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A manual method for the quantitative determination of urea in seawater was developed based on the reaction of urea with diacetylmonoxime. Critical factors include the treatment of collection bottles, sample storage, pre-filtration techniques, water purification, reaction temperature and reproducible cooling of samples during analysis. The limit of detection was 0·14 μg-at urea-N 1 −1 with Beer's Law being obeyed in the range tested of 0–15 μg-at urea-N 1 −1 . The precision (±1 SD) of replicate samples of 1,2 and 15 μg-at urea-N 1 -1 was 0·024, 0·019 and 0·03 μg-at urea-N 1 −1 ( n = 10) respectively.

The variability of primary productivity in surface waters was continuously recorded during spatial and sectional surveys of the Irish Sea in the summer of 1977. Localized patches of intense productivity were found, not in the frontal region of the western Irish Sea, but farther east. Subsurface distributions of physical, chemical and biological characteristics through a latitudinal section of the Irish Sea are reported. These data showed that the productivity peaks occurred at the boundaries of a water type exhibiting properties uncharacteristic of Irish Sea waters.

Distributions of physical, chemical and biological characteristics observed in the frontal region of the western Irish Sea during July 1977 are presented. High chlorophyll concentrations and phytoplankton cell numbers were associated with the lateral thermal discontinuity. The characteristics of the stratified and mixed waters varied along the length of the front, such that the changes recorded across each frontal transect were unique. The qualitative composition of the diatom population on either side of the front, however, was distinctive. Observations, concentrated on one transect, showed that the diatom population at the front was a mixture of the populations on either side, and that the high phytoplankton standing crop there was predominantly stimulated by nutrient complementation.