Potential biodiversity threats associated with the metal pollution in the Nile–Delta ecosystem (Manzala lagoon, Egypt)

Metal pollution may pose a high risk to the ecosystem stability. To evaluate the impact of the metal pollution on the biodiversity, metals concentrations in sediments, water, and molluscan shells were analyzed at sixteen sites in the Manzala lagoon, which is the largest in the Nile–Delta. The samples were collected along a gradient from the main agricultural drain, which brings mixed industrial, domestic, and agricultural wastes. An exponential trend of increasing metals concentration towards the main agricultural drain was observed. For sediment and faunal samples this trend was statistically significant in most cases, however, this trend was not found for all metals in water samples except for Zn. Partial Least Square (PLS) model confirmed that the distance to the drain is a second major predictor of the observed metal concentrations after sediment grain–size, while other physicochemical parameters have minor effects. Moreover, a significant correlation was found between most metals in the fauna and their respective sediments. In contrast, neither sediment nor faunal metal concentrations did correlate with water samples. Collectively, all pollution indices (enrichment factor, contamination factor, and geo–accumulation index) indicate high pollution by Cd and Pb and moderate pollution by Zn. UPGMA clustering and non–metric Multidimensional scaling (nMDS) classified the sites in the lagoon into three main categories: 1) highly polluted southeast corner around the drain, 2) moderately polluted northeastern corner, where the newly widened and deepened sea–outlet occur, 3) unpolluted northwestern part, which located away from the drain. Multiple regression analysis showed that the distance to the pollution source, salinity, and water depth are significantly predict the faunal diversity. Furthermore, the Pollution Load index (PLI) was significantly correlated with the diversity indices. Lower diversity and dominance of opportunistic taxa in the polluted sites are interpreted to be negative consequences of the metal pollution.