Abstract. A new water mass analysis technique is used to analyse the BATS oceanographic data set in the Sargasso Sea of 1988–1998 for changes in Labrador Sea Water (LSW) properties. The technique is based on a sequential quadratic programming method and requires careful definition of constraints to produce reliable results. Variations in LSW temperature and salinity observed in the Labrador Sea are used to define the constraints. It is shown that to minimize the residuals while matching the observed temperature and salinity changes in the source region the nitrate concentration in the Labrador Sea has to be allowed to vary as well. It is concluded that during the period of investigation nitrate underwent significant variations in the Labrador Sea.