Eurico J. D'Sa

Chromophoric dissolved organic matter (CDOM) variability in Barataria Basin using excitation―emission matrix (EEM) fluorescence and parallel factor analysis (PARAFAC)

Chromophoric dissolved organic matter (CDOM) variability in Barataria Basin, Louisiana, USA,was examined by excitation emission matrix (EEM) fluorescence combined with parallel factor analysis (PARAFAC). CDOM optical properties of absorption and fluorescence at 355nm along an axial transect (36 stations) during March, April, and May 2008 showed an increasing trend from the marine end member to the upper basin with mean CDOM absorption of 11.06 + or - 5.01, 10.05 + or - 4.23, 11.67 + or - 6.03 (m(-)(1)) and fluorescence 0.80 + or - 0.37, 0.78 + or - 0.39, 0.75 + or - 0.51 (RU), respectively. PARAFAC analysis identified two terrestrial humic-like (component 1 and 2), one non-humic like (component 3), and one soil derived humic acid like (component 4) components. The spatial variation of the components showed an increasing trend from station 1 (near the mouth of basin) to station 36 (end member of bay; upper basin). Deviations from this increasing trend were observed at a bayou channel with very high chlorophyll-a concentrations especially for component 3 in May 2008 that suggested autochthonous production of CDOM. The variability of components with salinity indicated conservative mixing along the middle part of the transect. Component 1 and 4 were found to be relatively constant, while components 2 and 3 revealed an inverse relationship for the sampling period. Total organic carbon showed increasing trend for each of the components. An increase in humification and a decrease in fluorescence indices along the transect indicated an increase in terrestrial derived organic matter and reduced microbial activity from lower to upper basin. The use of these indices along with PARAFAC results improved dissolved organic matter characterization in the Barataria Basin.

Bio-optical properties and ocean color algorithms for coastal waters influenced by the Mississippi river during a cold front

During the passage of a cold front in March 2002, bio-optical properties examined in coastal waters impacted by the Mississippi River indicated that westward advective flows and increasing river discharge containing high concentrations of nonalgal particles contributed significantly to surface optical variability. A comparison of seasonal data from three cruises indicated spectral models of absorption and scattering to be generally consistent with other coastal environments, while their parameterization in terms of chlorophyll (Chl) alpha concentration showed seasonal variability. The exponential slope of the colored dissolved organic matter (CDOM) averaged 0.0161+/-0.00054 nm(-1) and nonalgal absorption averaged 0.011 nm(-1) with deviations from general trends observed due to anomalous water properties. Although the phytoplankton specific absorption coefficients varied over a wide range [0.02 to 0.1 m2 (mg Chl)(-1) at 443 nm] being higher in offshore surface waters, values of phytoplankton absorption spectra at the Sea-Viewing Wide Field-of-View Sensor (SeaWiFS) wave bands were highly correlated to modeled values. Particulate scattering characteristics were similar to observations for other coastal waters, while backscattering ratios were on average lower in phytoplankton-dominated surface waters (0.011+/-0.003) and higher in low Chl near-bottom waters (0.0191+/-0.0045). Average percent differences in remote sensing reflectance Rrs derived from modeled and in-water radiometric measurements were highest in the blue wave bands (52%) and at locations with more stratified water columns. SeaWiFS estimates of Chl and CDOM absorption derived using regional empirical algorithms were highly correlated to in situ data.

Impacts of Loop Current Frontal Cyclonic Eddies and Wind Forcing on the 2010 Gulf of Mexico Oil Spill

Monitoring and M A Record-Breakin Geophysical Mon Copyright 2011 b 10.1029/2011GM The 2010 Deepwater Horizon Gulf of Mexico oil spill, the largest in U.S. history, highlights the environmental risks inherent in deepwater drilling. These risks were mitigated by rapid access to real-time satellite measurements from passive (optical, IR) and active (synthetic aperture radar, altimetry) sensors. This study employed satellite data, in tandem with in situ current and wind measurements, to track surface oil and to better understand the causes for observed large-scalemotions during the 84 day episode. The analysis revealed the merger of three cyclonic eddies along the Loop Current’s (LC’s) northern margin, ultimately forming a larger and more vigorous cyclonic eddy, measuring 280 130 km on 18 May. This larger cyclonic eddy, in tandem with a smaller anticyclonic eddy and a LCmeander, controlled the motion of the oil/dispersant mixture into deepwater (maximum current speed of 2.25 m s ), tripling the area of surface oiling from 9623 to 33,575 km. Two main events limited the flow of oil to the Florida Straits, the accumulation of oil within the merged eddy and the fact that this eddy did not move substantially for several months. The observed offshore entrainment of oil toward the LCwas successfully hindcast using a particle-tracking model based on geostrophic currents computed from satellite altimetry. This assessment of circulation processes may help to advance numerical circulation modeling efforts in this region of rapid current variability in support of safer deepwater drilling in the northern Gulf.

Colored dissolved organic matter in coastal waters influenced by the Atchafalaya River, USA: effects of an algal bloom

Colored dissolved organic matter (CDOM) optical properties were studied in the northern Gulf of Mexico coastal waters influenced by the Atchafalaya River in July 2005 during a period of low river discharge using both field and satellite ocean color data. Although a general conservative CDOM absorption-salinity relationship between the riverine and oceanic end members were observed, deviations in surface CDOM absorption from this relationship (CDOM loss) and average higher spectral slope S of the log-linearized absorption spectra in comparison to bottom waters suggested photodegradation of surface CDOM. Excess CDOM absorption in bottom waters at many stations were associated with low oxygen or hypoxic conditions suggesting a potential CDOM source due to remineralization of organic matter in the bottom waters. At a few stations anomalously high CDOM absorption could be linked to biological production associated with algal blooms. Absorption spectra of a surface sample at one such station deviated from standard exponential form in the 300-400m UV range probably due to the presence of photo-protective pigments. CDOM derived from the Sea-Viewing Wide Field-of-View Sensor (SeaWiFS) using a regional algorithm provided a synoptic view of surface CDOM variations and the extent of river influences into the northern Gulf of Mexico.

Short-term Influences on Suspended Particulate Matter Distribution in the Northern Gulf of Mexico: Satellite and Model Observations

Energetic meteorological events such as frontal passages and hurricanes often impact coastal regions in the northern Gulf of Mexico that influence geochemical processes in the region. Satellite remote sensing data such as winds from QuikSCAT, suspended particulate matter (SPM) concentrations derived from SeaWiFS and the outputs (sea level and surface ocean currents) of a nested navy coastal ocean model (NCOM) were combined to assess the effects of frontal passages between 23-28 March 2005 on the physical properties and the SPM characteristics in the northern Gulf of Mexico. Typical changes in wind speed and direction associated with frontal passages were observed in the latest 12.5 km wind product from QuikSCAT with easterly winds before the frontal passage undergoing systematic shifts in direction and speed and turning northerly, northwesterly during a weak and a strong front on 23 and 27 March, respectively. A quantitative comparison of model sea level results with tide gauge observations suggest better correlations near the delta than in the western part of the Gulf with elevated sea levels along the coast before the frontal passage and a large drop in sea level following the frontal passage on 27 March. Model results of surface currents suggested strong response to wind forcing with westward and onshore currents before the frontal passage reversing into eastward, southeastward direction over a six day period from 23 to 28 March 2005. Surface SPM distribution derived from SeaWiFS ocean color data for two clear days on 23 and 28 March 2005 indicated SPM plumes to be oriented with the current field with increasing concentrations in nearshore waters due to resuspension and discharge from the rivers and bays and its seaward transport following the frontal passage. The backscattering spectral slope γ, a parameter sensitive to particle size distribution also indicated lower γ values (larger particles) in nearshore waters that decreased offshore (smaller particles). The use of both satellite and model results revealed the strong interactions between physical processes and the surface particulate field in response to the frontal passage in a large river-dominated coastal margin.

Potential of MODIS EVI in Identifying Hurricane Disturbance to Coastal Vegetation in the Northern Gulf of Mexico

Frequent hurricane landfalls along the northern Gulf of Mexico, in addition to causing immediate damage to vegetation, also have long term effects on coastal ecosystem structure and function. This study investigated the utility of using time series enhanced vegetation index (EVI) imagery composited in MODIS product MOD13Q1 for assessing hurricane damage to vegetation and its recovery. Vegetation in four US coastal states disturbed by five hurricanes between 2002 and 2008 were explored by change imagery derived from pre- and post-hurricane EVI data. Interpretation of the EVI changes within months and between years distinguished a clear disturbance pattern caused by Hurricanes Katrina and Rita in 2005, and a recovering trend of the vegetation between 2005 and 2008, particularly within the 100 km coastal zone. However, for Hurricanes Gustav, Ike, and Lili, the disturbance pattern which varied by the change imagery were not noticeable in some images due to lighter vegetation damage. The EVI pre- and post-hurricane differences between two adjacent years and around one month after hurricane disturbance provided the most likely damage area and patterns. The study also revealed that as hurricanes damaged vegetation in some coastal areas, strong precipitation associated with these storms may benefit growth of vegetation in other areas. Overall, the study illustrated that the MODIS product could be employed to detect severe hurricane damage to vegetation, monitor vegetation recovery dynamics, and assess benefits of hurricanes to vegetation.

A semi-analytical total suspended sediment retrieval model in turbid coastal waters: A case study in Changjiang River Estuary

A simple semi-analytical model to estimate total suspended sediment matter (3S) was established for estimating TSM concentrations in Changjiang River Estuary. The results indicate that 3S model with near-infrared wavelengths provide good estimates of TSM concentrations in the study region. Furthermore, the applicability of 3S model was evaluated using an independent data set taken from Oujiang river estuary during September 2012. The results indicate that providing an available atmospheric correction scheme for satellite imagery, the 3S model could be used for quantitative monitoring of TSM concentration in coastal waters, even though local bio-optical information is still needed to reinitialize the model.

Effects of Hurricane Ike on the Louisiana―Texas coast from satellite and model data

The river-influenced Mississippi–Louisiana–Texas (MsLaTex) coast in the northern Gulf of Mexico was in 2008 impacted by Hurricane Ike that made landfall along the Texas coast on 13 September 2008, causing widespread damage due to storm surge and coastal inundation. We examine the effects of the hurricane on the MsLaTex coast using remote sensing, model simulation and field observations. Moderate resolution imaging spectroradiometer sea surface temperature imagery indicated an approximately 2–4°C decrease in temperature to the right of the hurricane track. The high-resolution (∼2 km) three-dimensional Navy Coastal Ocean Model simulation and tide data suggested the generation of a coastally trapped ‘barotropic Kelvin’ wave during the passage of Ike, which may have contributed to an earlier than expected surge in sea level. A distinct elevated band of suspended particulate matter (SPM) concentration extending from Louisiana to Texas (∼90.5–94.5°W) coast was observed offshore in sea-viewing wide field-of-view sensor-derived imagery of 17 and 25 September that appeared to be associated with the coastally trapped Kelvin wave that was enhanced west of the Mississippi Canyon. Two intermittent wind events on 15 and 21 September likely kept the SPM in suspension for an extended period. Subsequently, on 27 and 30 September, SPM decreased along the elevated band and near the landfall site but increased near the bays and passes due to receding flood waters. Off the Atchafalaya and Mississippi deltas a large pulse of river discharge containing elevated SPM levels caused a plume around the two river deltas and its offshore dispersion into the open Gulf of Mexico.

Seasonal variability in CDOM absorption and fluorescence properties in the Barataria Basin, Louisiana, USA.

Absorption and fluorescence properties of chromophoric dissolved organic matter (CDOM) along a 124 km transect in the Barataria Basin, a large estuary located in Louisiana, USA, were investigated during high and low flow periods of the Mississippi River in the spring and winter of 2008-2009. Mean CDOM absorption at 355 nm from the marine to the freshwater end member stations ranged from (3.25 +/- 0.56) to (20.76 +/- 2.43) m(-1) for the three month high flow period whereas it varied from (1.48 +/- 1.08) to (25.45 +/- 7.03) m(-1) for the same stations during low flow period. Corresponding salinity values at these stations indicated the influence of river and shelf exchanges in the lower basin and precipitation and runoff in the upper basin. An inverse relationship of CDOM absorbance and fluorescence with salinity observed in the basin could be a useful indicator of salinity. CDOM fluorescence also varied over a large range showing an approximately 8 to 12-fold increase between the marine and freshwater end members for the two flow seasons. Excitation-emission matrix spectral plots indicated the presence of various fluorescence components with highest being the A-peak, lowest the T-peak, and the C and M-peaks showing similar trends along the transect. During low flow season the A/C ratio were well correlated with station locations indicating increased terrestrial influence towards the upper basin. CDOM absorption and fluorescence at 355 nm were highly correlated and independent of CDOM sources suggesting that fluorescence could be used to characterize CDOM in the basin.

Atmospheric Correction and Vicarious Calibration of Oceansat-1 Ocean Color Monitor (OCM) Data in Coastal Case 2 Waters

The Ocean Color Monitor (OCM) provides radiance measurements in eight visible and near-infrared bands, similar to the Sea-viewing Wide Field-of-View Sensor (SeaWiFS) but with higher spatial resolution. For small- to moderate-sized coastal lakes and estuaries, where the 1 × 1 km spatial resolution of SeaWiFS is inadequate, the OCM provides a good alternative because of its higher spatial resolution (240 × 360 m) and an exact repeat coverage of every two days. This paper describes a detailed step-by-step atmospheric correction procedure for OCM data applicable to coastal Case 2 waters. This development was necessary as accurate results could not be obtained for our Case 2 water study area in coastal Louisiana with OCM data by using existing atmospheric correction software packages. In addition, since OCM-retrieved radiances were abnormally low in the blue wavelength region, a vicarious calibration procedure was developed. The results of our combined vicarious calibration and atmospheric correction procedure for OCM data were compared with the results from the SeaWiFS Data Analysis System (SeaDAS) software package outputs for SeaWiFS and OCM data. For Case 1 waters, our results matched closely with SeaDAS results. For Case 2 waters, our results demonstrated closure with in situ radiometric measurements, while SeaDAS produced negative normalized water leaving radiance (nLw) and remote sensing reflectance (Rrs). In summary, our procedure