Particle size spectra between 1 μm and 1 cm at Monterey Bay determined using multiple instruments

Particles are responsible for the vertical transport of material in the ocean. Size is an important characteristic of a particle, determining its fall velocity, mass content, scattering cross-section, and food value, as well as other properties. The particle size spectrum describes the distribution of particles in a volume of water as a function of their sizes. We measured particle size spectra in Monterey Bay, CA, using six different instruments that examined particles ranging from approximately 1 μm to 10 min. Before the results could be combined, they had to be adjusted for the different particle properties actually measured. Results from different optical instruments were similar, although the spectral values were sensitive to minor variations in the diameter assigned to particles. Sample volume was crucial in determining the effective upper size limit for the different techniques. We used fractal scaling to piece the results together, deriving fractal dimensions of 2.26-2.36. Diver observations of visible particles showed that they were composed mostly of aggregated diatoms. The particle size spectra n₁ were remarkably well fitted with a power law function n₁ = ad₁^-b₁, where d₁ is the image diameter and b₁=2.96-3.00. The equivalent slopes for particles measured with an aperture impedance instrument were 3.50-3.61. The particle volume distribution showed that most of the particle mass was in the 0.1-3 mm range. This volume distribution is consistent with theories that assume particle sizes are controlled by simultaneous coagulation and disaggregation.