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.

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Microstructure measurements along the axes of Monterey and Soquel Submarine Canyons reveal 200–300-m-thick well-stratified turbulent near-bottom layers with average turbulent kinetic energy dissipation rates 〈ɛ〉 = 4 × 10−8 W kg−1 and eddy diffusivities K = 16 × 10−4 m2 s−1 (assuming mixing efficiency γ = 0.2) to at least thalweg depths of 1200 m. Turbulent dissipation rates are an order of magnitude lower in overlying waters, whereas buoyancy frequencies are only 25% higher. Well-mixed bottom boundary layer thicknesses hN are an order of magnitude thinner than the stratified turbulent layer (hN ≪ hɛ). Because well-stratified turbulent layers are commonly observed above slopes, arguments that mixing efficiency should be reduced on sloping boundaries do not hold in cases of energetic internal-wave generation or interaction with topography. An advective–diffusive balance is used to infer velocities and transports, predicting horizontal upslope flows of 10–50 m day−1. Extrapolating this estimate globally suggests that canyon turbulence may contribute 2–3 times as much diapycnal transport to the World Ocean as interior mixing. The upcanyon turbulence-driven transports are not uniform, and the resulting upslope convergences will drive exchange between the turbulent layer and more quiescent interior. Predicted density surfaces of detrainment and entrainment are consistent with observed isopycnals of intermediate nepheloid and clear layers. These data demonstrate that turbulent mixing dynamics on sloping topography are fundamentally 2D or 3D in the ocean, so they cannot be accurately described by 1D models.

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