Graduation Year

2022

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Biology (Integrative Biology)

Major Professor

Susan S. Bell, Ph.D.

Committee Member

Bradley T. Furman, Ph.D.

Committee Member

David B. Lewis, Ph.D.

Committee Member

Luanna Prevost, Ph.D.

Keywords

Climate change, Crassostrea virginica, Ecosystem, Phase shift, Rhizophora mangle

Abstract

Ecological regime shifts are transitions in ecosystem state, which can occur either when external conditions gradually change or when large perturbations force a system into an alternative state. Such transitions can exhibit non-linear dynamics, where small changes in external conditions can result in abrupt transition and often occur with little warning. After crossing a tipping point, the system may be unable to revert to its original state if the new regime possesses strong, reinforcing feedbacks. Numerous marine ecosystems have experienced regime shifts due to a combination of stressors that have eroded ecological resilience, such as overfishing, nutrient loading, and climate change.

In the southeastern United States, a well-established regime shift exists between plant communities in tidal wetlands, where historically winter freeze events excluded woody mangrove species. However, mangroves are expanding increasingly poleward with climate change, at the expense of herbaceous tidal wetlands. Yet, other intertidal habitats are in jeopardy of mangrove displacement as well, such as oyster reefs comprised of Crassostrea virginica aggregations. Intertidal oyster reefs trap red mangrove (Rhizophora mangle) propagules, and if individuals can establish and persist on oyster reefs, complete conversion of the oyster reef to a mangrove “island” is possible. Although it is well understood that oyster reefs and mangroves play a role in geologic land-building in Southwest Florida, consideration of the two over ecological timescales remains largely uninvestigated.

In this dissertation, I investigate whether regime shifts occur between oyster reef and mangrove islands on the West Coast of Florida and identify key drivers that produce ecosystem change. To achieve this, I use a mixture of exploratory and experimental studies to advance our knowledge of the understudied system of oyster reefs within the red mangrove lined, subtropical estuary of Tampa Bay, Florida, USA. Using aerial imagery spanning 82 years, I found that >80% of oyster reefs converted to mangrove islands and rapid transition occurred following release from freezes below -7.3°C, the physiological tolerance limit for the red mangrove (Rhizophora mangle). Additional non-climate-mediated drivers of ecosystem change were also identified, including oyster reef exposure to wind-driven waves and propagule supply from adjacent tidal wetlands. Mean (± SD) time to conversion was 29.1 ± 9.6 years and these transitions are abrupt relative to radiocarbon dates from sediment cores which suggested oyster reefs existed for centuries prior to mangrove establishment. Climate projections near the mangrove range limit on Florida’s Gulf coast suggest that cascading regime shifts will begin to transform subtropical estuaries by 2070 if propagule supply keeps pace with predicted warming.

I also examined the survival and performance of early life stages of red mangroves when challenged with inundation levels coincident with the tidal position of oyster reefs in the study area. Following observations of intense herbivory, I caged red mangrove seedlings across the tidal gradient and examined whether biotic pressure contributed to red mangrove zonation in the lower intertidal. I found empirical support for a realized niche for R. mangle limited to above mean sea level (MSL) or ~58% inundation and maintained by local herbivory from a megaherbivore (Trichechus manatus latirostris) and burrowing into saplings by the isopod, Sphaeroma terebrans. Although I found two biotic interactions important for seedling and sapling survival, inundation stress independently restricted distribution of R. mangle to just below MSL (-0.019 m) or ~69% inundation. Based upon elevational surveys of oyster reef crests in Tampa Bay, most intertidal oyster reefs appear to be challenging settings for the R. mangle life stages examined here.

This dissertation produces a novel perspective of oyster reef-to-mangrove ecosystem transition, where elucidated dynamics suggest an ecological regime shift. If winter minimum temperatures remain consistently above -7.0 ºC, mangroves will expand at the cost of both herbaceous salt marshes and oyster reefs. However, if oyster reefs are exposed, subject to periodic wind-driven waves, propagules are unlikely to establish. Additionally, if oyster reefs remain below mean sea level, seedlings and saplings are unlikely to survive due to both physical stress and consumer pressure. Conversely, if oyster reef elevation is high in tidal position yet exposure is low, mangrove take-over of oyster reefs would seem likely. The climate models I provide here suggest that historic trends in oyster reef-to-mangrove transition dynamics observed in Tampa Bay should expand poleward this century. Mangrove release from extreme freezes are expected between Tampa and Cedar Key within the next 50 years, putting extensive oyster habitat in jeopardy. However, managers could intervene to prevent oyster reef-to-mangrove conversions by ‘weeding’ of early-established propagules or removal of saplings from reefs altogether. Additionally, the work here informs oyster restoration design by prioritizing the upper limit of natural oyster reef exposure or targeting lower salinity, subtidal reefs for priority in restoration.

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