Patrick Frouin

Effects of acidification on carbon and nitrogen stable isotopes of benthic macrofauna from a tropical coral reef

Stable isotope analyses are widely used to determine trophic levels in ecological studies. We have investigated the effects of carbonate removal via acidification on the stable carbon and nitrogen isotopic composition of 33 species of tropical benthic macrofauna, and we report guidelines for standardizing this procedure for higher taxa in tropical coral reef ecosystems. Many tropical benthic invertebrates are small in size, and therefore body tissue isolation (separating organic carbon from inorganic structures) is difficult and time-consuming. Literature reviews of invertebrate studies show a lack of consistent procedures and guidelines for preparation techniques, especially for carbonate removal via acidification of whole individuals. We find that acidification decreases the delta(13)C values of samples containing carbonate, with shifts ranging from 0.21 to 3.20 per thousand, which can be related to CaCO(3) content (assessed by a carbonate proxy), justifying acid pre-treatment. Carbonate-containing taxa benefiting from acidification included Amphinomida, Terebellida (Annelida), Anomura, Brachyura, Caridea, Amphipoda, Tanaidacea (Arthropoda) and Edwardsiida (Cnidaria). The delta(13)C shifts of samples containing no carbonate varied up to 0.02 +/- 0.20 per thousand. As this induced delta(13)C shift was lower than the range of an average trophic level shift (0.5 to 1 per thousand), we conclude that acid pre-treatment is unnecessary. Carbonate-free taxa consisted of Eunicida, Phyllodocida (Annelida) and Mollusca. We note minimal impact of acidification on delta(15)N values except for Brachyura, which showed a shift of 0.83 +/- 0.46 per thousand, which is still lower than a single trophic level shift (2.9-3.8 per thousand). We conclude that for trophic level studies, both the delta(13)C and the delta(15)N of carbonate-rich macrofauna can be determined from the same acidified sample.

Macrobenthic communities in a tropical lagoon (Tahiti, French Polynesia, central Pacific)

Abstract Soft bottom communities were sampled quantitatively in Tahiti lagoon (French Polynesia) at 18 stations in five zones around the island over 1 year. In addition, various environmental parameters (silt/clay fractions, organic content, chlorophyll and phaeopigment content) were sampled at the same stations over 2 years. The temporal and spatial variabilities of the macrobenthic communities are described and related to these environmental parameters. Each zone ran from the fringing reef to the inner flat of the barrier reef. The macrofauna exhibited a high richness (392 taxa) with an average mean biomass of 1.8 g AFDW m−2 (grams ash-free dry weight per square metre). These communities exhibited temporal but not seasonal fluctuations. The biomass of the macrofauna increased from the fringing to the barrier reefs, and the density of individuals was significantly higher on the fringing reefs. Ordination techniques highlighted four groups of stations characterised by distinctive species composition, density and biomass. The first group included stations located on the inner flat of the barrier reef and in the shallow lagoon area and was characterised by highly diverse communities dominated by polychaetes. The second group primarily included stations from the industrial and hotel zones. This group had the lowest diversity and was also dominated by polychaetes, especially the capitellid Dasybranchus sp. 1. The third group was dominated by gastropods and bivalves. The final group of stations was represented only by station 51, in zone 5, which was characterised by mobile soft sediments and wave action and was dominated by the decapod Hippa cf. pacifica. Presumably, these mobile sediments facilitate the development of this benthic community, which does not occur elsewhere in the lagoon. The diversity and biomass of these benthic communities are low compared with most other areas in the South Pacific.

Low community photosynthetic quotient in coral reef sediments

Fluxes of dissolved inorganic carbon and oxygen at the water-sediment interface were measured at eight coral reef stations (Indian Ocean) in summer and winter. The dark fluxes provided the community respiratory quotient (CRQ = dissolved inorganic carbon release / oxygen uptake) and the diurnal fluxes corrected from the dark fluxes gave the community photosynthetic quotient (CPQ = oxygen gross release / dissolved inorganic carbon gross uptake). The CRQ and the winter CPQ were not significantly different from 1. Summer CPQ (0.79; SD 0.02) was significantly lower than 1 due to the combined effect of the daily evolution of the community respiration and the discrepancy between the daily evolution in community oxygen respiration and community carbon respiration. These results highlight the importance of measuring simultaneously the benthic community production and respiration for long term integrated data sets, instead of the traditional daily or seasonal budget calculations from limited measures of community respiration.

Stable isotopes reveal spatial variability in the trophic structure of a macro‐benthic invertebrate community in a tropical coral reef

RATIONALE Studies of organic matter fluxes in coral reefs are historically based on physical and biogeochemical approaches. It is important to link these approaches to community analysis as the abundance and behaviour of species, populations or trophic groups can have a profound effect on nutrient budgets. METHODS We determined the carbon and nitrogen isotopic compositions of coral reef organic matter sources and macro-benthic invertebrate communities using a Europa Geo 20/20 isotope ratio mass spectrometer interfaced to an ANCA-SL elemental analyzer in continuous flow mode. Isotopic ecology metrics and a mixing model were used to analyze and interpret the data. RESULTS The coral reef macro-invertebrate community principally relies on detrital or recycled food sources. An increased reliance on reef nitrogen-derived sources was observed in the cold-dry season. The community food-web lengths differ noticeably across the coral reef and reflect the characteristics and origin of organic matter reservoirs. CONCLUSIONS Anthropogenic and terrestrial inputs lead to a loss of biological diversity. Exclusive dominance of suspension-feeding species is observed in areas receiving direct surface riverine particulate organic matter. The accumulation of sediment organic matter in eutrophic areas leads to dominance of deposit-feeding species. Copyright

Feeding habits and food partitioning between three commercial fish associated with artificial reefs in a tropical coastal environment

At Réunion Island (south-western Indian Ocean), artificial reefs were submerged in 2003 in a bay and were soon colonised by fish, among which were the highly abundant commercial species Lutjanus kasmira, Priacanthus hamrur and Selar crumenophthalmus. The high concentration and diversity of fish around the artificial reefs is surprising, considering the low abundance of potential benthic prey. We investigated the diet and food partitioning between the aforementioned species using stomach content and stable isotope analyses (δ13C, δ15N). Priacanthus hamrur and S. crumenophthalmus fed on a larger prey diversity and showed significant overlap in their diets, with crustacean larvae the dominant prey. Fish larvae dominated L. kasmiras diet, and δ15N values confirmed the species’ higher trophic level. Differences in δ13C between P. hamrur and S. crumenophthalmus indicated niche segregation, probably as a way to reduce competition, with P. hamrur being characterised by a smaller δ13C range and exhibiting a smaller isotopic niche than S. crumenophthalmus. There was a significant correlation between δ15N and fish standard length for the three species, suggesting that ontogeny partially explained the niche breadth. There was also a significant correlation between δ13C and length for L. kasmira, whereas individual specialisation was prevalent in S. crumenophthalmus.

AChE and EROD activities in two echinoderms, Holothuria leucospilota and Holoturia atra (Holothuroidea), in a coral reef (Reunion Island, South-western Indian Ocean)

AChE and EROD activities were investigated in two holothurian species, Holothuria leucospilota and Holoturia atra, from a tropical coral reef. These organisms were collected from 3 back-reef stations, where temperature and salinity were homogeneous. The activity levels of both AChE and EROD varied significantly between the two species, but were in the range of values determined in other echinoderm species. AChE activity levels were higher in the longitudinal muscle than in the tentacle tegument. Among the several tissues tested, the digestive tract wall exhibited higher EROD activity levels. Sex did not influence AChE and EROD activity levels in both species. Animal biomass and EROD activity levels were only correlated in the tegument tissue of H. atra, and we hypothesize a possible influence of age. EROD activity did not show intraspecific variability. A significant relationship was found between AChE activity and Cuvierian tubules time of expulsion in Holothuria leucospilota. Individuals collected at the southern site presented both lower AChE activity levels and Cuvierian tubules time of expulsion, indicating possible neural disturbance. More information on holothurians biology and physiology is needed to further assess biomarkers in these key species. This study is the first of its kind performed in the coastal waters of Reunion Island and data obtained represent reference values.

Clonal structure through space and time: High stability in the holothurian Stichopus chloronotus (Echinodermata)

Abstract Sea cucumbers are increasingly exploited for human consumption and for their curative properties, and many wild populations are now depleted or in danger of extinction. While aquaculture is seen as an alternative to fisheries and as a mean to restore wild populations, more knowledge is needed on their reproductive strategies to render this practice efficient, notably for fissiparous holothurians, which are some of the mobile animals able of asexual reproduction by transverse fission. Little information is available on their population genetic diversity and structure. Here, the clonal structure of populations of the fissiparous sea cucumber Stichopus chloronotus has been investigated using nine microsatellite loci and a random sampling, at different spatial (intra‐reef and inter‐reef) and temporal (inter‐season and inter‐year) scales. Our findings highlight the importance of asexual reproduction in maintaining these populations, and the prevalence of the “initial seedling recruitment” strategy (ISR), leading to a high stability of clonal composition over seasons and years. It also seemed that clonal propagation was limited to the reef scale (<10 km) while reefs were connected by sexual dispersal. This is the first time that clonal structure in sea cucumbers has been studied at such a fine scale, with a specific sampling strategy. It provides key findings on the genetic diversity and structure of fissiparous sea cucumbers, which will be useful for the management of wild populations and aquaculture.

Seagrass beds as feeding territory for farming Stegastes spp. (Pomacentridae)

Territorial Stegastes damselfishes play key trophic roles in benthic coral reef communities (Gobler et al. 2006). They typically occupy hard substrates such as live and dead corals, but may also use foliose macroalgae for cultivation of palatable epiphytes (Ceccarelli et al. 2005). Despite their widespread distribution and high abundance (Ceccarelli et al. 2001), Stegastes damselfishes inhabiting and using tropical seagrass beds have not been reported. Stegastes spp. are the dominant territorial herbivores in shallow inner reef flats of Reunion Island (South-West Indian Ocean), where they cultivate thick filamentous turf algae (e.g., Polysiphonia) on dead branching corals (Letourneur 2000). During monthly surveys in 2014 and 2015, territories of three species of Stegastes (S. limbatus, S. nigricans and S. punctatus) were observed in monospecific seagrass beds of Syringodium isoetifolium (0.5– 1.2 m depth) (Fig. 1). Their territories always included hard substrata used as shelter (Fig. 1a, d) and they were vigorously defended against conspecific intruders (see supplementary material). Seagrass leaves within their territories showed distinct epiphytic growth (Fig. 1b, c). Furthermore, the damselfishes were observed feeding and weeding on long turf algae on seagrass leaves (Fig. 1e, f). Seagrass habitats commonly present within or close to coral reefs may provide new surfaces for epiphytic communities farmed by damselfishes, extending their habitat and food resources. These observations suggest that the territorial and farming behaviours of Stegastes spp. may strongly influence seagrass bed trophodynamics, as has been found for coral reefs (Ceccarelli et al. 2001), and may disturb seagrass productivity. Indeed, they increase epiphyte loads that are known to reduce light and nutrient availabil i ty for seagrasses (Frankovich and Fourqurean 1997), as well as they exclude intruders and particularly associated herbivores, which are able to stimulate seagrass productivity (Valentine and Heck 1999). Further study of Stegastes spp. and seagrass interactions is needed in order to better understand the functional processes taking place between coral reefs and adjacent seagrass beds. Communicated by R. Thiel