 The intensity of a thin layer refers to the maximum concentration of the plankton within the layer relative to the background and the water column.Thin layer concentrations can range between three and 100 times more than the background and up to 75% of the total biomass in the water column.Thin layers of non-motile phytoplankton tend to collect at boundaries of strong vertical gradients in salinity (haloclines), temperature (thermoclines), and density (pycnoclines) which often coincide because they are directly proportional.