Climate change-induced salinity decrease is currently occurring in many estuarine coastal zones, due to increased outflow of freshwater. This freshening can be a problem for brackish-water animals, already living on the edge of their salinity tolerance. We measured oxygen consumption of common copepod Eurytemora affinis along a natural salinity gradient in the western Gulf of Finland. The salinity varied between 3 in the inner bay and 7 in the offshore area along the gradient, pH varied between 7.05 and 7.86. Our results show that respiration increased with decreasing salinity, as expected for a genus more commonly found in estuarine/saline waters, even if it has colonised brackish waters. Our results suggest that future decreasing salinity could enhance respiration rate, and so energy requirements, of large-bodied zooplankton in estuarine areas such as the Baltic Sea and could lead to lower food quality availability for coastal planktivorous fish, such as herring and sprat.