Growth Dynamics and Drivers of Deep-Water Seagrasses from the Great Barrier Reef Lagoon

Abstract

Seagrasses provide irreplaceable ecosystem services, yet in the Anthropocene, they are increasingly under threat from coastal development and climate impacts. Efforts to mitigate threats to seagrasses have led to investment and research into their distribution, ecological drivers and bioindicators of health. In the Great Barrier Reef (GBR), work continues to translate our mechanistic understanding of marine plants into impactful management of acute disturbances and chronic stressors. These applied outcomes have primarily focused on shallow seagrass communities, synthesising results and deriving relationships to be used by managers and regulators.

The goal of this thesis is to build our understanding of the dynamics and underlying drivers of GBR deep-water seagrasses for their better management and the communities they support. To achieve this, I (i) studied the seasonal patterns of deep-water seagrasses, characterising environmental parameters linked with growth and senescence; (ii) evaluated light and temperature as drivers of seagrass abundance and determined light thresholds for the dominant Halophila species; (iii) quantified seed banks over time and space, evaluating the role of seed stratification on germination; and (iv) investigated what role endogenous cues play in the phenology of a Halophilaspecies.

Deep-water Halophila species did not all follow the same growth patterns. Only Halophila decipiens had a true annual pattern, completing its life cycle in one growing season and depositing seeds for the subsequent year’s renewal. Deep-water GBR seagrasses grow near their physiological limits with small light reductions potentially leading to meadow-scale loss, and yet their physiological limits also vary among species. Limiting light led to decreased shoot density for both H. decipiens and H. spinulosa over different timeframes, yet neither were affected by increases in temperature irrespective of compounding low light stress. Variations in meadow reproductive output and seed banks critically structure deep-water meadows and underscore species-specific responses to environmental perturbations. Endogenous cues responsible for life stage transitions in terrestrial plants had not been studied before in seagrasses. The metabolomic profile, including key hormones, within the life stages of the H. decipiensgrowing cycle provided the first study linking metabolomic regulation with seagrass growth and development and underpins the ecological findings in this thesis.

This thesis contributes critical information on growth strategies that drive spatial and seasonal dynamics of tropical deep-water Halophilacommunities. It provides new insights and a gateway to explore emerging lines of research including greater use of ‘omics’ technology and integrating terrestrial plant research to further improve deepwater seagrass management.