Nutrient Sources in Support of Karenia brevis Blooms on the West Florida Shelf, a Review
P. H. LutherDuring the summer months, large algae blooms known as Red Tide form in the Gulf of Mexico along the West Florida Shelf. Caused by the dinoflagellate Karenia brevis, these blooms produce large volumes of toxins that are harmful to both humans and wildlife. Large blooms can lead to massive fish kills and aerosolized toxins that harm humans. The actual causes of these blooms and the sources of the necessary nutrients for bloom formation, specifically sources of organic nitrogen (N) and phosphorous (P), are not exactly known. In order to form and sustain blooms of the size seen on the West Florida Shelf, K. brevis requires high concentrations of these elements for sustained growth. This review looks to analyze recent research on the varied sources of these compounds and to critically analyze what effects these varying sources of nutrients have on the formation of K. brevis blooms.
The diazotroph Trichodesium erythaeum is a colonial organism found commonly in the Gulf of Mexico in the same waters as K. brevis. This organism is known to produce large amounts of organic nitrogen and ammonia, two resources that are critical in the formation of K. brevis blooms. Researchers have found that one of the limiting chemicals in the growth of and production of nitrogen compounds by Trichodesium is iron. The waters around West Florida Shelf do not normally contain enough iron to support large amounts of growth of the Trichodesium organism that are commonly seen. The source of the extra iron is a very interesting one. During the months of June and July, winds carry large amounts of iron rich dust across the Atlantic Ocean from the Sahara desert to the coasts of Florida. This iron is deposited in the water and stimulates the formation of Trichodesium blooms (Lenes et al, 2001, 1261-1277). Lenes et al. (2008) propose a three dimensional mathematical model based on existing data from the Gulf region that analyzes iron availability, Trichodesium bloom formation, the levels of dissolved nitrogen and phosphorous, and the ability for these to sustain a bloom of K. brevis. This model found that the deposition of Saharan dust provided an abundance of iron available for use by Trichodesium. The limiting nutrient in the growth of the Trichodesium then became the availability of phosphorous. The model showed that the trichodesium was able to sustain itself on preexisting sources of offshore phosphorous until winds caused the blooms to be blown to the coast where they could use rivers as a source for their nutrients. This model showed that this allowed the Trichodesium produce a high enough amount of dissolved nitrogen to support a bloom of K. brevis in the coastal waters of the Gulf of Mexico. This study does acknowledge that much of the assumptions it made about the levels of iron deposition are highly variable and can greatly affect the amount of Trichodesium growth. From the results of this study, one can see a clear source for the nitrogen involved in the formation of K. brevis blooms, but a significant source of phosphorous has yet to be seen. (1091-1110)
In review, Anderson, Gilbert, and Burkholder analyze the worldwide trend of an increase in the proliferation of harmful algal blooms, hereafter referred to as HABs, as it relates to an increase in nutrient loading of waters from human activities. While not looking specifically at blooms of K. brevis, this review does state that dinoflagellate species such as K. brevis tend to thrive when levels of nitrogen and phosphorous are higher in the water. This review states that a source of these nutrients may be runoff from human activities. It gives multiple examples of areas in which a large increase in the amount of runoff from point sources such as sewage and non-point sources such as agricultural runoff can be correlated to an increase in the blooming of HABs. The review also proposes natural nutrient sources such as ground water. This means that a possible source for the nutrients necessary to sustain a bloom of K. brevis may come from runoff into the Gulf from the many rivers along the coast of Florida (704-720).
Vargo et al. looks specifically at estuarine flux (aka. river flow) and its relation to bloom formation of K. brevis. Using data collected as part of the NOAA ECOHAB program, this study predicted the rates of nutrient uptake by K. brevis under a number of different conditions. The study also used data from a number of different sources to calculate the overall flow of nutrients from rivers into the coastal waters of the gulf. When this data was compared with the nutrient requirements of a K. brevis bloom of the size normally found in the Gulf, the study found that estuarine flux from rivers and estuaries near the West Florida Shelf was only able to support about 11-50% of the required nutrients for a bloom of K. brevis. This percentage only applies to areas of the gulf nearest to the coast where the flux has an effect. Therefor, estuarine flux, while supporting blooms as they move onshore, is not a major source for blooms as they from in the Gulf. This study also finds that the excretion of nutrients by other zooplankton could provide all of the necessary amounts of phosphorous and a good deal of the necessary amounts of nitrogen to support a bloom of K. brevis. The rates of zooplankton excretion are based on a previous study. These rates were compared to the nutrient requirements in the same manner as the rates of estuarine flux. In combination, zooplankton excretion and estuarine flux could support a bloom of K. brevis, especially in the coastal waters of the Gulf of Mexico (74-95).
While these sources of nutrients are able to sustain a bloom near the coast, blooms commonly form off the coast and then move inshore. Using satellite data to create a computer model, Stumpf et al. predicts that the formation of a bloom of K. brevis is related to a series of hydrodynamic events. They conclude that nutrients necessary for bloom formation are transported from the northern Gulf of Mexico in the nutrient-rich water of the Mississippi delta to the West Florida Shelf by northward summer winds that generate a circulatory pattern in the Gulf. These nutrients are transported to the shelf at below surface depths. The K. brevis organisms are then driven by chemotaxis to depths of up to 90m to access these nutrients. These waters contain enough nutrients for a small concentration of K. brevis to grow. When the winds change to a southward flow, upwelling occurs along the shelf and the once widespread cells are pushed into high concentration along the coast, thus forming a bloom. In this model, a high initial concentration of nutrients is not necessary because the K. brevis cells are initially widespread and are later pushed into high concentration. This model shows a source for nutrients that could easily support the formation of a bloom (189-210).
The formation of red tide blooms is an important topic today. These blooms have many harmful effects on both human and animal life. The mechanisms presented in this review are the most likely ways by which a bloom of K. brevis is able to from and sustain itself. It is quite possible that these nutrient sources combine to produce the large blooms that appear almost annually on the Gulf coast of Florida. There is still much research that needs to be done on the human impact on these nutrient sources and possible ways that this impact can be reduced to limit the scope and severity of these harmful blooms. This proves to be a bountiful subject of research for oceanographers and will most likely be for years to come.
Works Cited
Anderson, Donald M., Patricia M. Glibert, and Joann M. Burkholder. “Harmful Algal Blooms and Eutrophication: Nutrient Sources, Composition, and Consequences.” Estuaries 25.4 (2002): 704-26. JSTOR. Web. 05 Sept. 2014.
Lenes, Jason M., Brian P. Darrow, Christopher Cattrall, Cynthia A. Heil, Michael Callahan, Gabriel A. Vargo, Robert H. Byrne, Joseph M. Prospero, David E. Bates, and Kent A. Fanning. “Iron Fertilization and the Trichodesmium Response on the West Florida Shelf.” Limnology and Oceanography 46.6 (2001): 1261-277. JSTOR. Web. 27 Sept. 2014.
Lenes, J. M., B. A. Darrow, J. J. Walsh, J. M. Prospero, R. He, R. H. Weisberg, G. A. Vargo, and C. A. Heil. “Saharan Dust and Phosphatic Fidelity: A Three-dimensional Biogeochemical Model of Trichodesmium as a Nutrient Source for Red Tides on the West Florida Shelf.” Continental Shelf Research 28 (2008): 1091-115.Elsevier. Web. 04 Sept. 2014.
Stumpf, Richard P., R. Wayne Litaker, Lyon Lanerolle, and Patricia A. Tester. “Hydrodynamic Accumulation of Karenia off the West Coast ofFlorida.” Continental Shelf Research 28.1 (2008): 189-213. Elsevier. Web. 3 Sept. 2014.
Vargo, Gabriel A., Cynthia A. Heil, Kent A. Fanning, L. Kellie Dixon, Merrie Beth Neely, Kristen Lester, Danylle Ault, Susan Murasko, Julie Havens, John Walsh, and Steven Bell. “Nutrient Availability in Support of Karenia Brevis Blooms on the Central West Florida Shelf: What Keeps Karenia Blooming?” Continental Shelf Research 28.1 (2008): 73-98. Elsevier. Web. 4 Sept. 2014.