Connected by the ocean

Water, the ocean, the medium for life, connects us all. Let it not divide us. Dr Anjani Ganase, marine scientist, looks into appreciating water connectivity for maintaining healthy reefs around Tobago in the International Year of the Reef 2018. (Follow her on Twitter: @AnjGanase). First published in the Tobago Newsday, Thursday 12th April, 2018

The connectivity of the ocean facilitates processes such as exchange and transport of essential material (nutrients, oxygen and carbon dioxide), the migration of organisms and the dispersal of seeds and offspring between habitats. It allows marine turtles that hatch on our shores to hang out on reefs on other islands and to travel to other oceans. Whales, which prefer to raise their pups in warm waters, may grace us with Caribbean visits. Ocean connectivity allows coral reefs to be repopulated by fish and invertebrates from neighbouring reefs and from other habitats such as mangrove nurseries and seagrass beds.

Reef views of Angel reef off of Goat Island in Speyside, Tobago. How are these reefs connected to the adjacent reefs in Speyside and the rest of Tobago? What are their connections to the wider Caribbean? (Photos by Anjani Ganase)

The ocean is the major food source for humans, and we depend on the natural replenishment of much of what we extract for food. The ocean has provided major trade routes, but it has also been the medium for the spread of stressors, such as diseases and introduced species, and pollution. The Caribbean has a history of disease that has plagued our reefs, such as caused the mass die off of an important sea urchin (Diadema antillarum) around 1983 – 1984; it started in Panama, and spread across the rest of the Caribbean in the following year. In 1985, the lionfish was introduced off the coast of Florida, and 28 years later the population has spread up the northeast USA to Bermuda, and all the way to the shores of Trinidad and Tobago. 

The three-dimensional world of the ocean, although fluid, is far from homogenous; natural physical boundaries exist and have been observed. Variations in the physical conditions of the water, including light, depth, temperature and salinity, allow marine life to occur in unique habitats: coral reefs, mangroves, bays, sea grass beds, rocky intertidal zones and lowlight (mesophotic) reefs. Too warm or too cold waters or coastal waters too close to fresh water outflows may limit certain fish and coral life, while some waters prevent fish or coral colonies by too much wave action.

Many marine organisms (fish and invertebrate – coral, lobster, crabs, conch etc.) have a ‘bipartite life cycle,’ which means that they are born and disperse into the water column for the first part of their lives; the currents can keep larvae in the same area or carry them off to neighbouring reefs or other islands. Eventually, they settle on a specific reef and become residents as adults. Annual coral spawning events occur during the summer (July/ August) in the Caribbean with the synchronous release of eggs and sperms in calm waters which maximizes the fertilization success of offspring (van Woesik 2010). The drifting larvae are likely to repopulate their own reefs but may also contribute to other reefs or other marine habitats. This migration of new life among habitats is crucial for the replenishment of marine populations. Habitats that have been damaged by disturbances such as hurricanes, rely on this connectivity for recovery and renewal. Without these connections, our reefs and inhabitants  - the fish, conch, crabs, lobsters etc - would be depleted at a more rapid rate than already occurring.  

Reef views of Angel reef  and its residents off of Goat Island in Speyside, Tobago. 
Tobago at the southern end of the Lesser Antilles island chain, and more than 100 km away from nearest island neighbours Grenada and Barbados, is fairly isolated. As a Caribbean coral island, Tobago is the last stop on the connectivity train (Schill et al 2015). To add to this isolation, Tobago’s coral reefs are frequently washed by fresh water outflows of the Orinoco River which limits the number of coral species and other reef creatures that can successfully live on our reefs. Our closest potential supplier of new coral larvae is Barbados, which in turn may receive from Guadeloupe, Martinique, St. Lucia and St. Vincent and the Grenadines (Schill et al 2015). Continuing north up the island chain of the Lesser Antilles, the connections among corals get stronger and the sources of coral recruitment become more diverse. The repopulation of most Tobago reefs occur locally through self-recruitment. This means that Tobago’s reefs are unique but also vulnerable. We may have very limited reef recovery, following any island-wide disturbance to our coral reefs.

Photo 3. Strength of reef connections across the wider Caribbean region, based on modelled transported coral larvae (dispersal simulations between 2008–2011). The width and colour of the lines represent the strength of connection. The darker red and orange areas indicate high amounts of settled coral larvae transport (figure and caption sourced from Schill et al 2015).

What does this mean for us as our island’s stakeholders? It means that a larger effort is needed to ensure that the reefs are managed so that we do not dwindle our stocks. Loss of the coral reefs results in the loss of the fish and invertebrates (lobster) that live there. Our current poor management of our coastlines and the ongoing problem of coastal pollution will continue to degrade our reefs and kill coral stocks. Among the management needs, for example, there should be tighter regulation of the type and the timing of coastal construction and land clearing activities, especially during the rainy season and ecologically important habitats - mangroves and wetlands. These activities result in sediment run off to the sea, which smothers large sections of the reef. 

We are fortunate that Tobago’s location ensures lower exposure to hurricanes which do devastate reefs. Regional coordination of coral reef protection is crucial for the longevity of Caribbean reefs. Among the connected reefs in the Caribbean - especially those that are the biggest contributors of coral larvae to other reefs - 77 % of these coral reefs are not protected (Schill et al 2015). Connectivity is crucial for reseeding reefs destroyed by hurricane and coral bleaching events, both of which are expected to intensify in the future. The damage to the reefs of Puerto Rico and the Virgin islands by hurricanes Maria and Irma 2017 may prove a major blow to the reseeding of reefs of surrounding countries. Here in Tobago, let us make the effort to learn more about where our food comes from, and how to maintain and boost marine assets. Let us build the connections with the ocean and our reefs.

Video showing a 30-day visualisation of simulated coral spawning event based on NOAA’s Real-Time Ocean Forecast System (RTOFS) using ocean current data for three years 2009, 2010, 2011 (Schill et al 2015)


Schill SR, Raber GT, Roberts JJ, Treml EA, Brenner J, Halpin PN (2015) No Reef Is an Island: Integrating Coral Reef Connectivity Data into the Design of Regional-Scale Marine Protected Area Networks. PLoS ONE 10(12): e0144199. doi:10.1371/journal.pone.0144199

Van Woesik, R. "Calm before the spawn: global coral spawning patterns are explained by regional wind fields." Proceedings of the Royal Society of London B: Biological Sciences (2009): rspb20091524.


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