Quantifying direct and indirect linkages between seagrasses, environment and associated macrofauna in a temperate lagoon

Seagrass ecosystems play a vital role in marine environments, providing essential services, including physical habitat that supports a species diversity critical to robust trophic structures. However, seagrass loss due to various influences raises concerns about the persistence of these ecosystems. Seagrass-associated macrofauna serve as indicators of environmental health, noting change or stability. This study investigated the structural and environmental drivers influencing macrofaunal communities associated with seagrass (Zostera capensis) in a temperate lagoon. Variation in macrofaunal community structure across phenotypically distinct seagrass populations found large-leaved morphotypes in deeper intertidal stands to support higher species diversity and richness. Conversely, small-leaved populations in higher shore positions had a greater abundance of desiccation-resistant gastropods. Seasonal differences were significant, with higher abundances generally observed during spring and summer, while diversity and richness peaked in autumn and winter. Structural equation modelling revealed shoot densities and leaf width to have positive direct effects on macrofaunal abundance, while temperature and tidal exposure produced significant indirect effects. Temperature had a direct effect on seagrass structure and significantly influenced five out of six seagrass metrics measured. Observations of seagrass distribution in the lagoon indicate significant declines in large-leaved populations with likely adverse consequences for associated macrofauna and related trophic levels.