Response of subtropical submarine-cave ecosystem to Holocene cave development and Asian monsoon variability

A submarine cave is a unique environment that is dark, food limited, semi-isolated from the outside, and sheltered from wave action. However, our knowledge of the long-term change in submarine-cave ecosystems remains limited. We document here the community-scale responses toward long-term change in a submarine cave, Daidokutsu in Okinawa in southern Japan. Using both metazoans (ostracods and bivalves) and protozoans (larger benthic foraminiferans) in two sediment cores obtained from the cave, we reconstruct the faunal and diversity changes of the past 7 Kyr. All taxonomic groups showed long-term, gradual linear change of faunal composition from predominantly open-water taxa to predominantly cave taxa, and ostracods showed short-term variability of species diversity. The long-term faunal trend probably reflects gradual isolation of the cave ecosystem due to coral reef development (i.e., development of the cave ceiling) during periods of the Holocene transgression and subsequent sea-level highstand. The short-term diversity changes show substantial similarity to centennial- to millennial-scale Holocene Asian monsoon variability. Ostracod species diversity peaks tend to correspond with periods of strong East Asian winter monsoons. The results indicate that limestone submarine-cave ecosystems, an important cryptic habitat, developed gradually during the Holocene and may be sensitive to rapid climate changes

海底洞穴（submarine cave）是一类独特的生境，其环境黑暗、食物资源匮乏、与外界半隔绝且可免受波浪作用影响。然而，目前学界对海底洞穴生态系统的长期变化仍所知有限。本研究以日本南部冲绳地区的Daidokutsu海底洞穴为对象，揭示了其群落尺度上对长期环境变化的响应模式。通过对采集自该洞穴的两根沉积物岩芯（sediment cores）开展分析，本研究利用后生动物（metazoans，涵盖介形类ostracods与双壳类bivalves）以及原生动物（protozoans，以大型底栖有孔虫benthic foraminiferans为代表）作为生物指标，重建了过去7千年（Kyr）以来的动物群组成与多样性变化历史。所有分类类群的动物群组成均呈现长期、缓慢的线性变化趋势：从以开阔水域类群为主逐渐转变为以洞穴类群为主；介形类的物种多样性则表现出短期波动特征。这一长期动物群变化趋势，可能反映了在全新世海侵（Holocene transgression）及随后的海平面高位停滞期（sea-level highstand），珊瑚礁发育（即洞穴顶部的形成过程）导致洞穴生态系统逐渐与外界隔绝。介形类的短期多样性变化，与全新世百年至千年尺度的亚洲季风变化存在显著相似性。介形类物种多样性的峰值时段，往往与东亚冬季风（East Asian winter monsoons）增强的时期相吻合。本研究结果表明，作为一类重要的隐秘生境（cryptic habitat），石灰岩海底洞穴生态系统在全新世时期逐渐发育形成，并可能对快速气候变化较为敏感。