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The present study aims to examine physiological and biochemical response of three commercial fish species (Dicentrarchus labrax; Platichthys flesus and Solea solea), over contrasting environmental dynamics: an extremely dry (2012) and flood (2014) years, in a shallow temperate southern European estuary, the Mondego Estuary (Portugal). Physiological and biochemical biomarkers were evaluated by a principal component analysis (PCA), which allowed to conclude that severe climatic events affected the set of fish species analyzed, revealing two distinct annual brain antioxidant responses. The drought episode affected the physiological state of the organisms, as well as increased brain antioxidant potential, strongly associated with fluctuations in environmental drivers (salinity and dissolved oxygen), however, ROS have not been effectively neutralized by antioxidant defence system causing lipid peroxidation. During flood episode was stated a general depletion of the antioxidant potential in the analyzed fish species, affected by interactions with chemical compounds, increased by a combination of high precipitation and associated runoff, probably, increasing nutrient and contaminant load at the Mondego estuary. Nevertheless, lipid peroxidation remained low, related to the action of non-enzymatic antioxidants, since that the studied fish species had optimal physiological status and high nutritive reserves. According to the present work we consider that brain enzymatic depletion may be organ-specific, looking to the greater vulnerability of brain's proteins to degradation compared to lipids. The role of enzymatic mobilization on fish brains is not extensively yet known, but our results suggest that brain seems to be metabolic sensitive to salinity and dissolved oxygen fluctuations. This is the first approach made to evaluate the physiological and biochemical responses of the brain of aquatic organisms' to extreme climatic events and to establish reference values to determine the effects of extreme climate events to aquatic species.

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