TRENDS IN BIOEROSION AND ENCRUSTATION ON A HOLOCENE CARBONATE PLATFORM

Coralgal reefs are complex and dynamic ecosystems. Although characterized by long-term net accretion, reefs commonly reflect a delicate balance between constructive and destructive influences. Aside from skeletal growth, encrustation and bioerosion are important biological processes that influence reef systems. Although several studies have illustrated styles and intensity of bioerosion and encrustation at individual locations in modern systems, the nature of variability within and among Holocene carbonate platforms remains poorly constrained. To explore how trends in encrustation and bioerosion vary with geomorphic position and depositional setting on carbonate platforms, this study examines the type, rate and abundance of encrustation and bioerosion of experimental substrates (limestone blocks) across several depositional environments on Aranuka Atoll (Kiribati), equatorial Pacific. A year after deployment, encrusters on experimental substrates included red algae, encrusting foraminifera, and worms; bioeroders included algae, sponges, and worms. Experimental blocks reveal average linear bioerosion rates (per block) of 0.13-0.59 mm/yr (n=6, maximum local rate 4.07 mm/yr and minimum 0.03 mm/yr), whereas average linear encrustation rates vary from 0.11-1.11 mm/yr (n=6, maximum local rate 3.8 mm/yr and minimum 0.04 mm/yr). Both average linear bioerosion and average linear encrustation rate are positively correlated with distance from the platform margin (R² = 0.81 and 0.98, respectively), but include only ambiguous relations with water chemistry. Block average rates of linear bioerosion and encrustation are highest in the windward, southeast part of the atoll. These patterns reveal the quantitative linkage between geomorphic position and the biological processes of encrustation and bioerosion.