Mesophotic Coral Ecosystems

6.4. Sedimentation and pollution

Coral reefs normally require warm, clear, nutrient-poor water for development and survivorship (Veron 2000). For light- dependent organisms, anything that limits light penetration, suchas sedimentationand turbidity, is going tohave an impact on their survival. Sedimentation smothers corals causing energetic efficiency to be compromised (Riegl and Branch 1995), abrasion to coral tissues (Loya 1976b), impacts to calcification rates and coral growth, interference with feeding, photosynthesis of the symbiotic algae, respiration and settlement processes and fecundity of corals to be lowered (reviewed for shallow reefs

by Fabricius 2005). Sedimentation, nutrient enrichment and turbidity can also degrade both shallow and mesophotic coral reefs (Fabricius 2005, Vega-Thurber et al. 2013, Katz et al. 2015). MCE environments adjacent to shallow reef systems are usually influenced by sediment from the shallow reefs above (Figure 6.8). MCEs further offshore can also be impacted by sediment, especially from anthropogenic origins (e.g. sewage outflows and dredge dump sites; Armstrong and Singh 2012, Appeldoorn et al. 2015).

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Figure 6.8. Sedimentation on mesophotic reefs. (a) Large, easily disturbed sediment beds surround MCEs at 50 m off the south shore of Utila, Honduras (photo Ally McDowell). (b) Turbinaria sp. at 40 m off Eilat, Israel, with sediment deposited within the foliose structure, smothering the coral polyps (photo Gal Eyal).

Anthropogenic stressors on MCEs in Ponce, Puerto Rico

MCEs located in a narrow insular shelf off the south coast of Puerto Rico are subject to a range of stressors such as outflow from several rivers, a sewage treatment plant with a deep sewer outfall, three deep dredge dump sites and shipping activities.

of anthropogenic nitrogen) concentrations. The most impacted sites, which showed a progressive increase in sedimentation and a decline in live benthos, were located close to shore and associated with dredge disposal sites (Figure 6.9). Higher sedimentation rates and reduced light penetration resulted in poor MCE development, as well as shallower maximumdepths at whichmesophotic corals, algae and other photosynthetic organisms could survive.

A study in 2010 (Appeldoorn et al. 2015) found the MCEs off Ponce received high rates of sedimentation and enriched 15 N (an indicator

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Figure 6.9. Remotely operated vehicle video footage from the area off Ponce, Puerto Rico. (a) An impacted site close to shore at a former dredge disposal site, Ratones at 56 m, showing heavy siltation over the reef structure (photo Francisco Pagán). (b) A less impacted offshore site, Caja de Muertos at 62 m, showing large sponges, black corals ( Antipathes spp.) and an Agaricia sp. colony (photo Richard S. Appeldoorn).

MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS? 75