Nutrient limitation of phytoplankton growth in central plains reservoirs, USA

Historically, phosphorus has been considered to be the primary nutrient limiting phytoplankton growth in freshwater ecosystems. We tested this hypothesis in 19 Kansas reservoirs located within the Central Plains region, USA. Bioassay experiments were conducted to assess growth-rate limitation by nitrogen (N), phosphorus (P) and nitrogen and phosphorus (N and P). The addition of P alone rarely increased phytoplankton growth with only 8% of the total bioassay experiments indicating P-limitation. In contrast, limitation by N (16%) and co-limitation by N and P (63%) were more commonly observed. Results from the bioassay experiments were also used to test the hypothesis that nutrient limitation could be inferred from the water column total nitrogen : total phosphorus (TN : TP) ratio. We found that there was a classification range of TN : TP ratios that correctly predicted nutrient limitation in the majority of reservoirs. Generally, reservoirs that were N limited had water column TN : TP ratios <18 (molar); reservoirs that were co-limited by N and P had water column TN : TP ratios between 20 and 46; and reservoirs that were P limited had water column TN : TP ratios >65. Overall, these results suggest that management efforts should focus on both N and P decreases to control phytoplankton growth. Furthermore, the water column TN : TP ratio can be an effective tool for assessing potential nutrient limitation in the Central Plains region using the TN : TP classification values provided above.