Species composition and abundance of crustose coralline algae and formation of Xinfeng algal reef in Northwestern Taiwan

The composition and abundance of crustose coralline algae (CCA) have been documented in the Xinfeng algal reef (XAR). Eight CCA species were identified, including four in Phymatolithon (P. margoundulatum, P. taiwanense, P. variabile, and Phymatolithon sp.) and four only identified to the genus level (Chamberlainium sp. and three Dawsoniolithon spp.). Chamberlainium sp. was the most abundant species, while the three species of Dawsoniolithon were rarely observed. The CCA coverage varied from 18% ± 15% to 27% ± 21% in spring seasons of 2021–2024, but there were no significant differences. Based on carbon-14 dating, the CCA sediments in the XAR were estimated to be 5850–5990 years old. These CCA sediments were primarily composed of two paleo-CCA species, resembling modern Chamberlainium sp. and Dawsoniolithon sp. 1. However, the modern species of Phymatolithon, which are widespread and abundant in the algal reefs of northwestern Taiwan, were not observed in the CCA sediments. It is unclear whether their absence was due to fragile thalli after their death or their lack of occurrence, and this requires further investigation. The paleoenvironmental data and biogenic sediment composition analysis suggests that the XAR started to develop ~6000 years ago when the sea surface temperature began to decrease and seawater turbidity increased due to rainfall. Additional CCA sediment samples from this algal reef are needed to fully understand the historical changes in the paleo-environments in the XAR.