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This letter summarizes a regional assessment of radiometric data products from the Ocean and Land Colour Instrument operated onboard Sentinel-3A. The assessment is supported by in situ reference measurements from the Ocean Colour component of the Aerosol Robotic Network and the Bio-optical mapping of Marine Properties Program. Results indicate a systematic underestimate of the water-leaving radiance at the blue and red spectral bands. Conversely, the aerosol optical depth at 865 nm exhibits overestimate, while the Angstrom exponent shows a narrow distribution of values confined below a maximum of approximately 1.7. These findings suggest difficulty in separating water and atmospheric radiance contributions, which results in a poor determination of aerosol load and type and, consequently, an overestimate of atmospheric effects.

Standard ocean color data products from the Medium Resolution Imaging Spectrometer (MERIS) are compared with equivalent regional products in European seas exhibiting different bio-optical properties: the northern Adriatic Sea, the Baltic Sea and the Western Black Sea (ADRS, BLTS and BLKS, respectively). Investigated quantities are: 1) the algal-2 pigment index, alg2; 2) the composite absorption coefficient of yellow substance and non-pigmented particles at 442 nm, adg; and 3) the concentration of the total suspended matter, TSM. Regional data products are created using ocean color inversion schemes based on MultiLayer Perceptron (MLP) neural nets trained with field measurements from the Coastal Atmosphere and Sea Time Series (CoASTS) and Bio-Optical mapping of Marine Properties (BiOMaP) programs. MLP input is the remote sensing reflectance RRS at MERIS center-wavelengths specifically selected for different water types in view of minimizing the perturbing effects of inaccurate atmospheric correction on the retrieval of regional data products. A new method is also proposed to define the applicability of regional MLPs to input RRS. Results indicate that MERIS alg2 values tend to overestimate the equivalent quantity computed with MLP regional algorithms. The agreement between MERIS and regional TSM data products is significantly better than that reported for alg2 and adg, especially for BLKS. Findings highlight the relevance of using regional inversion schemes to evaluate standard products over extended oceanographic regions as a complement to the analysis of match-ups between marine products measured in situ and derived from space-born data.

Trends in the difference between primary satellite ocean-color products and in situ reference data at a coastal site are analyzed and presented. Investigated products are the normalized water-leaving radiance LWN and aerosol optical thickness τa from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) and the Moderate-Resolution Imaging Spectroradiometer onboard the Aqua and Terra platforms (MODIS-A and MODIS-T, respectively). In situ reference data are from the Ocean-Color component of the Aerosol Robotic Network (AERONET-OC). Restricting the period of investigation to May 2002-April 2010 for SeaWiFS and MODIS-T and July 2002-June 2008 for MODIS-A, results do not indicate any statistically significant trend in the bias of LWN. Biases, determined at around the middle of the considered period, exhibit values from -0.4% to +7.7% for SeaWiFS and from -1.9% to +4.6% for MODIS-T in the 412-555-nm spectral interval. Higher and systematically negative biases from -15.4% to -6.2% are observed for MODIS-A in the same spectral interval. Statistically appreciable trends are observed for τa from SeaWiFS at 443 and 490 nm (approximately +1% per year) and from MODIS-A at 667 nm (+4.7% per year). The biases are very high for both MODIS-A and MODIS-T τa products in the 412-555-nm spectral interval (on average, +21% and +16%, respectively) when compared to SeaWiFS (exhibiting values between +1% and +4%).

Multilayer perceptron (MLP) neural networks for regional satellite ocean color applications have been developed and assessed using in situ data from various European seas. Considered MLP products are chlorophyll a concentration ( Chl–a ), absorption by yellow substance at 412 nm ( a ys (412)) and concentration of total suspended matter (TSM), all determined from spectral remote sensing reflectance R RS . Investigated oceanographic regions comprise the Eastern Mediterranean Sea, the northern Adriatic Sea, the Western Black Sea and the Baltic Sea. The in situ measurements applied in the study were produced within the framework of the Coastal Atmosphere and Sea Time Series (CoASTS) and Bio-Optical Mapping of Marine Properties (BiOMaP) programs contributing to a unique dataset that represents different water types including chlorophyll a , yellow substance and sediment dominated waters. Performance analysis of the proposed regional MLPs indicates that Chl–a can be quantified with the highest accuracy in the Eastern Mediterranean Sea (with absolute percent difference of 14% with respect to in situ measurements). In the case of a ys (412), the most accurate determination is observed for the Baltic Sea waters (13%). Instead, TSM retrieval is the most accurate in the Black Sea (14%). The study demonstrated the limited generalization capability of regional algorithms. Within this context, saturation of MLP output occurring with input data not statistically represented in the training set has been investigated through cross-basin product analysis in view of proposing a practical solution to the problem.

The assessment of uncertainties and biases affecting remote sensing products is a pre-condition for an effective exploitation of these data in climate studies. Within such a context, this work presents and discusses the intra-annual variations of biases in primary satellite ocean color products (i.e., normalized water-leaving radiance and aerosol optical thickness) at a coastal site in the northern Adriatic Sea for the period 2002–2010. The analysis is performed using in situ data from the Ocean Color component of the Aerosol Robotic Network (AERONET-OC) and satellite data products from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) and the Moderate Resolution Imaging Spectroradiometer (MODIS-A and MODIS-T) onboard the Aqua and Terra platforms. Results indicate biases in the normalized water leaving radiance significantly increasing with the sun zenith angle and, to a lesser extent, also affected by variations in chlorophyll concentration and aerosol optical thickness.

Advanced remote sensing techniques and regional algorithms are combined to produce and analyze a 12-year multi-sensor time series of bio-optical properties for the Adriatic Sea. First, a multi-sensor series of normalized water-leaving radiance LWN is created with an optically-based merging technique applied to SeaWiFS and MODIS data, then distributions of inherent optical properties (IOPs) are obtained with a semi-analytical bio-optical model. Additionally, Chla is computed by two regional empirical algorithms associated with two classes of optical properties representative of open ocean waters and Adriatic coastal regions (Classes 1 and 2, respectively). The domain of applicability of each algorithm is dynamically determined on the basis of the LWN input spectrum by a novelty detection approach. The various satellite products are validated using a comprehensive bio-optical time series collected in the northern Adriatic. At the basin level, the frequency of occurrence of Classes 1 and 2 are 61% and 10%, respectively, with Class 2 waters particularly located in the northern part and characterized by higher levels of Chla and IOPs. The annual cycle of Chla over the 12-year period exhibits significant variations in amplitude and phase; moreover, the annual cycle shows a reduced amplitude during a period centered on 2004 for a time interval that varies with the region. In the northern Adriatic, the overall level of Chla appears lower between 2003 and 2008 compared to the beginning of the series.

The development and assessment of satellite ocean color products require quality assured in situ data representative of the variety of bio-optical regimes encountered in the different seas. The measurement program named Bio-Optical mapping of Marine Properties (BiOMaP) fulfills this requirement by using identical instruments and applying cross-site consistent methods for the characterization of seawater inherent and apparent optical properties in the various European seas. This work introduces the BiOMaP radiometric data and describes their application to the validation of primary ocean color products. Within this framework, the radiometric data are discussed through the spectral shape and amplitude of normalized water-leaving radiances ( L WN ). Specifically, the spectral shape is expressed through the Principal Component Analysis of L WN (λ)/ L WN (555) while the amplitude is represented by L WN (555). The resulting distribution of BiOMaP data in a three dimensional feature space demonstrates a continuity of cases across the investigated marine regions confirming a wide representativity of bio-optical regimes. The application of BiOMaP data to the validation of remote sensing reflectance from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) and the Moderate Resolution Imaging Spectroradiometer (MODIS), indicates improved performance of the SeaWiFS Data Analysis System (SeaDAS, version 6.1) atmospheric correction. In particular, the comparison of satellite and in situ matchups in the blue spectral region shows biases of a few percent with respect to the much larger reported in studies relying on earlier SeaDAS versions. Matchup analyses, restricted to the Eastern Mediterranean, Black and Baltic Seas, indicate marked regional differences likely explained by the diversity of water and aerosol types.

article i nfo The study presents and discusses the application of in situ data from the ocean color component of the Aerosol Robotic Network (AERONET-OC) to assess primary remote sensing products from the Moderate Resolution Imaging Spectroradiometer (MODIS) on the AQUA platform and from the Sea-viewing Wide- Field-of-view Sensor (SeaWiFS) on the OrbView-2 spacecraft. Three AERONET-OC European coastal sites exhibiting different atmospheric and marine optical properties were considered for the study: the Acqua Alta Oceanographic Tower (AAOT) in the northern Adriatic Sea representing Case-1 and Case-2 moderately sediment dominated waters; and, the Gustaf Dalen Lighthouse Tower (GDLT) in the northern Baltic Proper and the Helsinki Lighthouse Tower (HLT) in the Gulf of Finland, both characterized by Case-2 waters dominated by colored dissolved organic matter (CDOM). The analysis of MODIS derived normalized water- leaving radiance at 551 nm, LWN(551), has shown relatively good results for all sites with uncertainties of the order of 10% and biases ranging from − 1t o−4%. Larger uncertainty and bias have been observed at 443 nm for the AAOT (i.e., 18 and −7%, respectively). At the same center wavelength, results for GDLT and HLT have exhibited much larger uncertainties (i.e., 56 and 67%, respectively) and biases (i.e., 18 and 25%, respectively), which undermine the possibility of presently using remote sensing LWN data at the blue center wavelengths for bio-optical investigations in the Baltic Sea. An evaluation of satellite derived aerosol optical thickness, τa, has shown uncertainties and biases of the order of tens of percent increasing with wavelength at all sites. Specifically, MODIS derived τa at 869 nm has shown an overestimate of 71% at the AAOT, 101% at GDLT and 91% at HLT, respectively. This result highlights the effects of a limited number of aerosol models for the atmospheric correction process, and might also indicate the need of applying a vicarious calibration factor to the remote sensing data at the 869 nm center wavelength to remove the effects of uncertainties in the atmospheric optical model and the space sensor radiometric calibration. Similar results have been obtained from the analysis of SeaWiFS data. Finally, in view of illustrating the possibility of increasing the accuracy of satellite regional radiometric products, AERONET-OC data have been applied to reduce systematic errors in MODIS and Medium Resolution Imaging Spectrometer (MERIS) LWN data likely due to the atmospheric correction process. Results relying on MODIS match-ups for the Baltic Sites (i.e., GDLT and HLT) and MERIS matchups for the AAOT, have indicated a substantial reduction of both uncertainty and bias in the blue and red center wavelengths.

The ocean color component of the Aerosol Robotic Network (AERONET-OC) has been implemented to support long-term satellite ocean color investigations through cross-site consistent and accurate measurements collected by autonomous radiometer systems deployed on offshore fixed platforms. The AERONET-OC data products are the normalized water-leaving radiances determined at various center wavelengths in the visible and near-infrared spectral regions. These data complement atmospheric AERONET aerosol products, such as optical thickness, size distribution, single scattering albedo, and phase function. This work describes in detail this new AERONET component and its specific elements including measurement method, instrument calibration, processing scheme, quality assurance, uncertainties, data archive, and products accessibility. Additionally, the atmospheric and bio-optical features of the sites currently included in AERONET-OC are briefly summarized. After illustrating the application of AERONET-OC data to the validation of primary satellite products over a variety of complex coastal waters, recommendations are then provided for the identification of new deployment sites most suitable to support satellite ocean color missions.

Radiometric products determined from fixed-depth and continuous in-water profile data collected at a coastal site characterized by moderately complex waters were compared to investigate differences and limitations between the two measurement methods. The analysis focused on measurements performed with the same radiometer system sequentially deployed at discrete depths (i.e., 1 and 3 m) and successively used to profile the water column. Within the 412–683-nm spectral interval, comparisons show uncertainties of 2%–4%, 3%–5%, and 2% for the subsurface values of upwelling radiance, Lun, upward irradiance, Eun, and downward irradiance, Edn, all normalized with respect to the above-water downward irradiance. The related spectral biases vary from −2% to 1% for Lun, are in the range of 2%–3% for Eun, and are lower than 0.5% for Edn. Derived products like the irradiance reflectance, R, Q factor at nadir, Q, and normalized water leaving radiance, LWN, exhibit spectral uncertainties of 4%–6%, 2%–3%, and 2%–4%. The related spectral biases vary from 1% to 3%, 2% to 3%, and −2% to 1%, respectively. An analysis of these results indicates a general diminishing of uncertainties and biases with a decrease of the diffuse attenuation coefficient, Kd, determined at 490 nm, for most of the quantities investigated. Exceptions are Edn and Kd because an increase of Kd reduces the perturbations due to wave effects on downward irradiance measurements. An evaluation of the perturbing effects due to the presence of optical stratifications, which lead to a nonlinear decrease with depth of log-transformed radiometric measurements, shows an expected increase in uncertainty and bias specifically evident for Ku, Eun, Kl, and Lun, and derived quantities like R, Q, and LWN. Overall results, supported by a t-test analysis, indicate the possibility of using moorings in moderately complex coastal waters to determine LWN with a slightly higher uncertainty with respect to that achievable with continuous profiling systems.

A comprehensive set of bio-optical measurements collected at the Acqua Alta Oceanographic Tower site in the northern Adriatic Sea is used to assess satellite derived optical properties and concentrations of optically significant constituents. These include normalized water leaving radiance spectra LWN, absorption spectra due to phytoplankton, non-pigmented particles and chromophoric dissolved organic matter, back-scattering spectra, concentrations of chlorophyll a, Chla, and total suspended matter, TSM, and diffuse attenuation coefficients, Kd, obtained with a diverse set of algorithms. A total of 81 and 21 match-ups are found for SeaWiFS and MODIS LWN, respectively. For both sensor products, the match-ups show mean absolute percentage differences of approximately 30% at 412 nm, 20% at 443 nm, and 14% from 490 to 555 nm. Some dependence of these differences has been found with respect to the aerosol optical thickness and the single scattering albedo associated with the in-water constituents. However, the performance of the atmospheric correction scheme appears relatively robust with respect to angular and environmental conditions. The different Chla products generally show quite large uncertainties whereas a TSM product shows encouraging results. Three algorithms produce Kd (490) with a RMS uncertainty of 0.13 for log-transformed data. The comparison between in situ data and satellite derived absorption values yields varying levels of uncertainties for the three bio-optical algorithms considered here and for the different wavelengths. Preliminary improvements could be reached by reducing biases affecting the total absorption coefficient at various wavelengths. Another general result is that the bio-optical models are able to represent a larger part of the observed variability for phytoplankton absorption than for the absorption by dissolved and non-pigmented particulate matter. The assumed spectral shape of the latter absorption term appears logically as a strong determinant of the partition of the total absorption. The three sets of satellite derived back-scattering spectra compare favorably with in situ optical measurements, with mean RMS differences between 0.12 and 0.18. Importantly, the uncertainties obtained here for satellite match-ups of absorption coefficients are comparable to published estimates of the inherent uncertainties associated with the bio-optical algorithms.

A three-year time-series of radiometric data collected with an autonomous above-water system at the Acqua Alta Oceanographic Tower in the northern Adriatic Sea has shown its applicability for monitoring the trophic state of marine waters and its suitability for the validation of remote sensing products in coastal areas. Specifically, the radiometric data have been used to produce surface chlorophyll a concentration (Chla) by applying a regional algorithm proposed for the northern Adriatic Sea coastal waters. A comparison based on 41 match-ups between these Chla and reference values from high-performance liquid chromatography, has shown an average absolute difference of 32%. The comparison of Chla derived from remote sensing SeaWiFS and in situ above-water radiances has shown an average absolute difference of 21% for 183 match-ups, when the same regional algorithm is applied to both types of radiometric data.

Multilayer perceptron (MLP) neural network algorithms were developed to retrieve the absorption coefficient spectra of the colored dissolved organic matter (CDOM) and nonpigmented particulate matter (NPPM) from the remote sensing reflectance R/sub rs/ of optically complex waters. The two MLP algorithms, consisting of one hidden layer with ten neurons and requiring R/sub rs/ at 412, 490, and 665 nm as inputs, were trained with a comprehensive experimental dataset of the Northern Adriatic Sea coastal waters. The products of the proposed regional MLP algorithms showed higher accuracies than regional band-ratio algorithms, and exhibited average uncertainties of 20% and 25% in the determination of CDOM and NPPM absorption coefficients at 412 nm, respectively.

Wave perturbations induce uncertainties in subsurface quantities determined from the extrapolation of optical measurements taken at different depths. An analysis of these uncertainties was made using data collected in the northern Adriatic Sea coastal waters over a wide range of environmental conditions with a profiling system having a 6-Hz acquisition rate, ∼0.1 m s−1 deployment speed, radiance sensors with 20° full angle field of view, and irradiance collectors of ∼1-cm diameter. The uncertainties were quantified as a function of the depth resolution of radiance and irradiance profiles through the percent differences between the subsurface values computed from full and reduced resolution profiles (the latter synthetically created by removing data from the former). The applied method made the analysis independent from instrument calibration; from perturbations induced by instrument self-shading, deployment structure, and bottom effects; and from environmental variability caused by seawater and i...

The first three years of a time-series of marine bio-optical measurements performed from an oceanographic tower in the northern Adriatic Sea are used to derive empirical relationships for ocean colour applications in coastal waters. The site presents bio-optical characteristics pertaining to Case 1 and to Case 2 waters. Log linear relationships allow the diffuse attenuation coefficient for downwelling irradiance, KE d, at different wavelengths to be derived from its value at 490 nm. A local empirical algorithm making use of the remote sensing reflectance ratio R rs(490)/R rs(555) is shown to provide lower surface chlorophyll-a values (by a factor of 2 to 4) in the range 0.1–1.0 mg m−3 than the SeaWiFS OC2v4 algorithm.

The development and validation of ocean colour products require the availability of in situ atmospheric and marine data. Since 1995, the Coastal Atmosphere and Sea Time Series (CoASTS) Program ensures the collection of an in situ comprehensive set of measurements taken from an oceanographic tower in the northern Adriatic Sea. An example of a validation exercise is presented through the comparison of in situ and satellite-derived aerosol optical thickness at various wavelengths in the 443–865 nm spectral interval.

An evaluation of the accuracy of atmospheric and marine satellite-derived products is presented and discussed for the northern Adriatic Sea coastal region using match-ups of in situ and Sea-Viewing Wide-Field-of-View Sensor (SeaWiFS) data for the period September 1997-September 2001. The study, making use of a simple atmospheric correction scheme including a near-infrared (NIR) turbid-water correction, has shown mean relative percentage differences between in situ and satellite-derived aerosol optical thickness lower than 23% in the spectral range between 443 and 865 nm. By applying regional empirical bio-optical algorithms for chlorophyll a concentration (Chla), total suspended matter concentration (TSM), and diffuse attenuation coefficient at 490 nm (K/sub d/(490)), match-ups analysis has shown mean relative percentage differences of 40% for Chla, 28% for TSM, and 30% for K/sub d/(490). The analysis is supported by comparison of in situ and satellite-derived normalized water leaving radiances to highlight the importance of the NIR turbid-water correction and to discuss the intrinsic uncertainties due to the use of empirical algorithms.

An autonomous system for making above-water radiance measurements has been produced by adding a new measurement scenario to a CIMEL CE-318 sun photometer. The new system, called the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) Photometer Revision for Incident Surface Measurement (SeaPRISM), combines the normal CE-318 capability for measuring direct sun irradiance and sky radiance, with a new capability for measuring above-water radiance for the retrieval of water-leaving radiance. The system has been extensively tested during several measurement periods over a 1-yr time frame from August 1999 to July 2000 under various sun elevations along with different atmospheric, seawater, and sea-state conditions. The field assessment of the new instrument was conducted at an oceanographic tower located in the northern Adriatic Sea within the framework of measurement campaigns aimed at supporting ocean color calibration and validation activities. Sample data at 440, 500, 670, 870, and 1020 nm were collect...

The uncertainties associated with MERIS remote sensing reflectance (RRS) data derived from the SeaWiFS Data Analysis System (SeaDAS) are assessed with field observations. In agreement with the strategy applied for other sensors, a vicarious calibration is conducted using in situ data from the Marine Optical BuoY offshore Hawaii, and leads to vicarious adjustment factors departing from 1 by 0.2% to 1.6%. The three field data sets used for validation have been collected at fixed stations in the northern Adriatic Sea and the Baltic Sea, and in a variety of European waters in the Baltic, Black, Mediterranean and North Seas. Excluding Baltic waters, the mean absolute relative difference psi is between 10% and 14% for the spectral interval 490-560 nm, 16-18% at 443 nm, and 24-26% at 413 nm. In the Baltic Sea, the psi values are much higher for the blue bands characterized by low RRS amplitudes, but similar or lower at 560 and 665 nm. For the three validation sets, the root-mean-square differences decrease from approximately 0.0013 sr^-1 at 413 nm to 0.0002 sr^-1 at 665 nm, and are found similar or lower than those obtained for SeaWiFS or MODIS-Aqua. As derived from SeaDAS, the RRS records associated with these three missions thus provide a multi-mission data stream of consistent accuracy., JRC.H.3-Global environment monitoring

[1] The peculiar bio-optical properties of the northernmost waters of the Baltic Sea (Bothnian Sea and Bothnian Bay) are illustrated by exploiting unique in situ measurements of apparent and inherent optical properties collected in 2007. Specifically, this study shows that the high values of absorption by colored dissolved organic matter (ays(412) around 1.4 m−1) resulting from the presence of humic matter brought by the numerous Swedish and Finnish rivers alimenting the basin are not correlated with high concentrations of particles as generally observed in the southern Baltic Sea. As a consequence, extremely low values (i.e., less than 0.5) of the ratio of scattering to absorption are observed in the blue spectral region. The corresponding spectra of normalized water leaving radiance are close to satisfy the “black pixel” assumption in the blue and even in the green part of the visible spectrum. Consequently, the Bothnian Sea and most of all the Bothnian Bay, appear ideal candidates for studies on atmospheric corrections and vicarious calibration of ocean color sensors in coastal waters.

[1] SeaWiFS, MODIS and MERIS remote sensing radiometric products were assessed using in situ data collected at a coastal site in the northern Adriatic Sea from May 2002 to September 2005. The analysis was restricted to satellite and in situ data taken within one hour interval to minimize the effects of temporal variability of atmosphere and seawater around the measurement site. The comparison of SeaWiFS and MODIS normalized water-leaving radiances with in situ values showed averages of relative percent difference varying from 0% to −9% in the 443–555 nm spectral range. Higher values ranging from +15 to +42% were observed for the MERIS data in the equivalent spectral range (i.e., 443–560 nm).

Above- and in-water radiometric data were collected from two coastal platforms: a small boat and an oceanographic tower. The above-water data were processed with and without a correction for bidirectional effects (Q02 and S95, respectively). An intercomparison of water-leaving radiances over a wide range of environmental conditions showed (a) total uncertainties across the blue-green domain were to within 4%, (b) a convergence of the Q02 method with the in-water method (average Q02 intercomparisons were to within 4%), and (c) chlorophyll a concentrations derived from Q02 reflectances and the OC4V4 (Ocean Color 4 Version 4) algorithm agreed with independent high-performance liquid-chromatography determinations to within approximately 32%.

International audience; Whether for biogeochemical studies or ocean color validation activities, high-performance liquid chromatography (HPLC) is an established reference technique for the analysis of chlorophyll a and associated phytoplankton pigments. The results of an intercomparison exercise of HPLC pigment determination, performed for the first time on natural samples and involving four laboratories (each using a different HPLC procedure), are used to address three main objectives: (a) estimate (and explain) the level of agreement or discrepancy in the methods used, (b) establish whether or not the accuracy requirements for ocean color validation activities can be met, and (c) establish how higher order associations in individual pigments (i.e., sums and ratios) influence the uncertainty budget while also determining how this information can be used to minimize the variance within larger pigment databases. The round-robin test samples (11 different samples received in duplicate by each laboratory) covered a range of total chlorophyll a concentration, [TChl a], representative of open ocean conditions from 0.045 mg m(-3), typical of the highly oligotrophic surface waters of the Ionian Sea, to 2.2 mg m(-3), characteristic of the upwelling regime off Morocco. Despite the diversity in trophic conditions and HPLC methods, the agreement between laboratories, defined here as the absolute percent difference (APD), was approximately 7.0% for [TChl a], which is well within the 25% accuracy objective for remote sensing validation purposes. For other pigments (mainly chemotaxinomic carotenoids), the agreement between methods was 21.5% on average (ranging from 11.5% for fucoxanthin to 32.5% for peridinin), and inversely depended on pigment concentration (with large disagreements for pigments close to the detection limits). It is shown that better agreement between methods can be achieved if some simple procedures are employed: (a) disregarding results less than the effective limit of quantitation (LOQ, an alternative to the method detection limit, MDL), (b) standardizing the manner in which the concentration of pigment standards are determined, and (c) accurately accounting for divinyl chlorophyll a when computing [TChl a] for those methods which do not chromatographically separate it from monovinyl chlorophyll a. The use of these quality-assurance procedures improved the agreement between methods, with average APD values dropping from 7.0% to 5.5% for [TChl a] and from 21.5% to 13.9% for the principal carotenoids. Additionally, it is shown that subsequent grouping of individual pigment concentrations into sums and ratios significantly reduced the variance and, thus, improved the agreement between laboratories. This grouping, therefore, provides a simple mechanism for decreasing the variance within databases composed of merged data from different origins. Among the recommendations for improving database consistency in the future, it is suggested that submissions to a database should include the relevant information related to the limit of detection for the HPLC method. (C) 2003 Elsevier B.V. All rights reserved.

The predictions of a spectral light-photosynthesis model are compared with field data. The model calculations are based on pigment (chlorophyll and pheophytin) and temperature profiles and, when available, on irradiance recorded on deck. The agreement between computed and measured production values is satisfying over the full range (10(-4)-1 g C m-3 d-1 or 0.03-10 g C m-2 d-1). It is better when C-14 fixation has been measured via the in situ method; a small bias appears when production was measured on deck (simulated in situ method). In both cases however the standard deviation remains similar and computed and measured column production agrees within a factor of approximately-3. The same data set is also used to predict column production from pigment concentration within only the top layer, as supposedly remotely sensed. The model is run in combination with pigment profiles, which are ``reconstructed'' (in magnitude and shape) as a function of the upper layer concentration with statistical relationships previously established. The agreement between computed and measured production (within a factor 3.3 at 1 SD) is encouraging. The model uses mean and constant physiological parameters, which actually vary in the natural environment. Among these parameters, the Chl-specific absorption by phytoplankton algae and, to a lesser extent, the maximum quantum yield for growth are crucial. Very likely their variations are the main causes of divergence between predicted and field values.

The Aerosol Robotic Network (AERONET) was developed to support atmospheric studies at various scales with measurements from worldwide distributed autonomous sunphotometers [Holben et al. 1998]. AERONET has now extended its support to marine applications through the additional capability of measuring the radiance emerging from the sea with modified sun-photometers installed on offshore platforms like lighthouses, navigation aids, oceanographic and oil towers. The functionality of this added network component called AERONET - Ocean Color (AERONET-OC), has been verified at different sites and deployment structures over a four year testing phase. Continuous or occasional deployment platforms (see Fig. 1) included: the Acqua Alta Oceanographic Tower (AAOT) of the Italian National Research Council in the northern Adriatic Sea since spring 2002; the Martha s Vineyard Coastal Observatory (MVCO) tower of the Woods Hole Oceanographic Institution in the Atlantic off the Massachusetts coast for different periods since spring 2004; the TOTAL Abu-Al-Bukhoosh oil Platform (AABP, shown through an artistic rendition in Fig. 1) in the Persian (Arabian) Gulf in fall 2004; the Gustaf Dal n Lighthouse Tower (GDLT) of the Swedish Maritime Administration in the Baltic Sea in summer 2005; and the platform at the Clouds and the Earth's Radiant Energy System (CERES) Ocean Validation Experiment (COVE) site located in the Atlantic Ocean off the Virginia coast since fall 2005. Data collected during the network testing phase, confirm the capability of AERONET-OC to support the validation of marine optical remote sensing products through standardized measurements of normalized water-leaving radiance, LWN, and aerosol optical thickness, a, at multiple coastal sites.

The variability of the backscattering to scattering ratio of marine particles, bbp : bp, is examined from in situ measurements performed during the spring and early summer of 2004 in the eastern English Channel and southern North Sea. This area is characterized by a quasi-permanent background of mineral matter from direct inputs, or resuspension effects, and by relatively intense spring phytoplankton blooms (mainly diatoms and the prymnesiophyte Phaeocystis globosa). The bbp : bp surface values range between 0.0024 and 0.0417, with a mean value of 0.0138 6 0.0083. In order to interpret such a great variability, simultaneous water samples were collected for the biogeochemical characterization of the bulk suspended particle population. We show that the bbp : bp variability is related to the composition of the particulate assemblage expressed by Chl a, the POC : SPM and POC : Chl a ratios, where Chl a, POC, and SPM are the concentrations of chlorophyll a, particulate organic carbon, and suspended particulate matter, respectively. Low bbp : bp values are observed for a particle population dominated by low refractive index material such as phytoplankton, whereas high bbp : bp values are generally observed in presence of relatively high concentration of inorganic particles. The amount of organic material (both living and nonliving, including phytoplankton) relative to phytoplankton has a strong (and sometimes the greatest) effect on the backscattering-to-scattering ratio. Assuming that phytoplankton and detritus have similar refractive index, this pattern is interpreted as resulting from changes in the particle size distribution as well as by aggregation of mineral and nonliving organic detrital material. The backscattering coefficient (bb ;m 21) and the detailed understanding of its variability in natural waters are of fundamental importance for oceanographic sciences related to the knowledge of suspended marine particles. The backscattering coefficient is also of primary importance for remote sensing of ocean color, as the radiometric signal recorded by a sensor onboard an aircraft or a satellite is directly proportional to its intensity. However, ‘‘our presentday interpretation and detailed understanding of major sources of backscattering and its variability in the ocean are uncertain and controversial’’ (Stramski et al. 2004). Until the