Carbon, nitrogen and phosphorus budgets within a mesoscale eddy: comparison of mass balance with in vitro determinations

A comparison is made of in vitro estimates of nitrate, ammonium and phosphate uptake with changes in ambient nutrient concentrations within a mesoscale eddy in the northeast Atlantic in June 1996. The stability of the eddy resulted in greatly reduced exchange with adjacent water masses, and hence a mass balance calculation was possible for these three nutrients. Over a 5-day period, there was a combined decline in nitrate and ammonium concentrations of 2.22 mmol N m−3. In vitro estimates of nitrate and ammonium assimilation were estimated to be 1.69 mmol N m−3, i.e. the in vitro estimates are 76% of the observed decrease in nitrate plus ammonium. In the case of phosphate, in vitro experiments estimated phosphate uptake to be 0.092 mmol P m−3 over 3 days, and the measured change in dissolved phosphate concentration in the surface 12 m over the same period was 0.12 mmol P m−3, i.e. the in vitro determinations were 77% of the observed changes in phosphate concentration. Thus, this study provides evidence that the two approaches of a mass balance calculation and an in vitro determination give similar answers, within a factor of 0.3, to the estimation of nutrient utilisation in the surface mixed layer.