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Fm Page 22 Thursday, May 10, 2007 8:38 PM WILHELMINA R. 5 2 5 10 20 50 100 200 Particle size D (µm) Figure 10 Bias in scattering, γb (A, C), and backscattering, γbb (B, D). Results were derived as in Figure 9. Thick grey lines indicate where γ = 1. For the analysis conducted here two common PSDs previously used in other studies of IOPs are employed: 1. The power-law PSD (also referred to as the Junge-like PSD) is given by:  0, if D < Dmin or D > Dmax ;  f D = [ # m −3µm −1 ], −ξ D   n0  D0  , if Dmin ≤ D ≤ Dmax ( ) (27) where D0 [µm] is a reference diameter and n0 [ # m −3 µm −1 ] is the differential number concentration at the reference diameter.

This behaviour is similar to Qc for a polydispersion of spheres. However, the asymptotic value found in those studies was not always two, probably a consequence of the variety of means by which the ‘size’ of the particles of interest has been determined in the various studies (Aas 1984). , Kerker 1969). The resonance pattern in the VSF associated with a monodispersion of spheres disappears for even tightly sorted natural particles, in a similar manner as a polydispersion of spheres (Figure 16A,C).

Similarly, when microscopy is used in sizing, it is difficult to estimate the third (usually smaller) dimension. Thus, there are uncertainties in both measurements and modelling of IOPs of non-spherical particles. Several studies provide evidence that aquatic particles in general, and nonspherical particles in particular, are not optically equivalent to spheres. Kadyshevich (1977), Voss & Fry (1984) and Volten et al. (1998) measured the VSF and polarised scattering characteristics of oceanic samples and phytoplankton cultures, respectively, and found that the polarisation characteristics of scattering are not consistent with spherical particles.