Characterization of water quality in stratified nursery recycling irrigation reservoirs

• Vertical water quality variation characterized in recycling irrigation reservoirs (RIRs).
• Water quality variation associated with thermal stratification in RIRs.
• Implications of water quality variation for agricultural water management discussed.
• Support pH as an indicator for water quality monitoring in RIRs.

Recycling irrigation reservoirs (RIRs) are an emerging aquatic ecosystem of global significance. Building upon the recent discovery of thermal stratification in these novel systems and its impacts on the vertical distribution of dissolved oxygen and pH, this study investigated the dynamics of chlorophyll a (CHLA), oxidation reduction potential (ORP), electrical conductivity (EC), and turbidity in the water columns of eight RIRs in the Mid-Atlantic region in USA over a 3-year period from 2011 to 2014. The vertical distribution of CHLA was associated with thermal stratification while ORP and EC were directly affected by thermal stratification. Specifically, CHLA concentration was greatest in the middle thermocline, followed by lower hypolimnion during the stratification period. ORP level was higher at the surface than bottom with differences up to 505.67 mv in Reservoir VA23. EC level was higher in the bottom than at the surface with the maximum difference of 0.90 dS m−1 in Reservoir VA22. The top–bottom ORP and EC differences increased when water was stratified. Turbidity was indirectly affected by stratification. The correlation of turbidity with total precipitation was stronger during the non-stratification period from November to March than during the stratification period from April to October, mainly due to restriction on water mixing. The characterization of these additional water quality parameters in the water columns of RIRs provides fundamental knowledge about this novel water system and will help in developing sound water quality management practices for improved crop health and production while reducing agriculture's environmental footprint. This study also provides additional evidence supporting pH as a significant indicator for monitoring water quality in RIRs.