When designing a water system, it is important to consider the turnover rate. The turnover rates are affected by factors such as volume and residence time of the unit in which they occur. When tanks have long periods between refillings or ponds take longer for all organisms living within them to be filtered out than with shorter-term units like raceways, this leads to higher levels of nutrient concentrations that can lead to pollution problems due their current design limitations on how much waste material these systems can tolerate before eventually reaching critical loads where nutrients will start cycling back into waterways leading towards eutrophication.
The term "turnover" refers not only just an increase in numbers but also means 'real water exchange'.