Nutrient utilisation and particulate organic matter changes during summer in the upper mixed layer (Ross Sea, Antarctica)

The relationships among vertical stability, estimated nutrient utilisation and particulate organic matter in the Ross Sea are analysed from data collected during two cruises in the summers of 1987–1988 and 1989–1990. In the upper mixed layer (UML), identified through the vertical stability E(Z(UML)), nutrient consumption is calculated as the difference between the “diluted” nutrient value and the mean calculated from the integrated value in the UML. The nutrient utilisation ratio and E(Z(UML)) are linearly related for E(Z(UML))≤25, whereas for values > 25, the distribution pattern is more scattered and independent of E(Z(UML)). For E(Z(UML))≥25, utilisation values were ≥4, 0.4 and 10 mmol m−3 for nitrate, phosphate and silicate, respectively. Significant relationships between nutrient depletion and both particulate organic carbon (POC) and particulate protein/particulate carbohydrate ratios (PPRT/PCHO) are found. The analysis of particulate matter distribution vs nutrient utilisation shows that the stations could be divided into two groups having different characteristics. The first group includes coastal stations, where high nutrient utilisation, POC and PPRT/PCHO are typical of areas with high production. In the second group (pelagic stations), nutrient utilisation, POC and PPRT/PCHO are lower. The vertical stability can be used to discriminate among the factors that influence primary production.