Nitrate acute toxicity to post larvae and juveniles of Macrobrachium amazonicum (Heller, 1862)

The objective was to determine the median lethal concentration (LC₅₀) of nitrate for post-larvae and juveniles of Macrobrachium amazonicum exposed for 96 h, and to identify histopathological alterations in the gills of juveniles exposed to nitrate. Post-larvae and juveniles of M. amazonicum were exposed to seven different concentrations of nitrate (0, 50, 100, 500, 1000, 1500 and 2000 mg.L^-1) with four replicates (n = 10 prawn/experimental unit). The degree of damage to the branchial structure of juveniles was evaluated using standard histological processing with hematoxylin and eosin (H&E), subjected to the Organ Index (Iorg). LC₅₀ values at 24, 48, 72 were estimated by the Trimmed Spearman Karber Method software and were respectively 1574, 638, 237, and 194 mg.L^-1 for post-larvae and 1070, 286, 185 and 155 mg.L^-1 for juveniles, respectively. From these results, the safety level of nitrate was estimated at 157, 64, 24 and 19 mg.L^-1 for post-larvae and 107; 27; 18 and 16 mg.L^-1 for juveniles, respectively for 24 h, 48 h, 72 h and 96 h. Damage to the gills in treatments with 100% mortality for nitrate corresponded to the high occurrence of progressive, regressive, circulatory and inflammatory damages. The other treatments, which caused lower mortality, mainly resulted in inflammation and regressive damage, whose occurrence increased with increasing concentration of nitrate. However, in closed systems, the use of anaerobic denitrifying filters or aquatic macrophytes is necessary to provide suitable water quality for the best productive performance, avoiding the negative influence of the accumulated nitrate.