Remotely sensed imagery is proving to be a useful tool in estimating water depths in coastal zones. On the other hand, many coastal zone studies in the southern part of the Caspian Sea are only ...concerned with areas of shallow water and would benefit from easily updated bathymetric estimates. In this study, we tested three different methods for extracting bathymetry information from Landsat 5 data in the southeastern Caspian Sea, Iran. The first method used was a single band algorithm (SBA), utilizing either blue or red bands. The second method was principal components analysis (PCA), and the third method was the multi-layer perceptron (back propagation) neural network between visible bands and one output neuron (bathymetry). This latter MLP-ANNs method produced the best depth estimates (r = 0.94). The single band algorithm utilizing a red band also produced reasonably accurate results (r = 0.66), while the blue band algorithm and PCA did not perform (correlation between the estimated and measured depths 0.49 and 0.21, respectively). Furthermore, the shallow waters have negative influences on the accuracy of bathymetric modeling, thus the correction of data in these shallow waters is challenged by the presence of continental aerosols, bottom reflectance, and adjacency of land.
Technical advancements have widened the limits of remote sensing in mapping shallow water benthic habitats and bathymetry over the last decades. On the other hand, the needs of shallow water remote ...sensing have pushed instrument development. In this manuscript we provide 50-year retrospective of the developments in the field in terms of both instrumentation and methods. We also show that spectral features characteristic of the main benthic groups in shallow water are consistent from the tropics to sub-arctic regions and from salty to freshwaters. The fundamental limiting factor in both benthic mapping and bathymetry is absorption of light by water molecules. However, spectral absorption by water molecules is the key to bathymetry derivation. Variable backscattering by particles and absorption by dissolved organic matter is a confounding factor for all objectives. The combination of using the spectral and textural characteristics of bottom features and our knowledge about these features have now resulted in the ability to map habitats over large coastal systems. This manuscript has shown that optically shallow water remote sensing has reached levels where the satellite derived bathymetry and habitat maps are accepted by different end users (including the International Maritime Organisation) and are routinely used in ecological studies, monitoring and management of coastal environments.
•Historic review of 50 years development in shallow water remote sensing•Reflectance of broad habitat types are consistent over different aquatic environments.•SDB and habitat mapping are reaching maturity to be used by different end users.
We examined and parameterized chlorophyll-specific phytoplankton absorption coefficients (a*ph(λ)) for three turbid productive Estonian lakes on the basis of bio-optical measurements in 2005-2013. A ...new model parameterization was created that enables to reconstruct the spectra of a*ph(λ) for turbid productive waters with the higher reliability than previous parameterizations for ocean and coastal waters. The coefficients A(λ) and B(λ) of our model differ from those found in seas, coastal waters and other types of lakes. For any water type separately the increase of total chlorophyll concentration accompanied with the decrease of a*ph. Our results showed significant seasonal differences between the model parameters due to diversity of the phytoplankton assemblages. This suggests that season-specific models should be developed and validated. Improving the modelling of chlorophyll-specific phytoplankton absorption spectra for hypertrophic lakes is still pending on the availability of a larger dataset, which includes simultaneous measurements of chlorophyll concentrations, phytoplankton absorption coefficients and phytoplankton species composition. Our results implied that total chlorophyll concentration is not a universal predictor of the magnitude of chlorophyll-specific phytoplankton absorption coefficient. The aph(λ) models are also likely site and season dependent. Further research is needed for quantifying the role of accessory pigments and other optical constituents as well as the cell size of dominant algal species for considering their influence on the modelling outputs.
Given the importance of colored dissolved organic matter (CDOM) for the structure and function of lake ecosystems, a method that could estimate the amount of CDOM in lake waters over large geographic ...areas would be highly desirable. Satellite remote sensing has the potential to resolve this problem. We carried out model simulations to evaluate the suitability of different satellite sensors (Landsat, IKONOS, and the Advanced land Imager ALI) to map the amount of CDOM in concentration ranges that occur in boreal lakes of the Nordic countries. The results showed that the 8-bit radiometric resolution of Landsat 7 is not adequate when absorption by CDOM at 420 nm is higher than 3${\rm m}^{-1}$. On the other hand, the 16-bit radiometric resolution of ALI, a prototype of the next generation of Landsat, is suitable for mapping CDOM in a wider range of concentrations. An ALI image of southern Finland was acquired on 14, July 2002 and in situ measurements were carried out in 15 lakes (18 stations). The results showed that there is a high correlation$(R^{2}=0.84)$between the 565 nm/660 nm ALI band ratio and the CDOM absorption coefficient in lakes. Analysis of 245 lakes in the acquired satellite image showed a normal distribution of CDOM concentration among the lakes. However, the size distribution of lakes was highly skewed toward small lakes, resulting in the CDOM concentration per unit lake area being skewed toward high values. We showed that remote sensing enables synoptic monitoring of the CDOM concentration in a large number of lakes and thus enables scaling up to the level of large ecosystems and biomes.
Potentially toxic cyanobacterial blooms occur in the Baltic Sea in the middle of summer. Aggregations of cyanobacteria often form dense subsurface blooms or even surface scum, which are spatially ...extremely patchy
. It has been shown by using bio-optical modelling together with the hyperspectral sensor Hyperion image from the western part of the Gulf of Finland that chlorophyll concentration may vary between one and hundreds of mg m
−
3
within the 10-m scale during heavy bloom. The SeaWiFS, MODIS/Aqua and MERIS products over heavy bloom region are compared with each other, the modelling results, with the bio-optical modelling data and the available in situ data. In the early stage of bloom, these sensors give consistent results, which are in the same range as in situ data. During the heavy bloom of cyanobacteria, at the time of surface scum formation, results from different sensors are very variable, the values of chlorophyll concentration may exceed the standard processing limits and atmospheric correction often fails. The good capability of the MERIS and MODIS fine resolution bands to detect cyanobacterial bloom quantitatively is shown.
The importance of lakes and reservoirs leads to the high need for monitoring lake water quality both at local and global scales. The aim of the study was to test suitability of Sentinel-2 ...Multispectral Imager's (MSI) data for mapping different lake water quality parameters. In situ data of chlorophyll a (Chl a), water color, colored dissolved organic matter (CDOM) and dissolved organic carbon (DOC) from nine small and two large lakes were compared with band ratio algorithms derived from Sentinel-2 Level-1C and atmospherically corrected (Sen2cor) Level-2A images. The height of the 705 nm peak was used for estimating Chl a. The suitability of the commonly used green to red band ratio was tested for estimating the CDOM, DOC and water color. Concurrent reflectance measurements were not available. Therefore, we were not able to validate the performance of Sen2cor atmospheric correction available in the Sentinel-2 Toolbox. The shape and magnitude of water reflectance were consistent with our field measurements from previous years. However, the atmospheric correction reduced the correlation between the band ratio algorithms and water quality parameters indicating the need in better atmospheric correction. We were able to show that there is good correlation between band ratio algorithms calculated from Sentinel-2 MSI data and lake water parameters like Chl a (R2 = 0.83), CDOM (R2 = 0.72) and DOC (R2 = 0.92) concentrations as well as water color (R2 = 0.52). The in situ dataset was limited in number, but covered a reasonably wide range of optical water properties. These preliminary results allow us to assume that Sentinel-2 will be a valuable tool for lake monitoring and research, especially taking into account that the data will be available routinely for many years, the imagery will be frequent, and free of charge.
Monitoring and understanding the physical, chemical and biological status of global inland waters are immensely important to scientists and policy makers alike. Whereas conventional monitoring ...approaches tend to be limited in terms of spatial coverage and temporal frequency, remote sensing has the potential to provide an invaluable complementary source of data at local to global scales. Furthermore, as sensors, methodologies, data availability and the network of researchers and engaged stakeholders in this field develop, increasingly widespread use of remote sensing for operational monitoring of inland waters can be envisaged. This special issue on Remote Sensing of Inland Waters comprises 16 articles on freshwater ecosystems around the world ranging from lakes and reservoirs to river systems using optical data from a range of in situ instruments as well as airborne and satellite platforms. The papers variably focus on the retrieval of in-water optical and biogeochemical parameters as well as information on the biophysical properties of shoreline and benthic vegetation. Methodological advances include refined approaches to adjacency correction, inversion-based retrieval models and in situ inherent optical property measurements in highly turbid waters. Remote sensing data are used to evaluate models and theories of environmental drivers of change in a number of different aquatic ecosystems. The range of contributions to the special issue highlights not only the sophistication of methods and the diversity of applications currently being developed, but also the growing international community active in this field. In this introductory paper we briefly highlight the progress that the community has made over recent decades as well as the challenges that remain. It is argued that the operational use of remote sensing for inland water monitoring is a realistic ambition if we can continue to build on these recent achievements.
•Challenges, progress and issues motivating the special issue are outlined.•Special issue contributions are summarized.•Highlights from state-of-the-art in science, community coordination, future outlook
Eutrophication of inland waters is considered a serious global environmental problem. Satellite remote sensing (RS) has been established as an important source of information to determine the trophic ...state of inland waters through the retrieval of optically active water quality parameters such as chlorophyll-a (Chl-a). However, the use of RS techniques for assessment of the trophic state of inland waters on a global scale is hindered by the performance of retrieval algorithms over highly dynamic and complex optical properties that characterize many of these systems. In this study, we developed a new RS approach to assess the trophic state of global inland water bodies based on Moderate Resolution Imaging Spectroradiometer (MODIS) imagery and the Forel-Ule index (FUI). First, the FUI was calculated from MODIS data by dividing natural water colour into 21 indices from dark blue to yellowish-brown. Then the relationship between FUI and the trophic state index (TSI) was established based on in-situ measurements and MODIS products. The water-leaving reflectance at 645 nm band was employed to distinguish coloured dissolved organic matter (CDOM)-dominated systems in the FUI-based trophic state assessment. Based on the analysis, the FUI-based trophic state assessment method was developed and applied to assess the trophic states of 2058 large inland water bodies (surface area >25 km2) distributed around the world using MODIS data from the austral and boreal summers of 2012. Our results showed that FUI can be retrieved from MODIS with a considerable accuracy (92.5%, R2 = 0.92) by comparing with concurrent in situ measurements over a wide range of lakes, and the overall accuracy of the FUI-based trophic state assessment method is 80.0% (R2 = 0.75) validated by an independent dataset. Of the global large water bodies considered, oligotrophic large lakes were found to be concentrated in plateau regions in central Asia and southern South America, while eutrophic large lakes were concentrated in central Africa, eastern Asia, and mid-northern and southeast North America.
•Present a trophic state assessment method for global large inland waters based on MODIS.•Produce the trophic state map of global large inland water bodies.•Analyze the spatial distribution of the trophic states of global large inland water bodies.
The Sentinel-2A and 2B Multi-Spectral Instrument (MSI) offers a specification of potential value toward a number of objectives in remote sensing of coral reefs. Coral reefs represent a unique ...challenge for remote sensing, being highly heterogeneous at metre scales and occurring at variable depths and water clarity regimes. However, conservation initiatives, such as the United Nations Sustainable Development Goals, add urgency to the need for the large scale environmental monitoring information that remote sensing can provide. In the quest to meet this challenge a range of satellite instruments have been leveraged, from Landsat to high spatial resolution sensors such as WorldView-2, toward objectives such as: mapping of bottom types, bathymetry, change detection, and detection of coral bleaching events. Sentinel-2A and 2B offer a new paradigm of available instruments, with a 5-day revisit, 10 m multispectral spatial resolution and freely available data. Pre-launch simulation analyses by several of the authors suggested Sentinel-2 would have good performance for reef applications, in this paper we follow up on this study by reviewing the potential based on the substantial archive of actual data now available.
First we determine to what extent the World's reefs are covered by Sentinel-2, since the mission requirements do not by default include all reefs. Secondly we review how a 5-day revisit translates to a usable acquisition rate of clear images, given that cloud and surface glint are common confounding factors. The usable acquisition rate is the real determinant of the objectives to which the data can be applied. Finally we apply current processing algorithms to Sentinel-2 data of several sites over the Great Barrier Reef, including physics-based bathymetry inversion and object-orientated benthic mapping. Landsat 8 OLI is most comparable current sensor to Sentinel-2 MSI, so direct comparisons and the possibilities for data synthesis are explored.
Our findings confirm that Sentinel-2 has excellent performance for meeting several essential coral reef scientific and monitoring objectives. Taking into account cloud and sun glint, the usable acquisition rate for a large proportion of reefs is likely to be around 20 clear images a year on average, giving a new potential for evaluation of short time-scale disturbances and impacts. The spatial resolution of 10 m is a key threshold for delineating benthic features of interest such as coral structures, and there is evidence from image and field data that bleaching is detectable. Radiometrically Sentinel-2 data can support good results in physics-based methods, such as bathymetric mapping, comparable to Landsat 8 and WorldView-2. In addition the large scale acquisition area, provided by the 290 km wide swath, offers advantages over high spatial resolution imagery for mapping at multi-reef scales.
Sentinel-2 data can be immediately leveraged with existing methods, to provide a new level of reef monitoring information compared to that previously available by remote sensing. Combined with Landsat 8 and the historical Landsat archive, the data collected today will be invaluable for decades or even centuries to come. In this context, the main downside of the Sentinel-2 mission is that approximately 12% of the World's reefs currently lie outside the acquisition plan and are not imaged. Surprisingly, for a European initiative, coral reefs in European governed territories are among the worst served globally. These omissions, approximately only 1/200th of the currently imaged area, limit the global scope which otherwise would be one of Sentinel-2's greatest strengths.
•Despite being a land mission Sentinel-2 images nearly 90% of the World's reefs.•Sentinel-2 provides excellent results for bathymetry and benthic mapping.•Sentinel-2 has advantages over Landsat 8 and high spatial resolution sensors.•Coral bleaching is visible in Sentinel-2 images.•We recommend an acquisition plan review with the aim to cover all reefs globally.
PDF
