Silica production in the Monterey, California, upwelling; system

Silica cycling was examined during a major upwelling event in Monterey Bay, California. Strong upwcllingfavorable winds blew for 6 d with speeds up to 15 m s ’ 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 PM. Silicic acid concentrations decreased to a minimum of 15 @/I along the upwelling plume. Biogenic silica concentrations in the upwelling plume were generally between 2 and 5 pmol Si liter -I. Specific rates of biogenic silica production were CO.2 d-l in the freshly upwelled waters ald increased to > 1.0 d-l downplume. Kinetic experiments indicated that silicic acid concentrations throughout the upwelling plume supported maximal rates of silica production. Silica production rates were -1 pmol Si liter-’ tl I at the upwelling source, increasing to 7 pmol liter-’ dI 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 m2, w th 8 of 11 stations exhibiting concentrations > 100 mmol Si mP2. Specific production rates of biogenic silica wl:re generally > 1 d I, with production rates between 10 and 30 pmol Si literI d I. Integrated silica production rate:) averaged 205 mmol Si m-” d-l (range 13-1,140 mmol m 2 d-l), which is four times greater than the average rate observed for other coastal upwelling systems. The maximum value observed (I, 140 mmol m -2 d-l) 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 pmol Si liter I at stations where integrated production rates were >200 mmol Si rn--’ d-l. Ambient silicic acid concentrations in thcsc same waters were generally >8 PM. Kinetic studies showed that these silicic acid concentrations supported nearly maximal rates of sil ca production. Substrate limitation of silica production became readily detectable at 5 PM Si(OH),. By that time, 80 to >90% of the silicic acid and -90% of the nitrate in the upwellcd waters had been consumed, indicating that substrate limitation of silica production played only a minor role in controlling the magnitude of both net si ica production and new production by diatoms. Coastal upwelling systems support the highest rates of primary and new production in the ocean (Ryther 1969; Eppley and Peterson 1979). The nutrient-rich waters brought to the surface during upwelling events stimulate intense phytoplankton blooms that are initially dominated by diatoms. Unlike other phytoplankton, diatoms require silicon for growth (Lewin 1962), so that the magnitude of diatom blooms and their contribution to new and primary production may be controlled by the amount of silicic acid, Si(OH),, brought to the euphotic zone during upwelling. Upwelled waters off the coast of Peru (15”S), Baja California, and northwest Africa Acknowledgments We thank Linda O’Bryan and the crew of the RV Point SW foi technical assistance. This project was funded through the Laboratory-Directed Research and Development (LDRD) program at the Los Alamos National Laboratory. contain betwee:n 14 and 30 PM Si(OH), and support the highest silica production rates reported in the sea (Nelson et al. 1995). Nitrate (NO, ) is upwelled in approximately a 1 : 1 mole ratio with Si(OH), in those systems (Codispoti et al. 1982; Dugdale and Wilkerson 1989). That ratio is essentially the same as the N : Si mole ratio within nutrient-replete diatoms (Brzezinski 1985), implying that 100% of the new production in coastal upwelling systems could be carried out by diatoms. The actual contribution of diatoms to new production in an upwelling system where physical processes supply equal amounts of NC, and Si(OH), to the euphotic zone depends on the relative rates of recycling of particulate organic nitrogen (PON) and biogenic silica (BSiO,) in the surface waters. The rates of PON and BSiO, remineralization vary widely among upwelling systems. Off Peru, silica dissolution rates have been estimated to be < 10% of silica produc-