Sargassum

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Sargassum

Introduction

Sargassum is a type of brown algae[2] which often serves as a Macroalgal Bed in oceans. Sargassum algae serves several benefits to marine life, providing food, refuge, and breeding grounds to much fishlife. Due to impacts of climate change, in recent years sargassum has grown at an uncontrollable rate and beginning to threaten reefs.

Habitats and Location of Sargassum

Sargassum are generally found in “tidal splash zones, rocky marine pools, the intertidal zone, coral reefs and moderately deep coastal zone waters”[2]. Sargassum is found in throughout the Gulf of Mexico and North Atlantic[4]. Sargassum typically circulates in those regions and towards the Sargasso Sea in the North Atlantic Ocean, which is 1000 km wide and 3200 km long[5]. Scientists estimate that the Sargasso sea contains up to 10 million metric tons of Sargassum[5]. In the spring and summer, the seaweed typically arrives on shores in the Gulf of Mexico, the southern U.S. Atlantic coast and the northern Caribbean, crowding beaches[4]. As the seaweed has become more dense and widespread it washes up on these beaches more frequently, and has even been found on the Western Coast of Europe[6] where it is invasive and non-native to these coastal areas.

Biology

Appearance and Composition

Sargassum is a type of brown algae, genetically similar to yellow-green algae[2] which can appear anywhere from green to red in color. There are over 2800 estimated species, a number which has grown in recent years[2]. Brown algae are slender, and often branch with multiple thin tubes and large leaves[2]. Additionally, it often branches oxygen filled “berries”[1]. Sargassum may be macroscopic or microscopic[2]. Sargassum typically grows to about 2-3 meters, but there have been documented cases of Sargassum growing up to 16 meters[6]. A sargassum plant lives for approximately 3-4 years[6].

Reproduction

Sargassum has the ability to reproduce both sexually and asexually via floating fragments[6]. Sargassum’s flagellate spores alternate between releasing haploid and diploid gametes[2] during external fertilization. Sargassum’s ability to reproduce sexually has caused its spike in species variety. It is this variety which has allowed sargassum to survive in diverse environments throughout the Atlantic. If sexual reproduction continues at its current rate, it is expected that Sargassum will become more abundant in non-native regions. Sargassum lacks much scientific research due to its inability to sample [1], however scientists believe that “a decline in pH, increase in summer temperatures, and changes in the abundance” and distribution of Sargassum in an area lead to this instability in reproduction [7].

Ecology

Floating Sargassum beds serve critical ecosystem functions to a variety of marine species. These habitats contain a diverse assemblage of creatures including fish, sea turtles, and over 145 species of invertebrate, including sponges, fungi, bacteria, and protists. [7]

Ecological Roles

Sargassum beds can form floating rafts that extend for miles across the ocean. The majority of sargassum is found in the Sargasso Sea, known as the "golden floating rainforest" because of its rich biodiversity. [5] These rafts provide habitats, food, and breeding grounds for a variety of marine organisms, and some creatures, like the Sargassum fish, live their entire lives in sargassum. [1]

Nurseries

Sargassum beds provide a nursery area for many commercially important fish species such as mahi mahi, jacks, and amberjacks.[1] Additionally, juvenile sea turtles often hide from predators among the sargassum.[5]

Food

Sargassum rafts serve as feeding grounds for fish, seabirds, and other organisms in the open ocean. When the algae dies, it sinks and provides organic carbon to the water column and seafloor. It may act as a supplementary food source for bottom-dwelling organisms in the deep sea.[1]

Shelter

Floating sargassum rafts provide shelter in the otherwise completely open ocean. Juveniles from many species can mature to adulthood in the safety of the algae, and larger creatures such as dolphins and even whales use it as a shelter. Within the water column the sargassum creates layers in which different species occur. Smaller filefishes and triggerfishes are found within the algae network, while juveniles of larger fish species swim just below the raft and larger predators, like dolphins, are further down. [1]

Environmental Impacts

As sea-surface temperatures have been rising, winds in the subtropics have been growing weaker. These factors affect ocean currents, especially the North Atlantic Gyre that creates the Sargasso Sea. As a result of these changing currents, the Caribbean has experienced an influx of Sargassum in recent years. The algae has also begun to spread because of an increase in nutrients in the oceans due to human pollution. This overabundance of algae on beaches and among coral reefs has implications for the health of local and global environments. [5] Sargassum is generally harmless to humans and other organisms when it occurs naturally, and when it washes ashore many animals use it for food and shelter. However, in large amounts, the washed-up algae can prevent hatchling sea turtles from reaching the ocean. Additionally, concerns have arisen in many Caribbean communities about the affect Sargassum could have on the tourism industry, as many beach-goers are deterred by the unpleasant smell it creates. [5]

Effects on Coral Reefs

While many macroalgae have proven to be detrimental to the health of coral reefs, studies conducted on Sargassum reveal that the relationship between the organisms can be complex. Research done on Sargassum in the Great Barrier Reef determined that while conditions favorable to macroalgae are unfavorable for corals, the presence of the algae did not directly impact the reefs. [9] In some instances, the algae may in fact prevent coral bleaching by blocking sunlight and reducing water temperatures. [9] However, this same phenomenon also prevents that sunlight from being photosynthesized. While many algae compete with corals and cause reef degradation, non-allelopathic algae such as Sargassum may actually protect the corals from excessive stress, producing benefits that exceed the cost of competition. [10]

References

  1. 1.0 1.1 1.2 1.3 1.4 1.5 1.6 Sargassum: A Complex 'Island' Community at Sea: Tara L. Casazza and Steve W. Ross http://oceanexplorer.noaa.gov/facts/sargassum.html Cite error: Invalid <ref> tag; name "oceanexplorer" defined multiple times with different content Cite error: Invalid <ref> tag; name "oceanexplorer" defined multiple times with different content Cite error: Invalid <ref> tag; name "oceanexplorer" defined multiple times with different content
  2. 2.0 2.1 2.2 2.3 2.4 2.5 2.6 http://www.eoearth.org/view/article/150791/
  3. hhttp://www.bibliotecapleyades.net/ciencia/esp_bermuda_04.htm
  4. 4.0 4.1 http://www.newsweek.com/2015/07/10/sargassum-ruining-beaches-texas-tobago-347735.html
  5. 5.0 5.1 5.2 5.3 5.4 5.5 http://www.onecaribbean.org/wp-content/uploads/SargassumResourceGuideFinal.pdf
  6. 6.0 6.1 6.2 6.3 http://www.seaweed.ie/sargassum/
  7. 7.0 7.1 Pelagic Sargassum community change over a 40-year period: temporal and spatial variability: C. L. Huffard, S. von Thun, A. D. Sherman, K. Sealey, K. L. SmithJr. http://link.springer.com/article/10.1007%2Fs00227-014-2539-y Cite error: Invalid <ref> tag; name "springer" defined multiple times with different content
  8. https://en.wikipedia.org/wiki/Sargassum
  9. 9.0 9.1 Sargassum canopy decreases coral bleaching on inshore reefs :Jamal Jompa and Laurence McCook http://www.aims.gov.au/docs/projectnet/seaweeds.html
  10. Seaweed-Coral Interactions: Variance in Seaweed Allelopathy, Coral Susceptibility, and Potential Effects on Coral Resilience - Roberta M. Bonaldo and Mark E. Hay http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3899053/>