Date of Award
12-2023
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Ocean Engineering and Marine Sciences
First Advisor
Richard B. Aronson
Second Advisor
Steven Lazarus
Third Advisor
Robert van Woesik
Fourth Advisor
William F. Precht
Abstract
Coral-reef development is modulated by growth processes that deposit calcium carbonate onto the reef, such as calcification, and destructive processes that remove calcium carbonate from the reef, such as bioerosion. Wherever calcification rates exceed bioerosion rates, there is a net accretion of the reef through time, but where bioerosion rates exceed calcification rates, there is a net destruction of the reef through time. A reef’s framework is the legacy of the interaction of these processes throughout a reef’s history. Studying the fossil frameworks of reefs can shed light on the environmental factors that have controlled reef development in the past, which gives us a reliable insight into how certain reefs will respond to future variations in environmental conditions and disturbances. The goal of this dissertation is to determine how different drivers modulate the rates of carbonate production and bioerosion by quantifying their current and historical rates across environmental and ecological gradients. By determining how different factors influence the growth and destructive processes of reefs, we can accurately predict how a wide variety of reef systems will respond to future climate-change scenarios and prioritize our limited resources to protect the most vulnerable ones. I quantified the growth and destructive processes of reefs in the eastern Pacific to develop carbonate budget model and assess their potential to keep pace with future rates of sea-level rise under different upwelling regimes. To do so, I analyzed ecological surveys and push cores from reefs in the strong‐upwelling Gulf of Panamá (GoP) and the adjacent, weak‐upwelling Gulf of Chiriquí (GoC). Identifying the influence of upwelling on the persistence of reef development is necessary to identify environments that are suitable for reef development and in need of protection, and environments that are unsuitable for reef development and in need of restoration. In the Mexican Caribbean, I conducted ecological and sedimentological surveys of multiple reefs with varying ecological states to determine whether the sedimentary assemblage of each reef reflects the adjacent, living reef assemblage. Previous studies have suggested that coral-reef degradation is evident in the sedimentary record as a shift in dominance of sedimentary grains originating from coralline algae to sedimentary grains originating from corals. This shift in the sedimentary composition is hypothesized to be driven by elevated erosion rates of dead coral skeletons. I tested the relationship between the abundance of coral sand grains and the rates of bioerosion to determine whether sediment samples can be used to predict reef-wide rates of bioerosion. Sediment samples from reef cores could then be used to determine previous oscillations in bioerosion pressure that reefs might have experienced throughout their growth history. In the eastern Pacific, rates of vertical accretion during the Holocene have been similar in the GoP and the GoC; however, it seems that seasonal upwelling in the GoP exacerbated a climate‐driven hiatus in reef development during the late Holocene. The situation is now reversed and seasonal upwelling in the GoP currently buffers thermal stress, promoting coral growth and high accretion rates. On average, the GoP had significantly higher net carbonate production rates than the GoC. With an estimated contemporary reef‐accretion potential of 5.5 mm year−1, reefs in the GoP are projected to keep up with sea‐level rise if CO2 emissions are reduced, but not under current emissions trajectories. By contrast, with an estimated reef-accretion potential of just 0.3 mm year−1, reefs in the GoC are likely already unable to keep up with the contemporary sea‐level rise in Panamá (1.4 mm year−1). Whereas the GoP has the potential to support functional reefs in the near term, my study indicates that their long‐term persistence will depend on the reduction of greenhouse gases. The climate-driven hiatus in reef growth that Panamanian reefs experienced ~4100 years ago lasted ~2300 years, and it has been attributed to an increase in variability of the El Niño– Southern Oscillation (ENSO). One alternative hypothesis, however, suggests that the gap in the fossil record could have been a result of the loss of information to extensive bioerosion to the reef framework removing previously accreted material, and thereby creating a gap in the depositional record. I tested the alternative hypothesis that the hiatus was solely the result of bioerosion, assuming an acute disturbance halted coral growth 1800 years ago (the time at which reef accretion resumed after the hiatus) and that the entire framework remained in the taphonomically active zone at that time. I calculate that it would have taken 167–511 years for bioerosion to fully remove 2300 years-worth of framework growth under those circumstances. In fact, most of the reef framework in Panamá is stabilized in sediment that prevents the activity of bioeroders; only the upper ~1 m of open framework —several decades-worth of growth at most— would have been vulnerable to erosion, greatly increasing the time required to bioerode 2300 years of accumulation. I conclude that the hiatus was not solely an artefact of bioerosion; rather, a long-term increase in ENSO variability most likely suppressed coral growth and vertical reef accretion. In the Mexican Caribbean, the high abundances of coral grains in the sediments were associated with high rates of bioerosion pressure. Reef sediments may, therefore, reflect the degree of bioerosion pressure that reefs experience, and historical changes in bioerosion rates could potentially be assessed by examining the sediments from dated levels within cores of reef frameworks. Phenomena that promote sediment mixing, however, could easily dilute the signals of acute disturbances such as mass mortality events. Previous sedimentological studies have observed changes in the assemblages of reef sediments immediately after a hurricane, yet it is still largely unknown how long hurricane signals persist within reef sediments, and whether they are strong enough to preserve in the long-term sedimentary record of reef sediments. After a nine-year gap in hurricane activity, four hurricanes struck the Mexican Caribbean between 2020 and 2021. After the first 3 hurricane events in 2020, there was a significant decline in the living octocoral cover, coupled with an increase in octocoral sclerites in the sedimentary assemblage. By 2022, however, the abundance of octocoral sclerites in the sedimentary assemblage returned to their pre-disturbance levels, indicating a loss of the hurricane signal within a year of the last disturbance (Hurricane Grace in 2021). These trends suggest that short term (1–10 years) shifts in ecological and geological processes caused by acute disturbances are not preserved in the sedimentary record. Instead, reef sediments are likely time-averaged at a decadal to centennial scale, and do not record short-term shifts in reef processes. I demonstrate that some environmental controls that used to be detrimental to reef growth, such as strong upwelling, are now buffering the increasing thermal stress of climate change. Yet the reefs that are not protected by upwelling, such as the ones in the GoC, remained threatened by future thermal stress and they already cannot keep pace with current rates of sealevel rise. Although reefs in the GoC were able to recover from previous large-scale climatic events, declining coral cover and increases in bioerosion pressure threaten their resilience to climate change in the near future. To further understand the role of bioerosion on previous shutdowns of reef development, the sedimentary record could be used to track decadal- to centennial-scale oscillations in bioerosion pressure. Nevertheless, the reduction of greenhousegas emissions is essential to promote the recovery of degraded reefs, as well as to ensure the persistence of those that, for the moment, remain protected from thermal stress.
Recommended Citation
Rodriguez-Ruano, Victor, "Climate Change and The Shifting Geography of Coral-Reef Development" (2023). Theses and Dissertations. 1357.
https://repository.fit.edu/etd/1357