Defense Date

12-4-2023

Document Type

Thesis

Degree Type

Master of Science

Degree Name

Marine Science

First Advisor

Joshua Feingold

Second Advisor

David Gilliam

Third Advisor

Bernhard Riegl

Keywords

Tropical Eastern Pacific, bleaching, El Niño-Southern Oscillation, coral reefs and communities, free-living coral, Psammocora stellata, asteroids

Abstract

In the Galápagos Archipelago, the free-living nodular coral Psammocora stellata is a common species that does not form reef structures. However, it is a key component of the ecosystem by increasing habitat complexity and, consequently, species diversity. This species experienced a drastic decline in Devil’s Crown channel, Floreana Island, where it disappeared after the 1982-83 El Niño-Southern Oscillation, presumably by displacement of unattached colonies by strong surface waves and currents. This assemblage has now recovered to pre-impact levels. A combination of underwater photoquadrats from 2007 and 2009 and recent (2023) orthomosaics were used to determine changes in coral densities and size frequencies over time. Benthic community composition and coral-associated organisms were quantified based on depth and habitat use. Temperature and current velocity were investigated as potential contributors to community change. Coral density had a significant 47% increase from 2007 to 2023. Colony size distribution significantly changed over time: mean colony maximum diameters were larger in 2023 than in 2007. P. stellata had the highest densities at 11 m and 3 m depth. However, benthic community composition and sea stars distribution were most distinct between deeper and shallower areas. In this assessment, P. stellata distribution is related to underlying biotic and abiotic processes and described for the first-time using photogrammetry. A more in-depth understanding of this Psammocora dominated community will illustrate the importance of free-living corals in the Galápagos Islands.

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