Estuarbte, Coastal and Shelf Science 0982) 14, 589-598 Observations on the Surface Water Characteristics in the Western Irish Sea: July I977 J. Beardall,* P. Foster, D. Voltolina ~tarine Science Laboratories, J~lenai Bridge, Gwynedd, Wales, U.K. and G. Savidge Marine 13iology Station, Portaferry, N. Ireland, U.K. Rcccived 29 ffune 1981 and in revised form z 4 September 1981 Keywords: Bioassay; Irish Sea; nutrient distributions; phytoplankton; thermal discontinuity 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 pre- dominantly stimulated by nutrient eomplementation. Introduction A part of tile western Irish Sea is characterized by weak tidal energy. This affects the vertical mixing processes in the area, allowing the formation of a seasonal thermocline. The boundary between the stratified and the mixed region is marked by a lateral thermal discontinuity (Simpson, I97x; Simpson & Hunter, x974) which has been the object of continuing studies by the Marine Science Laboratories of the University College of North Wales. On a previous survey of the western Irish Sea front, carried out during tile summer of x974, it was attempted, using a combination of continuous on-line recording and numerical analysis tcchnlques, to explore tile relationships among the physical, chemical and bioa logical variables in the frontal zone. Although both the macro- and micro-variations of the different characteristics were successfully recorded and mapped, the numerical analysis did not generate any consistent relationship amongst the characteristics measut:ed. This was attributed to gradual changes in the properties of the waters at either side of the front, along its length (Foster et al., x976 ). In tile summer of 1977 wc undertook another survey of this area, with the same objectives but with a modified strategy. In particular, more emphasis was given to the characteristics of ~ address: Department of Biological Sciences, University of Dundee, Dundee DD/ 4HN, Scotland, U.K. 589 oa7z--77x,tlS21o6o~80+ xo $o't.oo/o ~) Io82 Academic Press Inc. (London'l Limited