Sensitivity of the copepods temora longicornis and tisbe battagliai to produced water from offshore oil fields.

Produced Water (PW), the main by-product of oil and gas extraction, poses potentially important ecological risks due to its chemical composition (including hydrocarbon residues, production chemicals, metals) and its direct discharge to the sea. This study examines PW toxicity from four offshore North Sea oil fields, using acute tests with the copepods Tisbe battagliai and Temora longicornis. It primarily addresses residual and persistent toxicity affected by storage conditions. A wide range of PW concentrations was used to determine median lethal concentrations (LC50). Temporal and spatial variability in PW toxicity reached up to 82% RSD, highlighting important differences in toxicity between oil fields. The highest toxicity resulted in an LC50 of ca. 2% PW and can potentially be explained by relatively higher concentrations of trace elements (i.e., B, Ba, Zn) and other unidentified dissolved compounds,even though a direct compound-driven contribution was not studied. Sample pre-treatments identified suspended particulate matter as a potential actor in mortality, particularly for T. longicornis. This species consistently showed greater sensitivity to PW than T. battagliai, underscoring its relevance for future risk assessments and the importance of multi-species testing.