Abstract:Dissolved organic matter (DOM) in seawater, consisted of a complex mixture ofcompounds and accounted for about 90% of marine organic carbon, is animportant part of the global carbon cycle and also a part of absolute salinity ofseawater. As the components of DOM are non-ionized in seawater, electricconductivity measurement cannot observe its contribution to salinity, whichcould lead to measurement error in salinity. Salinity measurement based on opticrefractive index could reveal contribution of all dissolved matters in seawaterincluding both non-ionized and ionized components. In this paper, acomparative study of DOM contribution to the salinity measurement by opticrefractive index method and electric conductivity method was carried out for thefirst time. Adding DOM, extracted from Aoshan Bay, Qingdao, China, intoChinese Standard Seawater, our experimental results showed an increase ofoptic refractive index at 1.19x10-4 + 4x10-6 per 1g/kg of DOM, whichcorresponds to a practical salinity increase of 0.697  0.036 PSU per 1 g/kg ofDOM. While, the conductivity of seawater measured by a conductivity-temperature sensor showed a decrease of 0.00065 + 0.00008 mS/cm per 1g/kg of DOM, which corresponded to a decrease of salinity at 0.00052 +0.00007 PSU per 1 g/kg of DOM. By comparing the optic and electricmeasurement results, the contribution of non-conductive components in thedissolved organic matters to salinity could be distinquished, which has greatsignificance for the measurement of absolute salinity, global carbon cycle etc.