Larger foraminifera distribution on a mesotrophic carbonate shelf in SW Sulawesi (Indonesia)

Larger symbiont bearing foraminifera typically live in shallow tropical seas. In this study the fauna composition of patch reefs scattered over the Spermonde Shelf (SW Sulawesi, Indonesia), a mesotrophic carbonate shelf, is examined. The foraminiferal fauna of the Spermonde Shelf is characterised by three assemblages, a seaward slope, a leeward slope and a reef base assemblage. This is similar to the fauna composition at Palau except that Palau’s reef flat assemblage is absent. The samples group in six clusters: the reef base, deep exposed slope, deep leeward slope, shallow leeward slope, shallow exposed slope and the southern near shore cluster. Of the four ecological zones that have been recognised on the Spermonde Shelf in corals and marine vegetation, we cannot distinguish between the two outer shelf zones, but we do recognise two near shore zones, a northern and a southern one. This latter part of the shelf within 4 km of the mainland coast has a less diverse foraminiferal fauna, in which the reef base fauna is absent. Compared to areas with a deeper euphotic zone, the deepest living species are absent whereas most other species occur in much shallower water. Neorotalia calcar and Calcarina gaudichaudii, however, have been found in deeper water, due to an absence of suitable habitat in the shallow areas. Other species, like Peneroplis planatus and Calcarina hispida have only been found in the deepest part of their depth range, as reported at Okinawa. These species show a greater flexibility in their habitat preference than has been observed so far. We used our results to test Hallock’s trophic resource continuum model with actual occurrences of larger foraminifera. The predictions of the model are met, but patterns, for example, in calcarinids and peneroplids show that not all taxa react in the same way to the described environmental parameters. Seasonal climatic conditions in the Spermonde Archipelago mainly affect species with a long life cycle and deep living species. A model is presented that explains why deep living species are more affected by seasonal changes in water transparency than shallow living species.