Nutrient limited does not mean that there is not enough nitrogen or phosphorus in the water or that water quality is good. Nutrient limited only indicates that the two nutrients are out of the appropriate balance (generally a nitrogen to phosphorus ratio of 16:1). Phosphorus limitation occurs when there is proportionally less phosphorus than nitrogen, i.e. there is excess nitrogen. Phosphorus limitation occurs in some locations in the spring when lots of nitrogen is available from stormwater flow. Nitrogen limitation occurs when there is proportionally less nitrogen than phosphorus, i.e. there is excess phosphorus. Nitrogen limitation often happens in the summer and fall when stormwater flows are lower (so less nitrogen is being added to the water) and some of the nitrogen has been used up by phytoplankton growth during the spring. If both nutrients are available in amounts well in excess of the needs of the phytoplankton, the system is nutrient saturated. Phytoplankton may also be nutrient saturated when light levels are too low. Phytoplankton growth is often nutrient saturated in the winter when light levels are low (due to high run off of sediments) and water temperatures are low (which slow phytoplankton growth). Phytoplankton growth is also usually nutrient saturated in upstream areas where conditions are naturally more turbid (low light levels) throughout the year.