The rate at which water masses are transformed from one density class to another is assessed in the shelf seas using the Walin (1982) framework. For a tidal-mixing front, the transformation is estimated using air-sea density fluxes and the diapycnal mixing diagnosed from a series of one-dimensional mixed layer models running across the shelf. These transformation rates diagnosed from the air-sea fluxes and diapycnal mixing agree with volume changes diagnosed directly from the model. The transformation from air-sea fluxes reaches a maximum amplitude typically twice that provided by diffusive mixing. This framework is extended to estimate the rate at which nutrients are converted in nutrient space including the effects of biological consumption. The transformation in density and nutrient space are broadly related in the spring when there is a relatively tight relationship between density and nutrient concentrations. For a shelf-break front in the Celtic Seas, the transformation is estimated from a combination of observed air-sea fluxes, remotely-sensed sea surface temperature and ship-based measurements of density and turbulent mixing. The transformation is controlled by diapycnal mixing along dense surfaces and by air-sea fluxes for lighter surfaces, and each contribution reaches comparable magnitudes over a six-month period.