Fringing Reefs of Reunion Island and Eutrophication Effects - Part 2: Long-Term Monitoring of Primary Producers
Studies 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.
Key words: Coral reefs, benthic community, subtidal, stability, algae,
coverage, diversity, biomass, cyanophytes, turf algae, encrusting corallines,
macroalgae, Lyngbya majuscula, Gracilaria canaliculata, Hypnea valentiae, sea
urchins, nutrients, herbivory.