Response of coral reefs and mangroves to Holocene sea-level changes: insights from Belitung Island, Indonesia

Coral reefs and mangroves are important yet vulnerable coastal ecosystems, especially given strong anthropogenic pressure such as the projected 21^st century sea-level rise. Despite the symbiotic relationship of reefs and mangroves within the context of sea-level changes, few studies provided a synchronous assessment of both systems, especially on timescales beyond a few decades. We focus on the response of coral reefs and mangroves in Belitung Island (Indonesia), where previous studies have already provided initial constraints on Holocene relative sea-level history. Along 3 different coastal transects within a distance of ~40 km, we surveyed and sampled 8 sedimentary cores of 1-3 m length, containing coral fragments, mangrove mud/sands and other types of coastal sediment. The three cross-sections evidence a surprisingly variable responses to the same sea-level history: 1) at Sidjuk, a mid-Holocene (~6 ka) coral reef within a former embayment was slowly filled up with sediments providing the substrate for estuarine mangrove development, 2) at Batu Itam, ~500 m of open coast mangroves prograded on top of a mid-Holocene reef, currently occupying around one third of the area within the intertidal zone and 3) at Mendanau, ~ 1 km of open coast mangroves prograded directly on top of basement rocks, with fossil corals seemingly sparse and isolated compared to the other sites. These results demonstrate how reef and mangrove response to sea-level changes may be extremely local in nature, and directly dictated by physical parameters such as sedimentation, wave erosion and reef construction rates. The spatial heterogeneity on the scale of a few 10s of kilometers emphasizes the need for tailored solutions with regards to future sea-level rise.