Based on density differences usually due to temperature resulting from atmospheric heat exchange and differential light pentration. Type of lake will determine when and where to sample. Stratification limits water exchange dividing the lake into two or more semi-isolated systems. Restricted flow of materials has tremendous biological consequences. In lakes, suspended particles are usually phytoplankton.
Measurements reflecting phytoplankton dynamics are extremely important in characterizing lakes.
And since phytoplankton production is a function largely of nutrient availabity and retention time, we have come full circle. Retention time is critical when understanding lake processes. In eutrophic lakes, the probability that use of the lake will be impaired is significant. Nitrogen availability limits plant growth in these situations. Pathogens - Fecal coliform bacteria 3.