Salinity and sigma-theta

Lophelia pertusa typically inhabits a narrow salinity band between 35 and 36‰ (Rogers, 1999). Though examples found in fjords may be subject to lower salinities due the input of freshwater from rivers (Rogers, 1999). In the Mediterranean, L. pertusa may be in the presence of salinities as high as 38.88‰ (Taviani et al., 2005). In shallow-water corals such as Montipora verrucosa, low salinities may reduce survival in elevated temperatures, whilst Stylophora pistillata experienced metabolic rate changes (Coles and Jokiel, 1978; Ferrier-Pages et al., 1999). The physiological responses may be the same in cold-water corals though no experiments have been published.

Salinity and temperature plays a role in the density of seawater (sigma-theta), of which L. pertusa only inhabits a small envelope. Sigma-theta is the measure of density calculated by potential temperature and salinity, ignoring compressibility. The sigma-theta (σΘ) value of water density is derived by this equation:

σΘ=1/(V(S,Θ,P))-1000 kg m^(-3)

V = Specific volume
S = Salinity
Θ = Potential temperature – Not taking into account the effect pressure has on temperature
P = Pressure at sea surface

L. pertusa reefs in the NE Atlantic are only found in a density ‘envelope’ of between 27.35 to 27.65 kg m-3 (Dullo et al., 2008). However tidal fluctuations were observed by Davies et al. (2009), due to down welling events to values as low as 27.23kg m-3 for short periods of time at Mingulay reef 1. Many studies show L. pertusa reefs exist only within the density layer, outside this layer only dead coral framework and mounts exist. Recent studies show L. pertusa reef formations only occur between 27.40 to 27.61 kg m-3 (Flögel et al., 2014), when reefs experiencing short term fluctuations were excluded. L. pertusa can occur outside the density belt, such as colonies in the Mediterranean, but not in reef formations, usually solitary polyps or patches (Flögel et al., 2014).

Low growth of L. pertusa reefs in the Oslo Fjords may be linked unfavourable water density (Flogel et al., 2014). Furthermore lower sigma-theta values have been observed in L. pertusa reefs in the Gulf of Mexico where growth is slow (Brooke and Young, 2009; Davies et al., 2010), and reefs are less impressive than the NE Atlantic reefs. Dullo et al. (2008) speculate that the density layer is essential for gamete or larval supply, due to their low concentration levels. Furthermore intermediate nepheloid layers which transport nutrients to reefs as observed by White et al., (2005), may be supported and formed by the density layer suggesting density may have a role to play in food supply. More research is needed to understand the role density has in habitat suitability. If the theory of its use for larval supply holds true, the sigma-theta layer would be essential for genetic diversity and the formation of reefs.

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