On the connectivity and “black water” phenomena near FKNMS:
A remote sensing perspective
Chuanmin Hu and Frank E. Muller-Karger
Institute for Marine Remote Sensing, College of Marine Science, University of South Florida
140 Seventh Ave. South, St. Petersburg, FL 33701, USA.
Phone: (727)5533987, Email: hu@seas.marine.usf.edu
, Internet: http://imars.usf.edu
Abstract
Satellite color images from SeaWiFS and MODIS for 1997-2004 were examined for dark water plumes or patches in the Southwest Florida coastal waters, particularly near the Florida
Keys National Marine Sanctuary. Most of these dark features appear to originate from coastal runoff from the Everglades Rivers and from the vicinity of Charlotte Harbor and Sanibel Island.
Because the FKNMS is located downstream of these areas, the dark waters, usually rich in nutrients and living and non-living materials, may present some stress to the local ecosystem.
Remote waters, such as those from the Mississippi River plume and from the Caribbean, may be brought to this region with ocean circulation (e.g., the Loop Current). Florida’s red tides, which occur every year along the west coast, may meander towards the Keys as they follow the dominant southward water movement. The connectivity of the FKNMS to local and remote systems requires an integrated observing system to better understand the effects of these dark patches on the delicate ecosystem.
Introduction
The Florida Keys National Marine Sanctuary (FKNMS) is one of the nation's premier marine protected areas, and it annually attracts 3 million tourists who spend 1.2 billion dollars (Causey,
2002). It is home to its world-renowned coral reefs and to many important commercial and recreational fisheries, and in particular is a critical location for early larval development of a number of marine species.
Similar to many other delicate marine ecosystems, the FKNMS is not isolated, but connected to the deep ocean, land, and the atmosphere through various processes including ocean circulation, terrestrial discharge, and atmospheric deposition ("Three Screen Doors", Jameson et al., 2002). The FKNMS is located in close proximity to an ever-growing urban South Florida environment and to the pollutant source of the commercial agriculture interests of central
Florida, therefore has been under significant environmental stress. Further, water-circulations may bring the Mississippi River water to the Keys (Ortner et al., 1995).
Recently, some “black water” events have been reported in the vicinity of FKNMS (Hu et al.,
2003; Hu et al., 2004; Neely et al., 2002; SWFDOG, 2002). These are characterized as discolored dark waters with high concentrations of tiny marine plants (phytoplankton) and yellow substance (colored dissolved organic matter, CDOM) where fish could not be found.
Adverse impact may occur to the benthic habitat after a prolonged “black water” event, even though the mechanism is not very clear (Hu et al., 2003).
The cause of “black water” is still a research topic and it requires an integrated study that combines multi-disciplinary observations and modeling. Because the FKNMS is located downstream of the Everglades discharge and several major rivers, one hypothesis is that it may
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be related to coastal runoff. As remote sensing has proven a powerful tool to study large-scale connectivity (Andrefouet et al., 2002), here we show several examples of how FKNMS is connected to other ecosystems. Specifically, we will show frequency of the dark water events and discuss future directions for an improved observing system.
Method
We use satellite remote sensing data to show the phenomena, as these are operationally available.
Color data are from the Moderate Resolution Imaging Spectroradiometer (MODIS, Esaias et al., 1999) and the Sea-viewing Wide Field-of-View Sensor (SeaWiFS, McClain et al., 1998), and sea surface temperature (SST) data are from MODIS and the Advanced Very High Resolution
Radiometer (AVHRR). These data were captured, processed, archived, and broadcasted in near real-time at the University of South Florida with standard algorithms as well as software developed in house. Of particular interest are the color data, which yield information on water constituents including phytoplankton abundance (through chlorophyll concentration), CDOM
(through its absorption), total suspended sediment concentration, and bathymetry. As the algorithms to retrieve these parameters from satellite measurements are not robust in the complex coastal environment, data were used in a relative sense. Specifically, coastal features were examined through three color bands (555 or 551, 490, and 443 nm) after atmospheric effects were removed. Fluorescence line height data from MODIS were examined to see if a certain feature is due to phytoplankton bloom. SST data were used mainly to see if a feature is associated with coastal upwelling.
We examined the daily color imagery (1-km resolution) from September 1997 (beginning of
SeaWiFS data collection) to date. Below are some examples, and a complete series is available upon request.
Results and discussion
Figure 1 shows some images where dark water plumes or patches can be found near the
Florida Keys. The colors do not reflect the real colors when viewed with a human eye, but were chosen to stretch the contrast between various features. For example, the dark color indicates high concentrations of phytoplankton and/or CDOM. Further, the various colors can be used as effective tracers to monitor water movement.
The images show the complexity of the water environment. Dark water plumes or patches are not uncommon in the south Florida coastal waters. Coastal runoff, upwelling, and red tides can all make water appear dark (their shades may be different, though). It is often (but not always) to trace the dark water’s origin with its distinctive color. For example, the images of 11/27/1997,
12/9/1997, 10/28/2000, and 11/30/2002 show that the dark/brownish “streamers” originated from the Everglades Rivers. The images of 10/30/1998, 8/30/2001, 2/15/2003, and 10/21/2003 show the dark water flows from the vicinity of Charlotte Harbor to the Dry Tortugas. The movement can be better revealed if a continuous series is animated. For example, the 4/20/2002 image shows that there is a wide band of dark water along the SW Florida coast. Image series shows that the water originated from the south near the Florida Keys, i.e., the water moved from the south to the north, as compared with the typical north-south water movement direction in this area.
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Figure 1. SeaWiFS images showing dark water plumes or patches near the Florida Keys. The RGB images were composed with water-leaving radiance at 555 nm (red), 490 nm (green), and 443 nm (blue) with land (gray) masked out. The RGB data were stretched to show various features in the water, therefore the appearance does not represent the real color of water when viewed with a human eye. The offshore blue color represents the dark blue ocean. The nearshore white color (if not clouds) is for shallow waters where bottom can be seen or high concentration of suspended particles present in the water. The brown-red color represents high concentration of phytoplankton or non-living particles (this type of water often carries high amount of CDOM). Dark color indicates high concentrations of phytoplankton and/or CDOM. In really the dark color may appear as dark greenish or dark brownish water, depending on the relative amount of the two substances. The images were arranged in the order of 3 panels/year from 1997 to 2003. See text for more details.
Coastal runoff alone can result in dark water patches, as shown in Hu et al. (2004). Extreme climate conditions, such as higher than normal rainfall or heavy storms from hurricanes, may cause excessive coastal runoff from either river discharge or land drainage, which contain high concentrations CDOM that strongly absorb sunlight and high concentrations of nutrients that may stimulate phytoplankton blooms. For example, the dark water patches of 10/24/1999 and
11/22/1999 were a result of excessive discharges from the Everglades Rivers after Hurricane
Irene. The image of 6/17/1998 was a result of higher than normal discharge of the Suwannee
River due to high rainfall in the 1997-1998 El-Nino period (upwelling on the west Florida shelf earlier this year also contributed partially to this dark plume).
Examination of the complete series shows that most of the dark patches occur in the fall, after the wet season of Florida, and they clearly show the coastal origin. However, there are some cases when the reason of dark water is not so clear. For example, the early 2002 dark water event, as shown in the 2/4/2002 image, was thought to be at least partially from the earlier extensive red tide along the SW Florida coast (SWFDOG, 2002). Indeed, red tide may appear very similar in satellite color imagery to other dark water patches that contain non-toxic phytoplankton blooms and/or high concentrations of CDOM (Hu et al., submitted). It is presently very difficult to differentiate what substance(s) a dark color patch may contain, due to technical difficulties in satellite data processing (i.e., the algorithms to obtain meaningful parameters from satellite measurements). Hu et al. (2004) showed that the use of the MODIS fluorescence data helped distinguish if a dark patch contains phytoplankton bloom (Figure 2). However, it is
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currently impossible to tell if a bloom is toxic (red tide) or not. Certainly, field samples under these circumstances can help tell the composition of the water.
83 o W 82 o W 83 o W 82 o W
Oct. 20, 2003
MODIS/Aqua
Enhanced
RGB
Oct. 20, 2003
MODIS/Aqua
Fluor Line
Height
(a)
83 o W 82 o
W
(b)
83 o W 82 o W
Figure 2. MODIS color imagery shows dark water plume from Charlotte Harbor to Dry Tortugas. (a) Enhanced
RGB composite showing the plume; (b) Fluorescence data showing high concentration of phytoplankton in the upstream of the plume. Fluorescence increases from dark blue, light blue, green, yellow, to red.
In addition to local connectivity, ocean circulations may bring “alien” waters to the Florida
Keys. The Loop Current (Fratantoni et al., 1998) may bring waters from the Caribbean to the
Florida coasts and it may also create local eddies to “pump” the bottom nutrients to the surface to stimulate phytoplankton blooms. Further, it may “drive” the waters from the Mississippi plume to the Keys (Figure 3). Similar river plume features can also be found in other years, for example, in 1998, 1999, 2000, 2001, 2003, and 2004, respectively. The year 2002 is the only year between 1997-2004 that did not show apparent river plume that reached the Florida Keys, but during spring-summer eastward and southeastward movement of the Mississippi plume near the Mississippi Delta was observed. This plume entered the west Florida shelf and dissipated gradually, and was believed to reach the Florida Keys eventually following the dominant southward current direction.
Episodic dark water events may present no harm to the Florida Keys ecosystem, but prolonged events, such as the one in early 2002 (Hu et al., 2003), may cause some stress. For
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example, the coral reef decline and benthic degradation, following the 2002 “black water” event, was thought to be related to the event, although the exact reason is unknown.
Figure 3. SeaWiFS color imagery on 4 August 2004 shows the Mississippi River plume that reaches the Florida
Straits, following the path of the Loop Current.
Certainly, remote sensing cannot provide all the information needed for this complex ecosystem. Understanding the connections of the Florida Keys ecosystem to nearby or remote regions via coastal and oceanic circulation patterns requires an approach that integrates various observing technologies. Of particular importance is field sampling, which can be used to study the vertical structure of the water mass, hydrography, composition of various phytoplankton species, nutrient availability. Hu et al. (2004) showed an example of how field information can be used to verify if a dark patch is from coastal runoff (characterized by low salinity) or generated by local upwelling. Details of field programs can be found elsewhere in this proceeding. Nevertheless, these observation/modeling efforts should be an integrated component in an effective management plan for coastal resources. This is particularly important not only for the FKNMS, but also for the entire SW Florida ecosystem, as during the next 10 to 20 years the
Everglades Restoration, an effort between Federal, State, County, and local organizations, has the potential to impact the entire South Florida coastal ecosystem, including coral reefs and fisheries.
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Conclusion
We examined the complete daily image series from SeaWiFS and MODIS covering the period of September 1997 – September 2004 for dark water patches in the SW Florida coastal waters near the Florida Keys. The dark color is an indication of high concentrations of phytoplankton and/or colored dissolved organic matter, rich in coastal runoff.
Dark water patches in this area are not uncommon, as revealed by the satellite imagery. Their appearance frequency, time, and spatial extent vary with time and space. Generally, more dark patches are found in the fall than in the spring, likely result from coastal runoff after the rain season. However, Florida’s late summer/fall red tides, which occur every year although their intensity and coverage vary from year to year, may also contribute to the dark color. The situation is further complicated when discharges from remote rivers such as the Suwannee and the Mississippi may reach the Florida Keys.
Clearly, the FKNMS is not isolated, but connected to other ecosystems (Lee et al., 2002). An integrated observing system that combines remote sensing, field observation, and modeling is important to understand the various processes that may affect the FKNMS. In particular, a fast response team to study anomaly events is required to better under stand the nature, origin, and impact of these events on the local ecosystem.
Acknowledgement
This work was supported by the National Aeronautics and Space Administration (NAG5-
10738). SeaWiFS data are property of Orbimage Corp., and their use here is in accordance with the SeaWiFS Research Data Use Terms and Conditions Agreement of the NASA SeaWiFS project. We thank the NOAA/AOML (Elizabeth Johns) and UM/RSMAS (Thomas Lee) for providing field hydrography data in this study.
Reference
Andréfouët, S., Mumby, P.J., McField, M., Hu, C., and Muller-Karger, F.E. (2002). Revisiting coral reef connectivity. Coral Reefs, 21, 43-48.
Causey, B. D., The role of the Florida Keys National Marine Sanctuary in the South Florida
Ecosystem Restoration Initiative, pp. 883-894. In J.W. Porter and K.G. Porter, Eds. The
Everglades, Florida Bay, and Coral Reefs of the Florida Keys, An Ecosystem Source Book.
CRC Press, Boca Raton, FL., 2002.
Esaias, W.E., Abbott, M.R., Barton, I., Brown, O.B., Campbell, J.W., Carder, K.L., Clark, D.K.,
Evans, R.H., Hoge, F.E., Gordon, H.R., Balch, W.M., Letelier, R., and Minnett, P.J. (1998).
An overview of MODIS capabilities for ocean science observations, IEEE Trans. Geosci.
Remote Sens., 36, 1250-1265.
Fratantoni, P. S., T. N. Lee, G. P. Podesta and F. Muller-Karger. The influence of Loop Current variability on the formation and evolution of cyclonic eddies in the Southern Straits of
Florida. J. Geophys. Res.
103:24,759-24,779, 1998.
Hu, C., K. E., Hackett, M. K. Callahan, S. Andréfouët, J. L. Wheaton, J. W., Porter, and F. E.
Muller-Karger, The 2002 ocean color anomaly in the Florida Bight: A cause of local coral reef decline? Geophys. Res. Lett.
30(3), 1151, doi:10.1029/2002GL016479, 2003.
8

Hu, C., F. E. Muller-Kager, G. A. Vargo, M. B. Neely, and E. Johns (2004). Linkages between coastal runoff and the Florida Keys ecosystem: A study of a dark plume event. Geophys. Res.
Lett. 31, L15307, doi:10.1029/2004GL020382.
Hu. C., R. Luerssen, F. E. Muller-Karger, K. L. Carder, and C. A. Heil. In search of red tides:
Observations on the west Florida shelf. Cont. Shelf Res.
Submitted.
Jameson, S. C., M. H. Tupper, and J. M. Ridley, The three screen door: Can marine "protected" areas be effective? Marine Pollution Bulletin 44:1177-1183, 2002.
Lee, T. N., E. Johns, D. Wilson, E. Williams, and N. Smith, Transport processes linking south
Florida coastal ecosystems. In J.W. Porter and K.G. Porter, Eds. The Everglades, Florida
Bay, and Coral Reefs of the Florida Keys, An Ecosystem Source Book.
CRC press, Boca
Raton, Fla., pp. 309-342, 2002.
McClain, C.R., Cleave, M.L., Feldman, G.C., Gregg, W.W., Hooker, S.B., and Kuring, N.
(1998). Science quality SeaWiFS data for global biosphere research. Sea Technology , 39, 10-
16.
Neely, M. B., E. Bartels, J. Cannizzaro, K. L. Carder, P. Coble, D. English, C. Heil, C. Hu, J.
Hunt, J. Ivey, G. McRae, E. Mueller, E. Peebles, and K. Steidinger (in press). Florida's black water event. Harmful Algae 2002. Proceedings of the Xth International Conference on
Harmful Algae. Steidinger, K. A., Landsberg, J. H., Tomas, C. R., and Vargo, G. A. (Eds.).
Florida Fish and Wildlife Conservation Commission, Intergovernmental Oceanographic
Commission of UNESCO, Florida Institute of Oceanography.
Ortner, P. B., T. N. Lee, P. J. Milne, R. G. Zika, E. Ckarke, G. Podesta, P. K. Swart, P. A. Tester,
L. P. Atkinson, and W. R. Johnson (1995). Mississippi River flood waters that reached the
Gulf Stream. J. Geophys. Res.
100:13595-13601.
SWFDOG, Satellite images track “black water” event off Florida coast,
EOS, Trans., AGU ,
83:281,285, 2002.
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