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Measuring the underwater light field is a key mission of the international Biogeochemical-Argo program. Since 2012, 0–250 dbar profiles of downwelling irradiance at 380, 412 and 490 nm besides photosynthetically available radiation (PAR) have been acquired across the globe every 1 to 10 days. The resulting unprecedented amount of radiometric data has been previously quality-controlled for real-time distribution and ocean optics applications, yet some issues affecting the accuracy of measurements at depth have been identified such as changes in sensor dark responsiveness to ambient temperature, with time and according to the material used to build the instrument components. Here, we propose a quality-control procedure to solve these sensor issues to make Argo radiometry data available for delayed-mode distribution, with associated error estimation. The presented protocol requires the acquisition of ancillary radiometric measurements at the 1000 dbar parking depth and night-time profiles. A test on &gt, 10000 profiles from across the world revealed a quality-control success rate &gt, 90% for each band. The procedure shows similar performance in re-qualifying low radiometry values across diverse oceanic regions. We finally recommend, for future deployments, acquiring daily 1000 dbar measurements and one night profile per year, preferably during moonless nights and when the temperature range between the surface and 1000 dbar is the largest.

Pearl River Delta (PRD) is the third largest and one of the most productive Deltas in south China, where long-term monitoring and its assessment of water quality are necessary to understand the distribution patterns to infer the coastal zone management. The studies on long-term monitoring in water quality variable concerning the changing climate and anthropogenic effects can be made possible by continuous coverage of Moderate Resolution Imaging Spectroradiometer (MODIS) sensor. This study attempts to study the long-term trends, quantitative measurements, and assessment of water quality variables including total suspended solids (TSS), diffuse attenuation coefficient for downwelling irradiance at 490 nm (Kd (490)), and sea surface temperature (SST) using 8-day composite and monthly data products of MODIS data for the period between July 2002 and August 2018 in the turbid waters of the PRD. Monthly products are used to quantify the variability of these water quality properties on long-term climatology, seasonal and as well as spatial distributions to interannual timescales. We observed a decreasing trend of TSS and Kd (490) between 2002 and 2018, though not significant, whereas, an increasing trend of SST has recorded for the PRD region during the same time. Seasonal patterns of TSS, Kd (490) and SST showed significant differences between the seasons. The high values of TSS (>15 gm−3) and Kd (490) (>2 m−1) were found in summer, autumn followed by winter, and low values (

One great advantage of optical hyperspectral remote sensing from unmanned aerial systems (UAS) compared to satellite missions is the possibility to fly and collect data below clouds. The most typical scenario is flying below intermittent clouds and under turbulent conditions, which causes tilting of the platform. This study aims to advance hyperspectral imaging from UAS in most weather conditions by addressing two challenges: (i) the radiometric and spectral calibrations of miniaturized hyperspectral sensors; and (ii) tilting effects on measured downwelling irradiance. We developed a novel method to correct the downwelling irradiance data for tilting effects. It uses a hybrid approach of minimizing measured irradiance variations for constant irradiance periods and spectral unmixing, to calculate the spectral diffuse irradiance fraction for all irradiance measurements within a flight. It only requires the platform's attitude data and a standard incoming light sensor. We demonstrated the method at the Palo Verde National Park wetlands in Costa Rica, a highly biodiverse area. Our results showed that the downwelling irradiance correction method reduced systematic shifts caused by a change in flight direction of the UAS, by 87% and achieving a deviation of 2.78% relative to a on ground reference in terms of broadband irradiance. High frequency (< 3 s) irradiance variations caused by high-frequency tilting movements of the UAS were reduced by up to 71%. Our complete spectral and radiometric calibration and irradiance correction can significantly remove typical striped illumination artifacts in the surface reflectance-factor map product. The possibility of collecting precise hyperspectral reflectance-factor data from UAS under varying cloud cover makes it more operational for environmental monitoring or precision agriculture applications, being an important step in advancing hyperspectral imaging from UAS. Innovation Fund Denmark/[7048-00001B]/IFD/Dinamarca Agricultural Water Innovations in the Tropics/[]/AgWIT/Canadá Universidad de Costa Rica/[805-C0-603]/UCR/Costa Rica UCR::Vicerrectoría de Docencia::Ciencias Básicas::Facultad de Ciencias::Escuela de Física

The diffuse attenuation coefficient of downwelling irradiance (Kd) is an essential parameter for inland waters research by remotely sensing the water transparency. Lately, Kd semi-analytical algorithms substituted the empirical algorithms widely employed. The purpose of this research was to reparametrize a semi-analytical algorithm to estimate Kd and then apply it to a Sentinel-2 MSI time-series (2017&ndash, 2019) for the Tr&ecirc, s Marias reservoir, Brazil. The results for the Kd semi-analytical reparametrization achieved good accuracies, reaching mean absolute percentage errors (MAPE) for bands B2, B3 and B4 (492, 560 and 665 nm), lower than 21% when derived from in-situ remote sensing reflectance (Rrs), while for MSI Data, a derived MAPE of 12% and 38% for B2 and B3, respectively. After the application of the algorithm to Sentinel-2 images time-series, seasonal patterns were observed in the results, showing high Kd values at 492 nm during the rainy periods, mainly in the tributary mouths, possibly due to an increase in the surface runoff and inflows and outflow rates in the reservoir watershed.

Phytoplankton primary production (PP) in lakes play an important role in the global carbon cycle. However, monitoring the PP in lakes with traditional complicated and costly in situ sampling methods are impossible due to the large number of lakes worldwide (estimated to be 117 million lakes). In this study, bio-optical modelling and remote sensing data (Sentinel-3 Ocean and Land Colour Instrument) was combined to investigate the spatial and temporal variation of PP in four Baltic lakes during 2018. The model used has three input parameters: concentration of chlorophyll-a, the diffuse attenuation coefficient, and incident downwelling irradiance. The largest of our studied lakes, Võrtsjärv (270 km2), had the highest total yearly estimated production (61 Gg C y−1) compared to the smaller lakes Lubans (18 Gg C y−1) and Razna (7 Gg C y−1). However, the most productive was the smallest studied, Lake Burtnieks (40.2 km2); although the total yearly production was 13 Gg C y−1, the daily average areal production was 910 mg C m−2 d−1 in 2018. Even if lake size plays a significant role in the total PP of the lake, the abundance of small and medium-sized lakes would sum up to a significant contribution of carbon fixation. Our method is applicable to larger regions to monitor the spatial and temporal variability of lake PP.

Monitoring the coastal water quality (CWQ) and determining the effects of meteorological factors (MF) on the long-term CWQ adjacent to urban agglomerations are key topics in coastal monitoring and management. This study used satellite-derived data to study the long-term CWQ around the Guangdong-Hong Kong-Macau Greater Bay Area (GBA) with indicators of chlorophyll-a concentration (Chla) and diffuse attenuation coefficient for downwelling irradiance at 490 nm (Kd(490)). The sea surface temperature (SST) and total precipitation accumulation (TPA) of the waters around the GBA were also studied from 2003 to 2019. Furthermore, statistical analysis was conducted to identify the impact of the MF on CWQ by quantifying trends and relationships of CWQ & MF. The results showed that (1) An increasing trend of annual Chla, Kd(490), and TPA were observed, whereas a decreasing trend of SST has been recorded from 2003 to 2019. Both CWQ indicators and meteorological variables showed significant differences among seasons, and the values of Chla, Kd(490), and SST were found higher in the coastal area but lower in the open waters area. (2) SST was the crucial factor of the high values of CWQ indicators in summer: the highest values of Chla and Kd(490) in summer are likely to be attributed to the proper SST (r = 0.87; r = 0.83) and higher TPA in spring (r = 0.95; r = 0.93). The precipitation played an uncertain role in driving the CWQ (r > 0.9 and r

Nilai koefisien diffuse atenuasi Kd(λ) yang berasal dari pengukuran downwelling irradiance Ed(λ) merupakan salah satu parameter penting dalam oceanografi yang memberikan informasi mengenai ketersediaan cahaya dan tingkat penetrasi cahaya di dalam kolom air yang memberikan gambaran mengenai tingkat kecerahan, fotosintesis dan proses biologi lainnya. Informasi mengenai koefisien diffuse atenuasi memegang peranan penting dalam pengembangan algoritma Bio-Optik pada data satelit Ocean Color. Tujuan dari penelitian ini adalah untuk menjelaskan variabilitas dari koefisien diffuse atenuasi di perairan dangkal Karang Lebar, pulau Air dan Panggang dengan menggunakan sensor irradiace hyperspectral radiometer TriOS-RAMSES dengan cakupan rentang panjang gelombang 320 hingga 950 nm dengan resolusi spektral 3.3 nm. Pengukuran in situ dilakukan dengan menurunkan sensor irradiance di setiap kedalaman secara vertikal pada kolom air. Secara keseluruhan hasil pengukuran menunjukkan bahwa nilai Kd(λ) memiliki pola dimana pada region panjang gelombang 380-480 nm akan menurun dan akan meningkat kembali hingga pada region panjang gelombang merah 560-760 nm. Nilai Kd(λ) di bagian dalam gobah ditemukan lebih tinggi dibandingkan di bagian luar gobah dengan perbedaan yang signifikan terjadi di region panjang gelombang merah, perbedaan signifikan nilai Kd(λ) juga terjadi di ketiga wilayah (F = 5.581 > F critical = 5.554) dimana masing-masing area memiliki karakteristik yang berbeda-beda. Secara dominan, nilai Kd(λ) dipengaruhi oleh serapan klorofil-a dengan R2 = 0.808 dibandingkan dengan hamburan dari muatan padatan terlarut dibuktikan dengan R2 = 0.043. Nilai Kd(λ) pada rentang panjang gelombang sinar tampak (400-700 nm) dapat memberikan gambaran mengenai jangkauan penetrasi cahaya sinar tampak yang bisa dideteksi oleh satelit dengan satuan satu kedalaman optik. Hubungan nilai Kd(λ) dengan satu kedalaman optik dijelaskan secara eksponensial dengan persamaan Kd(400-700nm) = 0.375*exp(-0.095*1ζ) dengan koefisien determinasi R2 = 0.97.

基于遥感手段准确估算水体漫射衰减系数Kd（490），可为有效评价近岸海域水质、准确估算真光层深度提供有力数据和方法支撑。利用2004~2012年在南海海域获得的9个航次的实测Kd（490）数据，通过建立实测与遥感产品的时空匹配数据对，获取与Kd（490）实测数据相对应的MODIS-Aqua遥感反射率（Rrs（λ））和叶绿素a浓度（Chl-a）产品，分别采用算法Muller，Werdell，Morel，Lee和MODIS业务化算法KD2M对南海海域Kd（490）遥感产品进行精度评估与对比；在此基础上，考虑到南海海域水体的特异性，基于相应的实测Rrs（λ）和Chl-a数据集，利用上述算法对Kd（490）反演产品再次进行精度评估与对比，进而探讨Kd（490）产品的误差来源和算法区域化修正的必要性。基于MODIS-Aqua遥感产品，利用上述五种算法获得的MODIS-Aqua Kd（490）产品值与实测值的平均相对误差（APD）分别是20.12%，29.22%，17.98%，22.61%和21.61%，其中，Morel算法最优，且相关系数最高达0.5；基于相应实测数据，利用上述算法反演获得的Kd（490）产品精度均有不同程度的提高，特别是算法Lee和Werdell，APD分别为17.86%和17.97%，提高了5%~11%，最优算法为Morel，其APD可达15.44%。合理选取评估算法及进一步开展算法的区域性修正对于准确估算南海海域Kd（490）产品是非常必要的。

The information capabilities of spectral measurements of underwater irradiance in the upper layer of Lake Baikal were demonstrated. It allowed to obtain the spectra of the vertical light attenuation coefficient, to restore the spectral structure of the incident solar radiation, including changes of photosynthetically active radiation with depth, and to calculate a number of primary bio-optical characteristics of water: spectra of the CDM absorption coefficient and the light backscattering coefficient, as well as the chlorophyll-a concentration.

Airborne lidar study of lake ecosystems is still a relatively unexplored field. In this paper we present measurements of the diffuse attenuation coefficient of downwelling irradiance ( K d ) obtained using a 532 nm airborne lidar in flights during 2004 and 2016 over Yellowstone Lake, Yellowstone National Park, Wyoming, USA. We compare the lidar measurements with MODIS K d data, discuss the impact that local weather and river inflows/outflows may have had on the data, compare to previous models of the diffuse attenuation coefficient, and examine several published relationships converting K d to Secchi disk depth.

A quasi-analytical algorithm (QAA)-based distribution and variability of particulate backscattering coefficient (bbp) was studied for Kerala coast, India. A total of 28 observations were made in the coastal stretch of about 410 km from Kasaragod to Ernakulam for up to 50 m depth. Optical data were collected using a hyperspectral underwater radiometer to evaluate the bbp, water-leaving radiance (Lw) and chlorophyll-a (Chl-a) concentration. We aimed to achieve three objectives, i.e. (1) QAA-based bbp calculation using underwater radiometer and its sensitivity to downwelling irradiance (Ed) and surface radiance (Es), (2) validation of the relationship between bbp and Chl-a concentration for inshore and offshore coastal waters and (3) the relationship of Lw with QAA-based bbp and in situ Chl-a. We observed that the range of bbp values varied between 0.07 and 0.002 m−1, with a maximum bbp value between 1200 and 1400 h for inshore waters. Ed and Es are independent variables and were placed at the denominator to calculate bbp, where Ed is found relatively more sensitive than Es. The correlation between bbp and Chl-a found growing with depth ( 20 m R2: 0.487), due to the increasing complexity of coastal waters (Case II). While relating the Chl-a and bbp with Lw, showed a poor corleation with a low R2 value of 0.229 and 0.203, respectively, signifying the maximum scattering due to other suspended matters with less contribution from Chl-a pigment in highly turbid coastal waters.

In this study, empirical and semianalytical algorithms are developed and compared for optically complex waters to retrieve the diffuse attenuation coefficient of downwelling irradiance (Kd(lambda)) from satellite data. In the first approach, a band ratio algorithm was used. Various sets of MERIS band ratios were tested to achieve the best estimates for K-d(490) based on the in situ dataset which was measured in Nordic lakes (oligotrophic to eutrophic conditions). In the second approach, K-d(490) was expressed as a function of inherent optical properties which were retrieved from MERIS standard products. The algorithms from both approaches were tested against an independent data set and validated in optically complex coastal waters in the Baltic Sea and in Nordic lakes with high concentrations of coloured dissolved organic matter (0.3 2.5 m(-1)). As a result, a combined band ratio algorithm was developed, which provides a promising approach R-2 = 0.98, RMSE= 17%, N = 34, p < 0.05) for estimating K-d(490) over a wide range of values (0.3-6.1 m(-1)).

Summary The mesopelagic habitat is a vast space that lacks physical landmarks and is structured by depth, light penetration, and horizontal currents. Solar illumination is visible in the upper 1,000 m of the ocean, becoming dimmer and spectrally filtered with depth—generating a nearly monochromatic blue light field [1]. The struggle to perceive dim downwelling light and bioluminescent sources and the need to remain unseen generate contrasting selective pressures on the eyes of mesopelagic inhabitants [2]. Hyperiid amphipods are cosmopolitan members of the mesopelagic fauna with at least ten different eye configurations across the family—ranging from absent eyes in deep-living species to four enlarged eyes in mesopelagic individuals [3–7]. The hyperiid amphipod Paraphronima gracilis has a pair of bi-lobed apposition compound eyes, each with a large upward-looking portion and a small lateral-looking portion. The most unusual feature of the P. gracilis eye is that its upward-looking portion is resolved into a discontinuous retina with 12 distinct groups, each serving one transverse row of continuously spaced facets. On the basis of eye morphology, we estimated spatial acuity (2.5° ± 0.11°, SEM; n = 25) and optical sensitivity (30 ± 3.4 μm 2 ⋅ sr, SEM; n = 25). Microspectrophotometry showed that spectral sensitivity of the eye peaked at 516 nm (±3.9 nm, SEM; n = 6), significantly offset from the peak of downwelling irradiance in the mesopelagic realm (480 nm). Modeling of spatial summation within the linear retinal groups showed that it boosts sensitivity with less cost to spatial acuity than more typical configurations.

Commonly we see the diffuse attenuation coefficient of downwelling irradiance (Kd) expressed as a sum of the contributions of various constituents. We show here that, both theoretically and numerically, because Kd is an apparent optical property (AOP), this approach is not consistent with radiative transfer. We further advocate the application of models of Kd developed in past decades that are not only consistent with radiative transfer but also provide more accurate estimates, in particular for coastal turbid waters.

Cast shadows caused by sparse clouds usually degrade spaceborne and airborne imagery. They result from the decrease of the direct solar beam due to the presence of a non-transparent cloud. The reduction of the downwelling solar flux density can be quantified during an air campaign, if the aircraft flies beneath the cloud and is equipped with an add-on instrument that measures the total downwelling solar irradiance. The objective of this work is to exploit such data for the de-shadowing of airborne hyperspectral imagery. Initially, the specific illumination and viewing conditions during the image acquisition, which allow the use of at-sensor downwelling irradiance data for the de-shadowing of airborne hyperspectral imagery, are outlined. Then a methodology is proposed that estimates the radiometric enhancement coefficients from the at-sensor irradiance data and correlates them with the image data using a shadow map. Improvements of the quality of the shadow maps are suggested. Performance assessment showed...

Despite the importance of diffuse attenuation coefficient of downwelling irradiance (K $ _{\rm d}$ ) to physical, chemical and biological processes in the euphotic zone, accurate determination of this apparent optical property from satellite ocean color data for coastal oceanic waters is still challenging with most of the existing algorithms. This paper presents a simple and robust algorithm developed using the relationship (R² = 0.995) of the ratio of remote sensing reflectances at 490 and 670 nm to the K $ _{\rm d}$ at a wavelength of 490 nm derived from a large number of radiometric profiles from various regional waters. When tested with three independent data sets from relatively clear and turbid waters, accuracy of K $ _{\rm d}$ (490) prediction by the present algorithm is statistically good compared to that of the existing algorithms. The effectiveness of the present algorithm is also tested on MODIS-Aqua images from complex waters around India. It is found that the present algorithm makes better retrievals of K $_{\rm d}$ (490) capturing the spatial characteristics of bloom and turbid water features. These results suggest that the present algorithm has the potential to provide more accurate K $_{\rm d}$ (490) from satellite ocean color data for coastal oceanic waters.

University of Massachusetts Boston; NASA Ocean Biology and Biogeochemistry and Water and Energy Cycle Programs; JPSS VIIRS Ocean Color Cal/Val Project; National Natural Science Foundation of China [41071223, 40976068, 41121091]; Ministry of Science and Technology of China [2013BAB04B00]

article i nfo Article history: The objectives of this study are to evaluate the applicability of three existing retrieval algorithms of diffuse atten- uation coefficients of downwelling irradiance at 490 nm, Kd(490), for turbid Case II waters, and to improve these existing models using a simple semi-analytical (SSA) model. In this study, based on comparison of the Kd(490) predicted by these models with field measurements taken in the Bohai Sea and Yellow Sea, it is shown that the SSA model provides a stronger performance than these three selected existing models. The atmospheric in- fluences on the MODIS data are removed using an improved shortwave infrared band-based (ISWIR-based) model, which is capable of retrieving spectral remote sensing reflectance within 19% uncertainty. The Kd(490) data was quantified from the MODIS images after atmospheric correction using the SSA model and Wang's model. The study results indicate that the SSA model produces 31.51% uncertainty in deriving Kd(490) from MODIS data, which is 12.1% higher than Wang's model. This study demonstrates the potential of the SSA model in estimating Kd(490) even in highly turbid coastal waters.

NASA; JPSS VIIRS; National Natural Science Foundation of China [41071223, 40976068, 41121091]; Ministry of Science and Technology of China [2013BAB04B00]; CNRS; INSU

Many deep-sea species, particularly crusta- ceans, cephalopods, and fish, use photophores to illuminate their ventral surfaces and thus disguise their silhouettes from predators viewing them from below. This strategy has several potential limitations, two of which are examined here. First, a predator with acute vision may be able to detect the individual photophores on the ventral surface. Second, a predator may be able to detect any mismatch between the spectrum of the bioluminescence and that of the background light. The first limitation was examined by modeling the perceived images of the counterillumination of the squid Abralia veranyi and the myctophid fish Cera- toscopelus maderensis as a function of the distance and visual acuity of the viewer. The second limitation was addressed by measuring downwelling irradiance under moonlight and starlight and then modeling underwater spec- tra. Four water types were examined: coastal water at a depth of 5 m and oceanic water at 5, 210, and 800 m. The appearance of the counterillumination was more affected by the visual acuity of the viewer than by the clarity of the water, even at relatively large distances. Species with high visual acuity (0.11° resolution) were able to distinguish the individual photophores of some counterilluminating signals at distances of several meters, thus breaking the camouflage. Depth and the presence or absence of moonlight strongly affected the spectrum of the background light, particularly near the surface. The increased variability near the surface was partially offset by the higher contrast attenuation at shallow depths, which reduced the sighting distance of mismatches. This research has implications for the study of spatial resolution, contrast sensitivity, and color discrimina- tion in deep-sea visual systems.

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 use of transparency as camouflage in the epipelagic realm is complicated by the presence of ultraviolet radiation, because the presence of UV-protective pigments decreases UV transparency and may reveal transparent zooplankton to predators and prey with UV vision. During July 1999, September 1999, and June 2000, transparency measurements (from 280 to 500 nm) were made on living specimens of 15 epipelagic (collection depth: 0–20 m, average: 11 ± 1 m) and 19 mesopelagic (collection depth: 150–790 m, average: 370 ± 40 m) species of transparent zooplankton from Oceanographer Canyon and Wilkinson Basin in the Northwest Atlantic Ocean. In addition, measurements of downwelling irradiance (from 330 to 500 nm) versus depth were made. The tissues from epipelagic zooplankton had lower UV transparency than those from mesopelagic zooplankton, while the average visible transparency (at 480 nm) of the two groups was not significantly different. Percent transparency was positively correlated with wavelength over most of the measured range, with a rapid decrease below a certain cutoff wavelength. In mesopelagic tissues, the cutoff wavelength was generally 300 nm. In epipelagic tissues, the cutoff wavelength was between 300 and 400 nm. Twelve out of 19 epipelagic tissues had transparencies at 320 nm that were half or less than their 480 nm transparency values, versus only 4 out of 21 mesopelagic tissues. The effects of UV absorption on UV visibility and minimum attainable depth were modeled using contrast theory and the physics of light attenuation. Because UV absorption was generally significantly greater in the UVB than in the UVA spectrum (where UV vision occurs), and because the highest UV absorption was often found in less transparent individuals, its modeled effects on visibility were slight compared to its effects on minimum attainable depth.

The influence of dimethylsulfide (DMS) and DOC concentrations and varying irradiance levels on the photolysis of DMS was evaluated. Laboratory experiments were conducted with 0.2 μm filtered aged seawater from the northern Adriatic Sea and artificial radiation. Photolysis of DMS followed pseudo first-order kinetics with the photolysis rate constant k directly dependent on irradiance intensity. Photolysis rates of DMS were also directly dependent on DOC concentration. In field experiments using natural solar radiation, DMS (5 nM initial conc.) was removed from freshly collected 0.2 μm filtered seawater at rates of 0.6±0.1 nmol l−1 h−1, equivalent to k=0.12±0.02 h−1. Based on these photolysis rates and using in situ profiles of downwelling irradiance, DMS and DOC concentrations, we calculated a water-column-integrated DMS removal rate due to photolysis of 165±20 μmol m−2 d−1. Averaged over the entire water column, the photochemical turnover time of DMS was 3.1±0.5 days. Most (88%) of the DMS was photolyzed in the top 10 m of the water column. Comparison of our data with photochemical and biological turnover rates published elsewhere indicates that photolysis may be an important sink of DMS in shallow coastal waters.

Primary production, pigment concentrations and spectral measurements of downwelling irradiance were made at four stations in four seasons (spring, summer, autumn, winter) during 1994 in the waters of the South Aegean Sea (Cretan Sea), Eastern Mediterranean. Rates of production were determined using in situ incubation techniques and included measurements at the surface micro- layer. Depth-integrated values averaged over season were 5.66 mg C nr2 h~' for primary production and the corresponding chlorophyll (Chi) a and phaeophytin (Phaeo) a values had means of 4.87 and 1.21 mg nr\ respectively. The assimilation ratio remained very low (mean over season: 1.19 mg C mg"1 Chi a h~'), as did the Phaeo a/Chl a ratio (mean over season: 0.24). The annual production for the area was estimated to yield 24.79 g C nr2 year"1. Primary production and Chi a estimates showed statistically significant seasonal, spatial and depth variations. The spectral values of the attenuation coefficient K6 (X) at 480 nm in the water column 0-50 m varied seasonally with higher values in spring (average 0.043 nr1) and winter (average 0.056 nr1). and lower values in summer (average 0.035 nr1) and autumn (average 0.026 nr1). The overall mean of this coefficient was 0.040 nr1, thus classifying the waters of the South Aegean Sea as Jerlov's Optical Type I. The results of the present investigation based on the productivity and light attenuation data demonstrate clearly that the South Aegean can be considered as one of the most oligotrophic areas of the Mediterranean Sea.

As it can strongly influence the availability of light and thus primary production, coloured dissolved organic matter (CDOM) affects the function of lake ecosystems. Therefore reliable methods are required for the monitoring of CDOM concentration. A new method using downwelling irradiance was tested for applicability in four selected lakes of the Bavarian Osterseen Lake District, which consists of 19 naturally connected freshwater lakes of different trophic level. The method separates between the direct and diffuse part of the incident light in order to handle the strong variability of the underwater light field. It is implemented in the software WASI, which is capable to retrieve water constituents by inverse modeling. During field campaigns downwelling irradiance measurements using RAMSES sensors were made in different depths. Simultaneously, water samples were taken in three depths (0.5 m, 2 m and Secchi disk depth), from which the absorption coefficient of CDOM, a Y , was derived in the range from 190 to 900 nm using photometric absorption measurements. Concentration (defined as a Y at 440 nm) ranged from 0.33 to 1.55 m -1 with a mean of 0.71 m -1 ± 0.04 m -1 , the spectral slope at 440 nm from 0.0120 to 0.0184 nm-1 with a mean of 0.0145 ± 0.0008 nm -1 . These laboratory measurements from water samples were compared to CDOM concentration obtained by inverse modeling of downwelling irradiance measurements using WASI. For sensor depths lower than 1 to 1.5 m large uncertainties were observed. The measurements in 2 m depth and at Secchi disk depth yielded good correlation between water sample and WASI derived data (R 2 = 0.87) with a mean standard deviation of 0.06 m -1 for the determined CDOM concentrations. This new method is an alternative to laboratory analysis of water samples from in situ measurements of CDOM concentration.

The vertical spectral diffuse attenuation coefficient of Kd is an important optical property related to the penetration and availability of light underwater, which is of fundamental interest in studies of ocean physics and biology. Models developed in the recent decades were mainly based on theoretical analyses and numerical (radiative transfer) simulations to estimate this property in optically deep waters, thus leaving inadequate knowledge of its variability at multiple depths and wavelengths, covering a wide range of solar incident geometry, in turbid coastal waters. In the present study, a new model is developed to quantify the vertical, spatial and temporal variability of Kd at multiple wavelengths and to quantify its dependence with respect to solar incident geometry under differing sky conditions. Thus, the new model is derived as a function of inherent optical properties (IOPs - absorption a and backscattering bb), solar zenith angle and depth parameters. The model results are rigorously evaluated using time-series and discrete in situ data from clear and turbid coastal waters. The Kd values derived from the new model are found to agree with measured data within the mean relative error 0.02~6.24% and R2 0.94~0.99. By contrast, the existing models have large errors when applied to the same data sets. Statistical results of the new model for the vertical spectral distribution of Kd in clear oceanic waters (for different solar zenith and inwater conditions) are also good when compared to those of the existing models. These results suggest that the new model can provide an improved interpretation about the variation of the vertical spectral diffuse attenuation coefficient of downwelling irradiance, which will have important implications for ocean physics, biogeochemical cycles and underwater applications in both relatively clear and turbid coastal waters. � 2013 Optical Society of America.

Regionally calibrated algorithms for water quality are strongly needed, especially for optically complex waters such as coastal areas in the Arabian Gulf. In this study, a regional qualitative algorithm was proposed to retrieve seawater transparency, with Secchi disk depth (SDD) as a surrogate, in the Arabian Gulf. A two-step process was carried out, first estimating the diffuse attenuation coefficient of downwelling irradiance at 490 nm (Kd_490) from MODIS/Aqua imagery and then SDD based on empirical correlations with Kd_490. Three satellite derived Kd products were tested and assessed against a set of in situ measurements, and one from a semi-analytical algorithm based on inherent optical properties gave the best performance with a R2 of 0.62. Comparisons between the performances of SDD models developed in this study and those established in other regions indicated higher accuracy of our proposed model for the Gulf region. The potential factors causing uncertainties of the proposed algorithm were also discussed. Seasonal and inter-annual variations of SDD over the entire Gulf were demonstrated using a 14-year time series of MODIS/Aqua data from 2002 to 2015. High SDD values were generally observed in summer while low values were found in winter. Inter-annual variations of SDD did not shown any significant trend with exceptions during algal bloom outbreaks that resulted in low SDD.

An improved, semi-automatic method for analysing the cards from the widely used Campbell-Stokes sunshine recorder is described. We show how the application of inexpensive commercial equipment and practices can simply and robustly build an archive of high-quality card images and manipulate them into a form suitable for easy further analysis. Rectified and registered digital images are produced, with the card's midday marker in the middle of the longest side, and with a temporal scaling of 150 px h−1. The method improves on previous, mostly manual, analyses by simplifying and automating steps into a process capable of handling thousands of cards in a practical time-scale. A prototype method of extraction of data from this archive is then tested by comparison with records from a co-located pyrheliometer at a resolution of the order of minutes. The comparison demonstrates that the Campbell-Stokes recorder archive contains a time series of downwelling solar-irradiance-related data with similar characteristics to that of benchmark pyrheliometer data from the baseline solar radiation network. A universal transfer function for card burn to direct downwelling short-wave radiation is still some way off and is the subject of ongoing research. Until such time as a universal transfer function is available, specific functions for extracting data in particular circumstances offer a useful way forward. The new image-capture method offers a practical way to exploit the worldwide sets of long-term Campbell-Stokes recorder data to create a time series of solar irradiance and atmospheric aerosol loading metrics reaching back over 100 yr from the present day.