Date of Award

5-2022

Document Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Ocean Engineering and Marine Sciences

First Advisor

Ricahrd B. Aronson

Second Advisor

Mark Bush

Third Advisor

Toby S. Daly-Engel

Fourth Advisor

Steven Lazarus

Abstract

Climate change is the primary threat facing coral reef ecosystems. How marginal reef systems responded to past climatic variability provides clues to their prospects for persisting through contemporary climate change. Reefs along the Pacific coast of Panamá are dominated by branching corals of the genus Pocillopora. These reefs experience a natural gradient of nutrients, pH, and temperature because of stronger seasonal upwelling in the Gulf of Panamá (GoP) than in the Gulf of Chiriquí (GoC). The shallow reefs within both of the gulfs of Panamá They are also strongly affected by climatic variability due to the El Niño–Southern Oscillation (ENSO). It is hypothesized that ENSO variability at the beginning of the Late Holocene ~4000 year ago caused a 2000-year hiatus in reef-building in Pacific Panamá. Nevertheless, the environmental conditions that drove the shutdown and continued to suppress reef development were inferred from the geochemistry of a limited number of coral samples. Foraminifera are good indicators of reef-ecosystem condition because they are abundant in reef settings, secrete calcium carbonate, host symbionts, respond rapidly to environmental variation, and persist in the absence of corals. In addition, unlike some other benthic taxa, the diversity of benthic Foraminifera and scleractinian corals reflect regional differences in oceanographic conditions in the ETP. Both taxa are approximately half as diverse in the ETP as they are in the Caribbean. The composition of contemporary foraminiferal assemblages and their geochemistry track environmental variations in the eastern tropical Pacific. Temperature loggers deployed from 2016 to 2019 showed that water temperatures were lower on average and more variable in the GoP due to stronger seasonal upwelling. To determine how regional oceanography and climatic drivers influence modern foraminiferal assemblages between the two gulfs, I examined benthic Foraminifera in surface sediments. Contemporary and subfossil samples from both gulfs were dominated by heterotrophic Foraminifera, which was likely the result of overall nutrient enrichment due to upwelling—even in the weakly upwelling GoC—combined with ENSO effects. However, the GoC had higher abundances of symbiont-bearing taxa than the GoP. Since the Gulf of Chiriquí experiences weaker upwelling than the Gulf of Panamá, it is more characteristic of an oligotrophic reef environment. Geochemical analysis of contemporary, symbiont-bearing miliolids, Sorites marginalis, revealed that foraminiferal Mg/Ca ratios were lower in the GoP than in the GoC. The elemental ratio of magnesium to calcium in tests of benthic Foraminifera has been used successfully as a proxy for ocean temperature because more magnesium is incorporated into the test during its construction with increasing temperature. The offset in foraminiferal Mg/Ca was consistent with the lower mean annual temperature observed in the GoP due to stronger seasonal upwelling. Since contemporary foraminiferal assemblages were shown to be modulated by environmental variations, I sought to test whether the climatic changes leading up to the shutdown of reef accretion also led to shifts in the foraminiferal assemblages over the same period. I sampled Foraminifera from two cores from reef-framework within the GoP (Saboga and Contadora) and one core from the GoC (Canales de Tierra). I also used Mg/Ca-based temperature reconstructions to evaluate past environmental change. Similar to assemblages found in contemporary sediments, heterotrophic foraminifers dominated at all sites, and Canales de Tierra was characterized by a greater number of symbiotic Foraminifera than those sites in the GoP. The density of symbiotic Foraminifera tracked Pocillopora growth through time at all sites, decreasing from ~15 individuals/gram of sediment to ~2 ind/g during the hiatus in reef growth (~4000 cal yr BP to ~2000 cal yr BP). I conducted geochemical analyses on foraminiferal tests within the cores to determine Mg/Ca ratios over time. At all sites, Mg/Ca ratios were more variable during the hiatus period. However, attempts to reconstruct temperature using Mg/Ca proxies from foraminiferal shells proved difficult in this study since shell Mg/Ca ratios from Contadora suggested an unrealistic temperature range of 8–64°C (107–247 mmol/mol) even after two rounds of analyses. At Saboga and Canales de Tierra, Mg/Ca ratios did not show as much variability as those from Contadora, ranging from 135 to 150 mmol/mol and 101 to 203 mmol/mol, respectively. Based on the cores from Saboga and Canales de Tierra, I hypothesize that the greater variability of Mg/Ca during is likely a result of higher variability in ENSO. Strong La Niñas at the inception of the hiatus would result in cooler water temperatures which would account for the lower Mg/Ca ratios. Stronger El Niño events during the hiatus are most likely represented by higher Mg/Ca ratios. Reef accretion resumed ~2000 cal BP at which time Mg/Ca-based reconstructions of temperature stabilized at all sites. A local effect on Mg/Ca concentrations in Contadora could suggest a reason for the increased concentrations of Mg/Ca during the hiatus but the cause of the anomalously high Mg/Ca values is unclear. Predicting the long-term behavior of ENSO with ongoing climate change is difficult due to conflicting evidence from paleoclimate proxies and model projections. However, the models that best capture key ENSO dynamics also tend to project an increase in future ENSO-driven sea-surface temperature variability and magnitude under ongoing warming particularly in the ETP. Based on the history of reef development in Pacific Panamá, it is likely that increased warming and ENSO variability could result in the collapse of Pacific reef ecosystems and a shift toward an increasingly heterotrophic state.

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