Molluscan Biofacies and their Environmental Implications, Nile Delta Lagoons, Egypt

Abstract

Molluscan assemblages and their distribution in surficial sediment of Idku, Burullus and Manzala lagoons and Mariut lake in the northern Nile delta, Egypt, are defined. Quantitative analysis is performed on thanatocoenoses comprising a total of 47 species using a principal component analysis ttl distinguish the molluscan biofacies. Cumulative dominance values of fresh-water, lagoon sensu stricto and marine settings, calculated for molluscan assemblages in each sample, reveal considerable faunal variability within and between the four Nile delta water bodies. Three molluscan biofacies are identified: those recording a marine, a fresh-water and a lagon sensu stricto influence. Biofacies indicate that, of the three lagoons, Manzala has the most fresh-water affinity and also receives the greatest incursion of marine waters via its outlet. Mariut lake biofacies, comparable to those of the Nile lagoons, likely record its history early in the last century when it was a lagoon connected to the Mediterranean. As a result of relatively high freshwater input, modern Nile molluscan biofacies comprise larger proportions of fresh-water species than in many other Mediterranean lagoons. The smaller number of molluscan biofacies and their more extensive geographic distribution in modern Nile lagoons, as compared to petrological subfacies are a function of sediment transport-related phenomena. The results of this study are applied to the fossil record in short cores from former Abu Qir lagoon, as old as 200 years, and older Holocene sequences in the underlying northern delta. Molluscan biofacies in these older units indicate settings that are somewhat less fresh-water and more lagoon sensu stricto influenced than those in modern Nile lagoons. Since molluscan faunas are sensitive markers of environmental conditions, it is recommended that their distributions in baseline studies be monitored in Nile delta lagoons and Mariut lake which are undergoing drastic modification by man.