Seagrass regulates elevated water column nutrients to promote leaf growth in a coral reef setting, Maldives

Abstract

ABSTRACT Nutrient pollution is a major driver of global declines in seagrass loss, yet in some oligotrophic settings, nutrient enrichment may initially promote growth. In the Maldives, some experimental and observational evidence suggests seagrasses are expanding, potentially in response to elevated nutrient leakage from anthropogenic island-based sources. Building on previous work showing that nutrient enrichment in surface sediments stimulated growth, here, we experimentally compared the response of Thalassia hemprichii to inorganic fertiliser diffused via the water-column or pore-water across two nutrient levels. Both diffusion methods enhanced leaf growth and new leaf production, but responses were significantly stronger under water-column diffusion. Isotopic analyses confirmed fertiliser incorporation into both the above- and below-ground tissues, with a clearer isotopic shift under below-ground enrichment. These findings highlight the sensitivity of seagrasses to nutrient addition, and their potential role as nutrient in complex coral reef settings. It also emphasises the importance of nutrient pathways when evaluating enrichment impacts from a nutrient-management perspective. Developing comprehensive nutrient budgets will be critical for managing eutrophication risks and safeguarding seagrass and coral reef ecosystems in the Maldives and for similar low-lying island contexts globally.