Fringing Reefs of Reunion Island and Eutrophication Effects - Part 2: Long-Term Monitoring of Primary Producers

International audienceStudies on the reef flat of Saint-Gilles La Saline between 1987 and 2009 compared spatio-temporal variations of primary producers on two sites, Toboggan and Planch'Alizés. Toboggan (Site-T) is an oligotrophic site characterized by Acropora corals, abundant sea urchins and few primary producers. Planch'Alizés (Site-P) is a heterotrophic site, characterized by massive corals, abundant primary producers and occasional sea urchins. From shore to outer reef front, the reef flat comprises three parts: the back reef (B), the coral zone with Large coral strips (L) and the coral zone with Narrow coral strips (N). This paper (which is Part 2 in a three part series) is divided into four sections. The first (1993, 1996, and 2002) focuses on four groups of primary producers: cyanophytes; turfs (including Stegastes epilithic algal communities); encrusting coralline algae; macroalgae. Their abundance and dominant species were recorded in the three subzones, B, L and N, using 50m-line intercept transects (LITs). The second section (1998 to 2009 inclusive, on the two same sites), reports on annually monitored turfs, encrusting coralline and macroalgae on two permanent 20m LITs. These annual records extend periodic records from these same LITs from 1987, 1993, 1996 and 2002. The third section describes species richness of primary producers at the infracentimetric level in zones B, L and N in 1994. In the final section, seasonal variability is documented as changes in the biomass of dominant macroalgae reported monthly over a period of seven months (1993-94). In 1993-2002, primary producers were dominant at P (average ~ 60%) but inconspicuous at T (~5%). In terms of relative cover, turfs and macroalgae were the dominant forms at both sites (75% turfs, 10% macroalgae at T ; 24% t. and 52% m. at P). On both sites, cyanophytes significantly decreased over time. Among turfs, fine filamentous turfs were prevalent on the back reef on both sites, and also on the narrow outer coral zone at P. Stegastes turf territories dominated both coral zones at Site-T (L and N), but only the L coral zone at Site-P. Encrusting coralline algae were sparse at Site-T but locally abundant at Site-P, where they increased significantly from 1993 to 1996 in both coral zones. There was a major decrease in macroalgae between 1993 and 2002 at P. There were marked differences in the trajectories of decline of the dominant soft macroalgae (Hypnea valentiae, Padina spp, Dictyota spp, Turbinaria ornata and Gracilaria canaliculata). Seventy six primary producer species were recorded in the infracentimetric study (52 species at T and 67 at P). Different algae dominated different areas at both sites. At Toboggan, locally dominant species were the small Gelidium pusillum, an unidentified Melobesiae, Jania adhaerens, the encrusting corallines Hydrolithon onkondes and Lobophora variegata, the turfs Polysiphonia mollis and Anotrichium tenue (mainly in Stegastes territories), Jania adhaerens and Dictyosphaeria verluysii. At P, the dominants were Mesophyllum erubescens encrusting dead hard substrates and decimetric bioclasts in the back reef (B), where they form numerous rhodoliths. In the P coral zones, the dominant taxa were Lyngbya majuscula (cyanobacteria) followed by Hydrolithon onkodes, Blennothrix cantharidosma and Lithophyllum kotschyanum. Jania adhaerens, Hydrolithon onkodes and Gelidiella acerosa were dominant in the narrow coral zone (N). The local composition of the algal community appears to be more related to geomorphological zones and the presence or absence of the territorial Stegastes fish than to environmental differences between T and P. There was strong seasonal change in macroalgae biomass in 1993-4. On the back reef of both sites, peak biomass was due to Hypnea valentiae in February, when it is significantly greater than it was in January. Dictyosphaeria verluysii was dominant and highly visible at Site-T, but extremely rare and hidden below the Hypnea valentiae at P. On the outer coral zone (N), biomass was significantly different between most months on both sites. At Site-T in summer (December to March) the peak biomass was due to the "sponge seaweed" Hydroclathrus clathratus (Phaeophyceae) which exploded in December, declined in January and disappeared in February. At Site-P, H.clathratus was almost absent, and instead, Gracilaria canaliculata dominated the community throughout the year. Dictyosphaearia verluysii and D. cavernosa were prevalent in winter, the former preferring intertidal locations. In autumn (April - June), Turbinaria ornata invaded the subtidal substrates on the outer N-zones. To conclude this Part 2 of the Reunion Island Fringing Reef series, we discuss the influence of nutrients and herbivores (especially sea urchins and the territorial damselfish Stegastes) on the abundance and distribution of primary producers