Red Tides on the Gulf

¶ … Red Tides on the Gulf of Mexico

The world's oceans form the basis for a food web that extends to virtually all living organisms, including human beings. The health of the oceans is therefore of critical importance to the future survival of mankind. One phenomenon in particular that threatens the health of the world's oceans is red tides. Although there are different types of red tides, the type that normally occurs in the Gulf of Mexico are caused by the algae, Karenia brevis. This paper provides a review of the relevant literature to describe the background and an overview of the problem, determine this organism's effect on benthic organisms, humans and marine animals that eat organisms with high levels of stored algae and how it affects other marine plants. A discussion concerning the differences in the red tide that occurs in the Gulf of Mexico and elsewhere is followed by a summary of the research and important findings in the conclusion.

Review and Discussion

Background and Overview.

In a marine environment, single-celled, microscopic, plant-like organisms referred to as phytoplankton or microalgae occur naturally in the surface layer of any body of water that receive adequate amounts of sunlight; such organisms provide the fundamental base of the food web that is responsible for the food that almost all other marine organisms rely (Landsberg, 2002). Approximately 2% of the more than 5,000 species of marine phytoplankton that are known to exist around the world have been documented as being harmful or toxic to some degree (Landsberg, 2002). Some of these organisms are also responsible for the red tides that have been documented in the Gulf of Mexico for centuries and occur on a regular basis (typically every year) and frequently last for several months (Woofter, Brendtro and Ramsdell 2005). These red tide events, or "blooms," are caused by Karenia brevis, a dinoflagellate that produces a family of neurotoxins which are collectively termed brevetoxins (Woofter et al. 2005). In those cases where algal blooms adversely impact humans or the environment, they are termed "harmful algal blooms" or HABs; scientists who investigate the interactions of aquatic environments and human health have cited HABs as one of the primary threats to human health (Fleming, Backer and Baden 2005). A recent study by Heil and Steidinger (2009) emphasizes that, "Annual blooms of the toxic dinoflagellate Karenia brevis in the eastern Gulf of Mexico represent one of the most predictable global harmful algal bloom (HAB) events, yet remain amongst the most difficult HABs to effectively monitor for human and environmental health" (611). In spite of the difficulties associated with their study, a growing body of research has been directed at these HABs. For instance, according to Backer, Kirkpatrick, Fleming, Cheng, Pierce, Bean, Clark, Johnson, Wanner, Tamer, Zhou and Baden (2005), "Karenia brevis (formerly Gymnodinium breve) is a marine dinoflagellate responsible for red tides that form in the Gulf of Mexico. K. brevis produces brevetoxins, the potent toxins that cause neurotoxic shellfish poisoning" (644). Besides neurotoxic shellfish poisoning though, K. brevis can have an adverse impact on a wide range of aquatic life and these issues are discussed further below.

Effect on Benthic Organisms.

According to Pillon, Boussioux, Escande, Ait-Aissa, Gomez, Fenet, Ruff, Moras, Vignon, Duchesne, Casellas, Nicolas and Balaguer (2005), benthic organisms are capable of ingesting environmental organic chemicals and releasing them directly back into the food web (278). Certain benthic organisms, such as bivalve mollusks, are the primary vectors of paralytic shellfish poisoning (PSP) in humans (Bricelj, Connell, Konoki, Macquarrie, Scheuer, Catterall and Trainer (2005). Moreover, bivalve mollusks demonstrate a wide range of inter-species variation in their ability to accumulate PSP toxins which can cause deaths in human beings through the blockage of sodium conductance in nerve fibers (Bricelj et al. 2005). Other researchers have investigated the sublethal effects of K. brevis on benthic organisms as well. For instance, Cohen, Tester and Forward (2007) studied grazing and mortality experiments with K. brevis cells and brevetoxins (PbTx-2) in an effort to identify toxicity routes for the copepods Acartia tonsa, Temora turbinata, and Centropages typicus. According to these researchers, "Subsequent behavioral experiments determined if copepod swimming and photobehavior, both behaviors involved in predator avoidance, were impaired at sublethal K. brevis and PbTx-2 levels. Copepods variably grazed toxic K. brevis and non-toxic Prorocentrum minimum at bloom concentrations" (2). Although these copepods developed concentrations of the K. brevis brevetoxins, the only mortality of significance was identified in T. turbinata and this was during the highest levels of concentration and Acartia tonsa demonstrated minimal sublethal behavioral impacts (Cohen et al. 2007). These researchers, though, did identify significant impacts on the photobehavior as well as swimming patterns of T. turbinata and C. typicus even at the lowest sublethal concentrations of K. brevis that were used (Cohen et al. 2007). Cohen and his colleagues conclude that, "While physiological incapacitation may have altered copepod behavior, starvation likely played a major role as well. These data suggest sublethal effects of K. brevis and brevetoxin on copepod behavior occur, and predicting the role of zooplankton grazers in trophic transfer of algal toxins requires knowledge of species-specific sublethal effects" (3). Likewise, Kubanek, Snell and Pirkle (2009) report that, "The red tide dinoflagellate Karenia brevis (ex Gymnodinium breve, ex-Ptychodiscusbrevis) is an episodically abundant phytoplankter in the Gulf of Mexico that is occasionally driven onshore by currents and winds, accumulating in dense blooms. Rotifers are likely to encounter this harmful alga in coastal waters and may be important in controlling, or failing to control, K. brevis blooms" (1026). The findings of the study by Kubanek and her associates (2009) confirmed the lethal effects of K. brevis on two species of rotifers. According to these researchers, "When K. brevis was offered as a sole diet, the feeding behavior of both rotifer species (B. plicatilis and B. ibericus) was almost completely inhibited. The failure of rotifers to feed on K. brevis translated into a failure to produce eggs and a population decline similar to that of starved rotifers" (1028). Unfortunately, efforts to control and remediate accumulations of K. brevis to protect humans may have an adverse impact on benthic organisms. In this regard, Haubois, Bricelj and Naar (2006) report that, "Sedimentation of toxic cells following inert clay application could reduce toxin incorporation by commercially important suspension-feeding bivalves and thus prevent direct public health impacts, but could potentially lead to brevetoxin (PbTx) accumulation by benthic deposit-feeders" (2003).

Effect on Humans and Marine Animals.

A recent study by Naar, Bourdelais, Tomas, Kubanek, Whitney, Flewelling, Steidinger, Lancaster and Baden (2002) investigated the impact of Karenia brevis on various marine organisms as well as humans. According to Naar and his associates, "The bloom-forming dinoflagellate Karenia brevis (formerly Gymnodiniuim breve) produces a family of neurotoxins known as the brevetoxins. These lipid-soluble polyether toxins, like the ciguatoxins, exert their toxicity by activating voltage-sensitive sodium channels" (179). When human eat shellfish that has been contaminated with the brevetoxins produced by K. brevis, it can result in a non-fatal type of food poisoning known as neurotoxic shellfish poisoning (NSP) (Naar et al. 2002). In addition, Woofter et al. note that besides NSP, "Brevetoxins produced by K. brevis blooms also pose a risk to human health. Aerosol forms of the toxin are produced by wind and wave action and move onshore, causing transient respiratory irritation in people that inhale the toxin" (11). A recent study by Fleming, Kirkpatrick, Backer, Bean, Wanner, Dalpra, Tamer, Zaias, Cheng, Pierce, Naar, Abraham, Clark, Zhou, Henry, Johnson, Van De Bogart, Bossart, Harrington and Baden (2005) notes that, brevetoxins have been shown to cause significant bronchoconstriction in experimental animals. According to these researchers, "A study of persons who visited the beach recreationally found a significant increase in self-reported respiratory symptoms after exposure to aerosolized Florida red tides. Anecdotal reports indicate that persons with underlying respiratory diseases may be particularly susceptible to adverse health effects from these aerosolized toxins" (Fleming et al. 2005:650).

When K. brevis creates red tide events known as "blooms," the elevated levels of brevetoxins contained in seawater can also adversely affect birds, fish, and marine mammals resulting in massive epidemics (Naar et al. 2002). In this regard, Woofter and his associates add that, "Brevetoxins from red tides are linked to deaths in marine mammals, including dolphins and manatees, which are intoxicated through both ingestion of organisms harboring high brevetoxin concentrations and inhalation of aerosolized brevetoxins" (11). Likewise, Baden, Bourdelais, Jacocks, Michelliza and Naar (2005) note that K. brevis-produced brevetoxins have been suspected of causing inhalation toxicity among humans and a growing body of research since 1981 has confirmed this causal relationship. According to Fleming and her colleagues, "Potential health effects associated with inhaled brevetoxins could extend beyond the reportedly transient respiratory irritation reported by humans exposed to Florida red tide brevetoxin aerosol to possible systemic chronic health effects" (618). Similar results have been reported by Freeman (2005) who notes that humans can become ill just by inhaling even minute amounts of airborne K. brevis and people with asthma are especially susceptible.

Moreover, K. brevis blooms are not a new phenomenon, and fish kills that result from red tides caused by K. brevis in the Gulf of Mexico have been described in the scientific literature since 1960 or so and have been reported anecdotally for more than two centuries (Naar et al. 2002). In this regard, Backer and her associates (2005) emphasize that, "The human health effects from consuming shellfish with high concentrations of brevetoxins in their tissues have been well documented. However, there is very little information describing human health effects from environmental exposures. It is ironic that we know the least about the aspects of the Florida red tide problem that poses the greatest public health hazard in terms of number of people affected" (645). Today, K. brevis blooms are monitored closely in order to mitigate the foregoing health hazards that are related to the consumption of shellfish and shellfish harvesting is banned in those instances where the densities of K. brevis exceed 5,000 cells per liter of seawater (Landsberg and Steidinger 1998).

A study by Brand and Compton (2007) analyzed K. brevis samples that were taken from the region near the southwest coast of Florida between Tampa Bay and Sanibel Island during the period between 1954 to 2002 to identify corresponding spatial and temporal patterns. According to these researchers, "K. brevis was found to be approximately 20-fold more abundant within 5 km of the shoreline than 20 -- 30 km offshore. Overall, K. brevis was approximately 13 -- 18-fold more abundant in 1994 -- 2002 than in 1954 -- 1963" (Brand and Compton 232). During the period 1954 to 1963, K. brevis blooms took place mostly during the fall months; by contrast, during the period between 1994 and 2002, blooms were prevalent during the fall months as well as winter and spring (Brand and Compton 2007). In this regard, Brand and Compton speculate that the increased concentrations of nutrients in the ecosystem are probably responsible for this increased prevalence of bloom events due in large part to the increased presence of humans and associated activities in this region over the past 50 years (Brand and Compton 2007).

Effect on Other Marine Plants.

Although the HABs caused by K. brevis have been shown to represent a serious health threat to benthic organisms as well as other aquatic and terrestrial life, including humans, less is known about the effect on marine plants. It is reasonable to suggest, though, that K. brevis can contribute to a toxic environment that may harm or even kill various marine plant species. In this regard, Woofter et al. (2005) emphasize that, "Aquatic species are of particular relevance because K. brevis is a fragile dinoflagellete that readily breaks, releasing toxin directly into the water or upon contact with inert or living objects" (12).

Differences between Red Tide in the Gulf of Mexico and Elsewhere.