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The use of the optical plankton counter (OPC) for processing zooplankton net samples is explored. A comparison of OPC counts and microscope counts from net samples was reasonable. Similarly, OPC biovolume and carbon had a ratio of 23:1 (biovolume;carbon). Coincidence was modelled using the Poisson distribution, and tested with a large diatom Coscinodiscus wailesii. Agreement appeared to he good up to 60 s 1 ; above this, the maximum response rate of the system became limiting.

Euphausiids were identified, measured and then dried during a research cruise to sites in the north-eastern Atlantic Ocean and shelf waters of the Celtic Sea in late summer. The lengths of furcilia larvae and later stages were measured in three ways, and regressions relating each pair of measures were calculated for different developmental stages of each species. Four hundred and ninety-three dried specimens of nine species were weighed and length-weight regressions were calcu- lated. A subset of 187 specimens was selected for analysis of carbon (C) and nitrogen (N) content soon after return to the laboratory. The C and N contents of a further 122 specimens were determined after the specimens had been stored for 9 years. No consistent differences in percentage C and N content were found between the two groups, indicating no systematic change during prolonged storage in desiccators. Calyptopes of Nyctiphanes couchi contained a lower percentage of both C (mean 36.2%) and N (mean 8.6%) than later stages (means: 40.6% C, 11.0% N). Carbon content and C:N ratios of a group of specimens of Thysanoessa longicaudata, including some juveniles, adult males and adult females, were distinctly higher (>46% and >5.0, respectively) than those of the majority of specimens of the same species.