Local human pressures modulate turf sediment loads in a warm-temperate oceanic island

Abstract

In the Anthropocene, algal turfs are expected to replace macroalgal forests and coral cover as the dominant benthic state. These turf seascapes play a key role in regulating ecosystem processes through sediment retention, which carries significant ecological and socio-economic implications. However, our understanding of sediments trapped by turf on coastal reefs, particularly in oceanic islands, remains limited. In this study, we quantified turf seascape architecture (algal composition and mean height) and sediment properties (total particulate load, grain size distribution, and organic content) across a warm-temperate oceanic island. We further decoupled the role of geomorphological, anthropogenic, and turf algae structural predictors in explaining spatial variation in turf sediment properties. Our results revealed significant spatial variation in turf sediment loads, varying by three orders of magnitude (∼1 g/m² to 2000 g/m²), while organic load varied by two orders of magnitude (∼1 g/m² to 100 g/m²). Human pressure and turf algal composition were the strongest predictors of turf sediment load, highlighting the role of local human stressors in modulating sediment dynamics in emerging turf seascapes. Our study provides baseline information on the patterns and drivers of turf sediments in oceanic islands, a critical area to develop management plans that target the resilience of core ecosystem functions under altered reef configurations in the Anthropocene.