In early 2002, a dark water event, also called "black water" by
the media, occurred off the coast of southwest Florida. Samples of
the water, which was described as dark brown-black-green, showed a
dominance of large, centric diatoms.
A dark water event, or "black water" as the media called it,
occurred off the coast of southwest Florida early in 2002. The
phenomenon was first observed in January 2002. On March 1, 2002,
the National Marine Fisheries Service (NMFS) reported the dark
water event to the Florida Fish and Wildlife Conservation
Commission's (FWC) Florida Marine Research Institute (FMRI). Close
observers characterized the water as dark brown-black-green. FMRI,
Mote Marine Laboratory, and University of South Florida personnel
analyzed a variety of samples, and the institute continued
monitoring the event until its conclusion. The chronology of the
Event Status was published on this Web site as information became
Microscopic algae (phytoplankton) identification and cell counts
indicate large, centric diatoms (Fam. Rhizosoleniaceae) dominated
the dark water. The Florida red tide organism, Karenia
brevis, and its toxin was either absent or found in
low-to-moderate amounts. Within the same region, a moderate red
tide occurred before and during the winter dark water event.
Brevetoxin, the toxin associated with Florida's red tide, can
linger in the environment for a period of time after Karenia
brevis cells return to background concentrations. This
prolongs the risk to marine animals.
Throughout 2001, southwest Florida faced climatic extremes. In
the winter and spring of 2002, severe drought conditions preceded
fewer and less severe weather fronts. While currents usually move
water in this region southward through the Lower Keys at this time
of year, the water was not undergoing a usual pattern of movement.
Oceanographic drifters indicate the water between Naples and the
Marquesas circulated in a gyre (spiral form) during the dark water
Researchers propose that, under these weather and water
conditions, the normal winter diatom bloom from western Florida Bay
shifted, becoming superimposed over a waning red tide from the West
Florida Shelf. Both blooms co-occurred with water that appeared
dark because of colored dissolved organic matter (CDOM) that was
primarily from rivers.
Investigators at one discolored water location found high
numbers of "comb jellies." Comb jellies are nonstinging, gelatinous
organisms that feed on small crustaceans-or microzooplankton-in the
water column. By eating the microzooplankton that help keep the
phytoplankton population in check, the comb jellies may have
contributed to the continuation of the phytoplankton bloom. Comb
jellies could affect the color of the water "seen" by satellites
and may also explain reports of jelly-like blobs in the water.
In summary, there were several oceanographically unique features
of this event:
- The magnitude and duration of the associated algal blooms
- The magnitude and duration of the stable water mass
- The climatic extremes in 2001 followed by a mild winter in
- The high densities of comb jellies observed
Thus, the early 2002 dark water event was a combination of a
large diatom bloom and a red tide, depending on the location.
Therefore, the timing of sampling could have influenced
interpretation of the results.
Stony coral cover decline and boring sponge loss reported at two
sites in Florida Bay have been associated with the March 2002 dark
water. Two important points must be considered for the proper
interpretation of this information:
- The majority of stony coral at Content Key had succumbed to
some event prior to 1996. Mean stony coral cover was only 1%
between 1996 and 2000; it declined to 0.5% by the summer of 2002.
Smith Shoal averaged 15%-20% stony coral cover between 1996 and
2001 but declined to approximately 5% by 2002. Because a red tide
co-occurred with the dark water at the Content Keys and Smith Shoal
areas prior to the annual coral and sponge assessment, the
definitive cause of the coral and sponge declines cannot be
determined after the fact. Florida's red tide can kill benthic
organisms like sponges and coral in addition to fish. Laboratory
bioassays to evaluate the effects of red tide and diatoms on corals
are being developed by FMRI staff.
- Between 1996 and 2002, data analyzed from all the Coral Reef
Monitoring Program stations show that mean stony coral cover in the
Keys has remained low but stable since 1999, ranging between 7.3%
and 7.5%. Sanctuary-wide stony coral declines between 1996 and 1999
are likely attributable to the passage of two hurricanes, a
bleaching episode, and disease.
A second event, also dubbed "black water" by the media, was
reported in August 2002 near Sanibel Island, Florida. No marine
organism mortalities were reported for this event. Data collected
during this smaller event indicated two separate algal blooms were
occurring: a cyanobacteria bloom at the mouth of the Caloosahatchee
River and a diatom-dominated bloom in the Ten Thousand Islands
area. All analyses performed to date identify river water as the
source of nutrients for these blooms. Nutrient input from rivers is
seasonally associated with diatom blooms and can contribute to
cyanobacterial blooms seen in estuaries.
Reports of discolored water near the Dry Tortugas in October and
November 2003 are also due to CDOM associated with riverine inputs.
Nutrient concentrations were low in the samples, similar to what is
typically found in this region. In contrast to earlier events, the
water contained little phytoplankton; although, diatom blooms were
coincident in Florida Bay and at the mouths of the Caloosahatchee
and Shark Rivers. There have been no reports of marine organism
mortalities associated with this event.
Prior to July 1, 2004, the
Fish and Wildlife Research Institute (FWRI) was known as the
Florida Marine Research Institute.