Paul R. Carlson

Satellite-based virtual buoy system to monitor coastal water quality

Abstract. There is a pressing need to assess coastal and estuarine water quality state and anomaly events to facilitate coastal management, but such a need is hindered by lack of resources to conduct frequent ship-based or buoy-based measurements. Here, we established a virtual buoy system (VBS) to facilitate satellite data visualization and interpretation of water quality assessment. The VBS is based on a virtual antenna system (VAS) that obtains low-level satellite data and generates higher-level data products using both National Aeronautics and Space Administration standard algorithms and regionally customized algorithms in near real time. The VB stations are predefined and carefully chosen to cover water quality gradients in estuaries and coastal waters, where multiyear time series at monthly and weekly intervals are extracted for the following parameters: sea surface temperature (°C), chlorophyll-a concentration (mgu2009m−3), turbidity (NTU), diffuse light attenuation at 490 nm [Kd(490), m−1] or secchi disk depth (m), absorption coefficient of colored dissolved organic matter (m−1), and bottom available light (%). The time-series data are updated routinely and provided in both ASCII and graphical formats via a user-friendly web interface where all information is available to the user through a simple click. The VAS and VBS also provide necessary infrastructure to implement peer-reviewed regional algorithms to generate and share improved water quality data products with the user community.

Vulnerability and resilience of seagrasses to hurricane and runoff impacts along Florida’s west coast

Many climate change models predict increasing frequency and severity of tropical cyclones (hurricanes) in the Atlantic Ocean, Caribbean Sea, and Gulf of Mexico. To assess this potential threat to seagrass communities in Florida’s Big Bend region, we performed a habitat change analysis based on aerial seagrass surveys performed prior to, and after, the extremely active Atlantic cyclone seasons of 2004 and 2005. To provide a regional context for changes in the Big Bend region, we also compared impacts there with changes in three other West Florida estuaries. Our analysis showed that storm impacts on seagrasses varied along Florida’s west coast. Physical disturbance caused minor losses in parts of Charlotte Harbor and the Big Bend region. However, heavy rainfall in Florida and Georgia associated with Frances and Jeanne combined with winter rains to cause complete loss of 1,500xa0ha of seagrasses and thinning of another 1,700xa0ha in the vicinity of the Suwannee River mouth. In Tampa Bay, Sarasota Bay, and Charlotte Harbor, despite localized losses, total seagrass area actually increased between 2004 and 2006. On the other hand, Tampa Bay, Sarasota Bay, and Charlotte Harbor all showed significant, and more pronounced, declines in seagrass cover as the result of another major rainfall and runoff event: the 1997–1998 El Nino event. Our results indicate that light stress, likely caused by suspended sediments, phytoplankton blooms, and dissolved organic matter, resulted in seagrass losses extending up to 40xa0km from the mouth of the Suwannee River. We conclude that water quality impacts, especially if they are persistent, can be more damaging than physical impacts of moderate (Category 1–3) tropical cyclones. We also conclude that runoff-related impacts on seagrasses vary depending on the timing, volume, and persistence of storm runoff in relation to normal seasonal runoff patterns and seagrass growth in each estuary.

Effect of El Niño on Demographic, Morphological, and Chemical Parameters in Turtle-Grass (Thalassia testudinum): An Unexpected Test of Indicators

We examined the response of demographic, morphological, and chemical parameters of turtle grass (Thalassia testudinum), to much-higher-than-normal rainfall associated with an El Niño event in the winter of 1997-1998. Up to 20 inches of added rain fell between December 1997 and March 1998. triggering widespread and persistent phytoplankton blooms along the west coast of Florida. Water-column chlorophyll concentrations estimated from serial Sea WiFS imagery were much higher during the El Niño event than in the previous or following years, although the timing and magnitude of phytoplankton blooms varied among sites. Seagrass samples collected in 1997, 1998, and 1999 provided an excellent opportunity to test the responsiveness of Thalassia to decline and subsequent improvement of water quality and clarity in four estuaries. Using a scoring technique based on temporal responsiveness, spatial consistency, and statistical strength of indicators, we found that several morphological parameters (Thalassia shoot density, blade width, blade number, and shoot-specific leaf area) were responsive and consistent measures of light stress. Some morphological parameters, such as rhizome apex density, responded to declines and subsequent improvement in water clarity, but lacked the statistical discriminating power necessary to be useful indicators. However, rhizome sugar, starch, and total carbohydrate concentrations also exhibited spatially and temporally consistent variation as well as statistical strength. Because changes in shoot density, as well as water clarity, affect rhizome carbohydrate levels, a composite metric based on Thalassia shoot density and rhizome carbohydrate levels together is probably more useful than either parameter alone as an indicator of seagrass health.

Alternative spatially enhanced integrative techniques for mapping seagrass in Florida's marine ecosystem

Seagrass is an important component of coastal marine ecosystems. Seagrass mapping provides a means for assessing seagrass health by monitoring the spatial distribution and density of seagrass habitat in coastal waters. Recent image processing and satellite technologies present the opportunity to leverage quantitative techniques that have the potential to improve upon traditional photo-interpretation techniques in terms of cost, mapping fidelity, and objectivity. Integrated spatial and spectral processing techniques were identified as an alternative method for mapping seagrass extent and density from an IKONOS satellite image of Springs Coast, Florida. These spatially enhanced integrative mapping techniques objectively standardize seagrass-monitoring efforts and enhance mapping capabilities by characterizing spatial seagrass density gradients. A combination of water column correction, pixel classification, and image segmentation techniques provided a seagrass density index map that represented seagrass density and distribution with high spatial detail and overall accuracy (77%) comparable to photo-interpretation techniques. Satellite imagery-based spatially enhanced image processing techniques were found to provide a consistent, quantitative, and cost-effective alternative for seagrass mapping in Springs Coast with the potential to be transferred to other parts of the world. A cost savings analysis concluded that there was a 13% cost saving using satellite photo-interpretation and a 47% cost saving using enhanced satellite classification when compared to aerial photo-interpretation.

Effects of Seagrass Die-Off on Benthic Fluxes and Porewater Concentrations of ∑CO2, ∑H2S, and CH4 in Florida Bay Sediments

Benthic fluxes and porewater concentrations of ∑CO2,∑H2S, and CH4 in living seagrass beds of Florida Bay (USA) are higher than in adjacent areas that have been recently denuded by disease. The enrichment and seasonal variation of porewater ∑CO2, ∑H2S, and CH4 in seagrass sediments suggest seagrasses contribute autogenic organic matter to the sediments. The mean stoichiometric ratio of porewater ∑CO2: ∑H2S in both the live and dead zone is greater than 5: 1, which is considerably higher than 2: 1 predicted by complete oxidation of organic matter via sulfate reduction. Precipitation of sulfides is not thought to be significant, and sulfide oxidation is only a partial explanation. Methane flux across the sediment/water interface ranged from 13.6 µmol m-2 d-1 from dead seagrass areas in the winter to 341 µmol m-2 d-1 from live seagrass areas in the fall; these rates are insignificant in a global context but are sufficient to calibrate an empirical sediment-water exchange model. The flux of sulfide from live-zone sediments was generally much greater than that from denuded areas. Our results demonstrate that seagrass beds influence the biogeochemistry of the carbonate sediments in Florida Bay.

Atmospheric Correction of AISA Measurements Over the Florida Keys Optically Shallow Waters: Challenges in Radiometric Calibration and Aerosol Selection

An Airborne Imaging Spectrometer for Applications (AISA) hyperspectral imager was deployed on a manned aircraft flown at 1305-m altitude to collect data over optically shallow waters in the Florida Keys with the ultimate goal of mapping water quality and benthic habitats. As a first step, we developed a practical atmospheric correction (AC) approach to derive surface remote-sensing reflectance (Rrs) from AISA measurements using radiative transfer simulations and constraints obtained from field spectral Rrs measurements. Unlike previously published method, the AC approach removes the surface Fresnel reflection and accounts for aircraft altitude and nonzero near-infrared (NIR) reflectance through iteration over the pre-established lookup tables (LUTs) based on MODTRAN calculations. Simulations and comparison with concurrent in situ Rrs measurements show the feasibility of the approach in deriving surface Rrs with acceptable uncertainties. The possibility of errors in the radiometric calibration of AISA is demonstrated, although a definitive assessment cannot be made due to lack of enough concurrent in situ measurements. The need for noise reduction and the difficulty in carrying out a vicarious calibration are also discussed to help advance the design of future AISA missions.

Physical and biological control of mangrove pore water chemistry.

Mangrove swamps comprise the largest fraction of tropical and subtropical intertidal wetlands, occupying over 20 million hectares worldwide (Chapman, 1976; McVey and May, 1987). Their importance as the basis of detrital food webs and as protected habitat for juvenile fish and shellfish has been well documented (Odum and Heald, 1972; Rodelli, Gearing, Gearing, Marshall and Sasekumar, 1984).

Short-term changes of remote sensing reflectancein a shallow-water environment: observations from repeated airborne hyperspectral measurements

ABSTRACT An atmospheric correction algorithm has been developed for the Airborne Imaging Spectrometer for Applications (AISA) imagery over optically shallow waters in Sugarloaf Key of the Florida Keys. The AISA data were collected repeatedly during several days in May 2012, October 2012, and May 2013. Non-zero near-infrared (NIR) remote-sensing reflectance (Rrs) was accounted for through iterations, based on the relationship of field-measured Rrs between the NIR and red wavelengths. Validation showed mean ratios of 0.94–1.002 between AISA-retrieved and in situ Rrs in the blue to red wavelengths, with uncertainties generally <0.003 sr–1. Such an approach led to observations of short-term changes in AISA-retrieved Rrs from repeated measurements over waters with bottom types of seagrass meadow, sand, and patch reef. Some of these changes are larger than twofold the Rrs uncertainties from AISA retrievals, therefore representing statistically significant changes that can be well observed from airborne measurements. Through radiative transfer modelling, we demonstrated that short-term Rrs changes within 1 hour resulted primarily from sediment resuspension, while tides played a relatively minor role due to the small variation in tidal heights. A sensitivity analysis indicated that although Rrs generally increases with decreasing tide height but increasing suspended sediments, more changes were observed over sandy bottom than over seagrass. The case study suggests that repeated airborne measurements may be used to study short-term changes in shallow-water environments, and such a capacity may be enhanced with future geostationary satellite missions specifically designed to observe coastal ecosystems.