Search

Understanding the variability of bio-optical properties in coastal seas is essential to assessing the impact of natural and anthropogenic activities on the quality of the coastal environments and their resources. This study investigated the vertical distribution of bio-optical properties and their potential driving forces in the Guangdong-Hong Kong-Macao Greater Bay Area (GBA) using a bio-optical dataset collected during the winter dry season. The hydrographic and biogeochemical properties observed across the GBA exhibited significant spatial variability, allowing the classification of the waters into three distinct regions: estuarine diluted water (EDW), Guangdong coastal current water (GCCW), and dense shelf water (DSW). Our findings show that EDW exhibited beam attenuation and optical backscatter coefficients an order of magnitude greater compared to the other two regions, which was attributed to factors such as higher concentrations of suspended particulate matter and organic material from estuarine sources. In contrast, the GCCW was characterized by lower salinity, temperature, and suspended particulate matter and displayed reduced turbidity near the coast, whereas nutrient-rich GCCW waters transported to the mid-shelf region supported increased phytoplankton biomass and a greater abundance of micro-phytoplankton. By exploring the bio-optical characteristics and their underlying processes in the GBA, this study enhances our understanding of the complex dynamics shaping the optical properties of coastal waters in this heavily urbanized region. [ABSTRACT FROM AUTHOR]

To examine the bio-optical response to coastal upwelling, we measured inherent optical properties (IOPs) and biogeochemical parameters simultaneously off Hainan Island in the northwest part of the South China Sea (SCS) during late summer 2013. Bio-optical relationships between IOPs and phytoplankton were used for calculating vertical profiles of the total chlorophyll a concentration (Chl-a) and the absorption by coloured detrital matter (CDM). These bio-optical properties, which showed distinct horizontal and vertical distributions across the continental shelf, were strongly influenced by upwelling processes, as well as the shelf topography. Phytoplankton biomass and CDM absorption in surface waters showed much higher values along the coast, with their spatial distributions related to topographic variability. Vertical distributions of phytoplankton were characterised by a subsurface chlorophyll maximum (SCM) layer. The strongest SCM (Chl-a = 4.22 mg m−3) was observed at 24 m depth in coastal waters near the northeast cape of Hainan Island. The depth of the SCM varied between 16 and 60 m at different stations, appearing to coincide with the isotherm of 22 °C. The SCM depth was inversely correlated with the magnitude of the SCM. Different shapes of Chl-a profiles were observed, which suggested that the vertical distributions of phytoplankton biomass were driven by different environmental factors. Elevated concentrations of CDM were mainly observed near the bottom, which suggest that the benthic nepheloid layer may be an important source of detrital material. The relationship between the absorption coefficient of CDM at 443 nm, aCDM(443), and Chl-a exhibited distinct differences between waters in upper ocean and in bottom layers, with the threshold depth being modulated by shelf topography. Our results highlight the utility of bio-optical observations with high resolution for better understanding the coupling between physical forcing and biogeochemical variability. [ABSTRACT FROM AUTHOR]

The composition and size distribution of particles in the ocean control their optical (scattering and absorption) properties, as well as a range of biogeochemical and ecological processes. Therefore, they provide important information about the pelagic ocean ecosystem's structure and functioning, which can be used to assess primary production, particle sinking, and carbon sequestration. Due to its harsh environment and remoteness, the particulate bio-optical properties of the Southern Ocean (SO) remain poorly observed and understood. Here, we combined field measurements from hydrographic casts from two research voyages and from autonomous profiling floats (BGC-Argo) to examine particulate bio-optical properties and relationships among several ecologically and optically important variables, namely the phytoplankton chlorophyll a concentration (Chl), the particulate absorption coefficient (ap), the particulate backscattering coefficient (b bp), and the particulate organic carbon (POC) concentration. In the clearest waters of the SO (Chl < 0.2 mg m−3), we found a significant contribution to absorption by non-algal particles (NAP) at 442 nm, which was up to 10 times greater than the absorption by phytoplankton. This makes the particulate bio-optical properties there remarkably different from typical oceanic case 1 water. A matchup analysis confirms the impact of this larger NAP absorption on the retrieval of Chl from satellite ocean colour observations. For waters with Chl > 0.2 mg m−3, no significant differences are observed between the SO and temperate waters. Our findings also demonstrate consistency in predicting phytoplankton carbon from either Chl or b bp, suggesting that both methods are applicable in the SO. [ABSTRACT FROM AUTHOR]

Understanding the variability of bio-optical properties in coastal seas is essential to assessing the impact of natural and anthropogenic activities on the quality of the coastal environments and their resources. This study investigated the vertical distribution of bio-optical properties and their potential driving forces in the Guangdong-Hong Kong-Macao Greater Bay Area (GBA) using a bio-optical dataset collected during the winter dry season. The hydrographic and biogeochemical properties observed across the GBA exhibited significant spatial variability, allowing the classification of the waters into three distinct regions: estuarine diluted water (EDW), Guangdong coastal current water (GCCW), and dense shelf water (DSW). Our findings show that EDW exhibited beam attenuation and optical backscatter coefficients an order of magnitude greater compared to the other two regions, which was attributed to factors such as higher concentrations of suspended particulate matter and organic material from estuarine sources. In contrast, the GCCW was characterized by lower salinity, temperature, and suspended particulate matter and displayed reduced turbidity near the coast, whereas nutrient-rich GCCW waters transported to the mid-shelf region supported increased phytoplankton biomass and a greater abundance of micro-phytoplankton. By exploring the bio-optical characteristics and their underlying processes in the GBA, this study enhances our understanding of the complex dynamics shaping the optical properties of coastal waters in this heavily urbanized region.

The composition and size distribution of particles in the ocean control their optical (scattering and absorption) properties, as well as a range of biogeochemical and ecological processes. Therefore, they provide important information about the pelagic ocean ecosystem’s structure and functioning, which can be used to assess primary production, particle sinking, and carbon sequestration. Due to its harsh environment and remoteness, the particulate bio-optical properties of the Southern Ocean (SO) remain poorly observed and understood. Here, we combined field measurements from hydrographic casts from two research voyages and from autonomous profiling floats (BGC-Argo) to examine particulate bio-optical properties and relationships among several ecologically and optically important variables, namely the phytoplankton chlorophyll a concentration (Chl), the particulate absorption coefficient (ap), the particulate backscattering coefficient (bbp), and the particulate organic carbon (POC) concentration. In the clearest waters of the SO (Chl < 0.2 mg m−3), we found a significant contribution to absorption by non-algal particles (NAP) at 442 nm, which was up to 10 times greater than the absorption by phytoplankton. This makes the particulate bio-optical properties there remarkably different from typical oceanic case 1 water. A matchup analysis confirms the impact of this larger NAP absorption on the retrieval of Chl from satellite ocean colour observations. For waters with Chl > 0.2 mg m−3, no significant differences are observed between the SO and temperate waters. Our findings also demonstrate consistency in predicting phytoplankton carbon from either Chl or bbp, suggesting that both methods are applicable in the SO.

Coral reefs inhabit clear oceanic-type waters to turbid coastal regimes. For shallow coastal water environments, the coral reefs from remote sensing will be substantially influenced by the complexity and heterogeneity of the optical properties of water. Through a set of bio-optical and chemical measurements in July 2019 around Xidao Island, Sanya, China, we explored spatial variations of water components' concentrations (including suspended solids, phytoplankton, and dissolved organic carbon) and the optical absorptions (i.e., particulate pigments, detritus, and colored dissolved organic matter) in waters over or around coral reefs; further analysis of their influences upon the remote sensing reflectance (Rrs(λ)) of water was performed. It was observed that the bio-optical and chemical properties of the waters over or around coral reefs were spatially and vertically heterogeneous for different sampling sites. As expected, the suspended solids dominated the optical properties of coral reefs waters in areas of Xidao Island, which evidently influenced the Rrs(λ), especially for the surface waters (Pearson r > 0.60, p < 0.01). In addition, the dissolved organic carbon concentrations exhibited significant relations to the Rrs(λ) both in surface and bottom water layers, whereas the colored dissolved organic matter showed a weak negative correlation with the Rrs(λ). These findings will support the mapping and monitoring benthic habitats with remote sensing imagery in coastal regions, especially when a removal of the influence by particulate sediments was available. [ABSTRACT FROM AUTHOR]

The Abrolhos Bank harbors the richest coral reef ecosystem in the South Atlantic Ocean. It exhibits unique geomorphologic structures, is localized in shallow depths, and is divided into two reef regions with an inner arc close to the coast (3–20 m depth) and an outer deeper arc (5–30 m depth). This study aims to describe some bio-optical properties of the Abrolhos Bank waters and to evaluate the performance of the inversion Hyperspectral Optimization Processing Exemplar (HOPE) model, developed to retrieve optical properties in shallow waters, in the region. To this end, measurements at 75 stations during two field campaigns conducted during the 2013 and 2016 wet seasons were analyzed, and the HOPE model was applied to both in situ remote sensing reflectance (Rrs) spectra and PRecursore IperSpettrale della Missione Applicativa (PRISMA) imagery. Significant differences in optical and biological properties were found between the two arcs. The empirical relationships between chlorophyll-a concentration (Chl-a) and absorption coefficient of phytoplankton at 440 nm (aphy(440)) diverged from Bricaud’s models, suggesting differences in phytoplankton diversity and cell size. In both arcs, total non-water absorption coefficient at 440 nm (aT-w(440)) was dominated by colored dissolved organic matter (CDOM) by ∼60%. Absorption coefficient by CDOM (acdom) presented a higher variability within the outer arc, with the lowest contribution from non-algal particles (NAPs), and the spectral slopes of aCDOM resembled those of the inner arc. The spectral slopes of the NAP absorption coefficient suggested a dominance by organic rather than mineral particles that probably originated from biological production. The HOPE model applied to in situ Rrs performed satisfactorily for depth in the Abrolhos Bank waters, although retrievals of aphy(440), CDOM plus NAP (adg(440)) and aT-w(440) were underestimated with a relative bias of −27.9%, −32.1% and −45.8%, respectively. The HOPE model retrievals from the PRISMA image exhibited low aphy(440) values over the whole scene and the highest adg(440) values in the Caravelas river plume. Very shallow depths (≤3 m), bottom substrate reflectance used as input in the HOPE model, model parametrization associated with the water complexity in the study site, and uncertainties associated to Rrs measurements used as input might be responsible for differences found when comparing HOPE retrievals with in situ measurements.

The ‘Estación Permanente de Estudios Ambientales’ (EPEA, 38° 28′ S-57° 41′ W, Argentina) is an ecological time series of in situ observations started in 2000 aiming to assess changes in the marine environment and plankton communities under a global change scenario. Bio-optical properties are studied at EPEA since the color of the ocean undergoes temporal fluctuations, both for natural and anthropogenic causes. Here we assessed whether bio-optical properties at EPEA have changed during 2000-2017, identifying the occurrence of special events and inter-annual trends in these properties. An increasing trend in chlorophyll-a concentration, possibly due to an increase in the smaller fraction of phytoplankton was observed. Although the absorption coefficient of phytoplankton did not follow a significant trend, it represented the occurrence of special events of high biomass suggesting that satellite information should be useful for the study site. The specific absorption coefficient of phytoplankton and the blue to red absorption ratio showed high values in summer and low in winter, according to the probable dominance of different size cells and their expected acclimation to the light regime. These results emphasize the relevance of periodic bio-optical in situ observations in understanding coastal ecosystems in a context of climate change.

The San Jorge Gulf, and the littoral to its north, is one of the most important fishing grounds for Argentina. Nevertheless, phytoplankton production has been scarcely studied. Here we analyzed during spring (2008) and summer (2009) the phytoplankton biomass, production, and the composition of phytoplankton and protozooplankton; their possible trophic relationships, and physical conditioners. At the south coast of the gulf during spring micro-nano-plankton (diatoms and dinoflagellates) were predominant and responsible for the maximum integrated production, comparable to that reported for the rich Argentinian shelf-break. Part of the organic carbon produced there was consumed by heterotrophic dinoflagellates, adding a trophic level to the food web. While at the center of the gulf, a conspicuous deep chlorophyll maximum would probably add organic matter to the bottom. During the following summer (2009), the ultrafraction represented the largest contribution to total phytoplankton biomass, and was dominated by Synechococcus sp. This, plus the abundance of ciliates, indicate the prevalence of a microbial food web during summer. It has been found that the frontal zones in the north and south of the gulf, favoring high phytoplankton biomass and its maintenance due to high primary production, provide a favorable food environment for impregnated female shrimp in spring, and for larvae during summer. [ABSTRACT FROM AUTHOR]

Vertical variability in the bio-optical properties of seawater in the northern South China Sea (NSCS) including inherent optical properties (IOPs) and chlorophyll a concentration (Chl) were studied on the basis of in situ data collected in summer 2008 using an absorption/attenuation spectrophotometer. An empirical model was developed to estimate Chl profiles based on the absorption line height at long wavelengths, with a relative root mean square error of 37.03%. Bio-optical properties exhibited large horizontal and vertical spatial variability. As influenced by coastal upwelling and the Zhujiang River (Pearl River) discharge, both IOPs and Chl exhibited high values in the surface waters of the inner shelf, which tended to decrease with distance offshore. Subsurface maximum layers of IOPs and Chl were observed in the middle and outer shelf regions, along with significantly higher values of attenuation coefficients beneath this layer that rapidly increased towards the bottom. In the open ocean, both IOPs and Chl exhibited consistent variability, with the subsurface maximum layer typically located at 34–84 m. Phytoplankton were found to be one of the major components in determining the vertical variability of bio-optical properties, with their vertical dynamics influenced by both physical forcing and light attenuation effects. The depth of the subsurface maximum layer was found to be closely related to the fluctuation of the oceanic thermocline and the depth of the euphotic zone, which also affected the total integrated biomass of the upper ocean. Typically high values of attenuation coefficients observed in the bottom waters of the continental shelf reflected the transport of particulate matter over the bottom boundary layer. Our results reveal large spatial differences in bio-optical profiles in response to complex marine ecodynamics in the NSCS. From the perspective of marine research, high-resolution optical measurements are clearly advantageous over conventional bottle sampling. [ABSTRACT FROM AUTHOR]

Continuous monitoring of blood-glucose concentrations is essential for both diabetic and nondiabetic patients to plan a healthy lifestyle. Noninvasive in vivo blood-glucose measurements help reduce the pain of piercing human fingertips to collect blood. To facilitate noninvasive measurements, this work proposes a Monte Carlo photon simulation-based model to estimate blood-glucose concentration via photoplethysmography (PPG) on the fingertip. A heterogeneous finger model was exposed to light at 660 nm and 940 nm in the reflectance mode of PPG via Monte Carlo photon propagation. The bio-optical properties of the finger model were also deduced to design the photon simulation model for the finger layers. The intensities of the detected photons after simulation with the model were used to estimate the blood-glucose concentrations using a supervised machine-learning model, XGBoost. The XGBoost model was trained with synthetic data obtained from the Monte Carlo simulations and tested with both synthetic and real data (n = 35). For testing with synthetic data, the Pearson correlation coefficient (Pearson’s r) of the model was found to be 0.91, and the coefficient of determination (R2) was found to be 0.83. On the other hand, for tests with real data, the Pearson’s r of the model was 0.85, and R2 was 0.68. Error grid analysis and Bland–Altman analysis were also performed to confirm the accuracy. The results presented herein provide the necessary steps for noninvasive in vivo blood-glucose concentration estimation.

Timely monitoring, detection and quantification of cyanobacterial blooms are especially important for controlling public health risks and understanding aquatic ecosystem dynamics. Due to the advantages of simultaneous data acquisition over large geographical areas and high temporal coverage, remote sensing strongly facilitates cyanobacterial bloom monitoring in inland waters. We provide a comprehensive review regarding cyanobacterial bloom remote sensing in inland waters including cyanobacterial optical characteristics, operational remote sensing algorithms of chlorophyll, phycocyanin and cyanobacterial bloom areas, and satellite imaging applications. We conclude that there have many significant progresses in the remote sensing algorithm of cyanobacterial pigments over the past 30 years. The band ratio algorithms in the red and near-infrared (NIR) spectral regions have great potential for the remote estimation of chlorophyll a in eutrophic and hypereutrophic inland waters, and the floating algae index (FAI) is the most widely used spectral index for detecting dense cyanobacterial blooms. Landsat, MODIS (Moderate Resolution Imaging Spectroradiometer) and MERIS (MEdium Resolution Imaging Spectrometer) are the most widely used products for monitoring the spatial and temporal dynamics of cyanobacteria in inland waters due to the appropriate temporal, spatial and spectral resolutions. Future work should primarily focus on the development of universal algorithms, remote retrievals of cyanobacterial blooms in oligotrophic waters, and the algorithm applicability to mapping phycocyanin at a large spatial-temporal scale. The applications of satellite images will greatly improve our understanding of the driving mechanism of cyanobacterial blooms by combining numerical and ecosystem dynamics models. [ABSTRACT FROM AUTHOR]

Natural and manmade disasters have occurred more frequently due mainly to climate change and human pressure for productivity. One of the world's vastest disasters in the mining industry occurred due to the collapse of the Fundao dam, Brazil, which discharged about 43 million m3 of iron tailings at the Doce River basin. Extreme natural events also affect this region and provoke substantial mass movement and substantial floods in the Doce River basin, and flow of anomalous volumes of sediments in its mouth. The extent of tailings and the sediment flow in these events were approached in previous research. However, their effects on the penetration of sunlight into the water column in the coastal region are unknown. Here, we evaluate the effects of an extreme natural event and a manmade disaster on the light regime of the water column at the Doce River mouth, using remote sensing data. In both events, the spatial and temporal distribution of suspended particulate matter (SPM), diffuse light attenuation coefficient (K d 490) and Euphotic Zone (Z eu) were analyzed. During the natural event, light penetration into the water column was strongly attenuated (K d 490: 0.35 m−1; SPM: 8.81 g/m3) but re-established after 1 month due to sediment deposition. In the case of the dam collapse, the attenuation of light penetration was also intense along the event (K d 490: 0.34 m−1; SPM: 13.87 g/m3); however, sediment deposition occurred sooner. Re-suspension of sediments due to wind action was recurrent after 8 months of the dam collapse, in contrast to the natural event, where re-suspension was not perceptible in satellite images. The results indicate that both events have considerable effects on the penetration of light in the water column, but with different intensity and length. Unlabelled Image • The Doce River basin, Brazil, has been impacted by life-threatening natural and manmade events between 2013 and 2015. • The consequences of the Fundao dam collapse and floods caused by extreme rainfall events are here assessed. • Research focus on the penetration of sunlight into the water column in the coastal region affected by the Doce River. • The natural event affected the water column light field extensively but during a shorter period. • The manmade disaster showed a protracted action and wind speed was a key factor for suspended sediment dispersion. [ABSTRACT FROM AUTHOR]

A wavelet analysis has been applied, for the first time, to 3-year high-frequency field observations of bio-optical properties (i.e. chlorophyll-fluorescence, beam attenuation and backscattering coefficients) in the northwestern Mediterranean Sea (BOUSSOLE site), in order to identify their dominant temporal patterns and evolution. A cross-wavelet and coherence analysis has also been applied to paired bio-optical coefficients time-series at the BOUSSOLE site, which allows identifying the temporal relationship between the cycles of the bio-optical properties. Annual, six- and four-month, intra-seasonal (i.e., mid- and short-terms) cycles are identified from the time-series analysis. The periodicities of chlorophyll-fluorescence, beam attenuation and particulate backscattering coefficients correlate well at different temporal scales and specific seasons. At annual, six- and four-month scales, different bio-optical properties follow rather similar patterns, likely driven by physical forcing. Intra-seasonal variability consists in both mid- and short-term variations. The former dominates during the winter and are related to episodic bloom events, while the latter variations (i.e. , diel) prevail during summer, in a stratified water column. • The Chl-Fluo variability has driven mostly by annual cycle. • The c p and b bp coefficients have driven mostly by 6-months cycle. • During winter, mid-term cycles (greater than 10 days) dominate the intra-seasonal signal for all parameters. • During summer, diel cycle has a strong impact on the intra-seasonal variability for all parameters. [ABSTRACT FROM AUTHOR]

Ocean processes that can influence rapidly changing ocean color include water-mass movement and bio-optical property changes in the water parcel. Traditionally, diurnal variability of bio-optical properties relies on daily time series at fixed locations by satellite sensors or in situ observations. There is a lack of an effective way to observe diurnal variation of bio-optical properties in a moving water parcel on a large scale. In this paper, we propose a new method to acquire diurnal variation of bio-optical properties in a moving water parcel. The novel approach integrates drifting buoy data and GOCI data. The movement of surface current was tracked by a drifting buoy, and its spatiotemporally matching bio-optical properties were obtained via the GOCI data. The results in the Yellow and East China seas during the summers of 2012 and 2013 show that the variation of time series following the movement of water parcel was obviously different from that obtained at fixed locations. The hourly differences of the former are 15.7% and 16.3% smaller than those of the latter for Chl a and total suspended sediment (TSS), respectively. The value of ag440 was more stable within the moving water parcel than in the fixed location. Our approach provides a simple and feasible way for observing diurnal variability of bio-optical properties in a moving surface water parcel.

• A novel blended algorithm was proposed to remotely observe lake POC across China. • POC concentrations in 450 large lakes were first mapped using OLCI/Sentinel-3 data. • Lake POC concentration across China was low in the west and high in the east. Remote sensing monitoring of particulate organic carbon (POC) concentration is essential for understanding phytoplankton productivity, carbon storage, and water quality in global lakes. Some algorithms have been proposed, but only for regional eutrophic lakes. Based on in-situ data (N = 1269) in 49 lakes across China, we developed a blended POC algorithm by distinguishing Type-I and Type-II waters. Compared to Type-I, Type-II waters had higher reflectance peak around 560 nm (>0.0125 sr−1) and mean POC (4.65 ± 4.11 vs. 2.66 ± 3.37 mg/L). Furthermore, because POC was highly related to algal production (r = 0.85), a three-band index (R 2 = 0.65) and the phytoplankton fluorescence peak height (R 2 = 0.63) were adopted to estimate POC in Type-I and Type-II waters, respectively. The novel algorithm got a mean absolute percent difference (MAPD) of 35.93 % and outperformed three state-of-the-art formulas with MAPD values of 40.56–76.42 %. Then, the novel algorithm was applied to OLCI/Sentinel-3 imagery, and we first obtained a national map of POC in 450 Chinese lakes (> 20 km2), which presented an apparent spatial pattern of "low in the west and high in the east". In brief, water classification should be considered when remotely monitoring lake POC concentration over a large area. Moreover, a process-oriented method is required when calculating water column POC storage from satellite-derived POC concentrations in type-II waters. Our results contribute substantially to advancing the dynamic observation of the lake carbon cycle using satellite data. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract This paper investigates the impact of wind-induced coastal upwelling on bio-optical properties over waters off the Kanyakumari coast for two consecutive winters (October 2011 to March 2012 and October 2012 to March 2013). Level-2 wind datasets from Oceansat-2 Scatterometer (12.5 km) have been used to estimate rate of upwelling using a published approach. Moderate-resolution Imaging Spectroradiometer on-board Aqua and Terra space-platform has been used to obtain bio-optical datasets (level-3, 4 km) that include chlorophyll- a (Chl-a) and particulate organic carbon (POC) concentration, absorption due to phytoplankton at 443 nm (aph_443), absorption due to detritus and gelbstoff at 443 nm (adg_443) and remote sensing reflectance (Rrs) at multiple wavelengths. Wind speed resulting in upwelling is found to be in range of 6.95 m s−1–18.67 m s−1. Phytoplankton bloom occurs when upwelling intensity is ranging between approximately 0.90 m/day – 3.50 m/day at a lag of about 2–3 days. Chl-a, aph_443, adg_443 and POC concentrations show simultaneous increment after upwelling and Chl-a is found to be the major parameter controlling other bio-optical properties of the study region. The waters belong to Case-1 type mainly co-dominated by phytoplankton and CDOM with very low concentrations of suspended particulate matter. Additionally, a noticeable drop in SST ranging between 0.55 °C to 1.50 °C has been observed at the same day of upwelling when upwelling intensity is around 1.5 m/day or above. Highlights • Wind speed resulting in upwelling ranges about 6.95 m s−1–18.67 m s−1 • Upwelling of around 0.90 m/day – 3.50 m/day results in phytoplankton bloom at an approximate lag of 2–3 days • Chlorophyll-a is the major parameter controlling other bio-optical properties • Waters are 'Case-1 type', co-dominated by phytoplankton and coloured dissolved organic matter • Upwelling of around 1.50 m/day or above results in sea surface temperature drop of about 0.55 °C to 1.50 °C [ABSTRACT FROM AUTHOR]

Strong seasonal upwelling, downwelling, changes in current patterns and the volume of freshwater discharge from Cochin Estuary defines the coastal waters off Cochin. These coastal waters were investigated through monthly sampling efforts during March 2015 to February 2016 to study the seasonal and spatial variability in bio-optical properties for the four different seasons mainly Spring Inter Monsoon (SIM), South West Monsoon (SWM), Fall Inter Monsoon (FIM) and Winter Monsoon (WM). The Barmouth region is the meeting place where freshwater from Cochin Estuary directly enters to the sea through a single narrow outlet, was dominated by highly turbid waters during the entire period of study. Among the four seasons, chlorophyll a (Chl_ a ) concentration showed a high value during SWM, ranged from 2.90 to 11.66 mg m −3 with an average value of 6.56 ± 3.51 mg m −3 . During SIM the distribution of coloured dissolved organic matter (CDOM) is controlled by decomposition of phytoplankton biomass and the river discharge, whereas during SWM the temporal distribution of CDOM is controlled only by river discharge. The highest value for CDOM spectral slope (S CDOM ) was observed during SWM, ranged from 0.013 to 0.020 nm −1 with an average value of 0.015 ± 0.002 nm −1 . During WM, the high S CDOM with lower a CDOM (443) indicates the photo-degradation affects the absorption characteristics of CDOM. The observed nonlinearity between Chl_ a and the ratio of phytoplankton absorption a ph (443)/ a ph (670) indicating the packaging effect and changes in the intercellular composition of pigments. During the study period, a ph (670) was strongly correlated with Chl_ a than a ph (443), which explains the accessory pigment absorption dominating more than Chl_ a in the blue part of the spectrum. Similarly, the results obtained from seasonal bio-optical data indicating that Chl_ a significantly contributes light attenuation of the water column during SIM, whereas detritus ( a d ) significantly contributes light attenuation during SIM and WM. During the study period, the relative absorption of detritus materials dominates the relative absorption of phytoplankton and CDOM at 443, 555 and 670 nm wavelengths. [ABSTRACT FROM AUTHOR]

Linear regression is widely used in applied sciences and, in particular, in satellite optical oceanography, to relate dependent to independent variables. It is often adopted to establish empirical algorithms based on a finite set of measurements, which are later applied to observations on a larger scale from platforms such as autonomous profiling floats equipped with optical instruments (e.g., Biogeochemical Argo floats; BGC-Argo floats) and satellite ocean colour sensors (e.g., SeaWiFS, VIIRS, OLCI). However, different methods can be applied to a given pair of variables to determine the coefficients of the linear equation fitting the data, which are therefore not unique. In this work, we quantify the impact of the choice of “regression method” (i.e., either type-I or type-II) to derive bio-optical relationships, both from theoretical perspectives and by using specific examples. We have applied usual regression methods to an in situ data set of particulate organic carbon (POC), total chlorophyll-a (TChla), optical particulate backscattering coefficient (bbp), and 19 years of monthly TChla and bbp ocean colour data. Results of the regression analysis have been used to calculate phytoplankton carbon biomass (Cphyto) and POC from: i) BGC-Argo float observations; ii) oceanographic cruises, and iii) satellite data. These applications enable highlighting the differences in Cphyto and POC estimates relative to the choice of the method. An analysis of the statistical properties of the dataset and a detailed description of the hypothesis of the work drive the selection of the linear regression method.

Mapping of Chlorophyll- a (Chl- a ) over the coastal waters of the Arabian Gulf and the Sea of Oman using the satellite-based observations, such as MODIS (Moderate Resolution Imaging Spectro-radiometer), has shown inferior performance (Chl- a overestimation) than that of deep waters. Studies in the region have shown that this poor performance is due to three reasons: (i) water turbidity (sediments re-suspension), and the presence of colored dissolved organic matter (CDOM), (ii) bottom reflectance and (iii) incapability of the existing atmospheric correction models to reduce the effect of the aerosols from the water leaving radiance. Therefore, this work focuses on investigating the sensitivity of the in situ spectral signatures of these coastal waters to the algal (chlorophyll: Chl- a ), non-algal (sediments and CDOM) and the bottom reflectance properties, in absence of contributions from the atmosphere. Consequently, the collected in situ spectral signatures will improve our understanding of Arabian Gulf and Sea of Oman water properties. For this purpose, comprehensive field measurements were carried out between 2013 and 2016, over Abu-Dhabi (Arabian Gulf) and Fujairah (Sea of Oman) where unique water quality data were collected. Based on the in situ water spectral analysis, the bottom reflectance (water depth < 20 m) are found to degrade the performance of the conventional ocean color algorithms more than the sediment-laden waters where these waters increase the R rs at the blue and red ranges. The increasing presence of CDOM markedly decreases the R rs in the blue range, which is conflicting with the effect of Chl- a . Given the inadequate performance of the widely used ocean-color algorithms (OC3: ocean color 3, OC2: ocean color 2) in retrieving Chl- a in these very shallow coastal waters, therefore, a new algorithm is proposed here based on a 3-bands ratio approach using [ R rs (656) − 1 − R rs (506) − 1 ] × R rs (661). The selected optimum bands (656 nm, 506 nm, and 661 nm) from this approach can be used to retrieve the Chl- a more accurately in these coastal Case II (turbid) waters which are close to the bands of the current missions such as Sentinel-3 OLCI (Ocean and Land Colour Instrument), MODIS, VIIRS (Visible Infrared Imaging Radiometer Suite) and LandSat 8. However, more uniformly distributed data over the Arabian Gulf is required to have a highly accurate regional model for Chl- a retrieval. [ABSTRACT FROM AUTHOR]

Sea eutrophication is a natural process of water enrichment caused by increased nutrient loading that severely affects coastal ecosystems by decreasing water quality. The degree of eutrophication can be assessed by chlorophyll-a concentration. This study aims to develop a remote sensing method suitable for estimating chlorophyll-a concentrations in tropical coastal waters with abundant phytoplankton using Moderate Resolution Imaging Spectroradiometer (MODIS)/Terra imagery and to improve the spatial resolution of MODIS/Terra-based estimation from 1 km to 100 m by geostatistics. A model based on the ratio of green and blue band reflectance (rGBr) is proposed considering the bio-optical property of chlorophyll-a. Tien Yen Bay in northern Vietnam, a typical phytoplankton-rich coastal area, was selected as a case study site. The superiority of rGBr over two existing representative models, based on the blue-green band ratio and the red-near infrared band ratio, was demonstrated by a high correlation of the estimated chlorophyll-a concentrations at 40 sites with values measured in situ. Ordinary kriging was then shown to be highly capable of predicting the concentration for regions of the image covered by clouds and, thus, without sea surface data. Resultant space-time maps of concentrations over a year clarified that Tien Yen Bay is characterized by natural eutrophic waters, because the average of chlorophyll-a concentrations exceeded 10 mg/m3 in the summer. The temporal changes of chlorophyll-a concentrations were consistent with average monthly air temperatures and precipitation. Consequently, a combination of rGBr and ordinary kriging can effectively monitor water quality in tropical shallow waters.

The Eastern Lagoon of New Caledonia (ELNC) is a semi-enclosed system surrounded by an extensive coral reef barrier. The system has been suffering impacts from climate variability and anthropogenic activities, including mining exploitation. Satellite monitoring is thus an essential tool to detect such changes. The present study aimed to assess the bio-optical variability of the ELNC and examine the applicability of ocean colour algorithms, using in situ bio-optical and radiometric data, collected during the March 2014 CALIOPE 2 cruise. The chlorophyll a concentration (Chla) varied from 0.13–0.72 mg·m−3, and the coastal stations were spectrally dominated by non-algal particles (NAP) and coloured dissolved organic matter (CDOM) (>80% of the total non-water absorption at 443 nm), due to the contribution of allochthonous sources. The phytoplankton specific absorption was generally lower (mean, 0.049 m2·mg Chla−1) than typical values observed for the corresponding Chla range, as well as the spectral slopes of the absorption of CDOM plus NAP (adg) (mean, 0.016 nm−1) and of the particle backscattering coefficient (bbp) (mean, 0.07 nm−1). The remote sensing reflectance obtained using two in-water approaches and modelled from Inherent Optical Properties (IOPs) showed less than 20% relative percent differences (RPD). Chla estimates were highly biased for the empirical (OC4 and OC3) and semi-analytical (GSM, QAA, GIOP, LMI) algorithms, especially at the coastal stations. Excluding these stations, the GSM01 yielded the best retrievals with 35–40% RPD. adg(443) was well retrieved by all algorithms with ~18% RPD, and bbp(443) with ~40% RPD. Turbidity algorithms also performed reasonably well (30% RPD), showing the capacity and usefulness of the derived products to monitor the water quality of the ELNC, provided accurate atmospheric correction of the satellite data. Regionally tuned algorithms may potentially improve the Chla retrievals, but better parameterization schemes that consider the spatiotemporal variability of the specific IOPs are still needed.

Measurements of absorption coefficients (a(λ), in m−1) collected by spectrophotometers in situ are overestimated due to the scattering of the reflecting tube absorption meter. Accurate correction of these data is essential in order to characterize water bodies bio-optically, as well as retrieve the remote sensing reflectance (Rrs, in sr−1), when applying a forward model. There are various methods of scattering error correction; however, they were all developed for clear water. In this research, different techniques were attempted in order to define the most appropriate method for correcting a(λ) values acquired by an absorption and attenuation spectral (ac-s) meter (WET Labs Inc., Philomath, OR, USA) in a tropical eutrophic reservoir. Three methods recommended by the manufacturer of the ac-s meter were tested: “flat” or “baseline”, “constant fraction”, and “proportional”. These methods were applied to two datasets that were measured in May and October 2014. The flat technique exhibited the lowest errors, with an average normalized root mean square error (NRMSE) of 7.95%, and a mean absolute percentage error (MAPE) of 29.26% for May. Meanwhile, proportional was the most suitable technique for most of the samples in October, with a mean NRMSE of 11.19% and a MAPE of 31.03% for October. In addition, the proportional method maintained the shape of the a(λ) values better than the other methods. Despite that, both the flat and proportional methods gave a similar performance statistically. Moreover, the flat method produced the best estimations of chla content for both datasets. Therefore, this method is recommended to correct ac-s data in retrieving such phytoplankton pigments.

The bio-optical changes of the water induced by red tides depend on the type of organism present, and the spectral characterization of such changes can provide useful information on the organism, abundance and distribution. Here we present results from the bio-optical characterization of a non-toxic red tide induced by the autotrophic ciliate Mesodinium rubrum . Particle absorption was high [ a p (440) = 1.78 m − 1 ], as compared to measurements done in the same region [ a p (440) = 0.09 ± 0.06 m − 1 ], with detrital components contributing roughly 11% [ a d (440) = 0.19 m − 1 ]. The remainder was attributed to absorption by phytoplankton pigments [ a ph (440) = 1.60 m − 1 ]. These a ph values were ~ 15 times higher than typical values for these waters. High chlorophyll a concentrations were also measured (52.73 μg L − 1 ), together with alloxanthin (9.52 μg L − 1 ) and chlorophyll c (6.25 μg L − 1 ). This suite of pigment is typical of the algal class Cryptophyceae , from which Mesodinium obtains its chloroplasts. Remote sensing reflectance showed relatively low values [ R rs (440) = 0.0007 sr − 1 ], as compared to other R rs values for the region under high bloom conditions [ R rs (440) = 0.0028 sr − 1 ], with maxima at 388, 484, 520, 596 and 688 nm. Based on the low reflection in the green-yellow, as compared to other red tides, we propose a new band ratio [ R rs (688)/ R rs (564)] to identify blooms of this particular group of organisms. [ABSTRACT FROM AUTHOR]

Remote sensing observations show optical signatures to conform to the physical oceanographic patterns in the Tasman Sea. To test the link between physical oceanographic processes and bio-optical properties we investigated an in situ bio-optical dataset collected in the Tasman Sea. Analysis of in situ observations showed the presence of four different water masses in the Tasman Sea, formed by the relatively warm and saline East Australia Current (EAC) water, a mesoscale cold core eddy on the continental slope, cooler Tasman Sea water on the shelf and river plume water. The distribution of suspended substances and their inherent optical properties in these water masses were distinctly different. Light absorption and attenuation budgets indicate varying optical complexity between the water masses. Specific inherent optical properties of suspended particulate and dissolved substances in each group were different as they were influenced by physical and biogeochemical processes specific to that water mass. Remote sensing reflectance signature varied in response to changing bio-optical properties between the water masses; thus providing the link between physical oceanographic processes, bio-optical properties and the optical signature. Findings presented here extend our knowledge of the Tasman Sea, its optical environment and the role of physical oceanographic processes in influencing the inherent optical properties and remote sensing signature in this complex oceanographic region. [ABSTRACT FROM AUTHOR]

Bio-optical characterization of water bodies requires spatio-temporal data about Inherent Optical Properties and Apparent Optical Properties which allow the comprehension of underwater light field aiming at the development of models for monitoring water quality. Measurements are taken to represent optical properties along a column of water, and then the spectral data must be related to depth. However, the spatial positions of measurement may differ since collecting instruments vary. In addition, the records should not refer to the same wavelengths. Additional difficulty is that distinct instruments store data in different formats. A data integration approach is needed to make these large and multi source data sets suitable for analysis. Thus, it becomes possible, even automatically, semi-empirical models evaluation, preceded by preliminary tasks of quality control. In this work it is presented a solution, in the stated scenario, based on spatial - geographic - database approach with the adoption of an object relational Database Management System - DBMS - due to the possibilities to represent all data collected in the field, in conjunction with data obtained by laboratory analysis and Remote Sensing images that have been taken at the time of field data collection. This data integration approach leads to a 4D representation since that its coordinate system includes 3D spatial coordinates - planimetric and depth - and the time when each data was taken. It was adopted PostgreSQL DBMS extended by PostGIS module to provide abilities to manage spatial/geospatial data. It was developed a prototype which has the mainly tools an analyst needs to prepare the data sets for analysis. [ABSTRACT FROM AUTHOR]

Recent advances in hydrologic optics provide a potential tool for extracting maps of optical properties of optically significant constituents. During summer experiments in the northern South China Sea, a procedure for inverting optical properties in optically complex coastal upwelling waters was performed. We tested an optical inversion model (OIM) that provides estimates of the absorption attributable to optically significant constituents (including phytoplankton, non-algal particles (NAP), colored dissolved organic matter (CDOM)), as well as the chlorophyll a concentration and the phytoplankton size class, from the total spectral absorption. The OIM provided reasonable estimates of these bio-optical products, and attempts to invert the bio-optical properties in the coastal upwelling were successful. Several bio-optical parameters displayed robust relationships to the hydrologic characteristics. The biomass was strongly enhanced in the upwelling, where the phytoplankton population was dominated by the microphytoplankton. The inherent optical properties in the surface waters delivered distinct responses to the coastal upwelling. The distribution of backscattering ratios and particulate size distribution slopes indicated that the offshore surface waters were mainly dominated by small organic NAP, while the upwelling surface water was characterized by larger particles. [ABSTRACT FROM AUTHOR]

Sea eutrophication is a natural process of water enrichment caused by increased nutrient loading that severely affects coastal ecosystems by decreasing water quality. The degree of eutrophication can be assessed by chlorophyll-a concentration. This study aims to develop a remote sensing method suitable for estimating chlorophyll-a concentrations in tropical coastal waters with abundant phytoplankton using Moderate Resolution Imaging Spectroradiometer (MODIS)/Terra imagery and to improve the spatial resolution of MODIS/Terra-based estimation from 1 km to 100 m by geostatistics. A model based on the ratio of green and blue band reflectance (rGBr) is proposed considering the bio-optical property of chlorophyll-a. Tien Yen Bay in northern Vietnam, a typical phytoplankton-rich coastal area, was selected as a case study site. The superiority of rGBr over two existing representative models, based on the blue-green band ratio and the red-near infrared band ratio, was demonstrated by a high correlation of the estimated chlorophyll-a concentrations at 40 sites with values measured in situ. Ordinary kriging was then shown to be highly capable of predicting the concentration for regions of the image covered by clouds and, thus, without sea surface data. Resultant space-time maps of concentrations over a year clarified that Tien Yen Bay is characterized by natural eutrophic waters, because the average of chlorophyll-a concentrations exceeded 10 mg/m3 in the summer. The temporal changes of chlorophyll-a concentrations were consistent with average monthly air temperatures and precipitation. Consequently, a combination of rGBr and ordinary kriging can effectively monitor water quality in tropical shallow waters. [ABSTRACT FROM AUTHOR]

Abstract: In this study, variations in the particulate organic carbon (POC) were monitored during a phytoplankton bloom event, and the corresponding changes in bio-optical properties were tracked at one station (114.29°E, 22.06°N) located in the Pearl River estuary. A greater than 10-fold increase in POC (112.29–1173.36mgm−3) was observed during the bloom, with the chlorophyll a concentration (Chl-a) varying from 0.984 to 25.941mgm−3. A power law function is used to describe the relationship between POC and Chl-a, and the POC:Chl-a ratio tends to change inversely with Chl-a. Phytoplankton carbon concentration is indirectly estimated using the conceptual model proposed by , and this carbon is found to contribute 47.21% (±10.65%) to total POC. The estimated carbon-to-chlorophyll ratio of phytoplankton in diatom-dominated waters is found to be comparable with results reported in the literature. Empirical algorithms for determining the concentrations of Chl-a and POC were developed based on the relationships of these variables with the blue-to-green reflectance ratio. With these bio-optical models, the levels of particulate organic carbon and Chl-a could be predicted from the radiometric data measured by a marine optical buoy, which showed much more detailed information about the variability in biogeochemical parameters during this bloom event. [Copyright &y& Elsevier]

Abstract: Photosynthesis–irradiance (P–E)-curves describe the photosynthetic performance of autotrophic organisms. From these P–E-curves the photosynthetic parameters α-slope, P max, and E k can be deduced which are often used to characterize and to compare different organisms or organisms in acclimation to different environmental conditions. Particularly, for in situ-measurements of P–E curves of phytoplankton the analysis of variable chlorophyll fluorescence proved its potential as a sensitive and rapid method. By using Chlorella vulgaris (Trebouxiophyceae), Nannochloropsis salina (Eustigmatophyceae), Skeletonema costatum and Cyclotella meneghiniana (Bacillariophyceae), the present study investigated the influence of cellular bio-optical properties on the correlation of the photosynthetic parameters derived from fluorescence-based P–E-curves with photosynthetic parameters obtained from the measurement of oxygen evolution. It is demonstrated that small planktonic algae show a wide range of cellular absorptivity which was subject to species-specifity, growth stage and environmental conditions, e.g. nutrient limitation. This variability in bio-optical properties resulted in a great deviation of relative electron transport rates (rETRs) from oxygen-based photosynthesis rates. Thus, the photosynthetic parameters α-slope and P max derived from rETRs strongly depend on the specific cellular absorptivity and cannot be used to compare the photosynthetic performance of cells with different optical properties. However, it was shown that E k is independent of cellular absorptivity and could be used to compare samples with unknown optical properties. [Copyright &y& Elsevier]

Abstract: Bio-optical properties of marine waters of the Cariaco Basin (southeastern Caribbean Sea) were assessed monthly between 1995 and 2005 as part of the CARIACO Ocean Time Series program. Temporal changes in light quality and penetration were caused by seasonal variation in the concentration of three major optical constituents, namely phytoplankton, detrital particles, and colored dissolved organic matter (CDOM). All constituents showed higher absorption coefficients during the upwelling season (January–May) compared to the rainy season (June–November). Both the absorption coefficient due to CDOM (a g (440)) and due to phytoplankton (a ph (440)) had similar contributions to total absorption of light during the upwelling season (a ph (440)=0.062±0.042m−1, a g (440)=0.065±0.047m−1). In contrast, a g (440) dominated light absorption during the rainy season (a ph (440)=0.017±0.011m−1, a g (440)=0.057±0.031). This led to an overestimate in SeaWiFS-derived chlorophyll concentrations during the rainy season, of between 7% and 45%. The detrital component, a d (440), typically showed the smallest contribution (a d (440)=0.021±0.014m−1 during upwelling and 0.007±0.001m−1 during the rainy season). There was no clear relationship between the various optically active components in time. During the upwelling season the chlorophyll-specific absorption coefficient, (440), was nearly half the value observed during the rainy season due to changes in the package effect (explaining ∼40% of the variability) and in accessory pigment composition as a result of species succession (explaining ∼60% of the variability). The euphotic zone depth (depth of the 1% photosynthetic active radiation (PAR) level) was typically shallower during the upwelling season (36.7±12.3m) than during the rainy season (47.9±13.5m) due to the onset of a shallower and stronger phytoplankton bloom. During upwelling, the highest chlorophyll-a concentrations (Chl>1mgm−3) were observed in the upper 25m with primary production rates exceeding 1800mgCm−2 d−1. During the rainy season, a deep chlorophyll maximum (DCM, concentrations between 0.2 and 0.8mgm−3) was observed between 35 and 55m, with low (<0.2mgm−3) Chl concentrations above this depth and primary production values of ∼990mgCm−2 d−1. The DCM occurred immediately above the seasonal thermocline and around the 1% PAR light level. During the upwelling season, no DCM was observed. [Copyright &y& Elsevier]

Abstract: The retrieval of chlorophyll-a concentration from remote sensing reflectance (Rrs) data was tested with the NASA OC4v4 algorithm on the inner New Caledonian lagoon (Case 2) and adjacent open ocean (Case 1) waters. The input to OC4v4 was Rrs measured in situ or modeled from water’s inherent optical properties (2001–2007). At open ocean stations, backscattering and absorption coefficients were correlated with chlorophyll (R 2 =0.31–0.51, respectively), in agreement with models for Case 1 waters. Taking spectrofluorometric measurement as reference, the OC4v4 model leads to an average underestimation of 33% of the chlorophyll concentration. For the lagoon waters, OC4v4 performed inadequately because the backscattering coefficient, highly correlated with turbidity and suspended matter (R 2 =0.98), was poorly correlated to chlorophyll (R 2 =0.42). The OC4v4 performance was better in deep lagoon waters for stations with a TDT index (Tchla ×depth/turbidity) higher than 19mgm−2 NTU−1 (R 2 =0.974, bias=10.2%). Global Imager Rrs provided a good estimate of Tchla (R 2 =0.79, N =28) in the deeper part of the lagoon. [ABSTRACT FROM AUTHOR]

Spatial and temporal patterns of bio-optical properties were studied in the Northern Gulf of Mexico during cruises in April and October of 2000, a year during which the discharge volume from the Mississippi River was unusually low. Highly variable surface Chl a concentrations (0.1 to 17 mg m−3) and colored dissolved organic matter (CDOM) absorption (0.07 to 1.1 m−1 at 412 nm) were observed in the study region that generally decreased with increasing salinity waters, being highest nearshore and decreasing at offshore stations. The optical properties of absorption, scattering, and diffuse attenuation coefficients reflected these distributions with phytoplankton particles and CDOM contributing to most of the spatial, vertical, and seasonal variability. The diffuse attenuation coefficient Kd(λ) and spectral remote sensing reflectance Rrs(λ) were linear functions of absorption and backscattering coefficients a(λ) and bb(λ) through the downward average cosine μd and the ratio of variables f/Q at the SeaWiFS wavebands for waters with widely varying bio-optical conditions. Although various Rrs(λ) ratio combinations showed high correlation with surface Chl a concentrations and CDOM absorption at 412 nm, power law equations derived using the Rrs(490)/Rrs(555) and Rrs(510)/Rrs(555) ratios provided the best retrievals of Chl a concentrations and CDOM absorption from SeaWiFS reflectance data. [Copyright &y& Elsevier]

Chlorophyll a concentrations (chl a) and the absorption coefficients of total particulate matter [ ap(λ)], phytoplankton [ aph(λ)], detritus [ ad(λ)], and colored dissolved organic matter: CDOM [ aCDOM(λ)] were measured in seawater samples collected in the subarctic North Pacific and the southern Bering Sea during the summer of 1997. We examined the specific spectral properties of absorption for each material, and compared the light fields in the Western subarctic Gyre (area WSG) with those in the Alaskan Gyre (area AG), and the southern Bering Sea (area SB). In the area WSG, the irradiance in the surface layer decreased markedly, indicating high absorption. In the area AG, the radiant energy penetrated deeply, and the chl a and absorption values were low throughout the water column. In the area SB, light absorption was high in the surface layer on the shelf edge and decreased with increasing depth; on the other hand, light absorption was low in the surface layer in the shelf area and increased with increasing depth. [ABSTRACT FROM AUTHOR]

The dynamics of phytoplankton across a seasonal frontal system formed in San José Gulf (SJG, Patagonia Argentina) and in neighbouring shelf waters was assessed based on bio-optical satellite data (2003–2018) and spring and summer in situ samplings. Bio-optical properties of the water masses on the eastern (ED) and western (WD) domains of the seasonal frontal system of SJG showed clear differences: the year-round-vertically-mixed waters from the WD, strongly connected with the adjacent shelf waters, evidenced a brief and strong single phytoplankton bloom, while those from the ED, showing lower exchange with shelf waters and a strong vertical stratification during the warm season, displayed an earlier and long-lasting spring phytoplankton bloom, followed by a late-summer and autumn bloom, both associated with the development and erosion of the seasonal thermocline. Waters from the entire system are optically influenced by the absorption of coloured dissolved organic matter and detritus (cdom + detritus), suggesting a strong sediment load contribution from the continent and the seabed. To remark, a strong correlation between satellite chlorophyll-a (Chla-sat) and absorption by phytoplankton (aphy 443) in the outer shelf waters differs from the weak correlation of those variables in the gulf's water masses, whose optical parameters are more complex. In situ Chla records may indicate wind-driven upwelling and downwelling areas in the northern and southern coasts of the ED. Dissolved nitrogen was identified as the limiting macronutrient for phytoplankton growth in the ED during summer. This work contributes relevant ecological information that may support management actions on the SJG shellfish artisanal fishery. [Display omitted] • A long-term bio-optical MODIS data in San José Gulf and adjacent waters were studied. • Additional spring and summer cruise data in the San José Gulf (SJG) was investigated. • Phytoplankton annual cycles respond to disparate nutrients/stratification/turbidity conditions. • East and west hydrographic domains in the SJG are evidenced for both data set. • Upwelling-downwelling structure in the east domain is driven by prevailing winds. [ABSTRACT FROM AUTHOR]

We describe the characteristics of surface water spectral reflectance variability and identify reflectance-based optical water types that are present in the southern South China Sea (SSCS) and the Straits of Malacca (SoM). Simultaneous measurements of in-water hyperspectral remote sensing reflectance (R rs), particulate backscattering coefficient (b bp) and in-water constituents were conducted at 473 stations from May 2009 to March 2019 during different monsoon seasons at both coastal and oceanic waters. Empirical orthogonal function (EOF) and hierarchical cluster analysis (HCA) were performed to determine the spectral variability of R rs and group the homogenous optical water type regimes. The results indicate that monsoons and regional hydrodynamic conditions have great influence on the optical properties. The SSCS exhibited strong seasonal patterns with maximum values of all the bio-optical parameters during the northeast monsoon and minimum values during the spring inter-monsoon. In contrast, the SoM waters are subjected to strong influence of freshwater discharges, thus resulting in optically complex water regimes. The EOF analyses clearly revealed two dominant modes of optical variability, with each mode corresponding to the optical conditions of SSCS and SoM, respectively. The correlation between EOF amplitude factors and optical parameters indicated that the R rs variability in SoM was primarily driven by the particulate backscattering and secondarily by light-absorbing components, while it was only minimally influenced by the absorption coefficients in SSCS. Hierarchical clustering revealed the presence of five distinct optical water types, which varied considerably according to the concentration of biogeochemical and optical properties. Classes 1 and 2, which were only found in SSCS waters, were defined by high magnitudes within the blue spectral region with very low concentrations of in-water constituents, indicating clear waters. Class 3 represents the optically transitional waters between coastal and open oceans, characterizing by a well-defined R rs peak at the blue-end of the spectrum with no single dominant constituent. Classes 4 and 5 are most representative of waters influenced by riverine plumes, particularly in the SoM, with the highest R rs magnitudes in the green and a well-defined absorption peak in the blue spectral region. Our findings confirmed that the performance of global Chl-a algorithms varied with the optical conditions present in the study area with the retrieval uncertainties increasing substantially from Class 1 to Class 5 water types. This finding will contribute significantly to the development of regional class-specific bio-optical models. • The first characterization of seasonal bio-optical variability of SSCS and SoM is presented. • Dominant modes of R rs variability and its driving factors are revealed. • Five dominant optical water types are identified. • The global Chl-a algorithms are strongly influenced by the optical water types. [ABSTRACT FROM AUTHOR]