Salinity controls the upper depth limit of black corals in Doubtful Sound, New Zealand

Abstract

The New Zealand endemic black coral Antipathes fiordensis is found in Fiordland in shallow depths (c. 5 m) compared with depths of other black corals worldwide. This is considered to be a result of the low light levels caused by the tannin‐rich, surface low‐salinity layers (LSLs) of water that are characteristic of the fiords. However, low salinity may also affect the distribution of black corals because of osmotic stress. We found a strong relationship between the upper limit of coral colonies and the salinity gradient along Doubtful Sound and no significant relationship between the upper depth limit of A. fiordensis and the underwater light field. This suggests that salinity, rather than light levels, plays a dominant role in controlling the upper depth limit of A. fiordensis. This result was supported by a salinity tolerance experiment in which coral colonies were held in situ in a range of salinities (6–32 psu). This experiment revealed that corals can tolerate salinities of between 20 and 30 psu for up to 6 h as long as lowered salinities were periodically interrupted by higher, less detrimental salinities (>32 psu). Although LSLs may reduce incoming irradiation, the extreme upper limit of A. fiordensis in Doubtful Sound appears to be controlled by the salinity field.