Virgin Islands National Park: Coral Reef: Recruitment Tiles

In this study, coral recruitment was measured on a kilometer-wide scale on
shallow (5–6 m depth) fringing reefs in St. John, US Virgin Islands, with the
objective of determining the extent to which variation in recruitment was affected
by biophysical coupling involving temperature and flow. Coral recruitment was
measured using settlement tiles deployed at 10 sites along 10 km of shore. The tiles
were first deployed in August 2006, and thereafter replaced every ≈6 months to
sample from either August to January, or January to August over 2 years. Seawater
temperature was recorded at the 10 sites using logging thermistors, and flow was
quantified using drogues. Overall, corals recruited at a rate equivalent to 76
corals m− 2 6 months− 1, and were represented mostly by poritids (43% of recruits),
agaricids (29%), faviids (17%) and siderastreids (7%). Although the density of
recruits differed among sites in a pattern that varied among periods and years,
there was a consistent trend for mean density to decline from ≈ 4 corals tile− 1 at
eastern sites, to ≤ 1 coral tile− 1 at western sites. One aspect of seawater
temperature – the daily range – differed among sites and was greater at western
compared to eastern sites, and while it was related inversely to recruitment over
one of the sampling periods, it was equivocal as a physical process affecting
recruitment. Instead, our results are consistent with biophysical coupling involving
patch depletion and downstream filtering, whereby patches of coral larvae are
delivered to the south shore of St. John and depleted of larvae through settlement
as the water progresses westward.

本研究针对美属维尔京群岛圣约翰岛沿岸水深5~6米的岸礁（fringing reefs）开展千米级空间尺度的珊瑚幼体补充量（coral recruitment）监测，旨在明确幼体补充量的变异在多大程度上受温度与水流介导的生物物理耦合（biophysical coupling）作用调控。研究采用珊瑚附著基片（settlement tiles）开展监测，在沿岸10千米范围内布设10个采样点位；该批基片于2006年8月首次布设，随后每间隔约6个月更换一次，在两年的监测周期内分别覆盖8月至次年1月、1月至8月两个采样时段。研究人员在10个采样点位利用自记热敏电阻温度记录仪（logging thermistors）记录海水温度，同时采用测流浮标（drogues）量化水流流速。整体而言，监测到的珊瑚幼体补充速率约为76个·平方米⁻¹·6个月⁻¹，附著的幼体以微孔珊瑚科（Poritidae，占总幼体数的43%）、石芝珊瑚科（Agariciidae，29%）、蜂巢珊瑚科（Faviidae，17%）及星群珊瑚科（Siderastreidae，7%）为主。尽管各采样点位的幼体密度在不同采样时段与年份间呈现出差异化的分布格局，但整体存在一致的变化趋势：幼体平均密度从东部点位的约4个·基片⁻¹逐步下降至西部点位的≤1个·基片⁻¹。海水温度的一项关键特征——日较差——在各采样点位间存在显著差异，西部点位的日较差显著高于东部点位；尽管在某一采样时段内，温度日较差与幼体补充量呈负相关关系，但该因子作为影响幼体补充的物理过程，其作用仍不明确。与之相反，本研究结果更符合“斑块耗竭与下游过滤”介导的生物物理耦合机制：珊瑚幼体斑块被输送至圣约翰岛南岸，随着水体向西流动，幼体通过附著过程逐渐被消耗殆尽。