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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