Steady Decline of Corals and Other Benthic Organisms in the SeaFlower Biosphere Reserve (Southwestern Caribbean)

Abstract

Coral reef decline persists as a global issue with ties to climate change and human footprint. The SeaFlower Biosphere reserve includes some of the most isolated oceanic coral reefs in the Southwestern Caribbean, which provide natural experiments to test global and/or basin-wide factors affecting coral reefs. In this study, we compared coral and other substrate cover (algae, cyanobacteria, and octocorals), along population densities of keystone urchin species from two atolls (Serrana and Roncador Banks), during 1995, 2003, and 2015/2016. We also surveyed benthic foraminifera as a water quality proxy for coral growth in the last period. A steady reduction in coral cover was clearly observed at Roncador's lagoon, but not at Serrana's reefs, with significant differences between 1995 and 2015/2016. Percent cover of fleshy algae decreased significantly also at Roncador between 1995 and 2003 but did not change notably from 1995 to 2016 at Serrana. However, both Banks exhibited a loss in crustose coralline algae from 2003 to 2015/2016. Likewise, a reduction in bottom complexity, measured as bottom rugosity, was evident between 1995 and 2003. Roncador Bank had unprecedented high octocoral densities, which increased almost threefold from 2003 to 2015. In contrast, urchin densities were low in Roncador; only Diadema antillarum increased from 2003 to 2016 in Serrana Bank. The Foraminifera in Reef Assessment and Monitoring (FORAM) Index (FI) in the two Banks was below the range expected for healthy coral reefs. Although both Banks follow a reduction in CCA and CA cover, Roncador Bank also faces an alarming decline in coral cover, urchins and bottom complexity (rugosity) in contrast to increases in octocoral densities and potential loss of resilience and eutrophication suggested by the FI index. These unexpected findings led us to consider and discuss potential outcomes, where these reefs deteriorate (i.e., erode and drown) providing ideal conditions for octocoral growth. Hence, it is of utmost urgency to start monitoring reef budgets, octocorals and nutrient sources.

Resilience of Octocoral Forests to Catastrophic Storms

“…Under the environmental and biotic conditions prevailing on present-day shallow Caribbean reefs, octocoral communities have exhibited resilience and some resistance 51,60 to major disturbances including hurricanes (present data), bleaching 61 , eutrophication 24 , diseases 62 and early signs of the effects of ocean acidification on seawater pH 63,64 . At high population densities, Caribbean octocorals form underwater forests 50 that alter environmental conditions, such as light, currents, and sedimentation, beneath their canopy, and create habitat utilized by many species 50,65,66 .…”

“…Understanding the stability of octocoral communities relative to disturbance is a critical component in assessing whether octocoral "animal forests" are the stable successor to the scleractinian dominated communities that preceded them. Although monitoring that also follows octocoral abundances is relatively rare, increased abundances of octocorals have now been documented on shallow reefs in the Caribbean such as the Florida Keys 23 , Virgin Islands 21,22 and the southwest Caribbean 24 . Disturbance events and whether octocoral communities are affected (i.e.…”

“…These losses typically have been recorded as stepwise incremental reductions in cover attributed to pulse disturbances caused by hurricanes 9 and bleaching 12,13 , that overlie the chronic effects of press disturbances such as diseases 14 , depressed rates of growth 15 , and reduced recruitment 16 .The declining abundances of scleractinians have generally been characterized as a trade-off in both the abundance and functional importance of scleractinians and macroalgae [17][18][19] . However, it has also become apparent that other sessile benthic invertebrates have increased in abundance 20 , and at some sites, where quantitative data are available, octocorals have increased as scleractinians have declined [21][22][23][24] . Octocorals have always been abundant on Caribbean reefs 25,26 , but unlike scleractinians, which have undergone marked declines in abundance, octocorals have increased in abundance on shallow reefs (i.e., <30 m depth) at sites ranging from the oceanic, southwest Caribbean 24 , to the U.S. Virgin Islands 22 , and the Florida Keys 23 .…”

Embryogenesis, polyembryony, and settlement in the gorgonianPlexaura homomalla

“…This is particularly important for taxa like Caribbean octocorals, which provide some of the same ecosystem services as scleractinian corals, such as three-dimensional structure for fish grazing (Tsounis et al, 2016) and the promotion of coral recruitment (Privitera-Johnson et al, 2015). While scleractinian cover on Caribbean reefs has declined (Hughes, 1994;Gardner et al, 2003), octocoral abundance has been increasing (Ruzicka et al, 2013;Lenz et al, 2015;Sánchez et al, 2019).…”

Corals in the Mesophotic Zone (40–115 m) at the Barrier Reef Complex From San Andrés Island (Southwestern Caribbean)

“…Shallow reefs in the SeaFlower Biosphere Reserve, even at the remotest bank atolls, are showing a steady decline in coral cover overall health condition during the last 20 years (Sánchez et al, 2019b). Mesophotic Coral Ecosystems (MCEs), located between 30 and >150 m of water depth, may act as a refuge of coral populations due to favorable conditions in this less altered environment (Bongaerts et al, 2010).…”