Defense Date

8-11-2022

Document Type

Dissertation

Degree Type

Doctor of Philosophy

Degree Name

Oceanography/Marine Biology

First Advisor

David Gilliam, Ph.D.

Second Advisor

Morgan Pratchett, Ph.D.

Third Advisor

Matthew Johnston, Ph.D.

Fourth Advisor

Rosanna Milligan, Ph.D.

Abstract

The persistence of coral reef communities is threatened by a suite of pressures operating at varying spatial and temporal scales. In general, acute disturbances (short term stochastic events such as marine heatwaves or hurricanes) and chronic pressures, such as ocean warming, have caused the most significant changes to stony coral assemblages (order Scleractinia) and continue to impair recovery potential. Additionally, many coral reefs are subject to local chronic anthropogenic pressures resulting in poor water quality or sedimentation, which further impact stony corals and shape benthic community structure, particularly near urbanized coastlines. For the viability of communities on coral reefs, a balance must be struck between loss following disturbance and recovery (i.e., resilience). The current scientific consensus is that under moderate disturbance regimes, locations subject to less local chronic anthropogenic pressure will be more resilient and the community will recover during inter-disturbance periods. However, given observed and predicted increases in the frequency and severity of acute disturbances under global climate change, resilience may be undermined regardless of contemporary differences in local chronic pressure. Florida’s Coral Reef (FCR), stretching 595km from St Lucie Inlet to the Dry Tortugas, exists along a gradient of chronic anthropogenic pressure and was impacted by multiple acute disturbances over the past two decades. This study explored benthic community dynamics and stony coral demographic processes, such as recruitment, growth and mortality, to assess the influence of acute disturbances and chronic pressures on FCR.

Spatiotemporal changes in the benthic community from 2004 to 2018 and stony coral resilience were assessed in relation to acute disturbances, in the three distinct coral reef regions on the FCR: the high-latitude, heavily urbanized Southeast Florida Coral Reef Ecosystem Conservation Area (ECA), the governmentally protected, but historically exploited Florida Keys and the comparatively remote, least exploited Dry Tortugas (Chapter 1). In the ECA, spatiotemporal variations in net growth rates and partial mortality were quantitively analyzed in the three most abundant coral species, Montastraea cavernosa, Porites astreoides and Siderastrea siderea from 2000 to 2020 (Chapter 2). The influence of water quality and temperature on benthic community composition and interannual changes in stony coral abundance, recruitment, mortality and diversity were analyzed in the ECA from 2018 to 2021, a period with no known acute disturbances (Chapter 3). Finally, spatiotemporal variations in recruitment, density and size structure of the contemporary stony coral assemblage were assessed in the ECA from 2018 to 2022 (Chapter 4)

Overall, stony coral resilience was found to be low across the FCR, regardless of differences in chronic pressure, with significant declines in cover during periods with acute disturbances, limited recovery during inter-disturbance periods and corresponding proliferation of macroalgae. High partial and whole colony mortality, predominately related to acute disturbances and stony coral tissue loss disease, constrained net growth rates in M. cavernosa, P. astreoides and S. siderea, such that colonies reached just a third of their potential size. Recruitment was generally low in the ECA, averaging 0.95 ±0.2 recruits m-2 (± SE). Siderastrea siderea recruitment was high at some inshore sites (~7 recruits m-2), but there was limited evidence these survived or grew into larger size classes. Spatial decoupling between recruitment, size structure and survival likely maintains a preponderance of small colonies in the ECA. Higher water temperature, particularly when annual mean water temperature was above 27 °C, was positively associated with stony coral recruitment, abundance and health, until threshold maximum temperatures were exceeded (>31 °C). The benefits associated with warmer temperatures were negated by poor water quality, as nutrient enrichment was related to increased macroalgae cover, reduced coral recruitment and higher partial mortality. At present the future for stony corals in Florida is bleak. High acute disturbance frequency, from a multitude of different stressors, and chronic environmental pressures were related to consistent declines in cover, high partial colony mortality which constrains growth and a degraded coral community composed of small colonies, primarily of weedy or resistant species. However, the data suggests reducing local chronic pressures in the ECA may limit increases in macroalgae cover and enhance stony coral recovery potential during inter-disturbance periods, particularly when temperatures are optimal. Urgent action to tackle global climate change and local anthropogenic pressures is therefore required for these coral communities to have any realistic prospect of recovery.

ORCID ID

0000-0002-2945-6576

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