Temporal dynamics of coral and algal cover and their drivers on a coral reef of Gorgona Island, Colombia (Eastern Tropical Pacific)

Abstract

Coral reefs are highly dynamic ecosystems often affected by diverse natural disturbances. However, dramatic declines in coral cover during recent decades raise the question of whether such declines are due to increasing anthropogenic impacts. To this end, the dynamics of coral and algal cover were studied based on data collected between 1998-2014 from 20 fixed 10-m transects at two depths in two sites of La Azufrada, a coral reef free of local human impacts, located at Gorgona Island, Colombia. Coral cover declined from 66.9 % in 1998 to 39.4 % in 2008, but then increased to 50.7 % in 2014. Fleshy and turf algae were low between 1998-2004, increased to 49.5 % in 2007, and then declined to < 33.0 %. Crustose coralline algae were lowest until 2009 when they increased as fleshy and turf algae decreased. Temporal variation was different between depths with shallow areas exhibiting major declines in coral cover until 2008 but a significant recovery since then. In contrast, coral cover in deep areas declined only slightly. Prolonged subaerial exposure of corals during extreme low tides appears to drive a cycle of coral disturbance and recovery. Coral growth makes the reef flat prone to subaerial exposure, after which corals bleach, die and are colonized by filamentous turf and fleshy algae. Sea urchins and herbivorous fishes increase their abundance in response to increased algal cover and control algal growth, leaving the substrate covered by crustose coralline algae and making it suitable for settlement by sexually-produced coral larvae. Fragmentation by both physical and biological agents (such as corallivores) enhances coral recruitment and the reef enters a coral recovery phase. Although this reef is resilient to subaerial exposure because it is protected from anthropogenic perturbations, increasing threats from global climate change may compromise its future resilience.