Stanford University

*** Ph.D. Thesis/ Oral Defense ***

Coral reef metabolism and resilience across scales: integrating biogeochemistry, hydrodynamics, and ecology

Alexy Khrizman

Monday, March 9, 10:00 AM

Shriram 262

Department of Earth System Science

Advisor: Dr. Rob Dunbar

Coral reefs are highly productive yet increasingly vulnerable ecosystems facing rapid environmental change. This dissertation examines how reef-scale biogeochemical processes respond to variability across space, from differences within a single reef to contrasts among reef habitats, and through time, from hourly fluctuations to multi-year change, as well as to ecological recovery following bleaching. Using multi-year in situ measurements of reef metabolism together with hydrodynamic and ecological observations, I quantify net community production (NCP) and net community calcification (NCC), two metrics that capture the balance between production and respiration, and between calcium carbonate formation and dissolution, across reefs in Palau (Pacific Ocean) and the Chagos Archipelago (Indian Ocean). In Palau, a three-year field study compares advanced in situ approaches, including gradient-flux and eddy-covariance techniques, to resolve fine-scale spatial and temporal variability in reef metabolism. Coupled with 3D reef reconstruction using Structure-from-Motion photogrammetry, these measurements demonstrate how benthic composition, flow regime, and episodic events shape ecosystem function even within a single reef system. In the remote Indian Ocean, I assess reef metabolism in two contrasting reef habitats - lagoon and forereef. Measurements on the forereef were repeated three and six years after a major mass-bleaching event, documenting substantial increases in production and calcification consistent with ecological recovery and increases in coral cover. Because internal waves are a dominant physical process on many exposed forereefs, particularly in the Chagos Archipelago, periodically transporting cooler, nutrient- and oxygen-variable waters onto reef slopes, I examine their spatial and temporal variability using multi-year temperature records (2016-2022). These records reveal strong variability in cooling intensity and frequency linked to thermocline depth and large-scale climate variability. An extreme internal-wave intrusion in December 2021 illustrates how basin-scale conditions can amplify subsurface cooling on exposed forereefs while simultaneously inducing hypoxia in semi-enclosed lagoons. Together, this work provides a cross-scale perspective on how physical forcing, ecological structure, and biogeochemical processes interact to govern reef productivity, calcification, recovery trajectories, and resilience in a changing ocean.