Geographic variations in the relationship between silicic acid and nitrate in the South Pacific Ocean

Abstract

Scatter plots of silicic acid vs nitrate concentration from stations in the South Pacific Ocean and the Pacific Sector of the Southern Ocean divide the area into zones of nutrient balance, excess silicic acid, or excess nitrate. The boundaries of the zones are similar to published surface-water mass maps. A T-S analysis of stations divided on the basis of nutrient character show that: (1) South Pacific Central Water is in nutrient balance; (2) Subantarctic Surface Water has actual excess nitrate and Antarctic Circumpolar Water (Surface) has potential excess nitrate; and (3) Ross Sea Water exhibits potential excess silicic acid. The actual nitrate excess in Subantarctic Surface Water is attributed to the circumpolar character of the water mass, to the extensive diatom growth in southern source water, and to the different mechanisms of regeneration of silicic acid and nitrate. The nutrient character of this water mass probably effects food web dynamics in its resident area and in coastal areas on which it impinges. The potential silicic acid excess of Ross Sea Water is attributed to the upwelling of silicic acid-enriched deep water at the Antarctic Divergence (≈ 70°S), which probably contributes to excess silicic acid in the North Pacific Ocean through Antarctic Bottom Water formation.