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Figure 4.13 Three steps to produce a zooplankton biomass size distribution from a) an image of zooplankton. The image is adjusted to a standard level b), and the areas of the blobs are determined and converted to equivalent spherical diameter (Area=rcr2) and displayed as a frequency histogram c) as numbers of bugs per mL of concentrated sample.

on the screen are counted and sized. A critical stage in this analysis is to optimise the appropriate sub-sample and dilutions, to prevent too much co-incidence and yet to have reasonable number of counts per grab. The actual particle concentration of each size category is determined by multiplying up from the sub-sample volume and the actual volume filtered.

The intercept and slope (negative) of the log-based biomass size distribution is a useful parameter of the plankton population dynamics (see Section 3.7). For analysis, any particular size intervals may be used, beginning at around three times larger than the net's mesh size. This is because many zooplankton species are shaped like an oblate spheroid, so that the smallest equivalent spherical diameter fully sampled by a certain mesh size is around three fold larger. Any size classes - linear or logarithmic -may be used, providing one converts the biomass to 'normalised biomass size spectrum' (NBSS, dividing the biomass concentration (mg.m-3) of each size class by the biomass size interval). For smaller estuarine zooplankton with 100 ^m mesh, we use 24 size limits set at 182 to 402 (that is, size intervals at 324, 361 etc to 1600 ^m equivalent spherical diameter).

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