The horizontal distribution of surface chlorophyll is continuous, with discontinuities associated with thermal gradients. Variations in chlorophyll levels are small in hydrographically uniform areas, and its distribution becomes more variable in hydrographically complex situations such as areas influenced by land run-off, strong tidal action and upwelled waters. Continuous measurements of chlorophyll and other biologically affected parameters are most useful in designing sampling programmes, relating discrete samples to the distribution of parameters in the field, or locating positions for special studies.