Simple community parameters, such as number of species caught and the species diversity of a planktonic group (in this case Ostracoda), have been found to be stable enough in time and space to detect shifts in these parameters likely to be generated by interannual and decadal variation, especially if sampling is conducted at mesopelagic depths. At this time scale, changes in spatial variability will be expressed most clearly as shifts in zoogeographical boundaries. The clearest boundary to be detected in the North-east Atlantic occurs in the vicinity of 40°N, across which is a rapid equatorwards reduction in the total available nutrient as a result of a reduction in the depth of the mixed layer in winter. The boundary at 40°N is marked in satellite images by a sharp reduction in seasonality of surface chlorophyll concentrations. In the planktonic ostracods there is a sharp decline in both species numbers and diversity on the poleward side to a depth of at least 2000 m. If these changes are related to the degree of seasonality in the production cycle, then they can be expected to be climatically controlled. Monitoring this boundary will give an unambiguous signal of any future climate change, natural or anthropogenic, having a significant biological impact on the oceanic ecosystem and on fluxes of material within the water column.