Date of Award

2026

Keywords

Thermal tolernace, Posidonia australis seedlings, biological levels, thermal safety margin, latitudinal gradient, regional and intra-regional comparision, mesocosm, curve fitting

Document Type

Thesis - ECU Access Only

Publisher

Edith Cowan University

Degree Name

Master of Science by Research

School

School of Science

First Supervisor

Marlene Wesselmann

Second Supervisor

Kathryn McMahon

Third Supervisor

Nicole Said

Abstract

Understanding species responses to climate change requires characterising thermal tolerance across life stages and biological levels, yet for seagrasses, studies have focused mainly on adults. While the thermal tolerance of early life stages remains poorly understood.

  1. Here, we present a comprehensive thermal tolerance assessment of Posidonia australis seedlings, quantifying responses across growth, photophysiology, metabolism, and survival from three regions spanning 800 km of Western Australia's coastline: warm population (Mid-West), central population (Perth Metropolitan), and cool population (Great Southern). Seedlings were exposed to five temperatures (21, 24, 27, 30 and 33°C) for three months, and thermal optima (Topt), thermal maxima (Tmax), and lethal thresholds (LT50) were derived from thermal performance curves.
  2. Population-level thermal tolerance showed no variation at sublethal levels across regional or local spatial scales for most response variables; however, regional differences were detected for survival with warm range populations (LT50: 30.9°C) exhibiting 0.8 to 1.6°C higher lethal temperature than central and cool population (30.1°C and 29.3°C respectively).
  3. At the sub-lethal level, thermal optima (Topt) for growth did not differ significantly across populations (warm range population: 24.3°C, central population: 23.8°C, and cool population: 24.1°C), except for at Topt for net production differed between regions (warm range population: 26.2°C, central population: 24.4°C, cool population: 26.6°C).
  4. Thermal maxima (Tmax) varied between biological levels: sublethal growth thresholds (Tmax: 28.0 – 28.7°C) occurred up to 2.2°C below lethal thresholds (LT50: 29.3 – 30.9°C), indicating that warming will impair seedling performance well before causing mortality.
  5. Despite this elevated tolerance, warm range populations displayed the narrowest thermal safety margins across all metrics: 1.8°C for growth, 2.5°C for metabolism, and 4.0°C for survival, consistently narrower than central and cool populations (up to 3.7 times for example in growth). Thermal safety margins revealed that these differences were predominantly driven by higher habitat temperatures at the warm range (difference of 2.0°C and 5.6°C compared to the central and cool population, respectively). Under high emissions scenarios for Western Australia (SSP5–8.5), warm range edge seedlings are projected to exceed sublethal growth thresholds before 2100, whereas warm and cool populations will not be impacted.
  6. Synthesis. Understanding thermal tolerance on early life stage and across multiple biological levels from contrasting environmental conditions is essential for accurately assessing their vulnerability and for informing future climate-adaptive restoration strategies.

Access Note

Access to this thesis is embargoed until 13th June 2029 

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Link to publisher version (DOI)

10.25958/qzpd-3m61