Spatial and temporal variabilities of nutrient limitation based on in situ experiments of nutrient enrichment...

In situ experiments of Nutrient Enrichment Bioassays (NEBs) were performed in a morphologically complex reservoir of a temperate region to identify primary limiting nutrient regulating phytoplankton productivity and determine a severeity of the nutrient limitation. Absolute nutrient contents and nutrient ratios of TN:TP and TDN:TDP in the ambient water indicated a potential limitation of phosphorus. This outcome agreed with the results of NEBs; Algal response in the P treatments, expressed as a ratio of CHLf:CHLi, showed significantly (p < 0.05) greater algal response than P + NH4-N or P + NO3-N treatments. The magnitude of the limitation, however, showed large spatial and temporal variations. The response in treatments enriched with phosphorus (P, P + NH4-N, and P + NO3-N) was greatest in the downlake zone and least in the point-source zone, while the response was greater during summer monsoon than any other seasons. Algal growth rate experiments showed that the response in treatments enriched with NO3-N and P + NO3-N never exceeded > 0.50 microg L(-1) per day and was significantly (p < 0.05) less than that in the three treatments with P (1P, 2P, and P + NH4-N), indicating a reduced response in the treatments enriched with NO3-N. The suppression of nitrate on algal growth seemed to be involved with inhibition effect of nitrogenase activity through low-nitrate uptake rate in the high nitrate-rich environment and large additions of potassium with phosphorus in spike. Regression analysis of log10-transformed CHLf:CHLi ratios against ambient nutrient contents showed that in situ algal response in the P treatments was inversely related (r = -0.77; p < 0.001) with ambient TP and positively correlated (r = 0.75; p < 0.001) with TN:TP ratios. This result suggests that in situ response depends largely on external loading of limiting nutrients.