Phosphorus geochemical forms and potential bioavailability in surface sediments from Edku Lagoon, Egypt, regarding diagnostic ratios (Fe:P, S:Fe, and OC:OP)

Sediments play an important role in the nutrient dynamics of shallow lakes. In this work, the geochemical characterization of phosphorus (P) in the surface sediments of Edku Lagoon, Egypt, was studied in Spring 2019. Spatial variation, geochemical forms of P, and the factors affecting them were discussed. The concentrations of four operationally determined bioavailable P fractions that can be extracted from the sediments were examined. These fractions were water-soluble P (WSP, F1), readily desorbable P (RDP, F2), algae-available P (AAP, F3), and NaHCO3-extractable P (Olsen P, F4). Thirteen surface sediment samples were collected from Edku Lagoon, and from feeder drains. Sediment geochemical properties, total P (TP), inorganic P (IP), organic P (OP), bioavailable P fractions, and diagnostic ratios (Fe:P, S:Fe, and OC:OP) were determined. The results were discussed through various statistical analyses. The ranges and relative percentages of these P forms of the total concentration in the lagoon sediment stations examined were in the following order: AAP (16.7–35.4; 25.7 ± 6.65%) > Olsen P (9.1–13.8; 11.3 ± 1.74%) > WSP (2.7–8.2; 6.0 ± 1.96%) > RDP (0.41–1.26; 0.89 ± 0.32%). The results showed that IP was the main form of TP in the sediments; its concentration was six times that of OP. The relative IP to TP ratio ranged between 69 and 98%, whereas OP accounted for 2–31% of TP. The sediments of Edku Lagoon were characterized by relatively large amounts of bioavailable P; the sum of P fractions exceeded 40% of the TP. The severe increase in AAP indicated very abundant vegetation. The diagnostic ratios (Fe:P, S:Fe, and OC:OP) were calculated. Most stations had Fe:P < 15 (high P internal loading). However, 85% of the studied sediment samples had S:Fe < 1.5 ratios, which showed high sediment potency to isolate Fe-associated P forms. The high OC:OP ratio along the studied lagoon and drains reflected the hypertrophic conditions and foreign input sources of organic carbon. The results reflected the ability of the studied P fractions to release easily from the lagoon sediments, which contributes greatly to the abundance of P from the sediments and its ease of access to surface waters, and therefore the sediments must be dredged in Edku Lagoon. The information collected will be useful in the management and development of the examined lagoon.