Spatial Patterns in the Distribution, Diversity and Abundance of Benthic Foraminifera around Moorea (Society Archipelago, French Polynesia)

Abstract

Coral reefs are now subject to global threats and influences from numerous anthropogenic sources. Foraminifera, a group of unicellular shelled organisms, are excellent indicators of water quality and reef health. Thus we studied a set of samples taken in 1992 to provide a foraminiferal baseline for future studies of environmental change. Our study provides the first island-wide analysis of shallow benthic foraminifera from around Moorea (Society Archipelago). We analyzed the composition, species richness, patterns of distribution and abundance of unstained foraminiferal assemblages from bays, fringing reefs, nearshore and back- and fore-reef environments. A total of 380 taxa of foraminifera were recorded, a number that almost doubles previous species counts. Spatial patterns of foraminiferal assemblages are characterized by numerical abundances of individual taxa, cluster groups and gradients of species richness, as documented by cluster, Fisher α, ternary plot and Principal Component Analyses (PCA). The inner bay inlets are dominated by stress-tolerant, mostly thin-shelled taxa of Bolivina, Bolivinella, Nonionoides, Elongobula, and Ammonia preferring low-oxygen and/or nutrient-rich habitats influenced by coastal factors such as fresh-water runoff and overhanging mangroves. The larger symbiont-bearing foraminifera (Borelis, Amphistegina, Heterostegina, Peneroplis) generally live in the oligotrophic, well-lit back- and fore-reef environments. Amphisteginids and peneroplids were among the few taxa found in the bay environments, probably due to their preferences for phytal substrates and tolerance to moderate levels of eutrophication. The fringing reef environments along the outer bay are characterized by Borelis schlumbergeri, Heterostegina depressa, Textularia spp. and various miliolids which represent a hotspot of diversity within the complex reef-lagoon system of Moorea. The high foraminiferal Fisher α and species richness diversity in outer bay fringing reefs is consistent with the disturbance-mosaic (microhabitat heterogeneity) hypothesis. Calculations of the FORAM Index (FI), a single metric index to assess reef vitality, indicate that all fore- and most back-reef environments support active carbonate accretion and provide habitat suitability for carbonate producers dependent on algal symbiosis. Lowest suitability values were recorded within the innermost bays, an area where natural and increasing anthropogenic influences continue to impact the reefs. The presence of habitat specific assemblages and numerical abundance values of individual taxa show that benthic foraminifera are excellent recorders of environmental perturbations and good indicators useful in modern and ancient ecological and environmental studies.

Fig 1. Location of the 45 sample station around Moorea, French Polynesia (for details see Table 1).

Fig 2. Scanning electron micrographs of selected species of indicator foraminifera characterizing the major cluster habitats around Moorea Island.

Species that bear symbionts: 1. Coscinospira hemprichii Ehrenberg; 2. Peneroplis pertusus Forskål; 3. Borelis schlumbergeri Reichel; 4, 5. Amphisorus hemprichii Ehrenberg (Scale bar is 200μm); 6, 7. Sorites orbiculus Ehrenberg; 8, 9. Parasorites orbitolitoides Hofker; 10. Monalysidium confusa McCulloch; 11. Assilina (Operculina) ammonoides Schröter; 12. Amphistegina lessonii d’Orbigny; 13. Heterostegina depressa, d’Orbigny; Species associated with the symbiont-bearing taxa: 14. Schlumbergerina alveoliniformis Brady; 15. Anomalinella rostrata Brady; 16. Eponides repandus Fitchel and Moll; Opportunistic species: 17. Ammonia tepida Cushman; 18. Bolivina striatula Cushman 19. Bolivinella elegans Parr; 20. Bulimina sp. 1 (Scale bar is 50μm); 21. Buliminella elegantissima d’Orbigny; 22. Elongobula spicata Cushman and Parker; 23. Elphidium oceanicum Cushman; 24. Fursenkoina schreibersiana Czjzek; 25. Hopkinsina pacifica, Cushman; 26. Loxostomina limbata, Brady; 27. Nonionoides grateloupi d’Orbigny; 28. Reusella pacifica Cushman and McCulloch; 29. Sigmavirgulina tortuosa Brady; 30. Trifarina bradyi Cushman; Heterotrophic species: 31. Sagrinella convallaria, Millett; 32. Wiesnerella auriculata Egger; 33. Quinqueloculina cf. Q. semireticulosa Cushman; 34. Quinqueloculina funafutiensis, Chapman; 35. Quinqueloculina exsculpta Heron-Allen and Earland; 36. Quinqueloculina eburnea d’Orbigny; 37. Quinqueloculina cuvieriana d’Orbigny. Scale bar is 100μm for all magnifications.

Fig 3. Abundance records of larger symbiont-bearing (LBF), Opportunistic (OPP) and Heterotrophic (HET) foraminifera within a) Opunohu and b) Cook’s Bay.

Fig 4. Increase in Fisher α diversity indices from the bays to the back-reef habitats.

Fig 5. Q-mode cluster diagram of sample sites exhibiting the presence of 6 major cluster groups.

Fig 6. Map showing locations of the clusters stations.

The symbols represent the clusters defined in Fig 5.

Fig 7. Principal Component Analysis (PCA) of the foraminiferafauna showing a) principal components 1 and 2, and b) principalcomponents 1 and 3.

The symbols represent the clusters as defined in Fig 5.

Fig 8. Ternary diagram showing percent abundances of wall structural types (porcellaneous, hyaline-perforate and agglutinated foraminifera) of individual sample sites around Moorea [55].

The symbols represent the clusters groups as defined in Fig 5.

Fig 9. FI values plotted along transects in Opunohu and Cook’s Bays showing indices rising from the innermost bays towards the reefal sites.