Mesophotic Coral Ecosystems

MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS?

MCEs off La Parguera are exposed to a highly energetic

thermocline and high-amplitude incident internal waves.

At 40 m, temperatures track seasonal surface temperatures,

varying between 26 and 29.8

C. Exceptions occur during

the passage of particularly large amplitude internal waves.

In one instance, a series of internal waves resulted in rapid

temperature drops at 40 m of up to 3

C as the peak of the wave

passed over the station. In contrast, at 80 m, temperatures are

almost always lower than surface temperatures, particularly

during the summer. During autumn and winter, however,

internal wave troughs can reach as deep as 80 m.

Both fish and benthic communities within MCEs are

biologically diverse. Benthic cover is typically dominated

by a diverse algal community, while sponges, scleractinian

corals, black corals and gorgonians provide larger scale

benthic structure. At 50–60 m, macroalgae comprise 55–60

per cent cover (Sherman et al. 2010). Among the macroalgae,

coralline red algae and Peyssonneliaceae generally provide

the most abundant cover. Corals and sponges only comprise

about 6 per cent and 16 per cent cover, respectively.

However, at 70 m, total algal cover is only 38 per cent (8 per

cent non-calcareous algae), but corals increase to 27 per cent

cover, with little change among sponges. In key locations,

coral cover can be extensive, due to large agariciid colonies

(Figure 2b), which are the dominant mesophotic corals off

La Parguera.

MCEs are unique communities and there is a clear shift in

species composition for both benthic organisms and fish

with depth. For algal species, there is a fairly high similarity

in species composition between 30 and 50 m (68 per cent).

The species similarity between 30 and 70 m falls to 54 per

cent, i.e., roughly half of the species are cosmopolitan at

these depths. Some 20 species of red and green macroalgae

are essentially restricted to the mesophotic realm, whereas no

brown macroalgal species occurring within southwest Puerto

Rico were observed to be depth restricted.

The composition of coral, octocoral and sponge species from 20

to 80 m changes as a function of depth. Shallower areas (shelf

edge) are dominated by the coral family Favidae (

Orbicella

species complex,

Colpophyllia

spp. and

Diploria

spp.), and the

deeper communities by Agariciidae and

Madracis

spp. A total

of 21 zooxanthellate scleractinian coral species were observed

in mesophotic habitats, plus two azooxanthellate coral species

(

Rhyzosmilia maculata

and

Astrangia solitaria

) and two

hydrocorals (

Millepora alcicornis

and

Stylaster roseus

)

All but

five coral species (

Agaricia undata, A. grahamae

Leptoseris

cailleti

A. fragilis

and

Mycetophyllia reesi

) are common shallow-

water dwellers. Coral diversity gradually drops with depth —

from 45 species at the shelf break to 24 species at mesophotic

depths. Of the species found at mesophotic depths (see Figure

3 for examples), species richness peaks between 40 and 60 m.

Mesophotic coral communities at depths greater than 50 m

in La Parguera are not well-connected to shallow coral

communities, as many shallow coral species decrease in

abundance or are absent at these depths (Table 1). The

major shallow reef-building corals, such as

Acropora

and

annularis

and

O. faveolata

, are not found at depths of 50 m or

greater;

Montastraea cavernosa

does occur at depths greater

than 80 m, but as small isolated colonies. In general, the coral

community at 50 m or deeper is very distinct, particularly so

for the dominant species, which are platy corals of the genera

Agaricia

and

Undaria.

For the shallow species present at

mesophotic depths, their generally small size and scattered

distribution further reduces the probability of spawning

and fertilization (in non-brooders). Furthermore, genetic

evidence based on the distribution of zooxanthallae clades

(the photosynthetic algae associated with scleractinian corals)

show distinct separation between corals from shallow and

mesophotic depths (Schizas unpubl.).

There is a rapid decrease in the proportion of shallow fish

species present with depth (Bejarano et al. 2014). Most of

these species disappear at or above 60 m. Table 2 lists the top

Figure 2.

Well-developed MCEs off of La Parguera. (a) Mixed community atop a deep buttress at 45 m. (b) Large colony of

Undaria

agaricites

(photos Héctor Ruiz).

(a)

(b)