Abstract
Although the role of deep-sea corals in supporting biodiversity is well accepted, their ability to recover from anthropogenic impacts is still poorly understood. An important component of recovery is the capacity of corals to grow back after damage. Here we used data collected as part of an image-based long-term monitoring program that started in the aftermath of the Deepwater Horizon oil spill to develop a non-destructive method to measure in situ growth rates of Paramuricea spp. corals and characterize the impact of the spill on growth. About 200 individual coral colonies were imaged every year between 2011 and 2017 at five sites (three that were impacted by the spill and two that were not). Images were then used to test different methods for measuring growth. The most effective method was employed to estimate baseline growth rates, characterize growth patterns, estimate the age of every colony, and determine the effects of impact and coral size on growth. Overall growth rates were variable but low, with average annual growth rates per site ranging from 0.14 to 2.5 cm/year/colony. Based on coral size and growth rates, some colonies are estimated to be over two thousand years old. While coral size did not have an influence on growth, the initial level of total impact in 2011 had a significant positive effect on the proportion of new growth after 2014. However, growth was not sufficient to compensate for branch loss at one of the impacted sites where corals are expected to take an average of 50 years to grow back to their original size. The non-destructive method we developed could be used to estimate the in situ growth rates on any planar octocoral, and would be particularly useful to follow the recovery of corals after impact or assess the effectiveness of Marine Protected Areas.
Highlights
Deep-sea corals are ubiquitous; they are found at all latitudes and are common at depths between 200 and 1000 m (Watling et al, 2011; Buhl-Mortensen et al, 2014). Octocorals, in particular, occur across a wide range of depths, including as deep as 6000 m (Grasshoff, 1981)
The non-destructive method we developed could be used to estimate the in situ growth rates on any planar octocoral, and would be useful to follow the recovery of corals after impact or assess the effectiveness of Marine Protected Areas
Gamete development and oocyte maturation in the octocoral species Ainigmaptilon antarcticum can take more than a year (Orejas et al, 2002), and low number of recruits and high recruit mortality have been recorded for Paragorgia arborea and Primnoa resedaeformis, respectively (Lacharité and Metaxas, 2013)
Summary
Deep-sea corals are ubiquitous; they are found at all latitudes and are common at depths between 200 and 1000 m (Watling et al, 2011; Buhl-Mortensen et al, 2014). Octocorals, in particular, occur across a wide range of depths, including as deep as 6000 m (Grasshoff, 1981). Deep-sea octocorals can form dense aggregations that harbor a high diversity and density of organisms (Krieger and Wing, 2002; Mortensen and Buhl-Mortensen, 2005; Buhl-Mortensen et al, 2010; De Clippele et al, 2015). Because they can live for hundreds to thousands of years (Risk et al, 2002; Roark et al, 2009; Prouty et al, 2014), corals sustain stable and longlasting ecosystems in the deep ocean. With the increasing number of threats to deep-water coral ecosystems resulting from human activities such as fishing (Koslow et al, 2001; Fosså et al, 2002; Hall-Spencer et al, 2002; Clark and Koslow, 2008), mining (Van Dover et al, 2017), oil extraction (White et al, 2012; Cordes et al, 2016), and climate change (Chen et al, 2017; Schmidtko et al, 2017), the need to understand their resilience and better inform conservation decisions has become acute
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