Vertical Attenuation Coefficient Research Articles

Caracterización limnológica, estructura del fitoplancton y batimetría de Laguna Negra, un lago coloreado del Parque Nacional Tierra del Fuego (Argentina)

The 4% of the Argentinian sector from Tierra del Fuego is covered with water. Colored lakes are among the most conspicuous water bodies in the Andean area, and Laguna Negra, in the National Park Tierra del Fuego, is one of them. The main physical and chemical variables were monitored and phytoplankton samples were obtained with a monthly frequency during the ice-free period, between September 2016 and March 2018 (n=15). Laguna Negra was deeper than expected, with a maximum depth of 15.5 m. In addition, this lake has high pH values (7.5±0.55), which were quite stable during the study (variation coefficient, VC=8.3%). These characteristics preclude us from considering this lake as a shallow peatland lake. Dissolved and total nutrients were low and did not show a temporal pattern of fluctuation during the study period. The same trend of no clear fluctuation was also shared by dissolved organic carbon concentration (DOC=7.7±0.78 mg/L). Taxa richness ranged between 6 and 13 throughout the study. The biovolume was inversely correlated with the vertical attenuation coefficient of light (Kd). Phytoplankton biomass (chlorophyll-a concentration, abundance, and biovolume) was generally low, with Dinobryon divergens, Mallomonas sp. and Parvodinium cf. inconspicuum being the dominant taxa. Depending on the classification used, Laguna Negra might be regarded as a humic or dystrophic lake because its color is at the edge of values used in the current literature to consider a lake as humic.

V. Relationships Between Bio-Optical Parameters of the Black Sea Waters and Spectral Reflectance Coefficient in Spring 2019

The paper presents an analysis of field measurements of the spectral reflectance of the Black Sea and the bio-optical and colorimetric characteristics calculated from it, as well as chlorophyll concentrations and the Secchi disk depth. For the regions of the Black Sea with different characteristics, the averaged values of the calculated characteristics are presented: the effective and dominant wavelength in the reflectance spectrum, the purity of the water color. Using the previously developed bio-optical algorithm, the concentration of chlorophyll, absorption by dissolved organic matter, backscattering by suspended matter, and the vertical light attenuation coefficient were calculated.Inverse relationships are established between the Secchi disk depth and the maximum reflectance, as well as between the Secchi disk depth and the dominant wavelength. Linear relationships with high correlation coefficients are obtained between the surface chlorophyll concentration and the effective and dominant wavelength. A significant relationship was also established between the surface chlorophyll concentration and the maximum reflectance. Among the relationships between bio-optical and colorimetric parameters, a high correlation is observed between the vertical light attenuation coefficient and color purity.Chlorophyll concentrations calculated from the data of the spectral reflectance coefficient using a semi-analytical algorithm are in good agreement with the data of field measurements. Comparison of calculated and measured chlorophyll concentrations with satellite products of optical ocean color scanners shows an overestimation of chlorophyll concentrations by satellite data processing algorithms. The obtained correlations can be used in the future to assess bio-optical parameters in the northern part of the Black Sea.

Modeling benthic solar exposure (UV and visible) in dynamic coastal systems to better inform seagrass habitat suitability

Seagrass meadows worldwide provide valuable ecosystem services but have experienced sharp declines in recent decades. This rapid loss has prompted numerous restoration efforts with variable levels of success, often depending on the suitability of the restoration sites. The selection of sites can be guided by simple habitat suitability models driven with environmental variables deemed critical to the successful growth of new transplants. Habitat suitability models typically consider the influence of bathymetry, sediment type, salinity, wave exposure, and water quality. However, they typically do not explicitly include benthic exposure to ultraviolet (UV) and commonly use depth as a coarse proxy for photosynthetically active radiation (PAR). Benthic exposure to UV and PAR are both key parameters for habitat suitability but can be challenging to determine, especially in coastal environments influenced by rivers and tides where they are extremely variable. Here, we demonstrate the development of a simple but effective model of spectrally-resolved benthic solar irradiance for a dynamic marsh-influenced mesotidal estuary in Massachusetts. In-situ measurements were used to develop and validate an empirical model predicting the UV–visible vertical diffuse attenuation coefficient spectra of downwelling irradiance, Kd(λ), from simple physical parameters about tides, river discharge and location. Spectral benthic solar irradiances (280–700 nm) were calculated hourly for 3 years (2017–2019) using modeled and validated cloud-corrected surface downwelling irradiances, estimates of water depth, and the modeled Kd(λ) spectra. The mapped irradiances were used to provide improved seagrass habitat suitability maps that will guide future restoration efforts in the estuary. We expect the approach presented here can be adapted to other dynamic coastal environments influenced by tides and rivers and/or applied to other light-dependent organisms and biogeochemical processes.

Phytoplankton Genera Structure Revealed from the Multispectral Vertical Diffuse Attenuation Coefficient

The composition of phytoplankton and the concentration of pigments in their cells make their absorption and specific absorption coefficients key parameters for bio-optical modeling. This study investigated whether the multispectral vertical diffuse attenuation coefficient of downward irradiance (Kd) gradients could be a good framework for accessing phytoplankton genera. In situ measurements of remote sensing reflectance (Rrs), obtained in an Amazon Floodplain Lake (Lago Grande do Curuai), were used to invert Kd, focusing on Sentinel-3/Ocean and Land Color Instrument (OLCI) sensor bands. After that, an analysis based on the organization of three-way tables (STATICO) was applied to evaluate the relationships between phytoplankton genera and Kd at different OLCI bands. Our results indicate that phytoplankton genera are organized according to their ability to use light intensity and different spectral ranges of visible light (400 to 700 nm). As the light availability changes seasonally, the structure of phytoplankton changes as well. Some genera, such as Microcystis, are adapted to low light intensity at 550–650 nm, therefore high values of Kd in this range would indicate the dominance of Microcysts. Other genera, such as Aulacoseira, are highly adapted to harvesting blue-green light with higher intensity and probably grow in lakes with lower concentrations of colored dissolved organic matter that highly absorbs blue light (405–498). These findings are an important step to describing phytoplankton communities using orbital data in tropical freshwater floodplains. Furthermore, this approach can be used with biodiversity indexes to access phytoplankton diversity in these environments.

River Streamflow, Remotely Sensed Water Quality, and Benthic Composition of Previously Undescribed Nearshore Coral Reefs in Northern Puerto Rico

Land-based sediment stress represents a threat to many coral reefs in Puerto Rico primarily as a result of unrestricted land cover/land use changes and poor best management practices. The effects of such stresses have been documented along most coasts around the island. However, little attention has been paid to reefs located on the north coast, and very little is known about their composition and current state. Here, we present a study characterizing riverine inputs, water quality conditions, and benthic composition of two previously undescribed coral reefs (Tómbolo and Machuca reefs) located just eastward of the Río Grande de Manatí outlet in north-central Puerto Rico. This study utilizes a time series of remotely sensed ocean color products [diffuse vertical attenuation coefficient at 490 nm (Kd490) and chlorophyll-a concentration (Chl-a) estimated with data from the Visible Infrared Imaging Radiometer Suite (VIIRS)] to characterize water quality in this coastal region. In general, the months with relatively high mean daily river streamflow also coincide with months having the highest proportion of eastward wave direction, which can promote the eastward influence of river waters toward the two coral reefs sites. Kd490 and Chl-a showed a higher riverine influence closer to the watershed outlet. Kd490 and Chl-a monthly peaks also coincide with river streamflow highs, particularly at those pixels closer to shore. Tómbolo Reef, located farther eastward of the river outlet, shows a well-developed primary reef framework mainly composed of threatened reef-building species (Acropora palmata, Pseudodiploria) and high coral cover (19–51%). The benthos of Machuca Reef, located closer to the river outlet, is dominated by macroalgae with a significantly lower coral cover (0.2–2.7%) mainly composed of “weedy” coral species (Porites astreoides and Siderastrea radians). Cover of major benthic components correlates with distance from the river outlet, and with gradients in Kd490 and Chl-a, with higher coral cover and lower macroalgal cover farther from the river outlet. Coral cover at Tómbolo Reef is higher than what has been reported for similar sites around Puerto Rico and other Caribbean islands showing its ecological importance, and as up until now, an unrecognized potential refuge of reef-building threatened coral species.

Colored dissolved organic matter absorption at global scale from ocean color radiometry observation: Spatio-temporal variability and contribution to the absorption budget

A semi-analytical model (CDOM-KD2) based on the light vertical attenuation coefficient (Kd(λ)) has been developed for estimating the absorption by colored dissolved organic matter, acdom(443), from ocean color remote sensing at global scale. The performance of this new inversion model together with that of former models by Shanmugam (2011) (S2011), Chen et al. (2017) (C2017) and Aurin et al. (2018) (A2018) was evaluated from in situ and matchup validation data sets gathering worldwide distributed samples. An overall consistency in the acdom(443) estimated from S2011, C2017 and CDOM-KD2 models with a slightly better performance of the latter method was observed (MAPD of 27.42% and 30.85% for open ocean with in situ and satellite data, respectively), emphasizing the possible specific assessment of acdom(443) dynamics from satellite remote sensing over the global ocean including the most oligotrophic waters. At 443 nm the global average relative contribution of acdom(443) to the absorption by colored detrital matter, acdm(443) is of 61% ± 14%, while the contribution of acdom(443) to the non-water absorption, anw(443), is of 35% ± 26%. Strong spatial disparities are however observed for both acdom(443) temporal dynamics and relative contribution in the absorption budget. A decoupling is observed between acdom(443) and particulate detrital (i.e. non-living) matter and phytoplankton in the gyre areas where a low temporal variability is globally observed. This is contrasting with water masses influenced by terrestrial inputs as well as in equatorial and subtropical areas impacted by main oceanic currents where CDOM loads and acdom(443) contribution in the water absorption budget are more variable.

Solar Radiation as an Isolated Environmental Factor in an Experimental Mesocosm Approach for Studying Photosynthetic Acclimation of Macrocystis pyrifera (Ochrophyta).

Solar radiation effects on the ecophysiology and biochemical responses of the brown macroalga Macrocystis pyrifera (L.) C. Agardh were evaluated using a mesocosm approach in Southern Chile. Treatments with different radiation attenuations were simulated with three vertical attenuation coefficients: (1) total (Kd = 0.8 m−1), (2) attenuated (Kd = 1.2 m−1), and (3) low (Kd = 1.6 m−1) radiation levels. Nutrient concentration and temperature did not show differences under the three light conditions. Photosynthetic activity was estimated by in vivo chlorophyll a (Chla) fluorescence under the three light treatments as an isolated physical factor in both in situ solar radiation in the field. This was achieved using a pulse amplitude-modulated (PAM) fluorometera—Diving PAM (in situ). Photosynthetic activity and biochemical composition were measured in winter during two daily cycles (1DC and 2DC) in different parts of the thalli of the plant: (1) canopy zone, (2) middle zone, and (3) down zone, associated with different depths in the mesocosm system. Nevertheless, the in situ electron transport rate (ETRin situ) was higher in the exposed thalli of the canopy zone, independent of the light treatment conditions. The concentration of phenolic compounds (PC) increases in the down zone in the first daily cycle, and it was higher in the middle zone in the second daily cycle. The Chla increased in the morning time under total and attenuated radiation in the first daily cycle. Solar radiation increasing at midday prompted the photoinhibition of photosynthesis in the canopy zone but also an increase in productivity and phenol content. Therefore, light attenuation in the water column drove key differences in the photo-physiological responses of M. pyrifera, with the highest productivity occurring in thalli positioned in the canopy zone when exposed to solar irradiance.

A New Algorithm to Estimate Diffuse Attenuation Coefficient from Secchi Disk Depth

The vertical diffuse attenuation coefficient Kd (PAR) is used for calculating the euphotic zone, the first optical depth that is important for primary productivity models. Currently, Kd (PAR) can be estimated using an irradiometer or a Secchi disk (SD). The main objective of this work is to define a model that can be applied to a wide range of optical marine conditions to estimate Kd (PAR) by SD. We used irradiance profiles and SD depth (ZSD) from 679 stations in various marine regions. Three parametric models were developed, and their statistical performance was evaluated in view of previous approaches reported and remote sensing data. The best results were obtained with an adaptive model representing three cases: clear-water, turbid-water, and a transition zone (R2 = 0.965, MAE = 0.083, RMSD = 0.239, BIAS = 0.01, and MPI = 0.854). Previous models considering a single optical depth figure at which the SD disappears did not capture the marine optical complexity. Our classification of 113 stations with spectral absorption data into Jerlov water types indicated that no unique correspondence existed between estimated Kd (PAR) and water type, making it ambiguous to associate compatible inherent optical properties and chlorophyll with ZSD. Although obtaining Kd (PAR) from ZSD is simple/low-cost, care should be taken in the methodology used to measure ZSD to ensure consistent results across different optical marine conditions.

Morphological differentiation in the widespread fish Galaxias maculatus: do darker environments imply bigger eyes?

The visual capacity of aquatic organisms is, in general, related to light penetration in the water. Therefore, aquatic environments that differ in color can potentially contribute to species polymorphism, especially in fish. In this study, we explore the relationship between light quality and intensity in water bodies and the eye and mouth size in Galaxias maculatus, one of the most widespread fish. Fish morphology was studied photographically, and the vertical attenuation coefficient of light (Kd) was measured in five aquatic systems: two humic lakes, two estuaries, and one river. Water color was also estimated as absorbance at 440 nm. In those environments with less light penetration and darker water color, we observed bigger eyes and, additionally, larger mouths. The darkness of water bodies was mainly related to water color associated with humic substances. As G. maculatus is known to be a visual predator, our results suggest that bigger eyes increase visual capacity in darker environments, which would result in improved feeding rates, also supported by larger mouth length.

Planktonic primary production in the western Dutch Wadden Sea

Pelagic primary production measurements provide fundamental information about the trophic status of a marine ecosystem. Measured carbon fixation rates generally have a limited temporal and spatial resolution, but can be combined with Earth Observation data to extrapolate the measurements. Here,P-Ecurves were fitted for 3 yr of14C incubation data from the western Wadden Sea, using 4 different models; 2 with and 2 without photo-inhibition. The curve-fit model by Jassby & Platt (1976) best fit the data. Applying this model showed that the photosynthetic parameters, normalised for chlorophyllaconcentration, of maximum production (PBmax) and initial slope of theP-Ecurve (αB) were correlated. Seasonality in photosynthetic parameters of this model and the relationship with environmental variables were explored, with a focus on variables that can be inferred from satellite algorithms. There were no significant correlations between αBand any of the environmental variables measured. WhilePBmaxcorrelated with sea surface temperature (SST), the vertical light attenuation coefficient, silicate and nitrate + nitrite concentration, the multivariate model that best explained the variation in estimates ofPBmaxwas a model that included SST and year. In the period from 2012-2014, daily and annual production ranged between 3.4 and 3800 mg C d-1and between 131 and 239 g C m-2yr-1, respectively. Comparison of these results with historical data (1990-2003) indicates that the decline in planktonic primary production that has been happening since the 1990s has halted. Although not tested, we believe that our approach is generally applicable to coastal waters.

Source‐independent waveform inversion for attenuation estimation in anisotropic media

ABSTRACTIn previous publications, we presented a waveform‐inversion algorithm for attenuation analysis in heterogeneous anisotropic media. However, waveform inversion requires an accurate estimate of the source wavelet, which is often difficult to obtain from field data. To address this problem, here we adopt a source‐independent waveform‐inversion algorithm that obviates the need for joint estimation of the source signal and attenuation coefficients. The key operations in that algorithm are the convolutions (1) of the observed wavefield with a reference trace from the modelled data and (2) of the modelled wavefield with a reference trace from the observed data. The influence of the source signature on attenuation estimation is mitigated by defining the objective function as the ℓ2‐norm of the difference between the two convolved data sets. The inversion gradients for the medium parameters are similar to those for conventional waveform‐inversion techniques, with the exception of the adjoint sources computed by convolution and cross‐correlation operations. To make the source‐independent inversion methodology more stable in the presence of velocity errors, we combine it with the local‐similarity technique. The proposed algorithm is validated using transmission tests for a homogeneous transversely isotropic model with a vertical symmetry axis that contains a Gaussian anomaly in the shear‐wave vertical attenuation coefficient. Then the method is applied to the inversion of reflection data for a modified transversely isotropic model from Hess. It should be noted that due to the increased nonlinearity of the inverse problem, the source‐independent algorithm requires a more accurate initial model to obtain inversion results comparable to those produced by conventional waveform inversion with the actual wavelet.

Propagation and attenuation characteristics of rockburst-induced shock waves in coal-rock medium

Avoiding resonance effect of roadway is very important to prevent the rockburst hazard triggered by shock wave interference in coal mines. Propagation and attenuation characteristics of shock waves in coal-rock medium at the working face at Zhuji Coal Mine were investigated by using dynamic analysis in a three-dimensional finite difference code, FLAC3D. We revealed the attenuation properties and resonant effect of shock waves at different frequencies. It was found from the numerical analysis that the intensity of shock wave attenuates exponentially with the increasing propagation distance, while the horizontal attenuation coefficients of stress are very small. The vertical attenuation coefficient of shock waves is much larger than the horizontal attenuation coefficient. Due to existing roadway, the attenuation coefficient of amplitude in x-direction is larger compared with that in y-direction, while the attenuation coefficients of vibration velocity in x-direction and y-direction are almost equal to each other. Along with the propagation of shock waves, the high-frequency components gradually attenuate, and the frequency moves to low-frequency band. When resonance occurs in 35 Hz, the intensity values of shock waves reach the maximum value. Therefore, the resonance phenomenon can stimulate the maximum vibration response of underground structures. The low plastic deformation around the roadway indicates that the surrounding rock mass may induce rockburst more easily.

Full-scale model testing on the dynamic behaviour of weathered red mudstone subgrade under railway cyclic loading

Although weathered red mudstone (WRM) is widely distributed in the southwest of China, its suitability as a fill material for the subgrade bed of a high-speed railway (HSR) has not been comprehensively investigated. This paper presents the results of a field full-scale model testing of the cyclic loading and response of a railway track-subgrade system for the Dazhou-Chengdu Railway, where some WRM has been used in a newly designed subgrade structure. A control section of the HSR was also built using a traditional subgrade bed (Group A&B material) to compare the performances of the different sections. To better understand the dynamic characteristics and cumulative deformation of the two types of subgrade, the dynamic actions of different axle loads and different train speeds were simulated using specifically designed track-cyclic-loading equipment. The transverse and vertical distributions of the dynamic stress, dynamic displacement and acceleration of the track-subgrade system were measured and evaluated. The influence of the wheel axle load on the growth factor of the dynamic parameters, the vertical attenuation coefficient of the dynamic stress, and the effect of using WRM in the subgrade on the post-construction settlements were investigated. The tests enabled the development of cumulative settlement laws with railway loading for the two types of subgrade. Although the dynamic parameters and cumulative settlement of the WRM subgrade are always greater than those of better-quality material (Group A&B subgrade), they comply with HSR regulations. In conclusion, the results demonstrated that weathered red mudstone can be used as a filling material in the newly designed subgrade structure for HSRs.

Recent Large Scale Environmental Changes in the Mediterranean Sea and Their Potential Impacts on Posidonia Oceanica

Climate related changes can have significant effects on Posidonia oceanica, an endemic seagrass species of the Mediterranean Sea (MEDIT). This seagrass is very important for many aspects of functioning of the sea but there is an increasing number of reports about the ongoing loss of its biomass and area coverage. We analysed multiyear data of the sea surface temperature (SST), sea level anomalies, ocean colour MODIS-A and ERA-Interim reanalysis. The results provide a description of current environmental conditions in the MEDIT and their spatial and temporal variability, including long-term trends. We defined regions where the extent of the P. oceanica meadows may be limited by specific environmental conditions. Light limitation is more severe near the northern and western coasts of the MEDIT, where the vertical diffuse attenuation coefficient is large. In the zone extending from the Gulf of Lion towards the south, significant wave heights reach large values. Wave action may destroy the plants and as a result the shallow water depth limit of P. oceanica meadows is most likely deeper here than in other regions. The highest SST values are documented in the south-eastern part of the Mediterranean Sea. In this area P. oceanica meadows are more endangered by the climate warming than in other regions where SSTs are lower. The absence of P. oceanica meadows in the south-eastern edge of the Mediterranean Sea can be attributed to high temperatures. Our conclusions are partly confirmed by the information about P. oceanica from the literature but more monitoring efforts are needed to fully describe current extent of the meadows and their shifts. Results presented in this paper can help with designing special programs to confirm the role of environmental conditions on the spatial distribution of P. oceanica and their future trends in the Mediterranean Sea.

Estimating Underwater Light Regime under Spatially Heterogeneous Sea Ice in the Arctic

The vertical diffuse attenuation coefficient for downward plane irradiance ( K d ) is an apparent optical property commonly used in primary production models to propagate incident solar radiation in the water column. In open water, estimating K d is relatively straightforward when a vertical profile of measurements of downward irradiance, E d , is available. In the Arctic, the ice pack is characterized by a complex mosaic composed of sea ice with snow, ridges, melt ponds, and leads. Due to the resulting spatially heterogeneous light field in the top meters of the water column, it is difficult to measure at single-point locations meaningful K d values that allow predicting average irradiance at any depth. The main objective of this work is to propose a new method to estimate average irradiance over large spatially heterogeneous area as it would be seen by drifting phytoplankton. Using both in situ data and 3D Monte Carlo numerical simulations of radiative transfer, we show that (1) the large-area average vertical profile of downward irradiance, E d ¯ ( z ) , under heterogeneous sea ice cover can be represented by a single-term exponential function and (2) the vertical attenuation coefficient for upward radiance ( K L u ), which is up to two times less influenced by a heterogeneous incident light field than K d in the vicinity of a melt pond, can be used as a proxy to estimate E d ¯ ( z ) in the water column.

Diffuse Attenuation Coefficient Retrieval in CDOM Dominated Inland Water with High Chlorophyll-a Concentrations

The kd(490) is a vertical light attenuation coefficient and an important parameter for water quality. The kd estimates are often based on empirical and semi-analytical algorithms, designed for oceanic and coastal waters. However, there is a lack of information about the performances of these models to inland waters dominated by chromophoric dissolved organic matter (CDOM). Therefore, to contribute to this investigation, nine empirical models based on the blue-to-green and blue-to-red ratios and chlorophyll-a (Chl-a) concentration were evaluated, as well as three semi-analytical models using bands from the Operational Land Imager (OLI)/Landsat-8. The errors (mean absolute percentage error, MAPE > 80%) presented by the empirical models confirmed that the blue-to-green ratio failed in retrieving kd(490) in an environment dominated by CDOM. Similar failures occurred with the models using the Chl-a concentration (MAPE ~60%) as input. A semi-analytical approach showed the lowest error (MAPE = 41.04%) in the estimate of the inherent optical properties for complex waters in order to reduce the errors above. After retrieval of kd(490) using the semi-analytical model, seasonal patterns were observed, and high values of kd(490) were detected in the dry season possibly due to the increase of the concentration of the optically-significant substances (OSS).

Parameterization of an optical model to refine seagrass habitat requirements in an urbanized coastline

Accurate simulation of the underwater light climate is a requirement to understanding and predicting the ecological benefits from initiatives to manage land-derived inputs of nutrients and suspended solids to the coast. The goal of this work was to derive an empirical light model to be used in the water quality module of a coupled physical-biogeochemical model, developed to estimate habitat suitability for seagrasses in a metropolitan region (Adelaide, South Australia) affected by wastewater and stormwater discharges. Paired measurements of water quality and the vertical attenuation coefficient (Kd) obtained at six fixed sites between summer and winter were used in a multiple regression analysis to optimize local specific attenuation coefficients for coloured dissolved organic matter (CDOM) (2.623 cm m−1, measured as absorption at 254 nm), chlorophyll a (0.071 m2 mg−1), suspended solids (SS) < 63 μm (0.032 m2 g−1) and >63 μm (0.001 m2 g−1). The fitted relation explained 65% of the variability in Kd, with a standard error of 0.10 m-1. This error is small considering a mean Kd value of 0.35 m-1, varying between 0.12 and 0.89 m-1. Fine SS contributed on average 6 times more light attenuation (34%) than coarse SS (6%). CDOM was the second largest contributor to Kd (36%). The contribution of chlorophyll a to Kd (13%) was only marginally higher than the background imparted by seawater (12%). In shallow waters (3–5 m), Kd values were >0.4 m-1 and heavily influenced by sediment resuspension. In deeper waters (12–15 m), Kd values decreased and were dominated by CDOM. The light climate in the north of the study region was also heavily influenced by CDOM inputs, not only from point sources but also from mangroves and seagrass beds. This analysis suggests the control of fine SS as the most powerful management option to deter further nearshore seagrass losses in the region, and promote seagrass restoration.

Estimation of underwater visibility in coastal and inland waters using remote sensing data.

An optical method is developed to estimate water transparency (or underwater visibility) in terms of Secchi depth (Z sd ), which follows the remote sensing and contrast transmittance theory. The major factors governing the variation in Z sd , namely, turbidity and length attenuation coefficient (1/(c+K d ), c=beam attenuation coefficient; K d =diffuse attenuation coefficient at 531nm), are obtained based on band rationing techniques. It was found that the band ratio of remote sensing reflectance (expressed as (R rs (443)+R rs (490))/(R rs (555)+R rs (670)) contains essential information about the water column optical properties and thereby positively correlates to turbidity. The beam attenuation coefficient (c) at 531nm is obtained by a linear relationship with turbidity. To derive the vertical diffuse attenuation coefficient (K d ) at 531nm, K d (490) is estimated as a function of reflectance ratio (R rs (670)/R rs (490)), which provides the bio-optical link between chlorophyll concentration and K d (531). The present algorithm was applied to MODIS-Aqua images, and the results were evaluated by matchup comparisons between the remotely estimated Z sd and in situ Z sd in coastal waters off Point Calimere and its adjoining regions on the southeast coast of India. The results showed the pattern of increasing Z sd from shallow turbid waters to deep clear waters. The statistical evaluation of the results showed that the percent mean relative error between the MODIS-Aqua-derived Z sd and in situ Z sd values was within ±25%. A close agreement achieved in spatial contours of MODIS-Aqua-derived Z sd and in situ Z sd for the month of January 2014 and August 2013 promises the model capability to yield accurate estimates of Z sd in coastal, estuarine, and inland waters. The spatial contours have been included to provide the best data visualization of the measured, modeled (in situ), and satellite-derived Z sd products. The modeled and satellite-derived Z sd values were compared with measurement data which yielded RMSE=0.079, MRE=-0.016, and R 2 =0.95 for the modeled Z sd and RMSE=0.075, MRE=0.020, and R 2 =0.95 for the satellite-derived Z sd products.

Multiple extreme environmental conditions of intermittent soda pans in the Carpathian Basin (Central Europe).

Soda lakes and pans represent saline ecosystems with unique chemical composition, occurring on all continents. The purpose of this study was to identify and characterise the main environmental gradients and trophic state that prevail in the soda pans (n=84) of the Carpathian Basin in Central Europe. Underwater light conditions, dissolved organic matter, phosphorus and chlorophyll a were investigated in 84 pans during 2009-2010. Besides, water temperature was measured hourly with an automatic sensor throughout one year in a selected pan. The pans were very shallow (median depth: 15 cm), and their extremely high turbidity (Secchi depth median: 3 cm, min: 0.5 cm) was caused by high concentrations of inorganic suspended solids (median: 0.4 g L-1, max: 16 g L-1), which was the dominant (>50%) contributing factor to the vertical attenuation coefficient in 67 pans (80%). All pans were polyhumic (median DOC: 47 mg L-1), and total phosphorus concentration was also extremely high (median: 2 mg L-1, max: 32 mg L-1). The daily water temperature maximum (44 °C) and fluctuation maximum (28 °C) were extremely high during summertime. The combination of environmental boundaries: shallowness, daily water temperature fluctuation, intermittent hydroperiod, high turbidity, polyhumic organic carbon concentration, high alkalinity and hypertrophy represent a unique extreme aquatic ecosystem.

Underwater Optics in Sub-Antarctic and Antarctic Coastal Ecosystems

Understanding underwater optics in natural waters is essential in evaluating aquatic primary production and risk of UV exposure in aquatic habitats. Changing environmental conditions related with global climate change, which imply potential contrasting changes in underwater light climate further emphasize the need to gain insights into patterns related with underwater optics for more accurate future predictions. The present study evaluated penetration of solar radiation in six sub-Antarctic estuaries and fjords in Chilean North Patagonian region (39–44°S) and in an Antarctic bay (62°S). Based on vertical diffuse attenuation coefficients (Kd), derived from measurements with a submersible multichannel radiometer, average summer UV penetration depth (z1%) in these water bodies ranged 2–11 m for UV-B (313 nm), 4–27 m for UV-A (395 nm), and 7–30 m for PAR (euphotic zone). UV attenuation was strongest in the shallow Quempillén estuary, while Fildes Bay (Antarctica) exhibited the highest transparency. Optically non-homogeneous water layers and seasonal variation in transparency (lower in winter) characterized Comau Fjord and Puyuhuapi Channel. In general, multivariate analysis based on Kd values of UV and PAR wavelengths discriminated strongly Quempillén estuary and Puyuhuapi Channel from other study sites. Spatial (horizontal) variation within the estuary of Valdivia river reflected stronger attenuation in zones receiving river impact, while within Fildes Bay a lower spatial variation in water transparency could in general be related to closeness of glaciers, likely due to increased turbidity through ice-driven processes. Higher transparency and deeper UV-B penetration in proportion to UV-A/visible wavelengths observed in Fildes Bay suggests a higher risk for Antarctic ecosystems reflected by e.g. altered UV-B damage vs. photorepair under UV-A/PAR. Considering that damage repair processes often slow down under cool temperatures, adverse UV impact could be further exacerbated by cold temperatures in this location, together with episodes of ozone depletion. Overall, the results emphasize the marked spatial (horizontal and vertical) and temporal heterogeneity of optical characteristics, and challenges that these imply for estimations of underwater optics.