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 S ound appears to be controlled by the salinity field.

Variations in Growth, Erosion, Productivity, and Morphology of Ecklonia Radiata (Alariaceae; Laminariales) Along a Fjord in Southern New Zealand1

“…The LSL in Doubtful Sound is thicker than that of other fjords in the region due the discharge of freshwater from the Manapouri hydroelectric power station. Incident light is strongly attenuated by the LSL, which, coupled with shading by the steep mountainous walls (< 2,000 m tall), results in a substantial reduction of available light in the inner regions by up to 95% of irradiance at the outer sites (Kregting and Gibbs 2006).…”

“…The LSL in Doubtful Sound is thicker than that of other fjords in the region due the discharge of freshwater from the Manapouri hydroelectric power station. Incident light is strongly attenuated by the LSL, which, coupled with shading by the steep mountainous walls (< 2,000 m tall), results in a substantial reduction of available light in the inner regions by up to 95% of irradiance at the outer sites (Kregting and Gibbs 2006). Inorganic N (NO 3 ) and NH 4 + ) in Doubtful Sound below the LSL where E. radiata resides shows little seasonal variation compared to other temperate regions: at 20 m depth, inorganic N ranges from 2 lM in summer to 7 lM in winter (Peake et al 2001).…”

Deep‐basin macrobenthos of Doubtful Sound, Fiordland, New Zealand

“…The 14 glacier-carved fjords within the Fiordland region of New Zealand's South Islandhave similarly received considerable attention. However, much of the current knowledge of New Zealand's fjord benthic communities is restricted to intertidal and shallow subtidal habitats (e.g., Batham 1965;Grange et al 1981;Witman & Grange 1998;Wing et al 2003;Kregting & Gibbs 2006). In contrast, little is known of the deep benthic communities of New Zealand's fjords, with only a few qualitative examinations of large-scale variations in the distribution of conspicuous species (Fleming 1950, Mollusca;Fell 1952, Echinodermata;Hurley 1964;McKnight 1968;McKnight & Estcourt 1978).…”

Near-surface water temperatures in Doubtful Sound and response to natural and anthropogenic drivers

“…Organisms are likely to be more tolerant of water temperature fluctuations, which more often can span an organism's range, than of salinity fluctuations, which span nearly the full spectrum of salinity between freshwater and seawater. Indeed, salinity has been identified as the key physical variable controlling the upper depth limit of black corals (Kregting & Gibbs 2006;Grange 1991), the vertical distribution of predators and grazers (Witman & Grange 1998;Lamare et al 2009), spatial patterns in community diversity (Smith & Witman 1999), and organism distribution and community composition within the intertidal zone (Boyle et al 2001) and shallow soft sediments (Tallis et al 2004;Rutger & Wing 2006;. A number of these studies have involved experiments to further confirm the effects of reduced salinity on mortality (Kregting & Gibbs 2006;).…”

“…Gradients in salinity associated with LSL have been identified as a primary driver of biological processes in the fiords, including the unique distribution of organisms along the steep rock walls (Witman & Grange 1998;Smith 2001;Kregting & Gibbs 2006). Maintenance of low-salinity water near the surface due to the tailrace discharge has also been linked to shifts in organism distribution and community composition within the intertidal zone along the rock walls (Boyle et al 2001) and within shallow (B4 m depth) sediments at the head of Doubtful Sound (Tallis et al 2004;Rutger & Wing 2006;.…”