Dark Water Events in Florida

Read summaries of several recent dark water events in Florida and learn how they can affect humans and ecosystems.

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.

Figure 1
Fig. 1 – The two satellite images show
the dark water event spreading along the
coast in November and December 2011.
Photo Credit: NOAA

Figure 2
Fig. 2 – A satellite image from October
2003 shows the plume of dark water
extending to the Dry Tortugas.
Credit: University of South Florida.

Figure 3
Fig. 3 – The 2001-2002 dark water
event extended from offshore of
Charlotte Harbor to the Florida Keys.
Photo credit: University of South Florida.

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 PDF icon

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 PDF icon.

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 PDF icon). 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.

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