Normalized Fluorescence Line Height Research Articles

First Light Demonstration of Red Solar Induced Fluorescence for Harmful Algal Bloom Monitoring

AbstractThe risks harmful algal blooms (HAB) pose to aquatic ecosystems, public health, and coastal economies necessitate supplementation of current observation strategies. Herein, we explore the use of red solar induced fluorescence (SIF) from the TROPOspheric Monitoring Instrument (TROPOMI), a remote sensing measurement retrievable in variable cloud conditions, for Karenia brevis detection. Along the West Florida Shelf from 2018 to 2020, we compare red SIF with normalized fluorescence line height (nFLH) from MODIS‐Aqua, a standard remote sensing HAB indicator limited to clear sky days. A strong positive linear relationship is found between nFLH and red SIF during severe bloom conditions in 2018 (N = 33,376, r‐value = 0.79, R2 = 0.63) permitting direct comparison. Red SIF provided nearly double the amount of spatiotemporal fluorescence information than nFLH. This work presents the first application of TROPOMI's red SIF for HAB monitoring and illuminates an approach for bolstering early warning systems for HABs beyond clear sky conditions.

Synoptic-to-intraseasonal atmospheric modulation of phytoplankton biomass in the inner sea of Chiloé, Northwest Patagonia (42.5°-43.5°S, 72.5°-74°W), Chile

The Inner Sea of Chiloé (ISC) in northwestern Patagonia has experienced large harmful algal blooms in the past decade, impacting human health and affecting the large aquaculture industry of the region. Thus, the investigation of factors favouring regional phytoplankton growth are of particular interest. Analysing the synoptic-to-intraseasonal variability, we explore changes in phytoplankton biomass in southern ISC (S-ISC, 42.5°-43.5°S, 72.5°-74°W) and their concurrent mesoscale and large-scale meteorological and oceanographic conditions. We use high-resolution satellite normalized fluorescence line height (nFLH) and chlorophyll-a (CHL-A) from the MODIS-Aqua sensor as proxies for phytoplankton biomass, besides oceanic and atmospheric variables derived from various remote-sensing sources and atmospheric fields from the ERA5 reanalysis. Specifically, we focus on high phytoplankton biomass events HBEs, which are defined as those cases when intraseasonal nFLH anomaly (nFLH’) exceeds the 95th percentile threshold. Each event was characterised by its first date of occurrence (called day 0). We detected 16 HBE between 2003 and 2019 in S-ISC. HBEs tend to occur under the influence of a mid-latitude migratory anticyclone that induce persistent cloudless conditions preceding day 0, leading to enhanced photosynthetically active radiation (PAR) starting around day -8, and positive sea surface temperature (SST) anomalies between days -4 and +4. We hypothesise that HBEs are mainly modulated by i) mixing and advection that could contribute to a greater availability of nutrients in the upper sea layers before the onset of the anticyclonic anomalies; and ii) increased thermal stratification related to positive PAR and SST anomalies that would promote phytoplankton growth during the anticyclonic regime. Furthermore, we show that the Madden-Julian Oscillation modulates the frequency of nFLH’ and thus of HBEs, a result that suggests an enhanced predictability of these cases.

HABs Karenia brevis and Pseudo-nitzschia pre- and post-Hurricane Michael.

Increased occurrences of harmful algal blooms (HAB) in the Gulf of Mexico, and even worldwide, yield concern for increases in brevetoxin exposure leading to respiratory illness or even death, highlighting the need for extensive scientific research and human health monitoring. It is known that major events such as tropical storms and hurricanes are followed by periods of increased red tides caused by HABs; however, the nature by which phytoplankton blooms proliferate following major events remains a topic of great interest and research. The impact of Hurricane Michael on October 10, 2018 on HABs in the Florida panhandle was examined by analyzing data from the Florida Fish and Wildlife Conservation Commission in coordination with Normalized Fluorescence Line Height (nFLH) data from the University of South Florida College of Marine Science. Results presented here demonstrate four phases of HABs during storm events: 1. Pre-storm concentrations, 2. Decreased concentration during the storm, 3. Elevated concentrations following the storm and 4. Recovery period. This time frame can serve to be important in understanding the health dynamics of coastal systems following major storm events.

Retrieval of Red Solar-Induced Chlorophyll Fluorescence With TROPOMI on the Sentinel-5 Precursor Mission

Solar-Induced chlorophyll Fluorescence (SIF) retrieved from spaceborne hyperspectral measurements can provide a synoptic perspective on the functioning of photosynthetic active vegetation and algae. The TROPOspheric Monitoring Instrument (TROPOMI) onboard the Sentinel-5 precursor mission provides data for the retrieval of SIF with an unprecedented resolution in the spatial and temporal dimensions. In this study, we propose an approach for the retrieval of the red SIF peak value (at around 685 nm) over both land and ocean from TROPOMI measurements. This approach exploits the solar Fraunhofer lines located in the spectral window of 669–686 nm to disentangle SIF from the solar radiation reflected by the surface–atmosphere system. First, through simulated TROPOMI-like datasets, we determine the proper parameter settings and demonstrate the feasibility of the approach for the red SIF retrieval from TROPOMI data. Then, the approach is applied to real TROPOMI measurements. The red SIF retrieved with the proposed approach for two months’ TROPOMI data is compared with another TROPOMI red SIF dataset and the marine normalized fluorescence line height (nFLH) product of the Aqua Moderate Resolution Imaging Spectroradiometer (MODIS). The comparisons display similar spatial distributions over both land and ocean for the weekly global composites. Especially, the comparison of the two red SIF datasets demonstrates good agreement overall, indicating consistency of the two retrieval approaches. The retrieval uncertainty for the weekly global composite is about 13% and 10% of the peak SIF value over land/ocean, respectively, which can be considered as satisfactory error thresholds for global SIF observations.

Global Retrievals of Solar‐Induced Chlorophyll Fluorescence at Red Wavelengths With TROPOMI

AbstractObservations of solar‐induced chlorophyll a fluorescence (SIF) from spaceborne spectrometers can advance our understanding of terrestrial and aquatic carbon cycles. Here we present the first global retrievals of SIF at red wavelengths from the TROPOspheric Monitoring Instrument (TROPOMI). Despite the weak signal level, considerable uncertainties, and subtle measurement artifacts, spatial patterns and magnitudes agree with independent data sets. Over land, spatial patterns of our red SIF estimates covary with the far‐red SIF data. Red SIF over the ocean is highly consistent with the normalized fluorescence line height (nFLH) inferred from measurements of the MODerate resolution Imaging Spectroradiometer (MODIS), even when comparing single days and fine spatial scales. Major advantages of our Fraunhofer line‐based SIF retrievals include the capability to sense SIF through optically thin cloud/aerosol layers and an insensitivity to ocean color. This opens up new avenues for studying ocean biogeochemistry from space.

Chlorophyll-a Concentration Assessment Using Remotely Sensed Data over Multiple Years along the Coasts of the United Arab Emirates

The United Arab Emirates (UAE) recently witnessed algal/phytoplankton blooms attributed to the high concentrations of Chlorophyll-a associated with the spread and accumulation of a wide range of organisms with toxic effects that influence ecological and fishing economic activities and water desalination along coastal areas. This research explores the UAE coasts as a case study for the framework presented here. In this research, we argue that advances in satellite remote sensing and imaging of spatial and temporal data offer sufficient information to find the best-fit regression method and relationship between Chlorophyll-a concentration and a set of climatic and biological explanatory variables over time. Three functional forms of regression models were tested and analysed to reveal that the Log-Linear Model found to be the best fit providing the most statistically robust model compared to the Linear and the Generalised Least Square models. Besides, it is useful to identify the factors Sea Surface temperature, Calcite Concentration, Instantaneous Photosynthetically Available Radiation, Normalized Fluorescence Line Height, and Wind Speed that significantly influence Chlorophyll-a concentration. Research results can be beneficial to aid decision-makers in building a best-fit statistical system and models of algal blooms in the study area. The study found results to be sensitive to the study’s temporal time-period length and the explanatory variables selected for the analysis.

Potential influence of the Deepwater Horizon oil spill on phytoplankton primary productivity in the northern Gulf of Mexico

Nine years after the Deepwater Horizon (DwH) oil spill (20 April–15 July 2010), the recovery of primary productivity at the ocean surface remains to be investigated. Here, we used the normalized fluorescence line height (nFLH) from the Moderate Resolution Imaging Spectroradiometer as an indicator of chlorophyll a concentration (Chl a). First, from the spatiotemporal variations of nFLH between 2001 and 2017, a reduction of nFLH after the DwH oil spill was observed (for a relatively long period, from 2011 to 2014). Second, a stepwise multiple regression model was used to examine which of the following environmental factors could explain the annual variations in nFLH: river discharge, total nitrogen load, total phosphorus load, photosynthetically available radiation, sea surface temperature and wind speed. Results show that river discharge, sea surface temperature and wind speed are the primary factors that regulated the annual nFLH variations in the DwH area during the pre-spill years. In contrast, this same model could not explain the reduction of nFLH for the four years after the DwH oil spill. After 2015, nFLH appears to have resumed to the pre-spill concentrations. Here we suggest that the nFLH reduction between 2011 and 2014 could have originated from the DwH oil spill, although the exact mechanism is yet to be determined.

Impacts of the decreased freeze-up period on primary production in Qinghai Lake

Although previous research has focused on the inundation changes in Qinghai Lake, the largest lake in China, few studies have investigated the variations in primary production and correlated these changes with environmental transitions. In this study, this knowledge gap was filled using multiple ocean color satellite missions between 2003 and 2017. The results indicated a substantial increase in phytoplankton growth over recent years, during which the normalized fluorescence line height (nFLH) and algal bloom index (ABI) increased by approximately 45% and 61%, respectively, from the first (2003–2012) to the second period (2013–2017). Such a remarkable increase is likely associated with a rapid decrease in the duration of the freeze-up period, for which the 2014–2017 mean was >2 standard deviations below that of the previous years. High temperatures and a large number of sunshine hours could possibly explain the elevated nFLH and ABI in 2013. A multiple general linear model revealed that the freeze-up period, number of sunshine hours, and temperature explained 76.1%, 5.6%, and 10.2%, respectively, of the long-term changes in primary production in Qinghai Lake during the observed period. This study not only provides the first comprehensive analysis of the biogeochemical properties of Qinghai Lake but also demonstrates the capability of multiple remote sensing products in addressing environmental problems. Further, the method here is easily extendable to similar water bodies worldwide to study their potential responses to climate variability.

Evaluating the seasonal and decadal performance of red band difference algorithms for chlorophyll in an optically complex estuary with winter and summer blooms

Standard ocean color chlorophyll algorithms, including normalized fluorescence line height algorithms, overestimate summer blooms and often miss large winter blooms in mid-latitude estuaries such as Long Island Sound (LIS). Thus, they are unable to accurately determine year-round chlorophyll concentrations. Here, we show that winter phytoplankton blooms cannot be retrieved using MODIS' 667 and 678 nm bands. Although MERIS' equivalent 665 and 681 nm bands are able to better characterize chlorophyll a concentrations at LIS compared to MODIS, we show that it is a red band difference (RBD) algorithm relying on observing the maximum spectral difference between Rrs681nm-Rrs665nm and Rrs709nm-Rrs665nm from the MERIS sensor that is able to successfully capture the seasonal chlorophyll dynamics measured in situ. Such switching algorithms are useful in areas where the shape of the reflectance spectrum changes throughout the seasons associated with changes in the phytoplankton community structure, and provide seamless implementation compared to algorithms based on seasonal tunings. We apply the switching RBD algorithm to the 10 years of available MERIS data, and results reveal important spatial and temporal patterns that cannot be assessed from in situ data alone. These include the observation that the decreasing trend in surface chlorophyll concentrations between 2002 and 2012 is localized, and may be related to changes in the flow of Atlantic waters into the estuary rather than a result of active management against eutrophication. Following from past studies, we demonstrate how high sensitivity red bands are valuable to successfully determine proxies for phytoplankton biomass in coastal areas where optical properties are dominated by dissolved organic matter, and where the fluorescence peak may not accurately capture the dynamics of productive waters. Such switching RBD algorithms applied to data from the Sentinel or PACE missions may allow distribution of near-real time chlorophyll a products for coastal waters across winter and summer seasons.

MULTI-TEMPORAL MODIS FOR DETECTION AND PUBLISHED LITERATURES FOR VALIDATION OF PHYTOPLANKTON BLOOMS IN SABAH AND SARAWAK, MALAYSIA

The coastal region of Sabah, Malaysia is mostly affected by harmful algal blooms (HABs) that often cause massive fish kills, and sometimes human casualties. Lack of a well-agreed, transparent and reproducible method, aperiodic nature and limited (technical) ability to monitor HAB at large regional scale, have all led to reinforced methodological improvement for bloom prediction, scientific management of coastal water resources, and regulatory attention. MODerate Resolution Imaging Spectroradiometer (MODIS), one of the well validated ocean colour sensors, enables acquiring high spectral resolution images, with short revisit time, freely accessible, and bay-wide coverage. Yet, the relative efficiency of MODIS-derived Chl-a (Chlorophyll-a), ABI (Algal Bloom Index), and nFLH (normalized Fluorescence Line Height) have not been compared in coastal regions of Malaysia. Fifteen MODIS Level 2 images acquired between 2005 and 2013 were considered as time series data that matched HAB events mentioned in previous studies. As historical real time in-situ data collection is often difficult (inaccessible), and thus unavailable; this project had to validate results obtained from literature, assuming that in-situ, would indicate HAB location at least during MODIS acquisition dates. Variations of HAB affected areas with temporal and spatial scales derived from bloom indices are shown in colour maps. Reliability of bloom information was measured by subjectively comparing HAB results provided by indices, and previously published in-situ results. ABI outperformed Chl-a and nFLH indices based on comparisons in both normal and HAB conditions occurring in the coastal waters of Sabah and Sarawak. The configuration and reliability retrieved from MODIS-ABI allowed their application in different likely tropical region as automated HAB monitoring systems and coastal water management programmes.

First assessment of MODIS satellite ocean color products (OC3 and nFLH) in the Inner Sea of Chiloe, northern Patagonia

The use of remote sensing has allowed enormous progress in our understanding of biophysical processes worldwide. Despite their importance, the use of satellite bio-optical products is still limited due to optical complexity. In this study we assess the performance of the Moderate Resolution Imaging Spectroradiometer algorithm for chlorophyll-a (MODIS-OC3) and of normalized Fluorescence Line Height (nFLH), in inner waters of northern Chilean Patagonia (41°-45oS). Satellite data were evaluated using a compilation of in situ chlorophyll-a data collected by the CIMAR-FIORDOS program during 2003-2012. During austral spring MODIS-OC3 showed a significant relationship with in situ measurements (R2 = 0.2, P = 0.03, F = 5.33) in comparison with winter when the relationship was non-significant. In contrast, nFLH explained a significant fraction of observed variance of Chl-a during austral winter (R2 = 0.54, P = <0.01, F = 10.68) and this relationship was not significant in spring. Our preliminary results offer an excellent chance to understand patterns of variability of the autotrophic biomass and physiological status in optically complex interior marine ecosystems.

Characterization of Karenia brevis blooms on the West Florida Shelf using ocean color satellite imagery: implications for bloom maintenance and evolution

Satellite ocean color remote sensing techniques, coupled with in situ data, were used to examine the spatial extent and evolution of four Karenia brevis blooms on the West Florida Shelf (WFS) in 2004, 2005, 2006, and 2011. Observations were obtained with the moderate resolution imaging spectroradiometer (MODIS-Aqua). These four blooms were delineated by combining remote-sensing reflectance at 555 nm and normalized fluorescence line height. In 2004 and 2005, the WFS was affected by several hurricanes, including the category 5 storm Hurricane Katrina. These hurricanes led to increased river discharge and vertical mixing which favored bloom intensification and dispersion. No hurricanes passed over the WSF in 2006; however, storms in south Florida may have aided bloom intensification via increased river discharge. In 2011, a bloom appeared off Venice, Florida, where several small creeks discharge. The bloom moved south toward Charlotte Harbor where it intensified and lingered for several months as it received nutrients from riverine discharge and upwelling events. While it is difficult to identify initiation stages of a K. brevis bloom (<∼50,000 cells L−1) using satellite imagery, the techniques used here provide information about bloom evolution (size, duration, and advection) and insight into factors affecting bloom dynamics.

Modified MODIS fluorescence line height data product to improve image interpretation for red tide monitoring in the eastern Gulf of Mexico

Several satellite-based methods have been used to detect and trace Karenia brevis red tide blooms in the eastern Gulf of Mexico. Some require data statistics and multiple data products while others use a single data product. Of these, the MODIS normalized fluorescence line height (nFLH) has shown its advantage of detecting blooms in waters rich in colored dissolved organic matter, thus having been used routinely to assess bloom conditions by the Florida Fish and Wildlife Conservation Commission (FWC), the official state agency of Florida responsible for red tide monitoring and mitigation. However, elevated sediment concentrations in the water column due to wind storms can also result in high nFLH values, leading to false-positive bloom interpretation. Here, a modified nFLH data product is developed to minimize such impacts through empirical adjustments of the nFLH values using MODIS-derived remote sensing reflectance in the green band at 547 nm. The new product is termed as an algal bloom index (ABI), which has shown improved performance over the original nFLH in both retrospective evaluation statistics and near real-time applications. The ABI product has been made available in near real-time through a Web portal and has been used by the FWC on a routine basis to guide field sampling efforts and prepare for red tide bulletins distributed to many user groups.

A novel method for characterizing harmful algal blooms in the Persian Gulf using MODIS measurements

Biophysical properties of water undergo meaningful variations under red tide (RT) outbreak. A massive Cochlodinium polykrikoids RT began in the eastern Persian Gulf (PG) in October 2008 and extended to the northern PG in December 2008. It killed large fish and hampered marine industries and water desalination appliances. Yet monthly averages of satellite-derived Chl-a (Chlorophyll-a), nFLH (normalized Fluorescence Line Height), and Kd490 (diffuse attenuation coefficient at 490nm) have not been compared in the PG. MODIS (MODerate Resolution Imaging Spectroradiometer) sensor provides global coverage, with short revisit time, and accessible, well validated ocean color products. This study compares the behavior of MODIS Chl-a, nFLH, and Kd490 in both normal and RT conditions. In doing so, their color maps are shown during normal and RT periods. Then, monthly variations of these products are shown as time-series between 2002 and 2008. HOCI (Hybrid Ocean Color Index) is defined by integrating these products to detect RT affected areas. The results gained from 100 locations in the PG show that HOCI >0.18mWcm−2μm−1sr−1mgm−4 and nFLH >0.04mWcm−2μm−1sr−1 discriminates non-bloom waters from algal blooms. Rrs(443)/Rrs(412)>1 is a proper statement to separate Trichodesmium erythtraeum from Noctiluca millaris, Noctiluca scintillans, and diatoms. Rrs(667)/Rrs(443)>1 can differentiate Cochlodinium polykrikoids from T. erythtraeum, N. millaris, N. scintillans, and diatoms as well. So, the combination of HOCI and Rrs(667)/Rrs(443) ratio is useful for detection and quantization of C. polykrikoids.

INVESTIGATION THE BEHAVIOR OF MODIS OCEAN COLOR PRODUCTS UNDER THE 2008 RED TIDE IN THE EASTERN PERSIAN GULF

Abstract. Biophysical properties of water undergo serious variations under red tide (RT) outbreak. During RT conditions, algal blooms spread out in the estuarine, marine and fresh waters due to different triggering factors such as nutrient loading, marine currents, and monsoonal winds. The Persian Gulf (PG) was a talent region subjected to different RTs in recent decade. A massive RT started from the Strait of Hormuz in October 2008 and extended towards the northern parts of the PG covering more than 1200 km of coastlines. The bloom of microorganism C. Polykrikoides was the main specie that generated large fish mortalities, and hampered marine industries, and water desalination appliances. Ocean color satellite data have many advantages to monitor and alarm RT occurrences, such as wide and continuous extent, short time of imagery, high accessibility, and appropriate estimation of ocean color parameters. Since 1999, MODerate Resolution Imaging Spectroradiometer (MODIS) satellite sensor has estimated satellite derived chlorophyll-a (Chl-a), normalized fluorescence line height (nFLH), and diffuse attenuation coefficient at 490nm (kd490). It provides a capability to study the behavior of these parameters during RT and normal conditions. This study monitors variations in satellite derived Chl-a, nFLH, and kd490 under both RT and normal conditions of the PG between 2002 and 2008. Up to now, daily and monthly variations in these products were no synchronously investigated under RT conditions in the PG. In doing so, the MODIS L1B products were provided from NASA data archive. They were corrected for Rayleigh scattering and gaseous absorption, and atmospheric interference in turbid coastal waters, and then converted to level 2 data. In addition, Enhanced Red Green Blue (ERGB) image was used to illustrate better water variations. ERGB image was built with three normalized leaving water radiance between 443 to 560nm. All the above data processes were applied by SeaDAS 7 software package. The Strait of Hormuz was selected as the study area in the eastern part of the PG. Images including high cloud coverage (&gt;50%) over the study area were filtered out. The classification maps of the above products were shown during RT and normal periods. Monthly variations of mentioned products were calculated for the dates before, during, and after RT appearance. The results were demonstrated as time-series diagrams. All the above calculations and presentations were performed in Matlab 7 software package. The results show that MODIS Chl-a, nFLH, and kd490 increased during the 2008 RT. Based on the feedback of these parameters under RT conditions, hybrid ocean color index (HOCI) is defined. HOCI is able to display better water variations during RT outbreak. High values of HOCI show RT affected areas.

VIIRS Observations of a &lt;italic&gt;Karenia brevis&lt;/italic&gt; Bloom in the Northeastern Gulf of Mexico in the Absence of a Fluorescence Band

The Visible Infrared Imager Radiometer Suite (VIIRS) is not equipped with a fluorescence band, which may affect its ability to detect and quantify harmful algal blooms (HABs) in coastal waters rich in colored dissolved organic matter. Such a deficiency has previously been demonstrated for a bloom of the toxic dinoflagellate Karenia brevis in the northeastern Gulf of Mexico (NEGOM) in summer 2014. Here, using data collected in the field and by VIIRS and Moderate Resolution Imaging Spectroradiometer (MODIS), we show that such a deficiency may be partially overcome using a red-green-chlorophyll-a index (RGCI). A relationship between near-concurrent (±4 hours) VIIRS RGCI (Rrs(672)/Rrs(551)) and field-measured chlorophyll-a (Chla; in mg m −3) was developed and evaluated using calibrated Chla obtained by a flowthrough system. A mean relative uncertainty, which was approximately twofold lower than VIIRS OC3M Chla, was obtained for VIIRS RGCI Chla (mean relative error: ∼56%) over a large range (0.5–20 mg m −3). Similar spatial patterns between near-concurrent MODIS-Aqua (MODISA) normalized fluorescence line height (nFLH) and VIIRS RGCI Chla imagery indicate that VIIRS RGCI may be used as a surrogate for MODISA nFLH in the absence of a fluorescence band. The success of this newly developed data product may be partially attributed to the 20-nm bandwidth of the VIIRS 672-nm band (662–682 nm) that covers a portion of the solar stimulated fluorescence region. However, whether such observations from a simple case study can be extended to other turbid coastal or inland waters still remains to be tested.

Evaluation and optimization of remote sensing techniques for detection of Karenia brevis blooms on the West Florida Shelf

We evaluated the performance of several published algorithms designed to detect Karenia brevis blooms on the West Florida Shelf using satellite imagery. Algorithms were tested using Moderate Resolution Imaging Spectroradiometer (MODIS) satellite images and historical ground-truth observations collected from August 2002 to December 2011. A total of 2323 phytoplankton cell count samples collected by the Florida Fish and Wildlife Conservation Commission (FWC) matched valid ocean color satellite observations over this period. Techniques were tested using the F-measure (FM) statistic. FM evaluates the proportion of image pixels that matches or misclassifies ground-based K. brevis observations. Several ocean color data products, such as normalized Fluorescence Line Height (nFLH), were also tested independently or combined to evaluate their effectiveness to detect K. brevis blooms. Techniques were optimized by altering their specific thresholds until the maximum FM was obtained. The historical techniques with the best performance were the Red Band Difference (RBD) and the Rrs–nFLH technique. This last method (Rrs–nFLH) is a combination of two standard ocean color data products: nFLH (> 0.033 mW cm− 2 μm− 1 sr− 1) and the remote sensing reflectance at 555 nm (Rrs555 < 0.007 sr− 1). Both techniques resulted in an FM of 0.62 and less than 3% false positives. The performance of most of the techniques improved significantly after optimization (the average FM increased from 0.47 to 0.57). The addition of a nFLH criterion improved the performance of most methods. We provide a list of recommendations on how to best apply the various techniques.

Global retrieval of marine and terrestrial chlorophyll fluorescence at its red peak using hyperspectral top of atmosphere radiance measurements: Feasibility study and first results

Chlorophyll fluorescence is directly linked to the physiology of phytoplankton or plants. Here, we present a new satellite remote sensing approach to retrieve chlorophyll fluorescence at its red peak (~685nm) by using measurements from the hyperspectral instruments SCanning Imaging Absorption SpectroMeter for Atmospheric CHartographY (SCIAMACHY) and Global Ozone Monitoring Experiment-2 (GOME-2). This method, which is based on the Differential Optical Absorption Spectroscopy (DOAS) technique, was used to exploit narrow spectral structures resulting from the filling-in of the Fraunhofer Fe I line, which originates from fluorescence. The reference spectra for chlorophyll fluorescence were calculated by the coupled ocean–atmosphere radiative transfer model SCIATRAN. We compared our results on marine chlorophyll fluorescence observations with the MODIS Terra normalized Fluorescence Line Height (nFLH) product for the average of years 2003–2011 and year 2009. Our method also enables the retrieval of chlorophyll fluorescence above land vegetation scenes. The results for the fluorescence observed above terrestrial vegetation for July and December 2009 were compared to MODIS Enhanced Vegetation Index (EVI). The comparisons show good spatial agreement between different retrievals providing evidence for the good performance of our algorithm. The method presented is generic and can be applied to other hyperspectral instruments in the future. Having established the retrieval technique, extensive studies of chlorophyll fluorescence will improve global knowledge on physiology and photosynthetic efficiency, in both the marine and terrestrial realms, and its dependence on environmental factors.

A harmful algal bloom of Karenia brevis in the northeastern Gulf of Mexico as revealed by MODIS and VIIRS: a comparison.

The most recent Visible Infrared Imager Radiometer Suite (VIIRS) is not equipped with a spectral band to detect solar-stimulated phytoplankton fluorescence. The lack of such a band may affect the ability of VIIRS to detect and quantify harmful algal blooms (HABs) in coastal waters rich in colored dissolved organic matter (CDOM) because of the overlap of CDOM and chlorophyll absorption within the blue-green spectrum. A recent HAB dominated by the toxin-producing dinoflagellate Karenia brevis in the northeastern Gulf of Mexico, offshore of Florida's Big Bend region, allowed for comparison of the capacities of VIIRS and Moderate Resolution Imaging Spectroradiometer (MODIS) to detect blooms in CDOM-rich waters. Both VIIRS and MODIS showed general consistency in mapping the CDOM-rich dark water, which measured a maximum area of 8900 km2 by mid-July 2014. However, within the dark water, only MODIS allowed detection of bloom patches—as indicated by high normalized fluorescence line height (nFLH). Field surveys between late July and mid-September confirmed Karenia brevis at bloom abundances up to 20 million cells·L−1 within these patches. The bloom patches were well captured by the MODIS nFLH images, but not by the default chlorophyll a concentration (Chla) images from either MODIS or VIIRS. Spectral analysis showed that VIIRS could not discriminate these high-phytoplankton water patches within the dark water due to its lack of fluorescence band. Such a deficiency may be overcome with new algorithms or future satellite missions such as the U.S. NASA's Pre-Aerosol-Clouds-Ecology mission and the European Space Agency's Sentinel-3 mission.

A simpler picture of satellite chlorophyll fluorescence

The Fluorescence Line Height (FLH) algorithm provides a standard method of measuring solar-stimulated chlorophyll fluorescence from coastal and ocean waters using the Moderate Resolution Imaging Spectroradiometer (MODIS), the Medium Resolution Imaging Spectrometer (MERIS) or other satellite sensors having the necessary spectral bands near 680 nm. In many coastal areas, this can provide superior estimates of chlorophyll concentration when compared to estimates derived from the standard green/blue algorithms. However, at least one significant data supplier normalizes the derived FLH values (to Normalized Fluorescence Line Height) on the assumption that the fluorescence signal is proportional to local solar irradiance. This is not the case, and normalization results in errors which make the fluorescence data much less useful. This paper demonstrates that FLH should be used as a measure of chlorophyll concentration without normalization.