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Southeast Asia's peatlands are a globally significant source of terrigenous dissolved organic carbon (tDOC) to the ocean, and field observations show that this tDOC is extensively remineralized within the shelf sea. Yet the processes that drive this remineralization remain unclear. Here, we combined incubation experiments and model simulations to quantify the rate and extent of photodegradation of tDOC in the Sunda Shelf Sea. During laboratory photodegradation experiments, 26%–74% of the peatland tDOC was photomineralized, but realistic in situ rates of photodegradation have not yet been estimated in this region. Based on spectrally resolved apparent quantum yields for tDOC remineralization calculated from experiments, modeled in situ solar irradiance, and measured in‐water inherent optical properties, we simulated peatland tDOC photomineralization for two coastal regions of the Sunda Shelf Sea. These simulations show that sunlight can directly remineralize 25 ± 9% of the tDOC input over its maximum 2‐year residence time in the Sunda Shelf Sea, accounting for 38% of the total tDOC remineralization. We also found that photobleaching can remove 54 ± 4% of colored dissolved organic matter over this time‐scale. We further derived a simplified photochemical decay constant ∅ref of 0.016 day−1 for Southeast Asia's peatland‐derived tDOC, which can be used to parameterize the recently proposed UniDOM model. We conclude that direct photodegradation may be a greater sink for tDOC in Southeast Asia's coastal ocean compared to higher latitudes, although it is insufficient to account for the total tDOC remineralization observed in the Sunda Shelf Sea. Plain Language Summary: Tropical peatlands in Southeast Asia contribute a large quantity of organic carbon to the coastal ocean. Field observations have clearly demonstrated that this organic carbon is rapidly decomposed to CO2, but the mechanism driving this remineralization is unclear. Organic carbon from peatlands appears to be easily decomposed upon sunlight irradiation, known as photodegradation. Here, we conducted experiments to measure the photochemical decay efficiency of peatland‐derived organic carbon and developed model simulations to calculate how much of the organic carbon is decomposed via photodegradation in natural coastal waters. We found that sunlight can directly remineralize 25% of the peatland‐derived organic carbon input to CO2 in the coastal ocean of Southeast Asia. This means that photodegradation decomposes more of the terrestrial organic carbon in Southeast Asia than in coastal oceans at higher latitudes, but photodegradation alone is still insufficient to account for the extent of all degradation observed in field data. Our data allowed us to derive region‐specific decay rates of photodegradation for coastal Southeast Asia, which can be used in large‐scale ocean carbon biogeochemistry models. Key Points: We combined incubation experiments and model simulation to quantify photodegradation of peatland dissolved organic carbon in coastal watersDirect photochemical remineralization can consume 25% of peatland dissolved organic carbon input in the coastal Sunda Shelf SeaUp to 74% of the Southeast Asian peatland‐derived dissolved organic carbon is photo‐remineralizable [ABSTRACT FROM AUTHOR]

High-quality ocean colour observations are increasingly accessible to support various monitoring and research activities for water quality measurements. In this paper, we present a newly developed regional total suspended solids (TSS) empirical model using MODIS-Aqua's Rrs(530) and Rrs(666) reflectance bands to investigate the spatial and temporal variation of TSS dynamics along the southwest coast of Sarawak, Borneo. The performance of this TSS retrieval model was evaluated using error metrics (bias = 1.0, MAE = 1.47, and RMSE = 0.22 in mg/L) with a log10 transformation prior to calculation, as well as a k-fold cross validation technique. The temporally averaged map of TSS distribution, using daily MODIS-Aqua satellite datasets from 2003 until 2019, revealed large TSS plumes detected particularly in the Lupar and Rajang coastal areas on a yearly basis. The average TSS concentration range of 15 - 20 mg/L was estimated at these coastal areas. Moreover, the spatial map of TSS coefficient of variation (CV) indicated strong TSS variability (approximately 90%) in the Samunsam-Sematan coastal areas, which could potentially impact nearby coral reef habitats in this region. Our findings on temporal TSS variation provide further evidence that monsoonal patterns drive the TSS release in these tropical water systems, with distinct and widespread TSS plume variations observed between the northeast and southwest monsoon periods. Map of relative TSS distribution anomalies revealed strong spatial TSS variations in the Samunsam-Sematan coastal areas, while 2010 recorded a major increase (approximately 100%) and widespread TSS distribution with respect to the long-term mean. Furthermore, our findings on the contribution of river discharge to the TSS distribution showed a weak correlation across time at both the Lupar and Rajang river mouth points. The variability of TSS distribution across coastal river points was studied by investigating the variation of TSS pixels at three transect points, stretching from the river mouth into territorial and open water zones, for eight main rivers. Our findings showed a progressively decreasing pattern of nearly 50% in relation to the distance from shore, with exceptions in the northeast regions of the study area. Essentially, our findings demonstrate that the TSS levels at the southwest coast of Sarawak are within local water quality standards, promoting various marine and socio-economic activities. This study presents the first observation of TSS distributions at Sarawak coastal systems with the application of remote sensing technologies, to enhance coastal sediment management strategies for the sustainable use of coastal waters and their resources. [ABSTRACT FROM AUTHOR]

Spectral measurements of remote-sensing reflectance (Rrs) and absorption coefficients carried out in three European estuaries (Gironde and Loire in France, Tamar in the UK) are presented and analyzed. Typical Rrs and absorption spectra are compared with typical values measured in coastal waters. The respective contributions of the water constituents, i.e., suspended sediments, colored dissolved organic matter, and phytoplankton (characterized by chlorophyll-a]), are determined. The Rrs spectra are then reproduced with an optical model from the measured absorption coefficients and fitted backscattering coefficients. From Rrs ratios, empirical quantification relationships are established, reproduced, and explained from theoretical calculations. These quantification relationships were established from numerous field measurements and a reflectance model integrating the mean values of the water constituents' inherent optical properties. The model's sensitivity to the biogeochemical constituents and to their nature and composition is assessed. OCIS codes: 010.0010, 010.4450, 010.7340, 000.2170, 120.0280.

Field determinations of the remote sensing reflectance signal are necessary to validate ocean color satellite sensors. The measurement of the above-water downwelling irradiance signal [E.sub.d](0+) is commonly made with a reference plaque of a known reflectance. The radiance reflected by the plaque ([L.sub.dspec]) can be used to determine [E.sub.d](0+) if the plaque is assumed to be near Lambertian. To test this assumption, basic experiments were conducted on a boat under changing sky conditions (clear, cloudy, covered) and with different configurations for simultaneous measurements of [L.sub.dspec] and [E.sub.d](0+). For all measurement configurations, results were satisfactory under a clear sky. Under cloudy or covered skies, shadow effects on the plaque induced errors up to 100% in the determination of [E.sub.d](0+). An appropriate measurement configuration was defined, which enabled [E.sub.d](0+) to be determined with an accuracy of better than [+ or -] 15% regardless of the sky conditions. [c] 2004 Optical Society of America OCIS codes: 000.2170, 010.4450, 120.4800.

Since the mid 1990s, Australia's Commonwealth Science Industry and Research Organisation (CSIRO) has developed a biogeochemical (BGC) model for coupling with a hydrodynamic and sediment model for application in estuaries, coastal waters and shelf seas. The suite of coupled models is referred to as the CSIRO Environmental Modelling Suite (EMS) and has been applied at tens of locations around the Australian continent. At a mature point in the BGC model's development, this paper presents a full mathematical description, as well as links to the freely available code and User Guide. The mathematical description is structured into processes so that the details of new parameterisations can be easily identified, along with their derivation. The EMS BGC model cycles carbon, nitrogen, phosphorous and oxygen through multiple phytoplankton, zooplankton, detritus and dissolved organic and inorganic forms in multiple water column and sediment layers. The underwater light field is simulated by a spectrally-resolved optical model that includes the calculation of water-leaving reflectance for validation with remote sensing. The water column is dynamically coupled to the sediment to resolve deposition, resuspension and benthic-pelagic biogeochemical fluxes. With a focus on shallow waters, the model also includes particularly-detailed representations of benthic plants such as seagrass, macroalgae and coral polyps. A second focus has been on, where possible, the use of geometric derivations of physical limits to constrain ecological rates, which generally requires population-based rates to be derived from initially considering the size and shape of individuals. For example, zooplankton grazing considers encounter rates of one predator on a prey field based on summing relative motion of the predator with the prey individuals and the search area, chlorophyll synthesis includes a geometrically-derived self-shading term, and the bottom coverage of benthic plants is generically-related to their biomass using an exponential form derived from geometric arguments. This geometric approach has led to a more algebraically-complicated set of equations when compared to more empirical biogeochemical model formulations. But while being algebraically-complicated, the model has fewer unconstrained parameters and is therefore simpler to move between applications than it would otherwise be. The version of the biogeochemistry described here is implemented in the eReefs project that is delivering a near real time coupled hydrodynamic, sediment and biogeochemical simulation of the Great Barrier Reef, northeast Australia, and its formulation provides an example of the application of geometric reasoning in the formulation of aquatic ecological processes. [ABSTRACT FROM AUTHOR]

Coastal water quality degradation is a global challenge. Marine pollution due to suspended sediments and dissolved matter impacts water colour, biogeochemistry, benthic habitats and eventually human populations that depend on marine resources. In Sarawak (Malaysian Borneo), peatland-draining river discharges containing suspended sediments and dissolved organic carbon influence coastal water quality at multiple locations along the coast. Optical remote sensing is an effective tool to monitor coastal waters over large areas and across remote geographic locations. However, the lack of regional optical measurements and inversion models limits the use of remote sensing observations for water quality monitoring in Sarawak. To overcome this limitation, we have (1) compiled a regional spectral optical library for Sarawak coastal waters, (2) developed a new semi-analytical remote sensing model to estimate suspended sediment and dissolved organic carbon in coastal waters, and (3) demonstrated the application of our remote sensing inversion model on satellite data over Sarawak. Bio-optical data analysis revealed that there is a clear spatial variability in the inherent optical properties of particulate and dissolved matter in Sarawak. Our optical inversion model coupled with the Sarawak spectral optical library performed well in retrieving suspended sediment (bias = 3% and MAE = 5%) and dissolved organic carbon (bias = 3% and MAE = 8%) concentrations. Demonstration products using MODIS Aqua data clearly showed the influence of large rivers such as the Rajang and Lupar in discharging suspended sediments and dissolved organic carbon into coastal waters. The bio-optical parameterisation, optical model, and remote sensing inversion approach detailed here can now help improve monitoring and management of coastal water quality in Sarawak. [ABSTRACT FROM AUTHOR]