The ecophysiology of the cold-water coral Lophelia pertusa (Scleractinia)
Lophelia pertusa is the world’s most common and widespread framework-forming cold-water coral. It forms deep-water coral reefs and carbonate mounds supporting diverse animal communities on the continental shelf and on seamounts. Cold-water corals are found throughout most of the world’s oceans at depths of around 200 to 1000 metres. Unlike their tropical counterparts, cold-water corals do not contain symbiotic algae. These recently discovered ecosystems have been damaged by deep-sea fishing and are threatened by climate change. Despite this, very little is known about the ecophysiology of L. pertusa and its likely response to environmental changes. The aims of this research were to investigate the physiology of L. pertusa and relate this to its environment. In particular, this research focused on the respiratory physiology, growth and feeding ecology of L. pertusa. This was investigated through a combination of aquarium experiments and analysis of field-collected samples. The majority of samples were collected from the Mingulay Reef Complex at 130 m water depth in the Sea of the Hebrides. This area has recently been mapped, revealing L. pertusa reefs aligned with seabed ridges. Samples were collected using a video-controlled van Veen grab. After collection the coral samples were maintained in a recirculating aquarium at SAMS laboratory. Lophelia pertusa is associated with water masses of oceanic origin where seawater temperatures are 4-12 oC and salinity is 35-37. The L. pertusa habitat is dynamic, often showing tidal fluctuation in currents, nutrient input and temperature. The small-scale variation in oxygen levels within the reef structure has not been investigated but seasonal falls of phytodetritus may locally decrease oxygen availability to the benthos. It was therefore of interest to investigate the effects of temperature and dissolved oxygen change on the oxygen consumption of L. pertusa. This study showed that L. pertusa could maintain respiratory independence over a range of PO2 illustrated by a high regulation value (R = 78%). The critical PO2 value of 9-10 kPa was very similar to the lower values of oxygen concentration recorded in the field. This suggests that oxygen level may be a limiting factor in the distribution of this cold-water coral. Lophelia pertusa survived periods of anoxia (1 h) and hypoxia (up to 96 h), by employing anaerobic metabolism. High Q10 values revealed sensitivity to short-term temperature changes (6.5-11 oC). The effect of temperature and dissolved oxygen change on the oxygen consumption of the cup coral, Caryophyllia smithii was investigated to allow comparisons with another cold-water scleractinian. Caryophyllia smithii was able to maintain respiratory independence throughout a range of PO2 and displayed a degree of regulation similar to L. pertusa. Q10 values suggest that C. smithii may be physiologically affected at high temperatures but may be more tolerant to temperature change than L. pertusa. Caryophyllia smithii was able to survive periods of both short-term anoxia and hypoxia and a substantial oxygen debt implied the use of anaerobic metabolism. Caryophyllia smithii also appeared to employ behavioural mechanisms at low and zero oxygen levels by inflating the polyp tissue. This behaviour has been reported in the field in sedimented areas but this is the first study that links the behaviour to oxygen availability. Lophelia pertusa was found to have low growth rates and low rates of oxygen consumption that appeared to vary geographically. There were also geographical variations in both the total amount of lipid stored and the lipid signatures. Analysis of lipid signatures suggested L. pertusa feeding has a strong link to surface production, with preferential feeding on copepod species. Calanoid copepods appeared to be the main zooplankton food source at the Mingulay Reef Complex, while cyclopoid copepods were dominant prey items in L. pertusa lipids from deeper sites at Rockall Bank and the New England seamounts. There were no seasonal differences in lipid signature or amounts suggesting either a lack of reproductive cycle or little seasonality in food input at the Mingulay Reef Complex. Climate change and fishing practices are causing changes in the distribution and abundance of these prey species, which is likely to affect L. pertusa. The opportunistic nature of feeding may imply an ability to adapt to changing conditions. This is the first study to investigate aspects of the physiology of L. pertusa and relate it to its natural environment. Lophelia pertusa appears to have slow physiological rates that vary geographically and are likely to be dependent on food sources. It also appears to be sensitive to small temperature changes and is at least partially limited in distribution by oxygen levels. This environment is likely to be threatened by changes in seawater temperature, ocean acidification, alteration of food webs and water masses as well as destructive fishing impacts. The results of this thesis form vital information that will underpin future research and allow us to predict the effects of anthropogenic impacts on L. pertusa.
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