Mechanisms for nutrient delivery to the inner shelf: Observations from the Santa Barbara Channel

Oceanic sources of nutrients to the kelp forests of the Santa Barbara Channel were diagnosed using time series from three moorings in 12- to 17- m water depth. An in situ nitrate autoanalyzer on the moorings provided the first high- resolution time series of nitrate + nitrite ( dissolved inorganic nitrogen, DIN) concentrations for this environment. Measurements between February 2001 and May 2003 show that the major mechanisms that supply DIN to the inner shelf of the Santa Barbara Channel are upwelling, diurnal internal motions, and storm runoff. These supply mechanisms vary in importance seasonally. Upwelling dominates increases of inner- shelf DIN concentration between March and May and accounts for more than half of the annual advective DIN transport to shelf reefs. In summer, baroclinic motions akin to internal waves are an important source of DIN because they occur when surface nutrient concentrations are depleted and other supply mechanisms are inactive. Brief episodes of upwelling become important in late autumn and early winter. DIN inputs from storm runoff, detected as salinity dilution at the moorings and estimated from measurements of stream discharge and nutrient concentration, are significant during winter runoff events.