C, N, and P contributions to sediments of two Venezuelan coastal lagoons and their relationships with the adsorption of P

In Unare and Píritu Coastal Lagoons, a study was carried out to assess the effect of C, N, and P load contributions on the capability of the sediments to immobilize phosphates. To achieve the later, the geochemical data of the sediments were coupled to the P-sorption index of Bache and Williams (IBW). In both lagoons, the sediments showed a pH > 7 because of calcareous sedimentation. The inorganic carbon values in the lagoons displayed a spatial distribution with higher concentrations toward the shores, that is defined by the carbonate lithology. On the contrary, the inner part of the lagoons was characterized by the presence of organic materials associated to the clay. The phosphorus content in the Unare Lagoon ranged from 290 to 625 mg kg⁻¹, whereas in the Píritu Lagoon the values fluctuated between 213 and 1013 mg kg⁻¹. The highest concentrations of phosphorus in both lagoons could be linked to sewage and runoff input from agricultural and livestock activities around the lagoon systems. The IBW displayed adsorption average values of 21.97 and 27.42 for Unare and Píritu Lagoon, respectively, corresponding to a rather low P sorption. In Unare Lagoon, the IBW showed positive correlations with C, N and Fe_labile but negative with P. However, in the Píritu Lagoon, despite the analogous lithology of the lagoons, a slightly positive non-significative correlation between IBW and IC was only found. Although the sediments adsorb P with a rather low capacity, they can mitigate the eutrophication process in the studied lagoons.