Transects of a region of the North Atlantic, conducted in spring 1997, provide simultaneous measurements of a number of biophysical variables. These data are used to quantify the degree of spatial heterogeneity in the horizontal distributions of zooplankton, phytoplankton and potential temperature. More specifically, the spectral slope is calculated for the distribution of chlorophyll a, potential temperature and individual size-classes of zooplankton. A comparison of the estimated slopes provides the first evidence, from continuous measurements, that the gradient of zooplankton spectra may vary according to the size of the organism and, furthermore, that it may be steeper than that for phytoplankton both as an individual size class (500–1000 μm) and as an ensemble (250–2000 μm). The latter result is the exact opposite of conventional wisdom founded on previous observations. Our results highlight significant deficiencies in current models for plankton patchiness.