Seasonal evolution of hydrographic properties in the Ross Sea, Antarctica, 1996–1997

Abstract

This paper briefly describes hydrographic and chemical results from four seasonal process cruises in the Joint Global Ocean Flux Study (JGOFS) Antarctic Environment and Southern Ocean Process Study (AESOPS) in the Ross Sea. The data include temperature, salinity, oxygen, nutrients (NO 3 −, NO 2 −, NH 4 +, PO 4 −3, Si(OH) 4), titration alkalinity (TA), and total inorganic CO 2 (TCO 2). In early spring (mid-October to early November 1996) ice cover was near 100%. The water column exhibited only small ranges of potential temperature, salinity, nutrients, TA and TCO 2. These nearly uniform conditions observed during this cruise were used as initial conditions from which to evaluate seasonal changes in biogeochemical properties. Later in the spring (November/December) of the following year (1997), an expanded polynya was present. In the summer (January) 1997, the sea-ice cover was minimal. Meltwater dilution and warming of exposed surface waters were at their maximum. Finally, in early austral autumn (April 1997) rapid cooling and freezing of surface waters and intensified vertical mixing of the water column resulted in a return toward winter conditions, but with significant depletion of nutrients still evident in surface waters. Modified circumpolar deep water (MCDW) was observed at ∼175°E., varying in location zonally with time. Beneath the MCDW, near the eastern end of the section at ∼180°, northward intrusion of ice shelf water (ISW) was persistent though variable. Among the biogeochemical changes observed along this section were: (1) Significant ammonium accretion at about 100 m; (2) appreciable nitrate drawdowns throughout the upper 20–40 m of the water column; (3) silicic acid depletions during the summer in the surface waters, which were largest at the western end of the study area; (4) an increase in TA largely due to the loss of NO 3 −; and (5) decreases in TCO 2 close to expected values for consumption of carbon and nitrogen in “Redfield” proportions. Consistent with previous observations, N, P and Si never approached limiting concentrations during any of the cruises.