Hydrography, nutrients and chlorophyll during El Niño and La Niña 1997-99 winters in the Gulf of the Farallones, California

摘要

Nutrient and chlorophyll concentrations were measured in January 1997, 1998 and 1999 in the Gulf of the Farallones, CA at locations stretching north/south from Point Reyes to Half Moon Bay, and seaward from the Golden Gate to theFarallon Islands. The cruises were all carried out during periods of high river flow, but under different climatologicalconditions with 1997 conditions described as relatively typical or ‘neutral/normal’, compared to the El Niño warmerwater temperatures in 1998, and the cooler La Niña conditions in 1999. Near-shore sea-surface temperatures ranged from cold (9.5–10.5°C) during La Niña 1999, to average (11–13°C) during 1997 to warm (13.5–15°C) during El Niño 1998. Nutrients are supplied to the Gulf of the Farallones both from San Francisco Bay (SFB) and from oceanic sources, e.g. coastal upwelling near Point Reyes. Nutrient supplies are strongly influenced by the seasonal cycle of fall calms,with storms (commencing in January), and the spring transition to high pressure and northerly upwelling favorablewinds. The major effect of El Niño and La Niña climatic conditions was to modulate the relative contribution of SFB to nutrient concentrations in the coastal waters of the Gulf of the Farallones; this was intensified during the El Niño winter and reduced during La Niña. During January 1998 (El Niño) the oceanic water was warm and had low orundetectable nitrate, that did not reach the coast. Instead, SFB dominated the supply of nutrients to the coastal waters Additionally, these data indicate that silicate may be a good tracker of SFB water. In January, delta outflow into SFBproduces low salinity, high silicate, high nitrate water that exits the bay at the Golden Gate and is advected northwardalong the coast. This occurred in both 1997 and 1998. However during January 1999, a La Niña, this SFB feature wasreduced and the near-shore water was more characteristic of high salinity oceanic water penetrated all the way to thecoast and was cold (10°C) and nutrient rich (16 μM NO3, 30 μM Si(OH)4). January chlorophyll concentrations rangedfrom 1–1.5 μg l 1 in all years with the highest values measured in 1999 (2.5–3 μg l 1) as a result of elevated nutrientsin the area. The impact of climatic conditions on chlorophyll concentrations was not as pronounced as might be expectedfrom the high temperatures and low nutrient concentrations measured offshore during El Niño due to the sustainedsupply of nutrients from the Bay supporting continued primary production.