Marginal coral reefs show high susceptibility to phase shift

Area of Science:

• Marine Biology
• Ecology
• Conservation Science

Background:

• Coral reefs in optimal conditions frequently undergo phase shifts due to degradation.
• Marginal reefs were historically considered more resistant to degradation, with fewer documented phase shifts.
• A lack of quantitative studies on marginal reef phase shifts necessitates further investigation.

Purpose of the Study:

• To quantitatively assess the geographic extent of phase shifts on marginal coral reefs in the Southwest Atlantic.
• To identify the drivers and patterns of phase shifts from coral to zoanthid and macroalgal dominance on these reefs.

Main Methods:

• Utilized existing metadata and a calculated phase shift index to identify phase shifts.
• Analyzed correlations between phase shifts and local human impacts (population size, urbanization, ports).
• Examined relationships between phase shifts and environmental factors (sea surface temperature, latitude, shore proximity).

Main Results:

• Phase shifts to zoanthid dominance positively correlated with proximity to large human populations, urbanized surfaces, and dredged ports.
• Zoanthid shifts negatively correlated with sustained sea surface temperature anomalies.
• Macroalgal shifts positively correlated with ports, urbanized surfaces, higher latitudes, and shore proximity, suggesting nutrient runoff links.

Conclusions:

• Marginal reefs exhibit a high frequency of phase shifts, indicating significant degradation.
• These findings challenge the notion of marginal reefs possessing higher natural resistance to human impacts.
• Human activities are significant drivers of coral reef phase shifts, even in marginal environments.