Semester of Graduation

Summer 2026

Degree

Master of Science (MS)

Department

Oceanography and Coastal Sciences

Document Type

Thesis

Abstract

Over the last fifty years, mangrove wetlands have been expanding and contracting near their northern range limit along the Mississippi River Delta Plain (MRDP), Louisiana, USA. Their resilience and persistence are regulated by interacting disturbances, including freezes, tropical cyclones, relative sea-level rise, and high subsidence rates. Although these disturbances follow a recurrent spatiotemporal pattern, no long-term studies have assessed their effects on net mangrove productivity, especially in coastal Louisiana, where disturbance interacts with human activities that have historically reduced space for wetland expansion. This study evaluated canopy litterfall net primary productivity (NPPL; leaves, wood, flowers, and propagules) in monospecific Avicennia germinans scrub (< 2 m), tall (2–3 m), and fringe (3–4 m) ecotypes at Port Fourchon from 2014 to 2026. NPPL was used to quantify disturbance magnitude, abruptness, resistance, duration, and recovery time in relation to phosphorus availability and salinity. Annual NPPL ranged from 1.4 to 8.2 Mg C ha⁻¹ yr⁻¹ and varied significantly among years, but not among mangrove ecotypes. Propagule production was a major contributor to annual NPPL in almost every year, indicating strong reproductive capacity in this deltaic environment. Spectral analysis identified a strong annual cycle in flower and propagule production, whereas foliar litterfall reflected both seasonal and lower-frequency disturbance signals. Foliar litterfall was higher during freeze periods, while wood litterfall was greater during tropical-cyclone periods. Following the 2018 freeze, foliar productivity retained 18.4% of its pre-disturbance baseline, but the 2025 freeze caused widespread canopy defoliation and initiated a new recovery trajectory. Although propagule production is absent after a January 2025 freeze event, coppice resprouting is producing new branches and leaves. Soil porewater soluble reactive phosphorus (SRP) was higher in fringe mangroves (138.78 ± 20.21 µM) than in scrub mangroves (85.00 ± 12.31 µM) and varied seasonally. SRP declined during peak flower and propagule production, suggesting strong plant uptake during reproduction. Soil total phosphorus (TP) was high across mangrove ecotypes, with higher concentrations in fringe soils at a depth of 5–15 cm. High phosphorus availability supports reproductive output, coppice growth, and canopy rebuilding after disturbance, long-term persistence will depend on the balance between recovery, disturbance, subsidence, and relative sea-level rise in the MRDP.

Date

7-17-2026

Committee Chair

Rivera-Monroy, Victor H.

LSU Acknowledgement

1

LSU Accessibility Acknowledgment

1

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