Current velocity

As a suspension feeding organism, Lophelia pertusa requires movement around them to supply food. For many years, it was unknown exactly what they eat and how they got their food.

In the late 90’s and early 2000’s, scientists believed that cold-water coral reef distribution was related to light seepage of hydrocarbons from cold seeps attracting primary producers, forming a theory called the “hydraulic theory” (Hovland and Thomsen, 1997; Hovland and Risk, 2003). Since, scientists have found that food supply relies on the physical oceanography in the local area. Internal waves, downwelling, tide currents, residual circulation amongst others are thought to supply food from outside the reef area and re-suspension of benthic material.

L. pertusa reefs are typically found in areas of accelerated current velocity at the sea floor (Thiem et al., 2006, Genin et al., 1986). Due to the complex hydrodynamic regimes associated with cold-water coral reefs and their topography, the current velocity is very variable. Over the space of a couple of days, the current velocity at the Mingulay reef may vary from around 0cm/s to nearly 50cm/s (Davies, unpublished). Roberts et al., (2005) also discovered currents of 70cm/s on the Galway carbonate mounds. However individual polyps are not adapted to such extreme flows, as demonstrated in several laboratory experiments. L. pertusa feed more efficiently under slow flow conditions near 2.5cm/s (Purser et al, 2010). Davies et al., (unpublished) further noted L. pertusa typically stop feeding after 10cm/s. Strong flows may bend the polyp out of shape or reduce the time the polyp has to contract with the food particle for consumption (Davies et al., unpublished). The optimum current speed is considered between 0 and 4cm/s (Mortensen, 2001). So here a paradox is presented, strong currents induced by a wide variety of oceanographic process are required to transport food to the immediate vicinity of the polyps for feeding, however, strong currents may inhibit feeding. The 3D reef structure may form a physical barrier alleviating the current velocity in the immediate vicinity of the reef, as observed in shallow-water coral reefs (Sebens et al., 1997). This allows currents to wash over cold-water coral reefs supplying food whilst weakening the flow for feeding.

Furthermore elevated current velocities are thought to be required to prevent sediment building up and silting over the coral (White et al., 2005). In past climates, high sedimentation 11,500 years ago is linked to the demise of L. pertusa from Mediterranean waters (Delibrias and Taviani, 1984; Remia and Taviani, 2005).

The broad importance of elevated flow can be seen in the difference between Norwegian margin reefs and Rockall trough reefs. Norwegian reefs are exposed to a unidirectional flow and therefore reefs are only found on the exposed side of a ridge or bank. The Rockall trough is subject to a diurnal tidal flow and therefore reefs can be found on both sides of ridges and banks.

 

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