Benthic assemblages for ecological evaluation of Lake Manzala, Mediterranean Sea, ‎Egypt.

Abstract

Lake Manzalah one of the northern Nile Delta in Egypt is the largest lake, which lies ‎between the lakes Borollus and Bardaweel and connects to the Mediterranean Sea. It ‎has an area of current 250 thousand acres. It contains many islands particularly along ‎the south-western region forming of semi-enclosed basins and it is characterized by dense ‎submerged hydrophytes as Pomatogeton pectinals; Ceratophyllum demersum and ‎floating hyacinth Eichhornia crassipes. Seven major drains are loading directly in the ‎south to the west. Lake Manzalah-bottomed shallow (0.5-1m), brackish water (3-18‰ ‎and is suffering from the phenomenon of high nutrition (eutrophication) due to increased ‎rates of nutrients and organic matter. So some sources of wastes discharges such as ‎sewage and industrial waste and agricultural activity pour directly into the lake, ‎especially the southern region (Bahr El Baqar Drain). The study aims to study the ‎distribution of benthic organisms in the lake and knowledge of the relationship between ‎the types and aggregates benthic food ecosystem as the important food sources' for ‎some aquatic organisms, especially economic fish and crustaceans in study stations. ‎Bottom sediment samples were collected from 11 sites by grab sampler covering ‎different environments lake and from 4 drains consists mostly of organic materials ‎‎(sludge) is made up of waste sewage, industrial, agricultural waste plants, and calcareous ‎shells empty. Results indicate for examining configuration qualitative macro benthic ‎recording the 16 species of aquatic plants and invertebrates where empty calcareous ‎shells were more frequent. The abundance of macrobenthic organisms at different ‎sampling sites along Lake Manzala can be ranked as follows: Ostracoda (45.4%, 1010 ‎ind/m2) > submerged plants (12.9%, 287 tufts/m2) > Amphipoda (9.2%, 207 ind ∕m2) > ‎Polychaetes (7%, 110 ind ∕m2) Bivalves (5.9%, 132 ind ∕m2). The biomass of benthic ‎assemblages at different sampling sites can also be ranked as follows: at ST5 (18.1%, ‎‎4433 ind ∕m2) > St6 (11.3%, 2772 ind ∕m2) > ST8 (9.8%, 2405 ind ∕m2) > ST3 (9.2%, 2247) ‎‎> ST7 (8.7%, 2122 ind ∕m2). The biomass of benthic assemblages at different sampling ‎sites can also be ranked as follows: echinoderms (52%) > molluscs (27%) > crustaceans ‎‎(16%) > polychaetes (2%) > other groups (3%). Due to the increase of pollutants ‎extensively loaded into the drains are the possible factors having affecting the ‎constituents’ structures of benthos. Sites of sampling study were evenly spread (J’ > 0.8) ‎only at sites 6 and 8, moderate diversity (H’ > 1.00) was at most sites and show lower ‎value at sites 3 and 11 and 0 at site 1. The abiotic environmental effects in the lake are ‎general to those generally observed in other areas influenced by organic wastes, namely, ‎changes in the physic-chemical properties of the sediments and low oxygen ‎concentrations in the bottom water due to the decomposition of organic materials.‎