HO26 Exploring abiotic factors as drivers of a unique Caribbean coral reef ecosystem
Dr Dan Exton, Operation Wallacea
The exact structure and appearance of a coral reef can vary in different areas of the world and based
on differing environmental conditions and anthropogenic impacts acting upon them. However, one of
the most common images of a coral reef is the clear blue water surrounding it; an image which is more
often than not accurate. The photosynthetic processes present on reefs, most importantly by
symbiotic microalgae called zooxanthellae which live within coral tissue and drive some of the
ecosystem’s most vital processes, makes high light availability an important component of reefs,
making clear water advantageous. Scleractinian corals, the ecosystem architects of complex reefs, also
thrive in low nutrient, or oligotrophic water, which limits macroalgal growth rates and helps corals to
dominate the benthic community. Clear water is a common symptom of low nutrient availability
through inhibition of phytoplankton growth. Finally, corals are particularly susceptible to the effects
of sedimentation, where particles settle onto the reef and smother coral polyps, meaning reefs
struggle to survive in sites with high sediment loading, for example near to riverine outputs or where
land runoff is high.
Clear water was therefore considered a pre-requisite for ecosystem health, and management targeted
such locations in order to maximise success. However, in recent years, there has been increasing
evidence that non-clear, or turbid, water could also provide a suitable environment for healthy coral
reefs, and in some instances even represent a beneficial alternative to clear water sites for long term
coral survival. One of the major current threats to Scleractinian corals is thermally induced bleaching.
Although increased temperature is the main driver of this form of bleaching, high light stress is also
known to play an important role, and therefore the reduced light quantity and quality caused by
reduced water clarity is believed to provide a useful buffer against thermal bleaching. This has been
supported by the discovery of several reefs in turbid water, where significant coral communities were
thriving. An example of this was in the Seychelles where, after the 1998 El Niño event caused mass
coral bleaching mortality in the region, large colonies of thermally sensitive coral species were found
successfully inhabiting turbid lagoons. An important consideration is the difference between turbidity
and sedimentation, two terms which are often confused. Turbidity refers to sediment in suspension
within the water column, whereas sedimentation describes the process of sediment deposition onto
the reef itself, which creates the impact of coral smothering.
The majority of Honduran coral reefs are representative of the Caribbean region; located in clear
water sites and with declines in Scleractinian coral cover leaving a regional average of approximately
23%. They have also suffered varying extents of macroalgal overgrowth, with the most heavily
impacted sites demonstrating macroalgal cover of over 50%. The reef system of Tela Bay, however,
boasts a significantly higher Scleractinian coral cover, with some sites approaching 70%, amongst the
highest in the entire Caribbean. However, historical fishing pressure has led to a complete collapse in
the reef fishery of the bay, making fish abundance and biomass low in contrast to such a healthy
benthic community. In many locations, such an extreme level of overfishing would inevitably lead to
the replacement of hard coral with macroalgae, and an eventual phase shift to an algal dominated
system. In Tela Bay, however, macroalgal cover is extremely low (<5%), a phenomenon which can
almost certainly be attributed to the unusually high population density of the sea urchin Diadema
antillarum, a Caribbean keystone species which under natural conditions provides the majority of
