Reverse flow injection techniques for the determination of nitrogen and phosphorus species in estuarine water of the Lagos Lagoon, Nigeria

Abstract

ABSTRACT The quantitation of nutrient species in the saline water of the Lagos Lagoon was carried out using flow injection analysis (FIA). Due to the Schlieren effect (SE), occasioned by the formation of frontal and rear negative peaks, modified reverse flow injection (rFI) methods were developed for the quantitation. Water samples were collected at 10 sites on and around the Lagoon from 2012 to 2015 and oxidised with acidic potassium persulphate. To eliminate the SE, acid-molybdate reagent was injected into a 36.4 g L−1 salinity carrier in the P manifold, and N-(1-naphthyl) ethylenediamine reagent (NINE) was injected into a 0.16 g L−1 salinity carrier in the N manifold. In both manifolds, the reagent-injection volume was 100 µL. Good turnover rates of 90 h−1 and 120 h−1 were attained for N and total-soluble/total P analysis, respectively. The method detection limit for phosphate-P analysis was 53 µg L−1 while that of nitrate-N analysis was 0.10 mg L−1. The accuracies were within 7% as the average recoveries of PO4 3–P and NO3 –N were 107% and 95%, respectively, in spiked blanks. At the top layer of the Lagoon water, the range of [NO3 −+NO2 −]-N was 0.16 to 0.50 mg L−1 while the soluble reactive P (SRP) was undetectable to 796 µg L−1. Total N (TN) ranged from 1.69 to 21.0 mg L−1 and total P (TP) was 0.13 to 3.94 mg L−1. The study showed that the nutrient limitation (N/P) in the Lagos Lagoon was controlled by rainwater runoff owing to the increased delivery of P to it in the wet season. The distribution of nitrate-N species lacked significant, spatial and temporal variation. Also, the increasing concentration of SRP species viewed against the population density of dwellers around the sites hints at anthropogenic influence.