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Dark Water Update—2004

Event History
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 available.

Findings
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 event.

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:

1. The magnitude and duration of the associated algal blooms
2. The magnitude and duration of the stable water mass
   
3. The climatic extremes in 2001 followed by a mild winter in south Florida
   
4. 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:

1. 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.



2. 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.