Phosphate and SFg (an inert tracer) were injected into the upper mixed layer in an area of 7.5 km2. 7.5 hours afcer the injection the chemically deter-mined orthophosphate had reduced to half its initial value and after 36 hours to <5nM. At the same time there was an increased turn-over time of biolo-gically available orthophosphate of more than one order of magnitude, sug-gesting that the phosphate had been removed by biological uptake. Primary productivity and phytoplankton biomass showed a nutrient-starved autotro- phic community, unaffected by the phosphorus addition in terms of biomass or productivity, their changes being rather attributed to small-scale spatio- temporal variability. There were complex changes in the detailed functioning of the microbial ecosystem which are being evaluated. There was a large reservoir of DOP in the photic zone (50-400 nM), with 15-25 nM of parti¬culate P and <5 nM of DIP. Only 17% of the DOP was UV-labile. It is now hypothesized that DOP may be important in providing the P substrate nee¬ded to remove excess nitrate from the surface waters between winter and ear¬ly summer and thus drive the system towards co-limitation.