Spatio-temporal variations of living coral coverage, species richness and diversity were studied on two fringing reef sites at Saint-Gilles La Saline on Reunion Island from 1987 to 2009. The Site-Toboggan (T) was characterized by oligotrophy, Acropora corals, abundant sea urchins and few primary producers. The Site-Planch’Alizés (P), was characterized by heterotrophy, massive corals, abundant primary producers and rare sea urchins. From the shore to the outer reef slope, both reef flats comprise the back reef at around 1.5m deep (‘B’), coral zone ‘L’ with large shore-normal strips of coral 0.8m deep, coral zone ‘N’ with narrow shore-normal strips of corals at around 0.4m deep, and an outer reef flat (<0.4m deep, with breaking surf - not studied).
Results are reported in three parts: (1) for 1993, 1996, and 2002, when the survey takes into account the reef flat as a whole on both sites ; (2) for 1987, 1993, 1996 and from 1998 to 2009, when the survey follows changes in two permanent transects on each site ; (3) for 1970 to 2009, in which species richness of Reunion in 2009 is compared to records of species richness over the last 40 years.
In the period 1993 to 2002, a total of 36 coral species was recorded (31 species at T, 19 at P). In 1993, after a 1992-bleaching event, Acropora coverage was low and only 3 species were recorded (A. muricata, A. cytherea, A. abrotanoides). By contrast, in 2002, when Acropora cover was much higher, there were 11 species, but only two at P in the 1993-2002 interval. The highest coral diversity (Shannon index, H’) occurred on the N coral zone at T in 1996, following an increase from 1993, and after which it declined, as the staghorn coral A. muricata strongly increased its cover. By comparison, non-Acropora coverage remained relatively stable at T. At P, coral coverage increased from 1993 to 2002 in both coral zones but only the increase of Porites (Synaraea) rus was statistically significant. Overall, trends in coral cover and diversity indicate both sites were in better health in 2002. Between 1987 and 2009, changes in the smaller fixed LITs were not typical of the overall trends. Among the three dominant species, Acropora muricata, Montipora circumvallata and Porites (Synaraea) rus there was no significant temporal variation at either site (reflecting the small sample size and high variance), although the P.rus coverage increased regularly.
The number of coral species on the reef flats may have slightly decreased in the last 40 years. Faure (1982, 2009) recorded 74 species in the1970s and 71 species in 2009, of which 36 species were recorded on inner reef flats and 62 species on outer reef flats. The number of species recorded by Faure on inner reef flats is the same as we recorded in survey 1. On Saint-Gilles La Saline, the genus Stylophora was totally absent from 2009 surveys (Stylophora pistillata was always rare on Reunion reefs but S. mordax was previously very common on the reef flats and outer slopes). At time of writing, the genus has only been recently observed on wave-exposed reef platforms (reef flats and and outer reef slopes). In 2009, the third species, missing from Faure’s records, is Favia rotumana. F. rotumana was very common in 1970s on the outer reef flats, but has not been sighted there.
This study suggests that Acropora abundance and diversity are reliable indicators of autotrophic functioning. Acropora muricata (formerly A. formosa) and A.digitifera are considered as the most eurytopic Acropora species present in Reunion, while Acropora austera is considered as environmentally sensitive and perhaps, one of the sentinel species on the reef flat. Many observations suggest that eurytopic Montipora circumvallata and Porites (Synaraea) rus may both be favored by nutrients and able to tolerate pollutants and large variation of abiotic factors such as temperature and salinity. Acropora, though becoming a rare genus in many areas, has been able to recover rapidly in Reunion, and are resilient in the period covered by the present study.