The presence of nutrients as well as coastal fronts, eddies, and upwelling zones greatly increase the persistence of thin layers. One of the main criteria for an aggregation of plankton to be considered a thin layer is that the increased concentration at a certain depth of the water column must appear in subsequently measured profiles.
The presence of nutrients, coastal fronts, eddies, and upwelling zones are key factors that contribute to the longevity of thin layers. The criteria for classifying a plankton aggregation as a thin layer involves consistent detection of increased concentration at a specific depth across multiple measurements.