Selective depressions of surface silicic acid within cyclonic mesoscale eddies in the oligotrophic western North Pacific

To reveal spatial dynamics of silicic acid [Si(OH)(4)1 in the poorly sampled oligotrophic western North Pacific, we investigated the surface distribution of Si(OH)(4) and associated biogeochemical parameters by using an underway survey system with a highly sensitive nutrient analyzer along the 138 degrees E transect (between 30 and 34 degrees N) and the 155 degrees E transect (between 10 and 35 degrees N) during the summers of 2007 and 2008. Surface Si(OH)(4) concentrations ranged from the detection limit (11 nmol L-1) to 2462 nmol L-1. High Si(OH)(4) concentrations ( > 1000 nmol L-1) and dynamic fluctuations were generally observed north of 23 degrees N, while consistently stable low concentrations of 415-751 nmol L-1 were observed south of 23 degrees N. Surface nitrate+nitrite (N+N) and phosphate (PO43-) were typically depleted to < 20 nmol L-1 except for PO43- in the area south of 16 degrees N. The majority of the study area was characterized by high-Si (OH)(4) and low-N+N and POI43-. However, submesoscale/mesoscale depressions of Si(OH)(4) were locally observed in the cyclonic eddy fields north of 23 degrees N. Among a total of six Si(OH)(4) depressions within the eddies, a complete Si(OH)(4) depletion ( < 11 nmol L-1) was observed on the cyclonic side near the Kuroshio axis (33.1 degrees N, 138 degrees E). This depletion was closely coupled with a diatom bloom, suggesting that Si(OH)(4) was exhausted by diatoms. All of the Si(OH)(4) depressions were selective and not accompanied by local depressions of N+N and POI43-. This unique phenomenon might be driven by biogeochemical processes such as selective Si export (Si pump), anomalous Si uptake associated with diatom physiology, and/or Si uptake supported by N-2 fixation. (C) 2014 Elsevier Ltd. All rights reserved.