Spatial and Temporal Variabilities of Nutrient Limitation Based on In Situ Experiments of Nutrient Enrichment Bioassay

Abstract

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 CHL _f  : CHL _i , 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 µg 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 CHL _f  : CHL _i 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.

Keywords:

• Nutrient limitation
• Nutrient enrichment bioassay
• Seasonality
• Algal growth

Acknowledgment

This research is part of my Ph.D. dissertation in the University of Missouri, Columbia, Missouri, USA.