Abstract. Penaeus setiferus postlarvae were exposed to acute levels of ammonia, nitrite, and to a mixture of both by a short‐term static method, The 24h, 48‐h and 72‐h LC50 values for un‐ionized ammonia were 1.49, 1.21 and 1.12 mg/L NH3‐N (un‐ionized ammonia as nitrogen), and 11.55, 9.38 and 8.69 mg/L ammonia‐N (un‐ionized plus ionized ammonia as nitrogen). The 24‐h, 48‐h and 72‐h LC50 values for nitrite were 268.06, 248.84 and 167.33 mg/L nitrite‐N (nitrite as nitrogen). Nitrite was much less toxic than ammonia. The joint effect of ammonia and nitrite on the postlarvae was synergistic at 48‐h exposure and antagonistic after 72 h. Postlarvae of P. setiferus may be considered as organisms sensitive to ammonia and nitrite.

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Abstract. The white shrimp Penaeus setiferus is a potential species for culture in the Gulf of Mexico. It has been demonstrated that postlarval P. setiferus is a species sensitive to ammonia and nitrite. In this study, the effect of ammonia and nitrite on the oxygen consumption of Penaeus setiferus postlarvae subjected to different dissolved oxygen concentrations was investigated. Un‐ionized ammonia tended to reduce the oxygen consumption of P. setiferus postlarvae. The addition of 0.4 mg/L un‐ionized ammonia‐N modified the response of the postlarvae to lower dissolved oxygen levels. Exposure to 0.7 mg/L un‐ionized ammonia‐N caused severe effects on the respiratory rate and it was lethal at low dissolved oxygen concentrations. Nitrite exposure decreased the respiratory rate of post‐larvae in high dissolved oxygen concentrations, whereas postlarvae exposed to nitrite under hypoxic conditions showed higher oxygen consumption than unexposed animals. The decreases of the oxygen consumption induced by the interactive effect of ammonia or nitrite and low dissolved oxygen concentration indicate a harmful condition. The results indicate that P. setiferus postlarvae is a sensitive organism to the combined effect of un‐ionized ammonia or nitrite and low dissolved oxygen concentrations.

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In this study, two Lactuca sativa crops grown in a net-protected site and an unprotected site were compared. The results showed metals were accumulated by Lactuca sativa. Higher metal accumulation in lettuce grown in the unprotected parcel was established, and the only significant differences in their physicochemical properties were cation exchange capacity and exposure to sunlight. Exposure to the sun may translate into higher soil-plant flux of water and/or higher decomposition rate of organic matter, which may augment the mobilization of metals into the plant, and therefore lead to higher noxious metal accumulation on the crops. Also, EDTA was explored as a possible extractant that could predict the bioavailablity of metals to the lettuce. Our results did not show a good agreement between the metal transferred from soil to leaves and the EDTA extracted metals, although some tendencies were found for the metal transfer from soil to roots. It is predicted that many developing countries will continue and/or increase the use of waste water for the irrigation of crops despite the health risks. Different solutions have been explored to diminish the availability of the metals in the soils. Our study shows that an action as simple as placing a net over the parcel could help diminish the accumulation of metals into the crops of a rapidly expanding peri-urban agricultural system.