Sargassum is a genus of brown macroalgae (seaweed) in the order Fucales of the class Phaeophyceae. This brown macroalgae is spread predominantly in tropical and subtropical areas. It comes from the Sargasso Sea, which was named because it hosts large amounts of Sargassum. Sargassum can extend for kilometers across the ocean surface, forming golden brown drifting mats that move with the currents and wind; in recent years, Sargassum has been known for arriving in large quantities throughout the Caribbean region. More recently, scientists have found using satellite data that a new hotspot has been forming from western Africa and extends into the Caribbean Sea, called the Great Atlantic Sargassum Belt (GASB).
Most of the macroalgae species inhabit shallow coastal waters and coral reefs, but some are planktonic (free-floating) or pelagic after being detached from reefs during storms. Sargassum includes two holopelagic species, Sargassum natans and Sargassum fluitans, with other species interconnected. These species are distinguished to reproduce solely by clonal reproduction through fragmentation. Sargassum plays an important ecological role in the open ocean because it provides a vital habitat for a variety of marine species, including fish, crabs, sea turtles, migratory birds and other organisms. However, when large amounts of Sargassum accumulate in the shore and start to decompose it forms what is known as Sargassum brown tides (SBT). SBT alters the water column by changing the appearance of the water and in addition alters the water quality by reducing light penetration, decreasing dissolved oxygen and changing the pH of the coastal waters. As a result, it affects seagrass meadows, corals, mangroves and coastal communities that depend on tourism.
History
Sargassum was named after sargaço, the Portuguese word for grape, after Portuguese sailors who found the floating macroalgae in the Sargasso Sea noticed the small air-filled bladders that keep the seaweed afloat.
The earliest mention in history of the Sargassum may be traced to the times of Cristobal Columbus in his maritime letters "Relaciones y Cartas" of the first voyage, where he mentions having encountered great masses of floating algae that made navigation difficult. Sargassum had, however, acquired a legendary reputation as it expands through the entire Sargasso Sea, and blocks ships movements, but since then, it has since been discovered that it happens in drifts only.
Taxonomy and classification Sargassum is a genus of over 300 species of brown macroalgae (seaweed) in the order Fucales of the class Phaeophyceae. It is one of the members of the proposed Chromista kingdom. Sargassum is spread predominantly in tropical and subtropical areas of the world. The species vary in size, shape and location. There are those that are attached (benthic) and others that are free floating (holopelagic). As mentioned before, the Caribbean receives two interconnected species, Sargassum natans and Sargassum fluitans.
Evolution Fossils of a floating brown algae reminiscent of modern Sargassum are common in certain layers of the Early Oligocene-aged Menilite Formation of Poland, in what was once the Paratethys Sea, suggesting that floating Sargassum mats have been a dominant feature of certain oceanic ecosystems for tens of millions of years. In addition to providing a minimum age for Sargassum, associated with these fossil algae are fossils of shallow-water fishes and crabs, despite these fossil deposits otherwise preserving deepwater fish fossils. This species assemblage closely resembles that found in the modern Sargasso Sea, and has been nicknamed a "quasi-Sargasso" assemblage.
Description
Species of this genus of brown algae may reach up to several metres in length. They are brown or dark olive green in color, and consist of a holdfast, a stipe, and fronds or branches. Oogonia and antheridia occur in conceptacles embedded in receptacles on special branches.
Most of the species of Sargassum have spherical, gaseous bladders, called pneumatocysts, which provide buoyancy to the fronds and keep them close to the surface so they can get the maximum amount of light to carry out photosynthesis. The fronds are frequently rough and slightly sticky in feel, and their structure is loose yet strong allowing the algae to endure the waves and the great force of the water currents. The morphological difference within the genus is also significant and tends to change according to the environmental factors, including the water flow, nutritional supply, and the light intensity.
Distribution and ocean currents The physical drivers behind Sargassum inundation events are prevailing winds and ocean surface currents. The Caribbean is located in a region heavily affected by trade winds. Trade winds are strong, consistent northeasterlies winds which blow dust-filled dry air from the Sahara across the Atlantic. Trade winds additionally play a critical role in the annual hurricane season in the Western Atlantic. The Caribbean Current and Antilles branch of the Atlantic North Equatorial Current are the major current transporters of Sargassum in the region.
Researchers have recently begun using Moderate Resolution Imaging Spectroradiometer satellite imagery and ocean current data to track and forecast inundation events with a high level of accuracy.
Ecology The Sargasso Sea has large pelagic mats of Sargassum, making it one of the few areas that can develop an ecosystem in the open-ocean region that has no land boundaries.US Department of Commerce, National Oceanic and Atmospheric Administration. (2013, June 01). What is the Sargasso Sea? Retrieved November 28, 2017 Drifting Sargassum patches acts as a refuge for many species in different parts of their development, but also as a permanent residence for endemic species that can only be found living on and within the Sargassum.Laffoley, D.d'A., Roe, H.S.J., Angel, M.V., Ardron, J., Bates, N.R., Boyd, I.L., Brooke, S., Buck, K.N., Carlson, C.A., Causey, B., Conte, M.H., Christiansen, S., Cleary, J., Donnelly, J., Earle, S.A., Edwards, R., Gjerde, K.M., Giovannoni, S.J., Gulick, S., Gollock, M., Hallett, J., Halpin, P., Hanel, R., Hemphill, A., Johnson, R.J., Knap, A.H., Lomas, M.W., McKenna, S.A., Miller, M.J., Miller, P.I., Ming, F.W., Moffitt, R., Nelson, N.B., Parson, L., Peters, A.J., Pitt, J., Rouja, P., Roberts, J., Roberts, J., Seigel, D.A., Siuda, A.N.S., Steinberg, D.K., Stevenson, A., Sumaila, V.R., Swartz, W., Thorrold, S., Trott, T.M., and V. Vats. (2011). The protection and management of the Sargasso Sea: The golden floating rainforest of the Atlantic Ocean. Summary Science and Supporting Evidence Case. Sargasso Sea Alliance, 44 pp. These endemic organisms have specialized patterns and colorations that mimic the Sargassum and allow them to be camouflaged in their environment. In total, Sargassum mats are home to more than 11 phyla and over 100 different species. There are also a total of 81 fish species (36 families represented) that reside in the Sargassum or utilize it for parts of their life cycles. Other marine organisms, such as young sea turtles, will use the Sargassum as shelter and a resource for food until they reach a size at which they can survive elsewhere. Below is a list of organisms that are associated with the Sargassum.
The Sargasso Sea plays a major role in the migration of catadromous eel species such as the European eel, the American eel, and the American conger eel. The larvae of these species hatch within the sea and as they grow they travel to Europe or the East Coast of North America. Later in life, the matured eel migrates back to the Sargasso Sea to spawn and lay eggs. It is also believed that after hatching, young loggerhead sea turtles use currents, such as the Gulf Stream, to travel to the Sargasso Sea, where they use the Sargassum as cover from predators until they are mature.
Sargassum also serve as a trophic link between the ocean surface and the seabed. Isopods of the species Bathyopsurus nybelini at depths of 5002–6288 m in the Puerto Rico Trench and Mid-Cayman spreading center have been observed consuming Sargassum. They even exhibit several adaptations for it, such as microbiomes capable of breaking down the seaweed and fixing nitrogen, specialized swimming strokes, and serrated, grinding mouthparts.
Organisms found in the pelagic Sargassum patches,
Arthropods Amphipods Skeleton shrimp Crabs Copepods Shrimp Sea Spiders Worms Annelid worms Flatworms Mollusks Nudibranchs Snails Squid Fish Sargassum fish Porcupinefish Triplefin Planehead filefish European eel American eel American conger eel Others Sea turtles Hydrozoans Sargassum species are found throughout tropical areas of the world and are often the most obvious macrophyte in near-shore areas where Sargassum beds often occur near coral reefs. The plants grow subtidally and attach to coral, rocks, or shells in moderately exposed or sheltered rocky or pebble areas. In some cases, these tropical populations often undergo seasonal cycles of growth and decay in concert with seasonal changes in sea temperature. In tropical Sargassum species that are often preferentially consumed by herbivorous fishes and echinoids, a relatively low level of phenolics and tannins occurs.
Biological impacts
Unprecedented Sargassum inundation events cause a range of biological and ecological impacts in affected regions. The decomposition of large quantities of Sargassum along coastlines consumes oxygen, creating large oxygen-depleted zones resulting in fish kills. Decomposing Sargassum additionally creates hydrogen sulfide gas (), which causes a range of health impacts in humans. During the Sargassum inundation event in 2018, 11,000 Acute Sargassum Toxicity cases were reported in an 8-month span on just the Caribbean islands of Guadeloupe and Martinique. Massive amounts of floating Sargassum present a physical barrier preventing corals and seagrasses from receiving sufficient light, fouling boat propellers, and entangling marine turtles and mammals. Sargassum influx imposes major problems of organic matter and nutrients in the offshore and inshore waters, changing the nutrient balance and possibly influencing the quality of oceanic waters. As much as these transfers might have short time positive effects on productivity, long term ecological impacts of these transfers are not well understood. Further investigations into the environmental causes, nutrient, and biogeochemical feed-backs of the Sargassum blooms are also necessary, since the occurrence has become more common and intense over the tropical Atlantic and Caribbean.
Nutrient factors The Sargasso Sea, a known source area for Sargassum blooms, is classified as an oligotrophic region. With warm, oxygen-poor waters and low nutrient contents, biomass production is limited by what little nutrients are present. Historically, low nutrient levels in the Sargasso Sea have limited Sargassum production. New influxes of nitrogen and phosphorus are driving factors in increased biomass production.