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Dark Water Events in Florida

Dark water, or black water, events occur occasionally along Florida’s coasts and are characterized by patches of water that appear dark or black. The dark color occurs when high concentrations of phytoplankton (microscopic algae) or colored dissolved organic matter in the water absorb the blue light.

Phytoplankton blooms, colored dissolved organic matter, river runoff and water circulation play varying roles in the formation of dark water patches, as highlighted by the following black water events in southwest Florida:

Winter 2011 to Spring 2012: This event began as a phytoplankton bloom in the Gulf of Mexico north of Charlotte Harbor in fall 2011. The dark water expanded and moved south, extending from the coast to the waters north of the Florida Keys by January 2012 and dissipating by April (Fig. 1). The size of the 2012 dark water event was much less than that of a similar event 10 years prior, covering on average about 500 square miles compared to approximately 2,300 square miles in 2002. The patches of dark water off southwest Florida contained relatively low concentrations of colored dissolved organic matter but high phytoplankton biomass, with varying concentrations of Karenia brevis (Florida red tide organism) and nontoxic mat-forming diatoms, which are microscopic phytoplankton with cell walls made of silica. To learn more about this event, read the following University of South Florida publication: Zhao et al. 2013.

Fall 2003: This dark water event originated near Charlotte Harbor following a period of higher than average rainfall. In mid-October, northeasterly winds pushed the plume of dark water offshore toward the Dry Tortugas (Fig 2). The cause of the dark water varied as the plume moved offshore. In nearshore waters, a dense phytoplankton bloom was the primary contributor; however, once offshore, phytoplankton biomass decreased and the dark water plume was sustained by colored dissolved organic matter. For more on this event, read Hu et al. 2004.

Fall 2002: Dark water near Sanibel Island was linked to two phytoplankton blooms – a cyanobacterial (blue-green algae) bloom at the mouth of the Caloosahatchee River and a diatom-dominated bloom in the Ten Thousand Islands area.

Winter 2001 to Spring 2002: This large and lengthy dark water event began offshore of Charlotte Harbor in late 2001 and spread south to the Florida Keys by March 2002 (Fig. 3). During this time period, the area covered by dark water ranged from more than 1,200 square miles to almost 3,700 square miles (Zhao et al. 2013). Nontoxic mat-forming diatoms were most abundant in the dark water, but a waning toxic red tide from the West Florida Shelf was also present. The co-occurring blooms combined with water containing colored dissolved organic matter from rivers to cause the dark color (Hu et al. 2003). Clockwise water circulation retained the dark water in early 2002, likely contributing to its size and duration (SWFDOG 2002).

Effects of Dark Water on Human Health and Ecosystems
Dark water events do not directly affect human health unless toxic phytoplankton such as K. brevis are present.

Dark water events can negatively affect ecosystems in several ways:

  • Discolored water reduces the amount of light reaching the sea bottom, which is detrimental to seagrasses that depend on sunlight for photosynthesis.
  • Bacterial breakdown of blooms can deplete oxygen in the water that bottom-dwelling organisms need to survive.
  • Co-occuring red tide toxins can poison fish and other organisms.


Hu C, Hackett KE, Callahan MK, Andrefouet S, Wheaton JL, Porter JW, Muller-Karger FE. 2003. The 2002 ocean color anomaly in the Florida Bight: A cause of local coral reef decline? Geophysical Research Letters 30 (3): 1151.

Hu C, Muller-Karger FE, Vargo GA, Neely MB, Johns E. 2004. Linkages between coastal runoff and the Florida Keys ecosystem: A study of a dark plume event. Geophysical Research Letters. 31 L15307, doi:10.1029/2004GL020382.

The South-West Florida Dark-Water Observations Group (SWFDOG). 2002. Satellite images track “black water” event off Florida coast. EoS Trans. AGU 2002, 83, 281–285.

Zhao J, Hu C, Lapointe B, Melo N, Johns EM, Smith RH. 2013. Satellite observed black water events off southwest Florida: Implications for coral reef health in the Florida Keys National Marine Sanctuary. Remote Sensing 5: 415-461.