High Macroalgal Cover and Low Coral Recruitment Undermines the Potential Resilience of the World's Southernmost Coral Reef Assemblages

Coral reefs are under increasing pressure from anthropogenic and climate-induced stressors. The ability of reefs to reassemble and regenerate after disturbances (i.e., resilience) is largely dependent on the capacity of herbivores to prevent macroalgal expansion, and the replenishment of coral populations through larval recruitment. Currently there is a paucity of this information for higher latitude, subtropical reefs. To assess the potential resilience of the benthic reef assemblages of Lord Howe Island (31°32′S, 159°04′E), the worlds' southernmost coral reef, we quantified the benthic composition, densities of juvenile corals (as a proxy for coral recruitment), and herbivorous fish communities. Despite some variation among habitats and sites, benthic communities were dominated by live scleractinian corals (mean cover 37.4%) and fleshy macroalgae (20.9%). Live coral cover was higher than in most other subtropical reefs and directly comparable to lower latitude tropical reefs. Juvenile coral densities (0.8 ind.m−2), however, were 5–200 times lower than those reported for tropical reefs. Overall, macroalgal cover was negatively related to the cover of live coral and the density of juvenile corals, but displayed no relationship with herbivorous fish biomass. The biomass of herbivorous fishes was relatively low (204 kg.ha−1), and in marked contrast to tropical reefs was dominated by macroalgal browsing species (84.1%) with relatively few grazing species. Despite their extremely low biomass, grazing fishes were positively related to both the density of juvenile corals and the cover of bare substrata, suggesting that they may enhance the recruitment of corals through the provision of suitable settlement sites. Although Lord Howe Islands' reefs are currently coral-dominated, the high macroalgal cover, coupled with limited coral recruitment and low coral growth rates suggest these reefs may be extremely susceptible to future disturbances.

珊瑚礁正面临日益加剧的人为及气候诱导的胁迫压力。珊瑚礁在受扰动后重新组装与再生的能力（即恢复力），很大程度上依赖于植食动物（herbivores）阻止大型藻类扩张的能力，以及通过幼体补充（larval recruitment）实现珊瑚种群更新的过程。目前，针对高纬度亚热带珊瑚礁的此类研究数据仍较为匮乏。为评估全球最南端珊瑚礁——豪勋爵岛（31°32′S，159°04′E）的底栖礁体群落潜在恢复力，本研究量化了其底栖生物组成、幼珊瑚密度（作为珊瑚幼体补充的替代指标）以及植食性鱼类群落结构。尽管不同生境与样点间存在一定差异，但底栖群落仍以活石珊瑚（scleractinian corals，平均盖度37.4%）和肉质大型藻类（20.9%）为优势类群。活珊瑚盖度高于多数其他亚热带珊瑚礁，且与低纬度热带珊瑚礁的水平相当。然而，幼珊瑚密度（0.8 个·m⁻²）较热带珊瑚礁的报道值低5至200倍，仅为其1/5至1/200。总体而言，大型藻类盖度与活珊瑚盖度及幼珊瑚密度呈显著负相关，但与植食性鱼类生物量无显著关联。植食性鱼类生物量相对较低（204 kg·ha⁻¹），且与热带珊瑚礁形成鲜明对比：其群落以大型藻类啃食型鱼类为主（占比84.1%），刮食型鱼类占比极低。尽管刮食型鱼类生物量极低，但它们与幼珊瑚密度及裸底盖度均呈显著正相关，这表明其可通过提供适宜的附着基，促进珊瑚幼体的补充。尽管豪勋爵岛珊瑚礁目前仍以活珊瑚为优势群落，但较高的大型藻类盖度、有限的珊瑚幼体补充以及较低的珊瑚生长速率，均暗示该礁体未来极易受到扰动的冲击。