Sedimentation‐driven environmental filtering shapes community assembly of cryptic coralline algae

Abstract Community assembly of cryptic crustose coralline algae (CCA) underlies the formation of algal reefs across the global ocean, yet the ecological mechanisms shaping these communities remain poorly understood. Sedimentation has been linked to variation in CCA community structure across the Taoyuan Algal Reef (TAR), the world's largest subtropical intertidal algal reef, which occurs in a high‐sediment environment. To understand how sedimentation affects CCA community assembly in the TAR, we compared the ecology, geographic distributions, and physiological responses of the CCA from the TAR with those in the New Taipei City Reef (NTCR), a nearby coral reef occurring in a low‐sediment environment. By analyzing the benthic cover data and the DNA barcoding data (from 632 CCA specimens) collected in this study and our previous study of the TAR, we found that the high intertidal CCA cover in the TAR was driven by two dominant species ( Harveylithon sp. and Sporolithon sp.), in contrast to the lower CCA cover in the NTCR occupied by a more diverse CCA species pool. Based on these observations, we hypothesized that CCA dominant in the TAR are more tolerant to sediment accumulation than the CCA dominant or common in the NTCR. To test this hypothesis, we conducted sediment burial experiments on Harveylithon sp. and Sporolithon sp., as well as Harveylithon catarinense and Lithophyllum sp., which were common in the NTCR but absent in the TAR. These experiments demonstrated that Harveylithon sp. and Sporolithon sp. exhibited higher physiological tolerance to prolonged sediment burial than H. catarinense and Lithophyllum sp. In addition, the occurrence data from our previous study (1736 CCA specimens collected around Taiwan and nearby areas) showed that H. catarinense and Lithophyllum sp. were widely distributed, suggesting that dispersal limitation unlikely explains their absence in the TAR. Together, these results provide strong evidence that heavy sedimentation acts as a powerful environmental filter in the TAR, favoring sediment‐tolerant CCA and promoting their ecological dominance. Our findings highlight the role of physiological tolerance in shaping CCA communities and in supporting the formation, resilience, and ecosystem functioning of algal reefs.