OBSERVATIONS OF THE INFLUENCE OF DIURNAL CONVECTION ON UPPER OCEAN DISSOLVED-GAS MEASUREMENTS

An important example of the interaction between biological productivit y and near-surface oceanography is the role of nocturnal convection an d diurnal restratification in modifying the environment in which photo synthetic activity takes place. In situ time series measurements of di ssolved oxygen reveal the effects of photosynthetic activity, respirat ion and redistribution by mixing. Moored thermistor time series and fr equent CTD casts show that restratification during the day is confined to a warmer shallow surface layer where most of the biological produc tion is expected to occur. The depth and rate of mixing is measured wi th neutrally buoyant floats which track the vertical excursions of con vecting water parcels. Early in the evening, at the onset of night tim e convection, this warm oxygenated water is mixed down and diluted by deeper less oxygenated water. The interpretation of oxygen time series at specified depths (here 21 m and 30 m) requires knowledge of this m ixing process. Use is made of in situ dissolved nitrogen time series t o infer that gas transfer at the surface is of secondary importance in determining the diurnal dissolved oxygen budget. A qualitative couple d biological/oceanographic model of the data is presented and discusse d. It is concluded that a serious overestimate of daily oxygen product ion can result from excluding diurnal convection from the interpretati on of oxygen time series.