An extensive die-off of bottom-dwelling species occurred in the summer of 2005 during a red tide offshore of west central Florida.
What happened to the reefs off Pinellas County?
Was this event related to the 2005 red tide?
What happened to the bottom communities?
What is hypoxia?
What is anoxia?
Why, when we have red tides almost every year, was anoxia unusually associated with the 2005 red tide?
Why was there a sulfur smell associated with this event?
Why did the jewelry that divers wore tarnish?
Has a bottom mortality event like this happened before?
Did the same bottom mortalities happen within Tampa Bay, since the red tide was there also?
How soon did the reefs recover?
Did pollution cause the 2005 red tide?
Why were the fish kills so bad in 2005?
Were these the worst fish kills in history?
Why did so many turtles die during the 2005 red tide?
Did this red tide affect turtle hatchlings?
Did this red tide affect manatees?
How did the benthic mortality event of August 2005 off west central Florida differ from the "dead zone" that occurs every year at the mouth of the Mississippi River?
References
What happened to the reefs off Pinellas County?
During the first week of August 2005, divers and scientists began documenting die-offs of bottom-dwelling (benthic) marine species at patch reefs offshore from Tarpon Springs to Sarasota. Divers with the FWC's Fish and Wildlife Research Institute, along with volunteers, found varied effects when they first inspected the reefs. They observed no live animals at the most heavily affected reefs. Some reefs showed some effect but had living organisms; others were unaffected.
Later, researchers from the University of South Florida documented the greatest effects on hard corals, porifera (sponges), and echinoderms (animals that include starfish, sea urchins, and sand dollars). Dupont et al. (2010) detailed a more than 50% decline in fish species richness (number of species) as well as significant reductions in the abundances of most species.
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Was this event related to the 2005 red tide?
Yes. The affected region lay within the central region of the large red tide that was both nearshore and offshore. Soft bottom habitats, not just reefs, were also affected.
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What happened to the bottom communities?
Scientists hypothesized that several factors may have contributed to the widespread animal mortalities (die-offs) observed, beginning in mid-July 2005, during a red tide that appeared earlier that year in a large coastal region to the west of central Florida.
The red tide dinoflagellate cells (microalgae known as Karenia brevis, or K. brevis for short) contain substances called brevetoxins that can kill fish and other marine life. Karenia brevis is phototactic, which means it senses light and swims up toward sunlight. Most red tides thus have cells most concentrated near the surface, but they can be distributed down to 100 feet. In well-mixed waters, K. brevis can swim at a rate of 3 feet per hour.
During the summer of 2005, intense heating of the coastal nearshore surface waters by the sun resulted in a "layering" of warmer, less dense seawater over a cooler, denser layer near the bottom. An abrupt vertical change in water temperature of 3-5°F over a small depth change can act as a physical barrier to red tide cells, trapping the cells in the cooler bottom layer. Bottom-dwelling animals that come into contact with red tide cells and their toxins may be killed.
Bacterial decomposition of dead animals and K. brevis cells decreases oxygen concentrations, which may further stress other animals, eventually resulting in mass mortalities and low oxygen (hypoxia) to no oxygen (anoxia) near the bottom. The large temperature difference between the two water layers further prevents dissolved oxygen from diffusing from surface layers to the bottom, worsening the oxygen depletion.
Fish killed on the surface by red tide toxin may also sink to the bottom during decay and contribute to a decrease in dissolved oxygen. The large quantity of red tide toxin that sinks also kills more bottom-dwelling animals.
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What is hypoxia?
Hypoxia refers to extremely low (<2.0 milligrams per liter) concentrations of dissolved oxygen in seawater. These are the concentrations at which animals become stressed.
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What is anoxia?
Anoxia refers to zero concentrations of dissolved oxygen in seawater. Most sea life cannot live under anoxic conditions.
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Why, when we have red tides almost every year, was anoxia unusually associated with the 2005 red tide?
Although red tides can start throughout the year, the normal "season" when most red tides start is from late August through November. Surface heating of seawater, which may trap red tide cells, is not characteristic of fall red tides. The red tide in 2005 also persisted over the same area for a longer period of time, allowing more toxins to build up in the system.
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Why was there a sulfur smell associated with this event?
The "rotten egg" smell of sulfur relates to changes in the chemistry of seawater when anoxia occurs. Normally, some bacteria use dissolved oxygen from seawater during decomposition, which yields carbon dioxide (COs), ammonia (NH3), and phosphate (PO4) as byproducts. When oxygen concentrations in seawater are low or zero, bacteria derive oxygen from other chemical compounds, including manganese dioxide (MnO2), nitrate (NO3), iron oxide (FeO3), and sulfate (SO4). From sulfate, this natural process produces sulfur, which is responsible for the smell.
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Why did the jewelry that divers wore tarnish?
The tarnishing of silver jewelry worn by divers who were documenting the event was probably the result of contact with sulfur underwater.
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Has a bottom mortality event like this happened before?
Yes, similar die-offs of bottom-dwelling animals occurred during a summer red tide in 1971. A large area of anoxia occurred offshore of Sarasota County in about the same area as the 2005 event when an abrupt change in water temperature (called a thermocline) trapped a red tide below it. Over an area of more than 580 square miles, researchers documented mortalities of bottom-dwelling animals including sponges, corals, molluscs, crabs, echinoderms, and fish on the patch reefs (Simon and Dauer, 1972; Smith, 1975).
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Did the same bottom mortalities happen within Tampa Bay, since the red tide was there also?
No. Nor did it happen in 1971, when red tide penetrated the upper reaches of Tampa Bay and an anoxic red tide event occurred offshore. Tampa Bay is shallower than the Gulf and tends to be well mixed from surface to bottom by winds and tides. Red tide cells concentrate in surface waters within the bay because the temperature layering observed offshore does not occur.
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How soon did the reefs recover?
The amount of time required for recovery of both the reef and the animals on the reef varies with the specific animals and benthic communities involved. Dr. Gregory Smith studied reefs affected by the 1971 red tide off Sarasota for more than five years and documented recovery of diversity of fish populations within 12 to 18 months of the end of the event. For the 2005 event, Dupont et al. (2010) documented recovery within two to three years for some species of slower-growing animals such as coral and sponges.
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Did pollution cause the 2005 red tide?
No, there was no evidence of a direct link between the 2005 Florida red tide and nutrient pollution. The red tide started approximately 20 miles offshore of Pinellas County, beyond the influence of coastal nutrients. Scientists know this from reports and water samples that anglers provided to FWRI in early January 2005, as well as from additional water sampling to verify satellite imagery of high-chlorophyll areas offshore.
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Why were the fish kills so bad in 2005?
The large number of fish mortalities relates to the extent and duration of the red tide. The Florida red tide organism, Karenia brevis, contains substances called brevetoxins that fish ingest by feeding on the cells directly, by absorbing the toxins from seawater across the gills, or by consuming accumulated toxins within their prey. Fish that swim in the upper surface areas are the first affected.
Early in a red tide, the amount of toxin dissolved in seawater is relatively low. Fish such as thread herring and mullet that feed on K. brevis and other microscopic algae may be relatively unaffected at first. Toxin concentrations in seawater increase in the later stages of a bloom, resulting in more fish kills. Toxins in dead animals and attached to particles start to sink to the bottom, where the effects are compounded on reef fish such as grouper and snapper. These species also prey on live fish that have consumed toxins.
With toxins widely distributed through the water column, conditions for these bottom-dwelling fish deteriorated in the summer of 2005. The combination of low dissolved oxygen, bacterial decomposition from dead animals and red tide cells, and high concentrations of brevetoxin and hydrogen sulfide contributed to large-scale fish kills involving a wide variety of fish species occupying different habitats.
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Were these the worst fish kills in history?
No. Large fish kills have been reported in southwest Florida coastal waters for hundreds of years. However, it is difficult to determine the total number of fish killed, because fish first sink and then float (when the air bladder fills) during decomposition. Wind and tides also may carry floating fish to other places. Determining the number of fish killed would be a massive, costly effort involving many simultaneous observations on the beach and water, as well as on the sea bottom for hundreds to thousands of square miles. Most observations of fish kills are thus descriptive and difficult to compare directly. However, the longer the red tide, the more fish are likely to be killed.
The 2005 red tide did cause a short-term decline in juvenile spotted seatrout, sand trout, and red drum population numbers within Tampa Bay (Flaherty and Landsberg, 2010). By the following year, juvenile fish population numbers had recovered to normal. The 2005 red tide also had a documented effect on adult spotted seatrout, described in the article 2005 Red Tide Impacts on Fish Spawning in Tampa Bay.
Quoted below are a few observations of fish kills from previous years' red tides:
1880: "The dead fish were most numerous on the outside beaches and on the inside beaches" (Walker, 1884).
1917: "Fishes of a great number of species were noted dead and dying" (Taylor, 1917).
1946-47: "In a preliminary note regarding the 1947 Red Tide, it was very conservatively estimated that 50,000,000 fishes were killed. … Since that time the area increased and a recheck of the area and the numbers of fishes seen floating and washed ashore leads to the corrected estimate that the fishes killed must have numbered in the neighborhood of a half billion" (Gunter et al., 1948). "Heavy masses of fish were reported 5 miles off Gulf Beaches. … Fish were reported 32 miles northwest of Clearwater beach and 19 miles out" (Feinstein et al., 1955). "Practically all species of fish, including such large forms as tarpon and [goliath grouper], were included in the victims of the red water" (Rounsefell and Nelson, 1966).
1953: "Millions of dead fish from the Dry Tortugas to Ft. Myers" and "Dead fish from red tide were stretched from Venice to Indian Rocks" (Feinstein et al., 1955).
1971: "Severe kill of reef fishes, 80-90 percent of offshore resident reef fish species perished" (Smith, 1975). "St. Petersburg city officials estimated that 2,367 tons of fish were removed during cleanup, costing approx. $155,763" (Steidinger and Ingle, 1972).
1995: "Extensive Fish Kills in more than 5,000 km2 of open water . . . between Sanibel and Tampa Bay" (Tester and Steidinger, 1997).
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Why did so many turtles die during the 2005 red tide?
Sea turtles are susceptible to red tide toxins. The number of sea turtles this bloom affected probably relates to its large size and the large number of resident sea turtles in the area. Because toxin was distributed into the bottom areas where these species feed and likely was in their prey (crabs, sponges, and molluscs), they were exposed to brevetoxins more frequently than normal. More than 100 loggerheads (Caretta caretta), green turtles (Chelonia mydas), and Kemp's ridley turtles (Lepidochelys kempii) (FWC, Sea Turtle Stranding and Salvage Network Data Base) died during this red tide (Landsberg et al., 2009).
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Did this red tide affect turtle hatchlings?
FWRI scientists have documented unusually high sea turtle mortality during many red tide events but no unusual hatchling deaths during the 2005 event or others. Several blooms occurred during the summer, when hatchling sea turtles were making their way from the beach to offshore water. Because hatchlings swim quickly offshore, they may not linger in areas of red tide long enough to incur any noticeable mortality.
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Did this red tide affect manatees?
Yes. Biologists responded to early reports of dead manatees throughout southwest Florida in areas where red tide counts were higher than normal for several months. As biologists recovered animal carcasses and conducted necropsies (nonhuman autopsies), it became obvious that these animals were dying as a result of exposure to red tide. Refer to the article Red Tide Manatee Mortalities and select 2005 for details.
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How did the benthic mortality event of August 2005 off west central Florida differ from the "dead zone" that occurs every year at the mouth of the Mississippi River?
The dead zone off the Louisiana coast each summer is directly associated with Mississippi River discharge. The river delivers the inorganic nutrients nitrate (NO3), ammonium (NH4), and phosphate (PO4) to the coast, causing large blooms of fast-growing, nontoxic diatoms. These blooms die and sink to the bottom, along with waste material from small animals feeding on the diatoms. Warmer surface waters act as a barrier, trapping this material in deeper, cooler waters, where it decomposes and uses up oxygen. Lower concentrations of oxygen stress animals and cause many to leave the area. Animals that cannot escape die, leading to more decomposition and oxygen reduction and, ultimately, anoxia.
The Florida red tide is not associated with the Louisiana dead zone. Red tide only rarely occurs off the Louisiana coast. Scientists have found that no single nutrient source--such as the Mississippi River in Louisiana--provides enough nutrients to support a red tide.
The area of anoxia off west central Florida resulted from an unusual summer bloom of toxic algae. Red tides in Florida usually start in fall, when waters are well mixed and oxygenated, and last for several months. The 2005 red tide lasted through summer, when the sun warms surface coastal waters much more than bottom waters.
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References
Dupont, J., P. Hallock, and W.C. Jaap. 2010. Ecological impacts of the 2005 red tide on artificial reef epibenthic macroinvertebrate and fish communities in the eastern Gulf of Mexico. Marine Ecology Progress Series 415:189-200.
Feinstein, A. A.R. Ceurvels, R.F. Hutton, and E. Snoek.1955. Red tide outbreaks off the Florida west coast. Florida State Board of Conservation, The Marine Laboratory, University of Miami, Coral Gables, Florida. 39 p.
Flaherty, K.E., and J.H. Landsberg. 2011. Effects of a persistent red tide (Karenia brevis) bloom on community structure and species-specific relative abundance of nekton in a Gulf of Mexico estuary. Estuaries and Coasts 34:417-439.
Gunter, G., R.H. Williams, C.C. Davis, and F.G.W. Smith. 1948. Catastrophic mass mortality of marine animals and coincident phytoplankton bloom on the west coast of Florida, November 1946 to August 1947. Ecological Monographs 18:309-324.
Landsberg, J.H., L.J. Flewelling, and J. Naar. 2009. Karenia brevis red tides, brevetoxins in the food web, and impacts on natural resources: Decadal advancements. Harmful Algae 8:598-607.
Rounsefell, G.A., and W.R. Nelson. 1966. Red tide research summarized to 1964, including an annotated bibliography. Special Scientific Report No. 535. U.S. Fish and Wildlife Service, Washington, DC.
Simon, J.L., and D.M. Dauer 1972. A quantitative evaluation of red tide induced mass mortalities of benthic invertebrates in Tampa Bay, Florida. Environmental Letters 3:229-234.
Smith, G.B. 1975. The 1971 red tide and its impact on certain communities in the mid-eastern Gulf of Mexico. Environmental Letters 9:141-152.
Steidinger, K.A and R.M. Ingle. 1972. Observations on the 1971 summer red tide in Tampa Bay. Environmental Letters 3:271-278
Taylor, H.F. 1917. Mortality of fishes on the west coast of Florida. Report of the U.S. Commissioner of Fisheries 848:1-24.
Tester, P.A. and K.A. Steidinger. 1997. Gymnodinium breve red tide blooms: initiation, transport and consequences of surface circulation. Limnology and Oceanography 42:1039-1051.
Walker, S.T. 1884. Fish mortality in the Gulf of Mexico. Proceedings of the U.S. National Museum 6:105-109.
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