A modular estuarine mesocosm

Abstract The need to simulate unique features of estuaries and the modes of entry of pollutants has led the authors to develop a modular approach to toxicity testing within an estuarine system. The approach incorporates replicate test systems that simulate the major aspects that are unique to estuarine systems. Initial validation of the design simulated a reference estuary on Wadmalaw Island, South Carolina. Replicate mesocosms were exposed to target concentrations 0, 2, and 8 μg/L of azinphosmethyl (AZM), an organophosphate insecticide. Single species toxicity tests were conducted within the systems in conjunction with the ecosystem simulations. High doses of AZM killed all animal life in the simulations, whereas lower concentrations showed a dose—response effect. Twenty‐four‐hour average AZM levels were used to calculate 24‐h and 96‐h LC50 values for Atlantic silverside minnows ( Menidia menidia ), mysid shrimp ( Mysidopsis bahia ), and grass shrimp ( Palaemonetes pugio ). Mysid shrimp were most sensitive to AZM (96 h LC50 = 0.811μg/L), whereas grass shrimp were least sensitive (96 h LC50 = 1.34 μg/L). Dissolved oxygen was reduced and ammonia concentrations increased from high doses of AZM. Increases in ammonia levels were not reduced by interstitial microbes, resulting in sustained high (>0.08 mg/L) ammonia concentrations. Lower doses were quickly reduced in the mesocosm. This study demonstrates that ecosystem level responses can be evaluated by the modular approach. This modular approach can be modified to simulate most aquatic ecosystems.