Silica production in the Monterey, California, upwelling system

Abstract

Silica cycling was examined during a major upwelling event in Monterey Bay, California. Strong upwelling-favorable winds blew for 6 d with speeds up to 15 m s-1 just prior to the study. A survey of the region near the end of the wind event showed newly upwelled water at the north end of the bay, with silicic acid concentrations up to 29.8 μM. Silicic acid concentrations decreased to a minimum of 15 μM along the upwelling plume. Biogenic silica concentrations in the upwelling plume were generally between 2 and 5 μmol Si liter-1. Specific rates of biogenic silica production were <0.2 d-1 in the freshly upwelled waters and increased to > 1.0 d-1 downplume. Kinetic experiments indicated that silicic acid concentrations throughout the upwelling plume supported maximal rates of silica production. Silica production rates were ~ 1 μmol Si liter-1 d-1 at the upwelling source, increasing to 7 μmol liter-1 d-1 downplume. The upwelling event was followed by several days of calm winds, creating ideal conditions for a phytoplankton bloom. Integrated biogenic silica concentrations between the surface and the 0.1% light depth during the calm period ranged from 56 to 566 mmol Si m-2, with 8 of 11 stations exhibiting concentrations > 100 mmol Si m-2. Specific production rates of biogenic silica were generally > 1 d-1, with production rates between 10 and 30 μmol Si liter-1 d-1. Integrated silica production rates averaged 205 mmol Si m-2 d-1 (range 13-1, 140 mmol m-2 d-1), which is four times greater than the average rate observed for other coastal upwelling systems. The maximum value observed (1,140 mmol m-2 d-1) is nearly four times greater than levels ever observed before in the sea. The high silica production rates seemed to result from an inefficient silicate pump. On average, 72% of the biogenic silica produced in the upwelling plume was retained in the surface waters, resulting in biogenic silica concentrations of 6.7-13.7 μmol Si liter-1 at stations where integrated production rates were > 200 mmol Si m-2 d-1. Ambient silicic acid concentrations in these same waters were generally > 8 μM. Kinetic studies showed that these silicic acid concentrations supported nearly maximal rates of silica production. Substrate limitation of silica production became readily detectable at 5 μM Si(OH)4. By that time, 80 to >90% of the silicic acid and ~90% of the nitrate in the upwelled waters had been consumed, indicating that substrate limitation of silica production played only a minor role in controlling the magnitude of both net silica production and new production by diatoms.