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MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS?
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The Gulf of Eilat/Aqaba (the Gulf) is a 180 km long, narrow,
blind-ended embayment connected to the Red Sea at its
southern end. On average it is 18 km wide (varying between
6 and 26 km), with a maximum depth of 1,825 m. The
circulation in the Gulf is driven by a combination of wind,
heat fluxes and tides. Wind-generated surface currents, and
what appear to be permanent anticlockwise gyres, move water
up the Jordanian coast and down the Israeli coast (Anati 1974,
Berman et al. 2000, Manasrah et al. 2006). Seasonal upwelling
events also bring water laden with numerous organisms from
the deeper reefs to the surface, and transport surface waters to
depth (Genin et al. 1995). Although described as oligotrophic,
these upwelling events can produce phytoplankton blooms in
spring and autumn (Labiosa et al. 2003). The Gulf is highly
saline — up to 41 parts per thousand in the north — and its
sea surface temperature varies from a minimum of 20°C in
the winter to a maximum of 28°C in the summer, with a deep-
water temperature (down to 1,825 m) constant at ca. 21°C
(NMP 2013).
The northern part of the Gulf is surrounded by arid
mountainous terrain, which provides a constant input of
wind-borne terrigenous sediment (Ben-Avraham et al. 1979).
On rare occasions, flash floods transport terrestrial sediment
into the deep waters of the Gulf via submarine canyons
(Katz et al. 2015). Despite this, the Gulf has unusually clear
water, with high levels of light throughout the year, even at
mesophotic depths (60–160 µmol m
-2
s
-1
at 36 m and 7.1–26.7
µmol m
-2
s
-1
at 72 m; Eyal et al. 2015). Surface levels of light
at Eilat are 40 per cent higher than those found at other reefs
in Heron Island, Australia; Puerto Morelos Quintana Roo,
Mexico; and Coconut Island, Hawai‘i (Winters et al. 2009).
The shallow coral reefs along the Gulf are among the world’s
most diverse in terms of average number of species per m
2
(Loya 1972). Due to the Gulf ’s unique geographic structure
and its extreme oceanographic conditions, a high proportion
of endemic species have evolved (Loya 2004). MCEs develop
to a depth of at least 150 m and occur almost continuously
along the coastline of Eilat, unlike the scattered nature of
shallow reefs in this area. The fauna comprises many unique
species found exclusively in mesophotic depths. So far,
taxonomic assessment of the mesophotic communities in the
Gulf has yielded 93 coral species (81 zooxanthellate and 12
azooxanthellate corals) from 13 families and two
incertae sedis
(meaning “of uncertain placement”) genera, three suspected
new coral species, eight new coral records to the Red Sea and
10 unidentified coral species (data from Tel Aviv University).
The mesophotic corals at one site in Eilat exhibited an average
coral cover of ca. 34 per cent compared with ca. 24 per cent
in the shallow reef (Table 1). Altogether, these parameters
indicate a healthy and flourishing MCE (Eyal 2012).
3.5.
Eilat, Red Sea, Israel
Gal Eyal
, Tel Aviv University and the Interuniversity Institute (IUI) for Marine Studies in Eilat, Israel
Yossi Loya
, Tel Aviv University, Israel
Figure 1.
Bathymetric map of the northern Gulf of Eilat/Aqaba. (a) Illustration of the Gulf of Eilat/Aqaba (based on Biton and Gildor
2011). (b) Magnification of the Gulf head in high-resolution bathymetry. The pink represents the mesophotic zone at depths of 30–
150 m (Background image based on Sade et al. 2008).