Quantifying epifaunal secondary production within tropical macroalgal meadows: Seasonality and sensitivity to canopy structure

Secondary production connects primary producers with higher-order consumers. The response of secondary production to seasonal variation in primary producers can influence trophic flows that underpin key ecosystem functions and services. Within the canopy-forming macroalgal meadows of Ningaloo Reef, Western Australia, we quantified the secondary production generated by Sargassum-associated epifauna across seasons and seascapes. We found a strong positive correlation between overall epifaunal production and Sargassum canopy size. Variation in epifaunal production was predominantly driven by seasonal changes in Sargassum canopy size. However, these seasonal effects were not uniform across the seascape. Key predictors of spatial and temporal variation in epifaunal production were the presence of invertivorous fish families, as well as daily and monthly fluctuations in sea temperature. Areal estimates of epifaunal productivity were much higher in summer than in winter, due to higher Sargassum canopy size and percent cover in summer. Epifaunal production was estimated to be more sensitive to Sargassum percentage cover than to canopy height. Modeling a 45% reduction in canopy height and percent cover of Sargassum associated with a marine heatwave event revealed a potential 81% drop in the areal rate of secondary production by Sargassum epifauna in this tropical fringing reef ecosystem. Disturbances to Sargassum canopy structure driven by global change can therefore significantly alter the productivity of these tropical macroalgal meadows and their ability to support higher level consumers within the food web, including important fisheries species. Our results highlight the importance of including epifaunal production estimates in predictive modeling when managing macroalgal-dominated marine ecosystems.