High‐resolution biogeochemical investigation of the Ross Sea, Antarctica, during the AESOPS (U. S. JGOFS) Program

Abstract

The results of high‐resolution biogeochemical measurements in the upper 200 m of the Ross Sea, Antarctica, obtained during the AESOPS (U. S. JGOFS) program using the Lamont Pumping SeaSoar (LPS) are presented. They consist of three west‐east transects from 170°E to 180° longitude along the AESOPS study line at 76.5°S and three short north‐south transects in the Ross Sea polynya during the initial and maturing stages of phytoplankton blooms in the austral spring and early summer of 1997. The LPS carried an in situ instrument array for measurement of temperature, salinity, fluorescence, photosynthetically active radiation (PAR), and dissolved oxygen. In addition, a high‐pressure pump mounted aboard the LPS fish delivered a continuous seawater sample stream to the shipboard laboratory for high‐speed analysis of its nutrient (nitrate plus nitrate, phosphate, and silicate) and total CO2 concentrations and CO2 partial pressure (PCO2). Vertical resolution of this sampling equaled or exceeded that of hydrostation‐style conductivity‐temperature‐depth (CTD) casts; horizontal resolution (nominally equal to a vertical cast every 3–5 km) exceeded station resolution by a factor of 10. While not perfectly synoptic, the 20‐hour duration of these transects is far shorter than the time typically taken to complete the line with conventional sampling methods. These surveys clearly identified three distinct deep water masses below about 100 m: High‐Salinity Shelf Water (HSSW) in the western end of the transects, Modified Circumpolar Deep Water (MCDW) in the middle of the transects, and Low‐Salinity Shelf Water (LSSW) to the east. The regions to the west were characterized by high biological productivity with high N:P and C:P uptake ratios, but little Si uptake, indicating that the production was dominated by Phaeocystis. To the east, biological productivity was lower than in the west, and low N:P and C:P uptake ratios and high Si uptake indicated the dominance of diatoms. The difference in uptake ratios appears to be entirely due to anomalously high P uptake by diatoms; N:C uptake ratios are similar in the two regions and very near canonical Redfield stoichiometry. The area in the center of the transects was characterized by high stratification and low, diatom‐dominated productivity; the reason for this low productivity is unclear but is speculated to be due to the short time which this water has been exposed to an ice‐free surface. The presence of strong variability at horizontal length scales of order 10 km is evident in nearly all fields especially in upper 100 m, although many of the features are resolved only by two or three LPS tracks (each separated by only a few kilometers). Physical, bio‐optical, and chemical variability is observed to extend vertically from sea surface to depths as deep as 140 m far below the 1% light level and the mixed layer depth. This may be attributed to downwelling of waters associated with eddies and/or meandering. A simple statistical measure presented to quantify the errors associated with the undersampling of such a highly variable field shows that biogeochemically important parameters like PCO2 and oxygen and chlorophyll concentrations are poorly resolved by a commonly used 50‐km hydro‐station spacing. Longitudinal averages of these parameters, however, are predicted fairly well at coarser resolutions.