Monday, December 29, 2014

Florida red tides (Karenia brevis): causes, effects and prevention techniques


Suppose you are on a summer vacation to Florida when you decide to visit the beautiful blue waters of the Atlantic Ocean. Upon your arrival you are alarmed to find the water is not blue at all but instead its bright red! For someone who has never witnessed a Florida red tide this phenomenon would undoubtedly cause some concerns and for good reason. Florida red tides are also known as a harmful algal blooms (HABs) and they are formed when concentrations of the phytoplankton, Karenia brevis, increase drastically (1). When these events occur, the ocean not only turns red but it becomes an extremely toxic environment for circumambient organisms such as dolphins, fish and humans. This blog post attempts to integrate some of the major studies on Florida red tides and their impacts on aquatic organisms as well as terrestrial animals such as humans.

Florida red tides occur primarily along the west coast of Florida and the Gulf of Mexico but have also been reported along the East coast of North Carolina (2). They are termed “harmful” algal blooms because K. brevis produces a suite of neurotoxins known as a brevetoxins (PbTx), which cause acute central nervous system damage in humans and other mammals (2). Some negative effects of HAB events include gross marine organism mortality rates, human neurotoxic shellfish poisoning and economical disruption of the fishing industry (2). There have been numerous studies of bottlenose dolphin mortality events coinciding with HABs. Fish communities were also studied during HAB events and the species richness and density were both found to be negatively affected by the events (3). A human impact of Florida red tides is the possibility of consuming shellfish highly concentrated with PbTx, causing neurotoxic shellfish poisoning. Although, the correlation between K. brevis blooms and brevetoxin poisoning may seem clear, only a small number of studies have been performed that actually test marine animals and their communities for this toxin after their death to determine if PbTx was indeed present.

K. brevis is a photoautotrophic dinoflagellate that occupies a planktonic and oceanic niche with optimal growth occurring in temperate to tropical waters (3). This microbe has two flagella, one wrapping around the cell and the other aiding in locomotion (1). The cells are pinkish/red in pigment, which is visible to the naked eye when their concentrations are high enough (1). They are ubiquitously found in low concentrations in the coastal waters of the Gulf of Mexico and Western Florida but concentrations drastically increase during HABs. This is when marine and terrestrial organisms are most threatened by the PbTxs (2). There are two types of brevetoxins, PbTx1 and PbTx2, each contributing a suite of derived compounds that form during HAB events and they differ in the number of carbon rings that each contains (2). These compounds contain a polycyclic ether ladder that binds to sodium pumps located on neurons, ultimately altering their function and inducing cell death (2). Brevetoxins are heat-stable as well as lipid soluble allowing for them to persist in high saline concentrations such as those of oceans. These factors are what make PbTxs so dangerous to the ecosystem and the surrounding life forms.

The frequency of HABs has notably increased over the last few decades and numerous fish kill events have been recorded in areas and times coinciding with these events (3). Occurrences of HABs are relatively sporadic but they are though to be primarily human induced (3). This is due to extra nutrition being added to the oceans that the phytoplankton can utilize and thrive on (3). Geographical location also is found to play a role in HAB occurrence (3). A study was conducted in order to determine if Florida red tide events were in fact affecting the surrounding fish communities, species diversity and population densities (4). They chose a system termed; catch per unit effort (CPUE), where they used a single large net and attempt to catch as many fishes as possible within designated areas during HABs and non-HABs. It was found that fish species richness and fish populations both decreased with the occurrence of HABs but replenished when there was no such event. This indicated that K. brevis blooms affect fish communities, their population densities and species richness in the surrounding coastal waters (4).

Numerous mortality events coinciding with HABs have been reported in bottlenose dolphins where hundreds of carcasses washed up onto the shores of Western Florida and the Gulf of Mexico (1). Samples were obtained from some of the dolphins found and high concentrations of brevetoxins were discovered (1). Some were also taken from dolphin prey species including different types of fish, which also contained high concentrations of brevetoxins (1). This suggested that the dolphins were getting sick from eating poisoned fish (1). In 1999/2000 152 bottlenose dolphins died followed a major HAB event and PbTxs were found in 52% of the samples tested. In 2004 105 bottlenose dolphins died without an apparent HAB event but all of the samples tested positive for brevetoxins in very high concentrations. It was concluded that there must have been a HAB effecting the bottlenose dolphin population that was not detectable with the current tools used to sense increases in K. brevis concentrations (1). Lastly, in 2005/2006 90 bottlenose dolphins died and 93% were positive for brevetoxins. These records indicate that HABs of K. brevis are responsible for the bottlenose dolphin mortality events that occurred along the west coast of Florida and the Gulf of Mexico from 1999 to 2006.

Filter feeders occupying a benthic niche, including mussels, clams and oysters are prone to high accumulations of PbTx (3). This is because they continuously feed on these toxic algae, which often settle in the benthic zone when dead (3). Neurotoxic shellfish poisoning in humans occurs through consumption of these contaminated shellfish (3). This directly leads to disruptions of the fishing industry and economy by contaminating their catches and limiting productivity (3). Brevetoxins excreted from K. brevis can also become aerosolized through regular oceanic currents and this can cause harm to humans and other mammals (3). Inhalation of PbTx is noted to cause burning of the nose and eyes, a choking cough and asthma attacks (3). Despite the clear negative effects that brevetoxins have on humans there has never been a reported death caused by brevetoxins (3).

Current preventative measures being taken to minimize casualties and other losses during Florida red tide events include satellite-imaging detection, which utilizes different mathematical algorithms to detect blooms (4). There are also multiple types of sensors that are used to sense color changes in the ocean and increases in K. brevis concentrations (4). It is well known that Florida red tides are harmful events, so it is comforting to know combative measures are being taken to prevent mass mortality events of aquatic organisms along with human and bird brevetoxin poisoning.

Literature Cited

1. Twiner, M. J., Flewelling, L. J., Fire, S. E., Bowen-Stevens, S. R., Gaydos, J. K., Johnson, C. K., … Rowles, T. K. (2012). Comparative analysis of three brevetoxin-associated bottlenose dolphin (Tursiops truncatus) mortality events in the Florida Panhandle region (USA). PloS One, 7(8), e42974. doi:10.1371/journal.pone.0042974

2. Pierce, R. H., & Henry, M. S. (2008). Harmful algal toxins of the Florida red tide (Karenia brevis): natural chemical stressors in South Florida coastal ecosystems. Ecotoxicology, 17(7), 623–631. doi:10.1007/s10646-008-0241-x.Harmful

3. Shen, L., Xu, H., & Guo, X. (2012). Satellite remote sensing of harmful algal blooms (HABs) and a potential synthesized framework. Sensors (Basel, Switzerland), 12(6), 7778–803. doi:10.3390/s120607778

4. Gannon, D., Berens McCabe, E., Camilleri, S., Gannon, J., Brueggen, M., Barleycorn, A., … Wells, R. (2009). Effects of Karenia brevis harmful algal blooms on nearshore fish communities in southwest Florida. Marine Ecology Progress Series, 378, 171–186. doi:10.3354/meps07853

5. Shen, L., Xu, H.,& Guo, X. (2012). Satellite remote sensing of harmful algal blooms (HABs) and a potential synthesized framework. Sensors (Basel, Switzerland), 12(6), 7778–803. doi:10.3390/s120607778

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