Oceanography – food supply

Many different types of hydrodynamic regimes can be important for food supply, the mechanisms are site dependant. Food is thought to be derived from benthic sediment re-suspension and surface primary production.

Internal waves are considered a common and important process for food supply to Lophelia pertusa reefs. Internal waves are waves generated under the sea and rarely if ever break the surface. The movement of the tide creates currents that flow over rough topography such as ridges and seamounts generating vertical motions creating internal waves in stratified waters. Internal waves are susceptible to breaking causing turbulence when they reflect off critical angles on slopes. This causes the wave to break and induces large scale mixing zones up to 20km on reefs around the Faroe Islands (Frederikson et al., 1992). Re-suspension of the bottom sediments caused by the turbulence may allow the coral to feed on dead organic matter, and may increase primary surface productivity which is also considered important for food availability.

Mingulay reef 1, demonstrates many types of oceanographic processes that provide food supply, internal waves, hydraulic jump, downwelling, upwelling and high current velocities. Tidally driven downwelling events are recorded to bathe L. pertusa reefs with warmer nutrient rich surface water on the turn of the tide at NE Atlantic reefs (Fig. 1o; Davies et al., 2009; Wagner et al., 2011; Findlay et al., 2013). Tidally driven currents follow the shape of the ridge, forming a static internal wave. The currents accumulate along the surface of the ridge, once past the tip of the ridge a hydraulic jump can be observed where the stronger currents follow the ridge down causing a depression of density downstream of the ridge. This depression of density is known as downwelling where surface waters are dragged deeper in the water column. Surface particles can be transported 130m deep in under one hour (Davies et al., 2009). As the tide turns, the packet of surface water that has been dragged down is moved towards the ridge and the reef. The packet of surface waters washes the reef with food. Davies et al. (2009) further observed high velocity currents as another mechanism of food supply for the Mingulay reef 1. At peak flood and ebb tides, where currents would be at their strongest, the strong bottom currents elevate turbidity and particles of silt and organic material are transported up the ridge. This upwelling supplies the reef with a secondary food supply of re-suspended sediment (Davies et al., 2009).

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Other complex oceanographic processes supplying food can be observed, residual circulation, Ekman drainage, nepheloid layers and many other types of internal waves. These will not be discussed in this blog, but the outcome is the same; food particles are transported from surface waters, pelagic zones and suspended sediment. Without a regular food supply, L. pertusa cannot grow or cope with extreme parameters that may affect the metabolism (ie temperature). Climate change may alter oceanographic processes which may affect cold-water coral reefs as changes in the past have coincided with coral extinctions in Norwegian reefs (Mikkelsen et al., 1982).

Finally: What does this all mean?

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