Divers chase a sea turtle at the wall break, 12 m deep, San Salvador, Bahamas.
Recently, there has been evidence of abnormal jellyfish abundance in numerous marine ecosystems worldwide (Hong et al., 2007). What has been causing the increased frequency of these jellyfish blooms? There have been a number of catastrophic events with the most recent one recalled in November 2007 where jellyfish blooms wiped out Northern Ireland's salmon farm, killing more than 100,000 fish (Smith, 2007). Abnormally rapid growths in populations have been seen around the world from Southwest Florida bays to Northeast Australia reefs (Mills, 2001). It is hypothesized that there are many factors contributing to jellyfish abundance such as drought, increased salinity on the coast, low levels of oxygen, eutrophication, overfishing and decimation of top predator species, and rising sea temperatures. Although the formation of blooms depends on natural forces such as ocean currents and seasonally temperature fluctuations, anthropogenic forcing is certainly a leading cause.
Jellyfish can be found in every ocean around the world. These marine invertebrates belong to the class Scyphozoa of the phylum Cnidaria. Their bell shape bodies are suspended by tentacles that are covered with cells that often produce a mild sting, although some stings such as the box jellyfish may be fatal. Jellyfish feed on small fish and zooplankton, a broad category of drifting organisms whose complete life cycle lie within plankton themselves. In recent events, jellyfish have been known to congregate into large swarms often called blooms, where high concentration of jellyfish are found in one area. There has been considerable research that looks at the anthropogenic changes and natural climate cycles have on our oceans. These changes to our oceans are having a profound effect on jellyfish populations. With a better understanding of this changing ecosystem, I am determined to connect some of these links between jellyfish and their environment. I believe that it is important to better monitor our seas and jellyfish blooms to prevent catastrophic events in the future. This abnormal abundance is also a sign of the health our oceans are in. We will have to learn to adapt to these changes we are creating in our oceans and may find ourselves eating down the food chain, reconsidering jellyfish as an edible meal. The jellyfish Rhizostome scyphomedusan is currently being harvested for food in Southeast Asia and is considered a delicacy with the appeal being in the crunch (Omori & Nakano, 2001). There are many variables that will be influencing research on jellyfish. Increased salinity has been associated with our current state of drought. Nutrient pollution has been traced to algal blooms, which are then traced to jellyfish blooms. Excess in nutrients such as phosphorus and nitrogen provide food for creatures the jellyfish feed on. Overfishing is a significant influence in that there is an inverse correlation between vertebrates and invertebrates. Global warming also contributes to its own set of environmental factors leading to changes in our oceans, our world's largest carbon sink.
There are a number of challenges facing the research on jellyfish blooms. Since there have been few records of jellyfish populations in the past, this lack of data makes it difficult to compare and measure current populations. Data that is currently available on jellyfish populations is provided by a research team called DockWatch. This organization collects environmental data and visual observations of jellyfish species and numbers to determine seasonality and distribution, if blooms are developing and where, and if blooms are local or transported (DockWatch, 2007). There are many organizations that have data as a result of by-catch records of the fishing industry. These include the U. S. National Marine Fisheries Service in Newport, Oregon and Alaska Fisheries Service Center. The data provided from the by-catch of jellyfish biomass has been a very valuable asset to jellyfish research.
It is particularly important to note that the causes of jellyfish increases are not known, in part, because little is known about the ecology of the polyps. It is necessary to be familiar with the polyp phase of a medusa’s life cycle to understand the true causes of jellyfish fluctuations. Analyzing jellyfish often prove to be particularly challenging. Their gelatinous bodies are impossible to preserve and their stinging tentacles make them hard to handle. Their life cycle also includes a stage where the polyps dwell in the deep sea, often inaccessible to researchers. Generalizing broadly from the results of studies could also lead to erroneous conclusions. Different species in the same environment may respond differently to changing conditions. Even responses to environmental conditions may differ among populations of the same species (Purcell, 2005).
There has been some speculation between climate change and jellyfish blooms. Are the rising sea temperatures influencing surges in jellyfish population? It is concluded that temperature has the most obvious effects on the population size of jellyfish and that warmer temperature is associated with greater abundance (Purcell, 2005). This increase in ocean temperatures could be the reason why some clusters are now surviving in the winter. Little is known about the ecology of the polyp, but it is understood that this is the only life cycle phase that is present during the cold season (Mills, 2001) as adult jellyfish usually die off in the winter. Since the polyps are experience warmer than average waters, this stage of the jellyfish cycle is thought to be one of the driving factors associated with climate change. Associated with this belief, jellyfish may have the opportunity to breed year round. Also, global warming is thought to result in expanded temporal and spatial distributions along with larger populations of jellyfish. Jellyfish are shown to prefer warmer water and some temperate species seem to appear to be limited by colder temperatures. For most temperate species studies, increases in ocean temperatures lead to larger numbers of jellyfish and increased sexual reproduction (Purcell, 2005). Richard Brodeur with the National Oceanic and Atmospheric Administration is a lead researcher ofjJellyfish in the Bering Sea. Brodeur speculates that the timing of the ice melts, which are resulting in greater ice cover in the Southeastern Bering Sea, are affecting jellyfish, at least indirectly. As ice melts phytoplankton grow. Since there has been a shift in the timing of ice melting to late spring there is more amplified and increased sunlight resulting in unprecedented surges of phytoplankton. This shift could be increasing plankton food for the jellyfish and providing them with a more stable, warmer, and productive surface layer after the ice has melted. (Brodeur, 1999) The Bering Sea’s native jellyfish, Chrysaora melanaster, is home to some of the most highly productive fish waters in the world. It accounts for up to five percent of the world’s total fishery production and 56 percent of the United States fishery production of fish and shellfish (National Research Council, 1996).
Although there is much debate over whether we are overfishing our seas, one thing we know for sure is in the absence of top predators, jellyfish are flourishing. Over the years, the decimation of certain species that once kept jellyfish populations in check are no longer present in the food web. The general reduction in marine vertebrates and overfished creatures has led to ecological niches for invertebrates to fill, particularly jellyfish. Some conclude that they are merely filling in the gaps. Since jellyfish feed on the same kind of prey (ichthyoplankton and zooplankton) as adult and young fishes, when fish are removed from the equation, jellyfish are more likely to move in (Whiteman, 2002). This particular nature of the jellyfish leads them to be both predators and potential competitors with other fish. The increase in jellyfish population has resulted in them vacuuming up plankton that feed young fish and may be contributing to the decreasing fish levels in our oceans. With the decimation of top predators that usually prey on jellyfish, there is one substantial predator left, the carnivorous jellyfish. It nearly exclusively preys on other jellies forming a somewhat independent food web named the ‘Jelly Web’ by B.H. Robison (Robison & Connor, 1999). This species of jellyfish was introduced in the Black Sea after a complete collapse of the fishing industry there. The Black Sea provides a graphic example of how anthropogenic alterations have altered a highly productive fishing region to succumb to sea of jellyfish – medusa and ctenophores. Due to the effects of overfishing and pollution, the jellyfish-eating mackerel Scomber scombrus has become uncommon. With the accidental introduction of the Atlantic American ctenophore, Mnemiopsis leidyi A. Agassiz in 1965, this nonindigenous species peaked in 1980 consuming nearly all of the zooplankton and plummeting fishing stocks to non-existent (Mills, 2001).
Eutrophication occurs when there is a significant increase in chemical nutrients, such as phosphorus and nitrogen in an ecosystem. Industrialization and agriculture has left a lasting impact on our oceans. High levels of nutrients from sewage plumes, fertilizers, storm runoff, and livestock fecal matter runoff has been polluting our oceans with organic and inorganic material. These nutrients provide food and nourishment to small organisms and creatures in the plankton family. With excessive nutrient loading, our oceans are now often experiencing massive algal blooms. Jellyfish thrive in nutrient rich waters feeding off plankton and zooplankton directly and indirectly through the food web (Mills, 2001). Often you find algal blooms where you find jellyfish blooms. Usually with eutrophication, low levels of oxygen will result.
Low Oxygen Concentrations
A significant fraction of jellyfish is water. Water accounts for more than 95 percent of its mass, much more than even humans. Dissolved within their fluid tissue is oxygen. This large source of oxygen enables them to survive in low oxygenated waters such as the Dead Zone. This unique metabolic trick allows them to survive in some of the most polluted waters in the world. The Dead Zone consists of over 7,000 square miles of the Gulf of Mexico. Every summer, the waters here become deprived of oxygen because it is being consumed by the decay of massive algal blooms. These algal blooms result from all the nutrient pollutants washed into the gulf from the Mississippi River and the Atchafalaya River (Whiteman, 2002). Since the jellyfish have such huge oxygen stores, they can thrive in the Dead Zone and feed off all the plankton that result from the algal blooms while other creatures, such as crab or shrimp, flee in order not to suffocate. It is also shown that in hyper-saturated waters with oxygen such as in the Northern Adriatic Sea, there has been a decrease in jellyfish (Mills, 2001).
The concentration of salt in the oceans has been positively correlated with jellyfish abundance. With the recent droughts in the U.S., Florida has seen massive explosions of jellyfish off of their coast lines and the news media has been recording many of these incidents in the past few years. Since there is a lack of fresh water flowing to the sea, due to drought, the coastal waters have become saltier. This spike in salinity levels on the coast are believed to have provoked massive jellyfish explosions. Although there are many claims that jellyfish like saltier water, I found little research to support that theory. According to Jennifer Purcell with Shannon Point Marine Center, salinities have significant effects on species living where salinity was less variable (Purcell, 2005). The seasonal changes in salinity are an influencing factor but more subtle than temperature. Iodide seems to be more directly related to jellyfish blooms. Since iodide is directly proportional to salinity, at very low salinities, iodide may be insufficient for strobilation. Strobilation is the process in which the polyp has developed enough that it can detach itself from its support and swim off. If the iodide is present in abundance, strobilation will be favorably successful. Levels of salinity vary in which jellyfish species can survive. There is defiantly a level where maybe not enough iodide is present to promote strobilation, which would be considered unfavorably low. There are three species that occur in very low salinity regions. These species of hydromedusae are indigenous to the Black Sea, Maeotias marginata, Blackfordia virginica, and Moerisia. These jellyfish are currently present in both San Francisco Bay and the Chesapeake Bay in North American (Rees & Gershwin, 2000). Although salinity could be low, there is a range that is necessary for growth as for the three species mentioned above.
The fishing industry has suffered significant economic damage from jellyfish blooms by reducing stocks of commercial fish. Since the jellyfish is both predator and competitor, they remove the eggs and larvae of fish, shrimp, and crabs from the water, as well as the plankton that these species eat. Monty Graham, a senior marine scientist at Alabama’s Dauphin Island Sea Lab, calculated that the spotted jellyfish in the Gulf of Mexico ate about 2,400 fish eggs daily (Whiteman, 2002). Another concern is that these invaders wreak havoc on fishing boats. Jellyfish were the cause of intermittently shutting down a $60 million shrimp fishery in the Gulf of Mexico because the nets were clogged with jellyfish. They weighed down the nets so much that the weight stops a boat like hitting a brick wall. Jellyfish that get stuck in nets often smother other fish to death. Also, poison from the tentacles can discolor the fish, reducing their market value.
Many beachgoers are stung every day by jellyfish. The increasing harassment of medusa is becoming more prevalent than ever. People often find themselves surrendering their favorite beaches to jellyfish or may take the alternate route and decide to put up with some level of the particular nuisance. There has also been a surge in medical liabilities for swimmers and scuba divers. Potential noxious effects are also a deterring factor to tourism.
Seawater Cooling Systems
In 1999, the Philippines were faced with a major dilemma. Enough jellyfish to fill fifty trucks were sucked from the ocean into an intake pipe of a local power plant’s seawater cooling system. The jellyfish clogged the filter screens and caused the power plant to shut down, plunging 40 million residents into darkness. Japan had experienced similar problems. They now have power plant staff prevent similar blackouts by collecting jellyfish that accumulate near intake pipes (Whiteman, 2002). The list goes on, shut-downs due to medusa have been reported in the Baltic region, Korea, India, Saudi Arabia, Australia and more (Moller, 1948).
It is important to understand why jellyfish populations are increasing. Increases in jellyfish have a host of negative effects such that they interfere with fisheries, negatively impact tourism, are a medical liability for swimmers, and clog seawater screens that factories use for their operations. In researching this topic it is difficult to directly link environmental changes to jellyfish blooms since their correlation is usually not primary and masked by other factors. “We’re pushing the oceans back to the dawn of evolution”, Jeremy Jackson a scientists said, “a half-billion years ago when the oceans were ruled by jellyfish and bacteria” (Weiss, A Primeval Tide, 2006). Is this evolution running in reverse? It is apparent that anthropogenic causes have brought many changes in the oceans that are thought to favor jellyfish.
There are many questions I came across while researching this topic that would provide a better understanding to what leads to destructive jellyfish blooms and how we can minimize possible catastrophes in the future. The formation of blooms is a complex process that depends on many variables. What are the factors directly related to jellyfish blooms? Why are we experiencing such an increase in jellyfish abundance? What impacts are these blooms having on the ecosystem? What are the short term and long term effects of such large numbers of jellyfish? Do blooms occur in one place and move with the current or is it multiple bloom development, a bloom from a pre-existing bloom? Where do nonindigenous species come from and how are they getting there?
Brodeur, R. D., & C. E. Mills, J. E. Overland, G. E. Walters & J. D. Schumacher, (1999). Evidence for a substantial increase in gelatinous zooplankton in the Bering Sea, with possible links to climate change. Fish. Oceanogr. 8:296-306.
Hong, J. H., C. He-Qin, X. Hai-Gen, F. Arreguin-Sanchez, M. J. Zetina-Rejon, P. Luna, W. Le Quesne, (2007). Trophic controls of jellyfish blooms and links with fisheries in the East China Sea. Science Direct. Ecological Modeling 212 (2008) 492-503.
Malakoff, D. (2001). Interest Blooms in Growing Jellyfish Boom. Science, 293(5527), 29. Retrieved December 1, 2007, from Academic Search Premier database.
McFarling, U. L. (2006). Altered Oceans. A Chemical Imbalance. Growing seawater threatens to wipe out coral, fish and other crucial species worldwide. Los Angeles Times. August 3, 2006.
Mills, C. E. (2001). Jellyfish blooms: are po;ulations increasing globally in response to changing ocean conditions? Hydrobiologia. Springer Netherlands. 10.1023 (55-68).
Moller, H. (1984). Effects on jellyfish predation by fishes. Proceedings of the Workshop on Jellyfish Blooms in the Mediterranean, Athens 1983. UNEP 1984: 45-59.
Moller, L. F., & RiisgĆrd, H. U. (2007). Impact of jellyfish and mussels on algal blooms caused by seasonal oxygen depletion and nutrient release from the sediment in a Danish fjord. Journal of Experimental Marine Biology and Ecology, 351(1-2), 92-105.
National Research Council, (1996). The Bering Sea Ecosystem: Report of the Committee on the Bering Sea Ecosystem. National Academy Press, Washington, D.C.: 307pp.
Omori, M. & E. Nakano, (2001). Jellyfish fishery in southeast Asia Hydrobiologia 451 (Dev. Hydrobiol. 155): 19-26.
Purcell, J. E. (2005). Climate effects on formation of jellyfish and ctenophore blooms. Western Washington University, Shannon Point Marine Center. Journal of the Marine Biological Association of the United Kingdom, 85(461-476).
Rees, J. T. & L. A. Gershwin, (2000). Non-indigenous hydromedusae in California’s upper San Francisco Estuary: life cycles, distribution, and potential environmental impacts. Sci. Mar. 64 (Suppl. 1): 73-86.
Smith, K. (2007). Jellyfish attack wipes out North Ireland salmon farm. Science. Thomson Reuters, Dublin 2008.
Weiss, K. R. (2006). Altered Oceans. A Primeval Tide of Toxins. Runoff from modern life is feeding an explosion of primitive organisms. This ‘rise of slime’, as one scientist calls it is killing larger species and sickening people. Los Angeles Times. July 30, 2006.
Whiteman, L. (2002). The Blobs Of Summer. Swarms of Jellyfish are Invading Coasts Around the world. Natural Resources Defense Council. On Earth, Environment, Politics, People. http://www.nrdc.org/onearth/02sum/jelly1.asp
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