significant seawater constituents have been derived and validated, and the results incorporated in Hydrolight radiative transfer models of both stratified and mixed water columns. The results, which confirm the significance of low concentrations of suspended minerals in degrading the performance of standard remote sensing algorithms for chlorophyll concentration and reducing water transparency, may be transferable to other shelf-sea regions. They also suggest that strategies for inverting ocean colour observations in optically complex (Case 2) waters can be targeted for different water types by relating spectral features to regional models of water column mixing.

SPECTRAL VARIATIONS IN THE NEAR-INFRARED OCEAN REFLECTANCE

DORON, Maéva¹; BELANGER, Simon²; DOXARAN, David³; BABIN, Marcel<sup>3

1</sup>LEGI - CNRS (formerly at LOV and ACRI-ST) LEGI - MEOM - BP53, Grenoble cedex 9, --, 38041, France; ²Université du Québec à Rimouski, Département de biologie, chimie et géographie, 300 allée des Ursulines , Rimouski, Québec, G5L 3A1, Canada; ³Laboratoire d'Océanographie de Villefranche, CNRS, Université Pierre et Marie Curie-Paris 6, Villefranche-sur-Mer, BP8, 06230, France

The optical properties of natural waters beyond the visible range, in the near-infrared (NIR, 700-900 nm), have received little attention because they are often assumed to be mostly determined by the large absorption coefficient of pure water, and because of methodological difficulties. It has recently been proposed that the variability in the shape of the surface ocean reflectance spectrum in the NIR is negligible in turbid waters. In the present study, we show, based on both in situ and remote sensing data, that the shape of the ocean reflectance spectrum in the NIR does vary in turbid to extremely turbid waters. Remotely sensed ocean reflectance data were collected using 3 different sensors (SeaWiFS, MODIS and MERIS) over the Amazon and Mackenzie turbid river plumes during extremely clear atmospheric conditions so that reliable removal of gases and aerosols effects on reflectance could be achieved. In situ NIR reflectance data were collected in different European estuaries where extremely turbid waters were found. In both data sets, a flattening of the NIR reflectance spectrum with increasing turbidity was observed. The ratio of reflectances at 765 and 865, for instance, varied from ca. 2 down to 1 in our in situ data set, while a constant value of 1.61 had been proposed based on theory in a previous study. Radiative transfer calculations were performed using a range of realistic values for the seawater inherent optical properties, to determine the possible causes of variations in the shape of the NIR reflectance spectrum. The most significant one was the gradual increase in the contribution of suspended sediments to the color of surface waters, which often leads to the flattening of the reflectance spectrum. Changes in the scattering and absorption properties of particles also contributed to variations in the shape of the NIR surface ocean reflectance spectrum. The impact of such variations on the interpretation of ocean color data is discussed.

SEDIMENT, STRESS, AND THE OPTICAL PROPERTIES OF A BOTTOM NEPHELOID LAYER.

Milligan, Timothy G.¹; Hill, Paul²; Law, Brent¹; Boss, Emmanuel<sup>3

1</sup>Fisheries and Oceans Canada PO Box 1006, Dartmouth, NS, B2Y 4A2, Canada; ²Dalhousie University, Halifax, NS, B3H 4J1, Canada; ³University of Maine, Orono, ME, 04469-5741 , United States

Suspended sediment in bottom nepheloid layers consists of organic and inorganic particles packaged into flocs that are much larger than the component particles. The size and composition of the component particles, the extent of repackaging, and floc size all vary with bottom stress. Given this complexity, it is surprising that a relatively robust correlation exists between beam attenuation and suspended particulate mass. Recent modelling of the optical properties of flocculated suspensions suggests that beam attenuation and suspended mass are linearly correlated because floc projected area is proportional to floc mass. This proportionality arises because flocs incorporate an increasing fraction of void space into their structure as they grow.

Models that link floc diameter, area, and mass require as inputs maximum floc size, the fractal dimension of the largest flocs, and the size and density of the component grains. In a bottom nepheloid layer, these variables all respond to changes in the boundary shear stress, but simultaneous measurements of optical properties, sediment properties and stress have been lacking. During the 2007 field program for the ONR-funded OASIS project carried out at the Martha?s Vineyard Coastal Observatory, simultaneous measurements of optical properties, in situ particle size distributions from a LISST and a Digital Floc Camera, and stress in the bottom boundary layer were made. In addition discrete water samples were filtered in situ for calibration of total suspended sediment concentration estimates from the optical instruments and for disaggregated inorganic grain size analysis using a Coulter Counter. To estimate fractal dimension we measured floc settling velocity as a function of size. In this study we explore how floc properties and component grain size and composition respond to changes in boundary shear stress. We use these data to predict how the relationship between beam attenuation and suspended sediment mass varies as a function of shear stress, and we compare the observations with predictions.