Bioavailability of phosphorus in Abu Qir Bay and Lake Edku sediments, Mediterranean Sea, Egypt

Abstract

The aim of this study is to investigate the distribution of phosphorus bioavailability (total phosphorus, organic, and inorganic phosphorus) in the sediments from two areas (Abu Qir Bay and Lake Edku) located east of Alexandria City. Abu Qir Bay is a very important productive area of the Mediterranean Sea off Egypt, since it receives nutrient-rich brackish water from Lake Edku as well as El-Tabia pump station. The release of phosphorus from the sediment is generally important for eutrophication, which influences the sea-water quality of the study area. Sediment samples from 10 sites of Abu Qir Bay and eight sites of Lake Edku were subjected to a seven-step extraction procedure to determine organic, inorganic, and total phosphorus as well as phosphorus bioavailability: water-soluble phosphorus (WSP), readily desorbable phosphorus (RDP), algal available phosphorus (AAP), and NaHCO3-extractable phosphorus. Our results indicate low recovery of available phosphorus extracted by CaCl2 for the sediments of the lake and the bay. The sediments of Lake Edku were richer in organic and inorganic phosphorus. The range and mean concentration (μg g−1) of the organic P were 17–260 (129) for Abu Qir Bay, and 84–930 (349 μg g−1) for Lake Edku. The accumulation of fresh macrophyte debris in Lake Edku sediments plays an important role in organic-matter enrichment (average 9%) and phosphorus bioavailability. The inorganic P was higher than the organic P for the study area, and its content (μg g−1) ranged from 127 to 446 (mean; 252) and from 515 to 947 (mean; 682) for Abu Qir Bay and Lake Edku sediments, respectively. The distribution of both total P and organic P content in the lake sediments revealed a decreasing level from the eastern to the western region, towards the opening of Boughaz El-Maadia, while the inorganic phosphorus increased from 598 μg g−1 in the eastern side to 875 μg g−1 in the western region of the lake. In general, the rank order of extraction efficiency was the same in the two regions, and the amount extracted was in the sequence AAP>Olsen-P>WSP > RDP.