A novel approach was used with data from the Moderate Resolution Imaging Spectroradiometer (MODIS) to characterize the intense blooms of cyanobacteria (primarily Microcystis aeruginosa) in Taihu ...Lake, China's third largest freshwater lake. The approach involves first deriving a floating algae index (FAI) based on the medium‐resolution (250 and 500 m) MODIS reflectance data at 645, 859, and 1240 nm after correction of the ozone/gaseous absorption and Rayleigh scattering effects and then objectively determining the FAI threshold value (−0.004) to separate the bloom and nonbloom waters. By definition, the term “bloom” or “floating algae” refers to bloom where cyanobacteria form floating scums on the water surface. The 9 year MODIS time series data showed bloom characteristics (annual occurrence frequency, timing, and duration) between 2000 and 2008. Assuming 25% area coverage as a gauge for significance, significant bloom events rarely occurred between 2000 and 2004 for the entire lake (excluding East Bay) or several lake segments (Northwest Lake, Southwest Lake, and Central Lake). In most lake segments, the annual frequency of significant blooms increased from 2000–2004 to 2006–2008, when they started earlier and had a longer duration. The year 2007 showed unique bloom characteristics due to conditions highly favorable for bloom development and proliferation. The results suggest that the long‐term bloom patterns are driven by both nutrients and climatic factors. The multiyear series of consistent MODIS FAI data products provide baseline information to monitor the lake's bloom condition, one of the critical water quality indicators, on a weekly basis, as well as to evaluate its future water quality trends.
Spectral remote‐sensing reflectance (Rrs, sr−1) is the key for ocean color retrieval of water bio‐optical properties. Since Rrs from in situ and satellite systems are subject to errors or artifacts, ...assessment of the quality of Rrs data is critical. From a large collection of high quality in situ hyperspectral Rrs data sets, we developed a novel quality assurance (QA) system that can be used to objectively evaluate the quality of an individual Rrs spectrum. This QA scheme consists of a unique Rrs spectral reference and a score metric. The reference system includes Rrs spectra of 23 optical water types ranging from purple blue to yellow waters, with an upper and a lower bound defined for each water type. The scoring system is to compare any target Rrs spectrum with the reference and a score between 0 and 1 will be assigned to the target spectrum, with 1 for perfect Rrs spectrum and 0 for unusable Rrs spectrum. The effectiveness of this QA system is evaluated with both synthetic and in situ Rrs spectra and it is found to be robust. Further testing is performed with the NOMAD data set as well as with satellite Rrs over coastal and oceanic waters, where questionable or likely erroneous Rrs spectra are shown to be well identifiable with this QA system. Our results suggest that applications of this QA system to in situ data sets can improve the development and validation of bio‐optical algorithms and its application to ocean color satellite data can improve the short‐term and long‐term products by objectively excluding questionable Rrs data.
Key Points
A QA system is developed for spectral remote‐sensing reflectance
The system consists of a reference and a score metric
It is applicable to both remotely sensed and in situ ocean color data
The newly developed semi-analytical scheme (Lee et al., 2015a) for remote sensing of the Secchi disk depth (ZSD, m) was modified and applied to Landsat-8 data to obtain high-spatial-resolution map of ...water clarity. In order to implement the quasi-analytical algorithm (QAA) for the derivation of absorption and backscattering coefficients from Landsat-8 data, which are key optical properties for the estimation of ZSD, the representative wavelengths of Landsat-8 bands in the visible domain are verified; so are the absorption and backscattering coefficients of pure water for these bands. This semi-analytical scheme was then applied to a dataset having both in situ measurements of ZSD (~0.1–30m) and remote-sensing reflectance and found that the estimated ZSD from remote sensing matches measured ZSD very well (R2=0.96, average absolute percent difference ~17%, N=197). This scheme was further applied to a Landsat-8 image collected in an estuary to obtain high-spatial resolution ZSD map, and the obtained spatial distribution of ZSD is found quite consistent with in situ measurements and visual observations. These results indicate an important application of Landsat data — to provide reliable high-resolution water clarity product of bays, estuaries, and lakes with a unified mechanistic system.
•Semi-analytical algorithm to derive water IOPs from Landsat-8 is developed.•Semi-analytical scheme to estimate Secchi depth is modified for Landsat-8 data.•Secchi depth derived from Landsat-8 band setting is verified.•High-spatial resolution water clarity can be mechanistically produced from Landsat-8.
Various empirical relationships have been developed in the past nine decades to link the Secchi-disk depth (ZSD) with the diffuse attenuation coefficient (K
PAR), the euphotic zone depth (Zeu), and ...chlorophyll (Chl) concentration, where the latter two are important for the quantification and evaluation of photosynthesis in aquatic environments. There was also a classical theory regarding Secchi-disk sighting, but large gaps existed between the observations and model outcomes. These gaps have puzzled the ocean community for>60 yr and have resulted in contradictory conclusions regarding the interpretation and usefulness of Z
SD. Here, we compare these measurements with data simulated based on an innovative theory and model regarding Z
SD, and we found remarkable agreements between theoretical predictions and these century-long observations. The results not only resolve the long-standing puzzles associated with these observations, but also unify the relationships published in the literature. In particular, the ratio of Zeu to Z
SD is found to be ∼ 3.5 for all waters, which is ∼ 45% greater than the consensus value of ∼ 2.4 suggested in the past for clear waters. In addition, the new model validates an empirical relationship between Z
SD and Chl developed for global oceanic waters, thus providing strong support for using historical Z
SD data to study changes of phytoplankton in global oceans in the past century.
Secchi disk depth (ZSD) is a measure of water transparency, whose interpretation has wide applications from diver visibility to studies of climate change. This transparency has been explained in the ...past 60+ years with the underwater visibility theory, the branch of the general visibility theory for visual ranging in water. However, through a thorough review of the physical processes involved in visual ranging in water, we show that this theory may not exactly represent the sighting of a Secchi disk by a human eye. Further, we update the Law of Contrast Reduction, a key concept in visibility theory, and develop a new theoretical model to interpret ZSD. Unlike the classical model that relies strongly on the beam attenuation coefficient, the new model relies only on the diffuse attenuation coefficient at a wavelength corresponding to the maximum transparency for such interpretations. This model is subsequently validated using a large (N=338) dataset of independent measurements covering oceanic, coastal, and lake waters, with results showing excellent agreement (~18% average absolute difference, R2=0.96) between measured and theoretically predicted ZSD ranging from <1m to >30m without regional tuning of any model parameters. This study provides a more generalized view of visual ranging, and the mechanistic model is expected to significantly improve the current capacity in monitoring water transparency of the global aquatic environments via satellite remote sensing.
•Caveats in the century-old underwater visibility theory are discussed.•A new theory for underwater visibility is proposed.•A new mechanistic model for Secchi disk depth is established.•Results from the new model are verified with wide range of measurements.
We extended the quasi-analytical algorithm (QAA) architecture to analytically derive absorption coefficient of chromophoric dissolved organic matter (ag). Specifically, we used an empirical formula ...based on total absorption and particle backscattering coefficients to estimate and then remove detritus absorption coefficient (ad), and developed a scheme to use absorption coefficients at three wavelengths (412, 443, and 490nm) for the separation of ag and aph (absorption coefficient of phytoplankton). The algorithm was tested using an in situ data set collected in the South China Sea and the Taiwan Strait and a global in situ data set—the NASA Bio-Optical Marine Algorithm Data set (NOMAD). Our results indicated that this new analytical algorithm for retrieving ag performed reasonably well with a mean absolute percentage error of approximately 45% for ag(412), while it also presented a satisfactory performance for aph and ad in both coastal and oceanic waters. Furthermore, the applicability of this new algorithm for general oceanographic studies was briefly illustrated by applying it to MODIS measurements over the Taiwan Strait and the shelf region near the Mississippi River delta. Nevertheless, more independent tests with in situ and satellite data are needed to further validate and improve this innovative approach.
► The QAA extended to analytically derive absorption coefficient of CDOM (ag). ► Three wavelengths (412, 443, and 490nm) used for component separation. ► Assessed using two regional and global scale in situ datasets. ► A mean absolute percentage error of ~45% for ag(412) found. ► Applied to MODIS measurements over the Taiwan Strait and the Mississippi River delta.
Himawari-8 (H8) is a geostationary meteorological satellite launched by JAXA (Japan Aerospace Exploration Agency) and is now operated by JMA (Japan Meteorological Agency). It takes measurements at a ...temporal resolution of 10 min for full disk view. Although designed as a meteorological satellite, the Advanced Himawari Imager (AHI) onboard H8 has three visible (460, 510 and 640 nm), one near infrared (860 nm) and two shortwave infrared bands (1610 and 2257 nm) to observe the Earth system. In this study, the Floating Algae Index (FAI) developed for ocean color satellites (Hu, 2009) is adapted to process AHI data for the first time and applied for waters of Lake Taihu, China. For a total of 18 near-cloud-free images, a correlation coefficient (r) of 0.92 was obtained between the algae area derived from AHI FAI and that from the Moderate Resolution Imaging Spectroradiometer (MODIS) FAI, and the mean percentage difference is ~5% in algae coverage. More concurrent images (n = 80) were collected for a comparison between AHI and Geostationary Ocean Color Imager (GOCI), as GOCI is also a geostationary satellite, resulting in a correlation coefficient of 0.91 and percentage deviation of ~8% in observed algae coverage. These results indicate that H8/AHI can obtain reliable observations of floating algae at ultrahigh temporal resolutions (10 min). Especially, such ultrahigh-frequency measurements show that part of Lake Taihu (e.g., Meiliang Bay) experienced more frequent events of floating algae (mostly >60%) than that observed by GOCI (generally <40%). These results indicate that ultrahigh-frequency measurements are important not only for efficient environmental monitoring but also for the scientific understanding of algae dynamics.
•Distribution of floating algae in Taihu Lake is derived from H8-AHI.•Algal patch distribution from AHI is consistent with that of concurrent MODIS and GOCI.•Results show importance of high-frequency sampling on environment monitoring/study.
The spectral absorption coefficient of pure seawater (a
(λ)) in published studies differ significantly in the blue domain, yet the impacts of such discrepancies on the inherent optical properties ...(IOPs) derived from ocean color have been scarcely documented. In this study, we confirm that changes in a
(λ) may have significant impacts on retrieved IOPs in oligotrophic waters, especially for the phytoplankton absorption coefficient (a
(λ)). Two sets of a
(λ) data, a
_PF97 (Appl. Opt. 36, 8710, 1997) and a
_Lee15 (Appl. Opt. 54, 546, 2015), were selected for optical inversion analysis. It is found that a
(λ) retrieved with a
_Lee15 agree better with the in-situ measurements in oligotrophic waters. Further applications to satellite images show that the derived a
(λ) using a
_Lee15 can be up to 238% higher than the retrievals using a
_PF97 in the core zone of the subtropical ocean gyres. Given that a
_PF97 is commonly accepted as the "standard" a
(λ) by the ocean color community in the past decades, this study highlights the need and importance to update a
(λ) with a
_Lee15 for IOPs retrievals in oligotrophic waters.
Secchi disk depth (ZSD) and Forel-Ule index (FUI) are the two oldest and easiest measurements of water optical properties based on visual determination. With an overarching objective to obtain water ...inherent optical properties (IOPs) using these historical measurements, this study presents a model for associating remote-sensing reflectance (Rrs) with FUI and ZSD. Based upon this, a scheme (FZ2ab) for converting FUI and ZSD to absorption (a) and backscattering coefficients (bb) is developed and evaluated. For a data set from HydroLight simulations, the difference is <11% between FZ2ab-derived a and known a, and <28% between FZ2ab-derived bb and known bb. Further, for a data set from field measurements, the difference is < 30% between FZ2ab-derived a and measured a. These results indicate that FZ2ab can bridge the gap between historical measurements and the focus of IOP measurements in modern marine optics, and potentially extend our knowledge on the bio-optical properties of global seas to the past century through the historical measurements of FUI and ZSD.
Euphotic zone depth (Z
eu) products from ocean color measurements are now produced from MODIS ocean color measurements, one of which is based on inherent optical properties (IOP-approach) and the ...other is based on chlorophyll-a concentration (Chl-approach). For the first time, the quality of these satellite Z
eu products is assessed with extensive field-measured Z
eu (in the China Sea), where 78% of the measurements were made on the continental shelf (≤
200
m). For the data with matching location and time window, we have found that the overall average difference (ε) between satellite and
in situ Z
eu is 21.8% (n
=
218, Z
eu ranges from 4 to 93
m) with a root mean square error in log scale (RMSE) of 0.118 by the IOP-approach, while it is 49.9% (RMSE
=
0.205) by the Chl-approach. These results suggest that 1) MODIS Z
eu products for waters in the China Sea are robust, even in shelf waters; and 2) Z
eu produced with IOPs are more reliable than those produced with empirically derived Chl. Spatial and seasonal variations of Z
eu in the China Sea are briefly described with Z
eu products generated by the IOP-approach. These results will facilitate further research on carbon cycling and environmental changes on both local and global scales.
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