Nutrient cycling in ecological aquaculture wastewater treatment systems: vertical distribution of benthic phosphorus fractions due to bioturbation activity by Tegillarca granosa

Abstract

Fractionation of sediment phosphorus (P) was conducted to examine the distribution and bioavailability of P fractions in a blood clam Tegillarca granosa-based ecological aquaculture wastewater treatment system. In a laboratory experiment, either a low or high density of clams, or no clams (control), were added to a polypropylene chamber containing aquaculture wastewater and an 8 cm sediment layer. Distribution and bioavailability of inorganic P (Inorg-P)—which included exchangeable P (Ex-P), metal oxide-bound P (NaOH-P), calcium-bound P (HCl-P), redox-sensitive P (BD-P), and detrital P (De-P)—and organic P (Org-P) in the sediment were investigated after 30 d using a sequential extraction procedure (SEDEX). Results showed that T. granosa bioturbation activities markedly changed the contents of P fractions, especially Ex-P, NaOH-P, BD-P, and Org-P (p < 0.05). Total Inorg-P varied from 1.83 to 6.75 µmol g-1, with an average of 4.29 ± 1.37 µmol g-1 among the 3 groups. Org-P ranged from 5.01 to 9.76 µmol g-1 and accounted for 22 to 41% of total P (TP). Despite some variation in contents at different depths, the average proportions of P fractions to TP were, in ascending order, Ex-P, BD-P, NaOH-P, HCl-P, De-P, and Org-P. Inorg-P had the highest proportions (73.83%) of TP. Bioavailable P (Ex-P, NaOH-P, and BD-P) proportions were 61, 58.23, and 44.44%, respectively, relative to the control. The variant contents of the P fractions were most probably due to enhanced organic matter consumption and mineralization, metal oxide dissolutions, and the increased bioavailable P in the bioturbated sediment. This study suggests that T. granosa can modify the concentrations, distribution, and bioavailability of sedimentary P, as well as reduce Org-P. Our study further advances the knowledge on P cycling and provides theoretical assistance for monitoring P in aquatic environments.