Mesophotic Coral Ecosystems

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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).

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 o 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 o 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 O. 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

MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS? 46