Surface enrichment of inorganic nutrients in the North Pacific Ocean

Abstract

Vertical profiles of the inorganic nutrients phosphate, silicate and nitrate show frequent surface enrichment (i.e. a sub-surface minimum) throughout the historical California Cooperative Oceanic Fisheries Investigations (CalCOFI) data set and in hydrographic sections across the North Pacific Ocean. Although present in the open ocean, it is especially pronounced in the offshore California Current and near the Hawaiian Islands. Its consistent occurrence over time, space, and methods argues that it is a real feature of the upper ocean ecosystem, rather than a result of some methodological problem. In the California Current, surface enrichment is spatially and temporally rare at nearshore stations. When nearshore enrichment does occur, minimum values are shallow (10–30 m). The subsurface minima become more frequent and deeper (eventually occurring at >100 m) the farther offshore one goes. A similar trend exists alongshore: the minima are deeper and more frequent towards the south. In all data, the enrichment is usually clearest and most frequent in silicate, weakest and least frequent in nitrate. Surface enrichment can contribute up to 100% of a specific inorganic nutrient available above the nutricline. The cause of surface enrichment is unknown: possible explanations include (1) nutrient scavenging and vertical transportation by rising particles or bubbles; (2) Langmuir circulations; (3) nutrient transport by vertically migrating organisms; (4) atmospheric inputs; (5) remineralization related to distributions of organisms with depth; or (6) photochemical processes. Because the three nutrients remineralize at different rates, this feature might be useful as a potential tracer of past or ongoing physical and biological processes in the mixed layer.