Microbiological and zooplankton activity at a front in Liverpool Bay

Shallow sea fronts, zones of transition between mixed and stratified waters, are generally characterized by accumulation of biomass1,2. Various mechanisms can be suggested for this accumulation. Pingree et al.1 considered that phytoplankton are limited on the stratified side by nutrient supply and on the mixed side by light intensity, whereas at the front itself, recently stabilized mixed water provides conditions for rapid growth. Adjacent water masses may also complement each other's nutrient deficiencies. At the western Irish Sea front for example, silica has been limiting on the mixed side and combined nitrogen on the stratified side, so that the front supported more algal growth than either water separately2. Another possibility, as fronts are regions of convergence, is advection of suspended matter, which may include detritus, phytoplankton and zooplankton3. This is unlikely to end with the passive accumulation of material at the front: bacterial mineralization of organic detritus will stimulate phytoplankton growth, bacterial and phytoplankton growth will support increased zooplankton activity and this in turn will increase the rate of recycling. Although little investigated, this mechanism seems to be important in a front existing in Liverpool Bay, as we report here.