Variations of chlorophyll-a in the northeastern Indian Ocean after the 2004 South Asian tsunami

Analysis of satellite remote sensing data has revealed changes in distribution of chlorophyll-a (Chl-a) and sea surface temperature (SST) in the Indian Ocean during the South Asian tsunami in December 2004. Chl-a data derived from Moderate Resolution Imaging Spectroradiometer (MODIS) and Sea-viewing Wide Field-of-view Sensor (SeaWiFS) images were examined for the period from 1998 to 2005. Around the epicentre of the Sumatra earthquake, the Chl-a concentration was found to increase prior to the main event on 26 December 2004 and then decrease during the tsunami event, while a high SST (～30–31°C) was observed in and around the epicentral region. Chl-a concentrations in the coastal waters of the Southeast Asian countries were remarkably low during and after the tsunami. Similar but relatively small variations in Chl-a and SST were observed during the second earthquake on 28 March 2005. Analysis of Chl-a, SST, wind and upwelling water has provided information for understanding the changes in Chl-a concentration during the tsunami. A very large offshore phytoplankton bloom (～300 km²) appeared to the southeast of Sri Lanka about 3 weeks after the tsunami; this might have been caused by a tropical storm that could be responsible for the enhancement of nutrients.     

Variability of bio-optical properties at sampling stations and implications for remote sensing: a case study in the north-east Gulf of Mexico

Variations of bio-optical properties at oceanographic sampling stations, although important for satellite data validation and algorithm development, have rarely been documented or studied. Using flow-through data and water samples collected from the flow-through system and Niskin bottles at ～260 stations between summer 1998 and spring 1999 in the north-east Gulf of Mexico (27.5° to 30.4°?N, 90° to 80°?W), we study the variability of several properties, including chlorophyll-a concentration and Gelbstoff absorption, at the sampling stations. It is found that the standard deviations for both Gelbstoff and chlorophyll are less than 10% of the mean values for more than 90% of the stations, including the coastal stations where water is turbid or Case II. High variations are found in the frontal regions near river plumes. At several stations chlorophyll-a and Gelbstoff vary by nearly two-fold due to spatial and/or temporal variations of the properties near the plume waters. This suggests that for water samples collected from moderately coloured waters (chlorophyll-a >0.25?mg?m^?3) or coastal river plume waters, special care should be taken to validate the sample data by using multiple samples, a continuous flow-through system, or a concurrent satellite data product map. Otherwise large uncertainties are likely to occur when these data are used to validate satellite estimates.     

A new regional algorithm for estimating chlorophyll-a in the Alboran Sea (Mediterranean Sea) from MODIS-Aqua satellite imagery

The utility of three different algorithms for retrieving surface chlorophyll-a values from satellite images of MODIS-Aqua is tested in the northern Alboran Sea. The available global algorithm to calculate chlorophyll-a from reflectance of MODIS-Aqua (OC3M) overestimates the surface chlorophyll-a in the study area. Another regional algorithm specifically developed for the Mediterranean Sea (MedOC3) improves the estimates although the best outcome is obtained with OC5, which was developed for Atlantic coastal waters. The three tested algorithms perform worse at in situ chlorophyll-a concentrations higher than 1 mg m^?3 and exhibit uncertainty levels higher than 35% for this range of concentrations. A new algorithm (ALBOC3) is proposed which produces a good estimation of the in situ chlorophyll-a for the whole range of concentrations normally registered in the study area (0.1–3.5 mg m^?3). We hypothesize that the particular bio-optical features of the northern Alboran Sea phytoplankton explain the poor functioning of the published algorithms that have been tested in this work.     

Variability of chlorophyll-a off the southwest coast of India

Coastal upwelling off the southwest coast of India during the southwest monsoon is a well-known phenomenon that enhances the chlorophyll-a (chl-a) biomass. The present study explores this property and examines the variability of surface chl-a using satellite data obtained from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) for the period from September 1997 to December 2010. Spatial variability showed substantial cross-shore as well as along-shore gradients during the southwest monsoon. Temporal variability in chl-a was studied in conjunction with satellite observations on sea-surface temperature, sea-surface height anomaly, winds, and currents. The results revealed the dominant influence of the West India Coastal Current on chl-a variability during the upwelling and downwelling periods. Moreover, noticeable intra- and inter-annual variability was observed in the parameter. Therefore, an empirical mode decomposition (EMD) method was used to identify the oscillations influencing variability. SeaWiFS chl-a data for the period 2008–2010 were omitted from this analysis due to gaps in the record. EMD analysis revealed oscillations ranging from seasonal to a five-year periodicity. Quasi-biennial oscillations are identified as the dominant factor causing inter-annual variability in chlorophyll in the study area, compared with the El-Niño Southern Oscillation and Indian Ocean Dipole. Chl-a was also studied in two smaller grids of size 0.5° × 0.5° separated by around 300 km and representing coastal and offshore areas, to understand the nature of variability in these areas. The annual range of variability was high (0.1–8.0 mg m^?3) near the coast consequent on high upwelling intensity, and very low (about 0.1 mg m^?3) in the offshore grid due to the absence of upwelling.     

Interannual variability of chlorophyll-a concentration along the southwest coast of India

The interannual variability of chlorophyll concentration along the southwest coast of India is studied using remote-sensing data from SeaWiFS. The data are analysed in conjunction with satellite-measured sea surface winds. The satellite-measured chlorophyll data for a period of 10 years from 1998 to 2007 were made use of for indexing the maximum offshore extent of chlorophyll along the coast for each month. From the empirical orthogonal functional analysis of chlorophyll data, it is observed that the dominant mode is annual. Interestingly, intraseasonal variability and the influence of climatic events like El Niño are observed in the secondary principle component of the time series. The variability of chlorophyll coincided well with variability of Ekman transport all along the coast with higher chlorophyll (>1 mg m^?3) when the Ekman transport is greater than 1000 kg/m/s. During the years 2005–2007, reduction in the meridional (along shore) component of wind resulted in reduction of Ekman transport, the phenomenon which leads to a decrease in chlorophyll. This is due to the reduction in the amount of nutrients that entrained to surface layers during upwelling of the southwest monsoon. The chlorophyll-a is minimum when Ekman transport is less than 0.5 kg/m/s on the normalized scale. For higher values of chlorophyll, the Ekman transport is higher, indicating the contribution of wind in enhancing the already upwelled chlorophyll production. The smaller value of R ² infers that there exist other forces as well involved in augmenting the surface chlorophyll. The enhanced knowledge on the offshore extent and the intraseasonal and interannual variability of chlorophyll can provide valuable inputs on fisheries and primary productivity for this region.     

Remote sensing of coastal waters by airborne lidar and satellite radiometer Part 1: A model study

Abstract

Radiative transfer calculations for remote sensing of coastal waters by airborne lidar and satellite radiometer have been compared in order to answer the question, whether an airborne lidar may be used instead of in situ measurements from ships to calibrate a satellite radiometer. The radiative transfer of laserlight measuring the Raman-scattering of water molecules, the fluorescence of chlorophyll-a and the fluorescence of yellow substance or Gelbstoff is simulated by the lidar equations while the radiance to a satellite radiometer is calculated with an ocean-atmosphere model based on the matrix-operator method. Including multiple scattering in the lidar equations, an eigenvalue analysis shows that three oceanic constituents (chlorophyll-a, nonchlorophyllous particles and Gelbstoff) can be separated measuring the backscattered laserlight at three wavelengths from a height of 100 to 200m. Changes in the concentration of all three substances are detected with higher accuracy with an airborne lidar than with a radiometer even at the same height. A comparison of different algorithms indicates that the common blue-green algorithms fail in coastal waters due to the variability of several oceanic constituents, which influence the colour of sea water. In this case, algorithms using the sun-induced chlorophyll-a fluorescence at 685 nm, are superior to blue-green algorithms. Airborne lidar measurements of the chlorophyll-a fluorescence at 685 nm, normalized by the Raman-signal at 650 nm, are as good as in situ data and can be used to calibrate satellite measurements of chlorophyll-a.     

Spatio-temporal distribution of chlorophyll-a concentration derived from MODIS satellite remote-sensing and in situ observations in Sanya Bay,China

This study examined satellite chlorophyll-a (chl-a) concentration and in situ observations in Sanya Bay (SYB). In situ observation of chl-a was conducted four times per year at 12 sampling stations in SYB from January 2004 to October 2008. Monthly satellite chl-a was derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) during 2000–2012. This study compared satellite chl-a values to in situ measurements in SYB. The two data sets match well in the whole region except for two estuaries. Results show that the average in situ chl-a was 1.49 mg m^?3 in SYB. Chl-a was relatively higher (>2 mg m^?3) and more variable in coastal areas, with a tendency to decrease offshore (<0.4 mg m^?3). The chl-a level in summer displayed obviously vertical stratification, with higher values at the bottom and lower values at the surface. Analysis of monthly mean chl-a showed that the highest level (>2 mg m^?3) appeared in December, with the lowest in March (<1 mg m^?3). The gradients are ranked winter, autumn, summer and spring. There was higher chl-a in autumn and winter, which may be associated with the stronger wind monsoon then. Annual mean chl-a from 2000 to 2012 varied from 1.17 to 2.05 mg m^?3, with the minimum in 2001 and the maximum in 2005. The chl-a level presented a roughly increasing tendency from 2000 to 2012, which may be related to the increasing nutrients associated with the development of tourism and fishery.     

Validation of SeaWiFS-derived chlorophyll for the Rio de la Plata Estuary and adjacent waters

The South Atlantic Ocean near the Río de la Plata Estuary is a highly dynamic region that encompasses four different water bodies: the Río de la Plata, the continental shelf and the Brazil and Malvinas currents. Bio-optical measurements obtained during 6–11 November 1999 provided the initial field data for validating Sea viewing Wide Field of view Sensor (SeaWiFS)-derived chlorophyll imagery for this region. The turbid waters of the Río de la Plata Estuary showed the highest variability and complexity in water optical properties, while the offshore waters of the Brazil Current had the lowest. In most cases, the estimates of chlorophyll using the SeaWiFS algorithm were higher than in situ measured values.     

Evaluation of spatio-temporal variations in chlorophyll-a in Lake Naivasha,Kenya: remote-sensing approach

Restoration of the ecosystem services and functions of lakes requires an understanding of the turbidity dynamics in order to arrive at informed environmental management decisions. The understanding of the spatio-temporal dynamics of turbidity requires frequent monitoring of the turbidity components such as chlorophyll-a concentration. In this study, we explored the use of Moderate Resolution Imaging Spectroradiometer Aqua (MODIS-Aqua) satellite data in studying the spatio-temporal changes in chlorophyll-a concentration in Lake Naivasha, a turbid tropical system. The temporal trend of chlorophyll-a concentration over the study period in the lake was also evaluated. The temporal trend assessment was achieved through the removal of periodic seasonal interference using Seasonal-Trend decomposition based on the LOESS (Local Regression) procedure. The resultant chlorophyll-a concentration maps derived from MODIS-Aqua satellite data give an indication of the monthly spatial variation in chlorophyll-a concentration from 2002 to 2012. The results of regression analyses between satellite-derived chlorophyll-a and in situ measurements reveal a high level of precision, but with a measureable bias with the satellite underestimating actual in situ measurements (R² = 0.65, P < 0.001). Although the actual values of the chlorophyll-a concentrations are underestimated, the significant relationship between satellite-derived chlorophyll-a and in situ measurements provides reliable information for studying spatial variations and temporal trends. In 2009 and 2010, it was difficult to detect chlorophyll-a from the MODIS-Aqua imagery, and this coincided with a period of the lowest water levels in Lake Naivasha. An inverse relationship between de-seasoned water level and chlorophyll-a concentration was evident. This study shows that MODIS-Aqua satellite data provide useful information on the spatio-temporal variations in Lake Naivasha, which is useful in establishing general trends that are more difficult to determine through routine ground measurements.     

Comparisons of chlorophyll variability between the four major global eastern boundary currents

The first two years of SeaWiFS (Sea viewing Wide Field of view Sensor) data (1997–1999) are used to document the variability of large-scale surface chlorophyll patterns within the coastal region along the full latitudinal extent of each of the four major global eastern boundary currents; the California, Humboldt, Benguela and Canary Currents. Seasonal chlorophyll patterns are compared to coincident seasonal cycles of Ekman transport calculated from satellite scatterometer data. In all four regions, maximum chlorophyll concentrations are generally temporally and latitudinally coincident with the seasonal maximum in upwelling (offshore Ekman transport) over most of their latitudinal range, but exceptions are documented. Interannual differences are evident in each region, most notably in the two Pacific regions where the 1997–1998 chlorophyll seasonality was affected by El Niño conditions. Significant differences between previously published chlorophyll seasonality deduced from the relatively sparse coverage of the Coastal Zone Color Scanner (CZCS) and the more complete coverage of SeaWiFS in both Southern Hemisphere regions are evident.     

Anthropogenic impact on spring bloom dynamics in the Yangtze River Estuary based on SeaWiFS mission (1998–2010) and MODIS (2003–2010) observations

Nutrient output from the Yangtze River to the sea has increased dramatically since the 1960s, and over the past 50 years more than 50,000 reservoirs on the Yangtze River basin have had little impact on water discharge, but have drastically reduced the annual river-to-sea sediment flux, especially after 2000. This can be presumed to have a close link with the 73% (accumulated incidences) of algal bloom reported on China's eastern coast which have taken place in the Yangtze River Estuary (YRE) and its adjacent waters from 2000 to 2009. A conceptual view explains that the algal bloom zone varies between the YRE and mid-shelf waters of the East China Sea, where the optimum balance of light availability and nutrient supply exists. A reduction in turbidity with declining river-to-sea sediment load around the YRE would provide a deeper euphotic layer for the growth of phytoplankton, which is stimulated by eutrophication following increased river-to-sea sewage. Although the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) represents an immature state of ocean colour remote sensing, especially in coastal waters, in general, SeaWiFS serves as a useful tool to highlight the geographical spread and intensity of spring bloom dynamics at the basin scale in the YRE. We suggest that the problem of skin effect bias in spaceborne observation is minimal and that satellite-derived pigment data should be mainly from phytoplankton production when estimating Chl-a during the spring bloom season in the YRE by analysis of bio-optical properties of spring bloom waters. Comparisons between in situ and simultaneous SeaWiFS-derived Chl-a suggest that SeaWiFS standard Chl-a data show comparable results with ship survey data, with a mean ratio (in situ to satellite ratio) of 1.28 ± 0.78 (R ² = 0.71, n = 14, p < 0.001) in spring blooms. We took the seasonal average Chl-a in our defined multi-year highly productive zone in the YRE (28°30′–32°N, 122°–123°30′°E, A) compared with its neighbouring region B from 1998 to 2010, and determined a linear least-squares fit to the trend line. Further analysis in the form of t-tests was run for four seasonal periods, March–April, May–June (spring bloom season), July–August, and September–October. The SeaWiFS-derived 13-year Chl-a mean slope in the spring bloom season is significantly (P = 0.012) increasing by 0.212 mg m^?3 per year. However, such a trend in other seasons is not significant and there is also no significant variation in B over all seasons. Our results show that there is a positive relationship between the annual mean of Chl-a in the spring bloom season in A and annual sewage water discharge at the Yangtze River basin, and similarly a negative relationship between Chl-a and annual river sediment flux was found from 1998 to 2010. Variation in 3-year sediment flux, sewage water discharge, and Chl-a was –25% (±18%), 15% (±5%), and 14% (±6%), respectively. This result supports the findings of previous studies that human activity has a measurable effect on coastal phytoplankton biomass and that the eutrophication effect seems to stimulate increased Chl-a in the spring bloom season, but not on the enhancement of annual Chl-a levels in the YRE.     

Validation of chlorophyll-α concentration maps from Aqua MODIS over the Gulf of Gabes (Tunisia): comparison between MedOC3 and OC3M bio-optical algorithms

Few studies have focused on the use of ocean colour remote sensors in the Gulf of Gabes (southeastern Tunisia). This work is the first study to evaluate the ocean colour chlorophyll-a product in this area. Chlorophyll-a concentrations were measured during oceanographic cruises performed off the Gulf of Gabes. These measurements were used to validate satellite data acquired from the Moderate Resolution Imaging Spectroradiometer (MODIS) on the Aqua satellite. First, two atmospheric correction procedures (standard and shortwave infrared) were tested to derive the remote-sensing reflectance, and then a comparison between two bio-optical (OC3M and MedOC3) algorithms were realized using the in situ measurements. Both atmospheric correction procedures gave similar results when applied to our study area indicating that most pixels were non-turbid. The comparison between bio-optical algorithms shows that using the regional bio-optical algorithm MedOC3 improves chlorophyll-a estimation in the Gulf of Gabes for the low values of this parameter.     

Detection of total suspended sediments in the North Sea using AVHRR and ship data

This study reports on a combined investigation between in situ measurements of total suspended sediments, collected by ship, and remotely sensed data provided by the Advanced Very High Resolution Radiometer (AVHRR) during the winter-summer period of 1989. It is meant to be a case study stating the problems inherent to coastal waters and proposing a methodology to understand them. Ship and satellite data are compared in order to detect suspended sediments in case 2 waters in the North Sea through a linear regression analysis. The results show a wide range of coefficient of determination (R²) values. The highest values correspond to summer dates, while the lowest belonged to the winter and spring period. It was found that for the summer dates the relatively still atmospheric and water conditions were suitable for comparison with satellite data providing good values of R². In winter and spring, unsettled sea water conditions complicated the comparison of the data. It can be concluded that the seasonal stratification of the water column during summer time allows a better correlation between in situ and remotely sensed data than the typically well-mixed waters during winter.     

Monsoon and eddy forcing of chlorophyll-a variation in the northeast South China Sea

The physical and biological environments of the northeast South China Sea (SCS) were investigated using 11 year satellite and reanalysis data, including ocean chlorophyll-a (Chl-a) concentrations, sea surface wind (SSW) values, sea surface temperatures (SSTs), sea surface height anomalies (SSHAs), etc. The findings reveal that ocean Chl-a concentrations west of the Luzon Strait have the most significant annual cycles in the SCS. The dominant forcing mechanisms are monsoon winds and mesoscale eddies studied by multiple regression analysis. In the offshore regions, strong winds directly caused the enhancements by local vertical mixing and entrainment. As in the near-shore regions, the alongshore winds indirectly caused the enhancements by inducing coastal upwelling. Although SST is highly correlated with Chl-a concentration, SST cooling is difficult to observe in the bloom region. It is considered a consequence of monsoon and eddy forcings. Other mechanisms, such as local Ekman pumping by the wind stress curl and the geostrophic potential vorticity, have little effect on Chl-a seasonal variations.     

Asterionella blooms in the northwestern Bay of Bengal during 2004

Asterionella glacialis Castracane, a diatom species of marine phytoplankton, bloomed in the coastal waters of the northwestern Bay of Bengal during 24 March to 4 April 2004. This species dominated by 99% in cell concentration on 26 March 2004. During the bloom period, phytoplankton population density increased by three orders of magnitude, while the chlorophyll a concentration increased by 70 to 80 times relative to the pre‐bloom situation. The blooming was associated with coastal upwelling and influenced water quality. Corresponding to the blooming period, satellite sensor‐derived products of sea surface temperature (SST) and chlorophyll a data were analysed to understand the coverage and dynamics of phytoplankton in the area.     

Seasonal and interannual variability of chlorophyll-a in the Gulf of Oman compared to the open Arabian Sea regions

Field sampling, remote sensing and modelling were employed to understand the seasonal and interannual changes of chlorophyll-a concentrations in the Gulf of Oman in comparison to open sea regions. In these regions, maximal chlorophyll concentrations were reported during the summer monsoon (with peaks in June and August), while in the Gulf of Oman, the chlorophyll maximum was observed during the winter monsoon (February–March). From 1997 through to 2008, the interannual variability in chlorophyll-a concentrations in the Gulf of Oman has not exhibited pronounced trends and neither have the other two (oceanic) regions in the western Arabian Sea. However, an increase of the annual variation in chlorophyll concentrations over the years was noticed. The diatom biomass decreased two-fold from 1997 to 2007. Nitrate concentration and mixed-layer depth also declined. In comparison to the seasonal blooms driven in the Gulf of Oman by the dinoflagellate Noctiluca scintillans, the year 2008 was markedly different. The summer bloom was shifted to September; it was gradually extended in time and formed by the other species. An applicability of the concept of ecosystem regime shift is discussed.     

Bio-optical properties of east coast Malaysia waters in relation to remote sensing of chlorophyll

Surface water samples collected during the monsoon and inter-monsoon seasons of 2009 off the east coast of Peninsular Malaysia have been analysed for concentrations of total chlorophyll, suspended particulates and coloured dissolved organic matter (CDOM). Spectral absorption coefficients of dissolved and particulate materials have also been measured. Significant seasonal variabilities in concentrations and optical properties were reported with high concentrations of all parameters during the northeast monsoon (NEM) season and low during the southwest monsoon (SWM) and inter-monsoon seasons. Contrary to previous reports on the oligotrophic nature of the waters during the inter-monsoon season, relatively high concentrations of chlorophyll (>3 mg m^?3) were observed at offshore stations in the study area in the spring and fall inter-monsoon months. The chlorophyll-specific absorption spectrum changes with the seasons with the greatest absorption per unit chlorophyll during the SWM and the least during the inter-monsoon seasons, probably in response to seasonal changes in phytoplankton community and cell size structure. The water is classified as optical case 2. At the blue end of the spectrum (440 nm), light absorption by non-phytoplankton materials (CDOM and detritus) accounts for nearly 70% of the total non-water absorption regardless of the season. At the wavelength (676 nm) of the secondary chlorophyll absorption peak in the red part of the spectrum, light absorption by chlorophyll contributes 80–90% to total non-water absorption at most stations and this may provide the basis for remote sensing of phytoplankton chlorophyll in these waters.     

On a 16° N chlorophyll plume along the east coast of India

The plume rich with chlorophyll‐a concentration (>0.3 mg/l) observed on 22 March 2003 along the 16° N in the western Bay of Bengal off the Krishna–Godavari river delta was studied. Relatively high sea surface temperature (>30° C) observed in the plume area indicated their origin in the coastal waters. The bloomy plume was found spreading 400 to 500 km offshore in the form of an offshore jet extending as far as 86° E in the Bay of Bengal. An offshore flow was observed with geostrophic velocity exceeding 50 cm/s with a cyclonic eddy on its north around 17° N, 82° E and an anti‐cyclonic eddy around 14° N, 83° E to its south. The hydrographic data of the area were studied with the observations made by GTS data buoy indicated coastal upwelling. Similar plumes were also observed during the years 2004 and 2005.     

Interannual variability of the chlorophyll-a and sea surface temperature of the Black Sea

A comparative analysis of the Black Sea surface temperature and chlorophyll-a concentration was based on the satellite data (PODAAC JPL AVHRR Pathfinder and SeaWiFS) for 1998–2005. The fields of dispersion and spatial gradient were analysed as well. On the interannual scale, no statistical relationship was found between the field of sea surface temperature and chlorophyll-a. The mesoscale variability (which masked the statistical assessments of interannual coupling) was well pronounced in the fields of both parameters. For the deep part of the sea, a sign of the wave-type origin of the variability of chlorophyll-a and sea surface temperature was evaluated.     

SeaWiFS validation in European coastal waters using optical and bio-geochemical measurements

The National Aeronautics & Space Administration (NASA) Sea viewing Wide Field of view Sensor (SeaWiFS) began operational measurement of ocean colour in September 1997. Upgrades to the SeaWiFS data processing system (SeaDAS) have occurred frequently and the effects of these revisions on the remotely sensed estimates of chlorophyll-a concentration (chl-a) have been significant. Measurements of chl-a from research work in the Bay of Biscay and Gulf of Cadiz during 1998–1999 are used to validate the SeaWiFS chl-a product generated using the current version of SeaDAS (version 4.1). The validation data cover coastal and offshore waters, including those dominated by inorganic suspended sediment, and an intense dinoflagellate bloom where shipboard chl-a measurements exceeded 50?mg?m^?3. The standard SeaWiFS chlorophyll algorithm (OC4v4) generally performed well, but significantly over-estimated chl-a where inorganic suspended sediment was present. The algorithm is only applicable to chl-a values up to 64?mg?m^?3, which was less than chl-a at the centre of the bloom. A novel algorithm for chl-a, which first estimates the inherent optical properties of the water, was applied to the SeaWiFS measurements but failed on over 90% of the pixels, perhaps because SeaWiFS is under-estimating water reflectance at the extreme blue end of the visible spectrum.