Graduation Year

2023

Document Type

Thesis

Degree

M.S.

Degree Name

Master of Science (M.S.)

Degree Granting Department

Biology (Integrative Biology)

Major Professor

Susan S. Bell, Ph.D.

Co-Major Professor

Peter D. Stiling, Ph.D.

Committee Member

Andrew M. Kramer, Ph.D.

Committee Member

Paul Camilo Zalamea, Ph.D.

Keywords

Displacement, Encroachment, Propagule Establishment, Regime Shift

Abstract

Mangroves are invading oyster reefs in areas no longer impacted by winter freezes and completely transforming them into mangrove islands. The mechanisms influencing the transition between these two habitat states remain unclear. Here, I test experimentally whether oyster reef (Crassostrea virginica) structure directly facilitates red mangrove (Rhizophora mangle) propagule capture via differences in propagule retention and successful mangrove establishment. In addition to oyster reef structure, I compared physical reef features of size, elevation, and wind-wave exposure to uncover whether these traits influenced mangrove retention. Recently- abscised red mangrove propagules were inserted onto 21 oyster reefs across 5 sites in Tampa Bay in 2022 in a manner akin to natural propagule capture. Each reef had manipulated plots of oyster cover mimicking low to high levels of shell structure representing possible capture points a propagule might encounter during dispersal. After 24h, plots with high and intermediate shell cover retained a mean of 73% of propagules while low shell plots retained a mean of 66%; however, no significant differences in retention between structure levels were detected. A corresponding field survey during 2021-2022 on the same oyster reefs illustrated that after naturally occurring propagule delivery, successful propagule establishment and their further growth was a rare occurrence. Even when subjected to intense levels of propagule pressure- up to 267 new recruits observed on an individual reef- no more than 5 mangrove propagules successfully established on any of the 21 reefs during the survey and no more than 4 seedlings, the next stage of growth, were observed on a single reef. No significant association between reef physical features and short-term propagule retention or successful propagule establishment were detected. This combined information implies low rates of propagule establishment even after high (or experimentally enhanced) recruitment and reveals a need to understand better the factors influencing how mangrove propagules survive to a seedling stage on oyster reefs. Importantly, our findings also indicate that humans could effectively protect at-risk oyster reefs with a program of intermittent manual removal of mangrove seedlings, given their low abundance, thereby discouraging a potentially undesired regime-shift towards a mangrove dominated ecosystem.

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