Effect of Water Column Stability on Surface Chlorophyll and Time Lags under Different Nutrient Backgrounds in a Deep Reservoir

Min Zhang,Qinghua Cai,Yaoyang Xu,Xiaodong Qu,Zhipan Niu

doi:10.3390/w11071504

Abstract

Hydrodynamic conditions are considered to be very important in the control of algal blooms. Weekly or daily measurements may miss some important events in the hydrodynamic process, resulting in inaccurate evaluations of the impacts of hydrodynamics on phytoplankton. In this study, high-frequency (15-min interval) measurements were used to analyze the effect of water column stability on surface chlorophyll a (Chl a) and lag time under different nutrient backgrounds during a cyanobacterial bloom in the Three Gorges Reservoir, China. Cross-correlation analysis between the relative water column stability (RWCS) and Chl a was performed at different stages. The results showed that the RWCS above the euphotic depth influenced the surface Chl a concentration most significantly. A lower RWCS (<20) limited the increase in the Chl a concentration, and a higher RWCS caused a significant increase in Chl a only when nutrients were not limited (TN/TP < 29) and light and temperature conditions were suitable. It took a short time for a higher RWCS to significantly increase the surface Chl a concentration compared with a lower RWCS. When the waterbody had a very low Chl a concentration (almost 0), approximately 2 days were needed to significantly increase the Chl a concentration, while approximately only half an hour was needed when the background concentration of Chl a was slightly higher. During the bloom period, a decline in the RWCS significantly decreased the Chl a in a very short time (approximately half an hour). Reducing the water column stability could be a good approach to control cyanobacterial blooms.

Highlights

Environmental factors such as nutrients, light, and hydrodynamic conditions [1,2,3] impact the general composition of phytoplankton through their synergistic effects
Because the effects the relative water column stability (RWCS) above the 3-the and3-m water columns usually theand mostChl a, prominent, we showed theofrelationships between and Chlwere a, 5-m prominent, we showed the relationships between the 3-m RWCS and chlorophyll a (Chl a), 5-m RWCS and Chl a, and 12-m RWCS and Chl a in different stages (S1–S4 and the whole period (All)) (Figure 7)
In S1 and S3, the surface Chl a concentration was most significantly influenced by the stability of the water column above 5 m, which concentration was most significantly influenced by the stability of the water column above 5 m, which helped us to conclude that only the water stability in the upper water column affects the algal blooms

Summary

Introduction

Environmental factors such as nutrients, light, and hydrodynamic conditions [1,2,3] impact the general composition of phytoplankton through their synergistic effects. High concentrations of nutrients are critical for the formation of algal blooms but are not always the determining factor [4]. Algal blooms always occur or reach a peak in response to increased water temperature and the presence of stratification [7,8], especially. Some studies have reported that blooms do not break out in the littoral zone with high levels of nutrients, but rather in the pelagic zone with weak water disturbance [13]. These researchers concluded that nutrients in the water column will not stimulate strong algal blooms unless vertical diffusivity is very low

Methods

Results

Discussion

Conclusion