Exploring the Scale of the Caribbean Sea Marine Ecosystem

The Wider Caribbean Region (WCR) is a region that runs along east coast of North and South America from Cape Hatteras in the north to the Amazon River mouth in Brazil in the south. The Caribbean Sea is one of four large marine ecosystems within this region. The three other LMEs include the Gulf of Mexico LME, the North Brazil Shelf and the Southeast US Continental Shelf LME. The Caribbean has a geophysical nature of being semi-enclosed in the western Atlantic Ocean and this has been a major factor in determining the LME boundaries within the WCR. Central and South America make up the major land boundaries of this LME and the Antilles Islands bound the opposite sides. The coastal region of the Caribbean Sea LME includes environments such as sandy beaches and rocky shores but is mostly characterized by coral reefs, mangroves and seagrasses.

The Caribbean seafloor has underwater ridges and sills that separate it into 5 basins, namely, Grenada, Venezuela, Columbia and Yucatan Basins and the Cayman Trough. The Cayman Trough is roughly 7500m in depth while the average depth of the Caribbean Sea is 2400m. The region of the Caribbean LME has highly variable oceanography both spatially and temporally with an overall circulation in the clockwise direction and some of it in the form of meanders and mesoscale eddies.

The two largest river systems in the world are on the north coast of South America and dominant this region of the LME. These include the Amazon and the Orinoco Rivers. These two rivers combine when entering the Caribbean Sea and create a freshwater plume which in turn creates a freshwater lens at the top of the ocean thus leaving saltier water below it. There is a gradient of salinity that is formed due to this plume and causes the water column to be highly stratified. This stratification occurs mostly in the upper 1200m of the water column and can also be attributed to the prevention of flow in the deep water caused by the sill depth of the Antilles Islands Arc. The stratification which occurs in this area is important for reducing the efficiency of hurricane-force winds which prevents cooler water being brought up from the deep and prevents the sea surface temperature from cooling drastically. This plume, thus, is believed to provide sustained heat and moisture which intensifies hurricanes and tropical cyclones which is known to occur over the Caribbean Sea.

Scale of the Caribbean Sea

The Caribbean Sea receives waters from both the North and South Atlantic. To determine where these waters came from, the temperature, salinity and dissolved oxygen must be measured and examined. Water that originates from the South Atlantic will be less saline and will have more dissolved oxygen at the same density as waters that originate from the North Atlantic. The waters that flow through into, and are found in, the Caribbean are time dependent as it has been observed that waters entering the Caribbean are more likely to be of South Atlantic origin but seem to contain North Atlantic waters in some surveys and data analyzed by Wust.

The Caribbean Sea is considered to be a hot-spot, globally, for marine biodiversity and is stated to contain the largest concentration of marine species in the Atlantic Ocean. Coral reef development is distinct in the Wider Caribbean Region thus majority of the corals and the associated species are endemic, therefore this is a region of significant global biodiversity. Due to Orinoco River plume, there has been an increase in plankton abundance at the edge of the low-salinity lens upwelling regions. Over 500 fish species are supported by the Caribbean Sea and several other on-fish taxa.

The South-western part of the Caribbean Sea, the seagrasses are dominated by Thalassia testudinum (turtle grass), and they have been observed to have an intermediate productivity and biomass. Syringodium filiforme (manatee grass) can be found to be intermixed with turtle grasses. In this area the dominant species of corals are Agaricia agaricites, Porites astreoides, Siderastrea siderea and Montas-traea faveolata. The reefs that inhabit these species are generally found to be surrounded by terrestrial mud and resuspended substrates due to currents and strong waves. The seagrasses provide food and shelter to many of the reef dwellers as well as regulatory services for the ocean such as climate and nutrient regulation. They are responsible for the absorption of high amount of carbon dioxide which is used in photosynthesis and thus produces organic matter that is eventually transported and provides food to offshore ecosystems.

Mangroves found this is region have been observed to be expanding due to low human impact. They are dominated by Rhizophora mangle (red mangrove), Avicennia germinans (black mangrove), Laguncularia racemose (white mangrove) and Conocarpus erectus (buttonwood). The Mangroves have regulatory function in the ocean by protecting the coast and participating in waste regulation. They help decrease the wave and current forces towards land and they help to buffer the inland effect of waste onto the coral reefs and seagrasses. Mangroves also provide food for the commercial fish in the region.

Before the mass mortality in the Caribbean region in 1983, the species of sea urchin D. antillarum was a key herbivore species. Another key herbivore that has been observed in more recent studies is the sea urchin E.viridis, and the studies show that the populations vary during different times of the year due to the changes in the algal density. Green, red and brown algae have all been observed in all parts of the Caribbean Sea and are important features of the region’s underwater plant life. Three groups of Protoctista dominate the Caribbean, namely, Foraminifera, Dinoflagellata and Amoebozoa which make up majority of the primary producers. Mollusca, Crustacea and Tetrapods are the most diverse groups found in the Caribbean Sea, in terms of number of species and are found to make up 60% of the total biota.

Belize is the second largest reef in the world, after The Great Barrier Reef, and is found in the North of the Caribbean Sea. Some of the top predators that are found in the Caribbean Sea, and more so in the Belize, are the bottlenose dolphin (Tursiops truncatus), the nurse shark (Ginglymostoma cirratum), the Caribbean reef shark (Carcharhinus perezi) and the blue-ringed octopus (Hapalochlaena lunulata) which is has a venom that is fatal. Some creatures that are found in the Caribbean Sea are on the list of the most endangered species in the world and these include the Cocqui llanero frog and the West Indian manatee.

The coastal ecosystems are highly productive in the Caribbean Sea LME due to the ratio of the continental margin being larger to that of the open ocean area. This means that the geological, physical and biological processes have a higher importance in the coastal regions compared to those in the oligotrophic offshore waters. These offshore waters have a very high water clarity which is influenced by the oligotrophic conditions and ocean masses in the region.

The North Equatorial current that flows through the passage between the Lesser Antilles into the Caribbean Sea tends to be nutrient-poor which adds to the oligotrophic conditions of the offshore waters. It is the Lesser Antilles Islands that are more so characterized by coral reefs, mangroves and seagrasses at shallow depths, and only if the island has a wide enough shelf, which not all of them do.

Depending on the season, the Caribbean Sea can be considered mesotrophic rather than oligotrophic. The Orinoco River plume increases the primary productivity in Autumn by introducing large amounts of dissolved organic matter and nutrients from land masses such as Puerto Rico. During winter and spring, there are trade winds that cause coastal upwelling on the coast of Colombia and Venezuela and this brings nutrients to the surface which increases the phytoplankton biomass and thus increases primary production. Organic suspended matter also have enhanced transportation due to zooplankton production in the southeastern Caribbean. This organic matter is usually in the form of fecal pellets and due to its fast sinking rates, it is closely coupled with the benthic assemblages in the Venezuela Basin.

Coral reefs, together with seagrasses and mangroves, support the rich biodiversity of the Caribbean Sea which are important for food chains and tourism. The Caribbean Sea is also a highly sought-after spot for natural gas and oil. However, climate change poses detrimental effects on the entire large marine ecosystem as many animals in the Caribbean Sea have specialized temperature ranges in which they live and with increasing temperatures, stress on these animals increases. Along with increasing temperatures, the frequency of hurricanes and tropical cyclones will increase which poses a threat to the structure of the coral reefs (i.e. bleaching which leads to micro-algal toxins in commercial fish), mangroves and seagrasses.

The Caribbean Sea comprises of many complex systems that feed into one larger ecosystem, namely the Wider Caribbean Sea ad all work and function together. The LME consists of many primary producing organisms that feed higher key herbivores which, in turn, feed the higher trophic level fish and top predators of the system. The Caribbean is a key ecosystem that contains fish that are very important for some of the North Atlantic fisheries as well as a major food source for the surrounding countries and islands themselves. Commercial species such as rock lobster and conch have seen a decline in the stocks due to the are not being able to support the demand at times. Human health has been threatened as well as the character of the Caribbean that vacationers seek out. Destruction and reduction of the coastal habitats make the countries and communities more vulnerable to climate change impacts which leads to devastating results. With the data that is collected the Caribbean Time Series, more can be understood about how the river systems and humans are impacting this large marine ecosystem and what measures we can put into place to protect and conserve the structures and biodiversity within it.