Análise sedimentológica de recifes do gênero Phragmatopoma Mörch, 1863 (Annelida: Polychaeta) da Região Metropolitana de Fortaleza (CE)

Abstract

Annelids of the genus Phragmatopoma are marine animals with benthic sessile habit found in nearly all Brazilian coast, where they build reefs via aggregating coastal sands. Their habitat and preservation of reefs aid Holocene studies on relative sea-level variation, besides other biological, oceanographic and even economic aspects, arousing great interest in their biosedimentary buildings. This work analysed sedimentology of seven reefs of the genus Phragmatopoma from different places in the coastal zone of the Metropolitan Region of Fortaleza (MRF), by comparing their properties between themselves, with sands of the local beaches and with an region free from urban influence (São Miguel do Gostoso, State of Rio Grande do Norte). The analyses included a biological zoning of a natural rocky prominence, in addition to counting of the main detrital components of the sands by microscopy, quantification of the gross content of calcium carbonate and organic matter, grain size analyses and heavy minerals petrography. The results suggest there is not preference for specific types of encrusting hardened substratum. In the MRF, the biological zoning was similar to the Brazilian Southeast coast for half of the organisms in the infralitoral zone, being the difference centred in the major diversity of algae in the MRF. Regarding organic matter, grain size and heavy minerals, the reef sands and beach sands of the same places were very similar. Higher values in the calcium carbonate content and in the quantity and diversity of bioclasts in the reef sands compared to the beach sands from the same places point to a clear predilection of the polychaetes for calcareous biodetritic components of the sands, either by a matter of shape or hydrodynamic availability. Overall, the result may be regarded as resemblant to preliminary studies performed in the Atlantic coasts of North America and Southeast Brazil for polychaetes of the same genus.