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