Nutrients in the Southern Ocean GLOBEC Region: Variations, Water Circulation, and Cycling

Yulia M. Serebrennikova and Kent A. Fanning

As part of the US Southern Ocean Global Ecosystem Dynamics (GLOBEC) study on the West Antarctic Peninsula continental shelf, four survey cruises were conducted along a grid of close-spaced stations lying within an area known as Marguerite Bay and on the adjacent continental margin. The cruises occurred in the autumn and winter of 2001 and 2002. Repeated sampling of the stations on the grid permitted a delineation of several important features and processes. Upper ocean spatial variations were strongest for composite nutrient profiles; deeper waters varied much less by comparison, especially far down the continental slope. Waters in Marguerite Bay and adjacent shelf areas were clearly enriched in ammonia (e.g., 4.5 μm) relative to other parts of the grid, especially in the autumn of 2001. Offshore-onshore variations in nitrate and phosphate were approximately the reverse of those of ammonia while those of silica tended to duplicate the ammonia pattern. These results are consistent with the occurrence of blooms of diatoms away from the coastline. Nearshore mixed-layer ammonia enrichments existed as maxima that were co-located with a coastal current, at least in year 1 of GLOBEC. Analysis of differences in mixed-layer nutrient concentrations and standing stocks indicated that the annual cycles of nitrate, phosphate, and silica appear to be invariant in time, while ammonia showed strong, erratic seasonal and year-to-year declines. An inverse -3 mol:mol relationship between nitrate and ammonia existed throughout all GLOBEC cruises, by comparison, and for an autumn JGOFS Ross Sea study and a Bering Sea study also support the concept of an inverse relationship. Silica was an excellent tracer of upwelling and horizontal injection of offshore waters onto the West Antarctic Peninsula Continental Shelf.

STATUS UPDATE

FINAL VERSION READY (hard copy + electronic version)