Mesophotic Coral Ecosystems

MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS?

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).

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).

6.4.

Sedimentation and pollution

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.

A study in 2010 (Appeldoorn et al. 2015) found the MCEs off Ponce

received high rates of sedimentation and enriched

N (an indicator

Anthropogenic stressors on MCEs in Ponce, Puerto Rico

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.

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).

(a)

(b)