Determining the main driver of hypoxia potential in freshwater inland lakes

Abstract

This study aimed to determine the potential for hypoxia in inland freshwater lakes via examining water quality variables, mainly dissolved oxygen (DO). To this end, field studies including environmental surveying and water column sampling at six stations along two routes in Lake Zarivar, Iran, together with laboratory analysis were conducted. Then, a stepwise multivariate regression analysis was employed to identify existing patterns and effects among variables. To secure the evolved model generalizability, the range of data was adjusted using available measurements from several similar lakes/wetlands with respect to their climate and sampling season. Finally, the evolved models’ reliability was examined using validation samples as well as a comparative performance appraisal with a benchmark model attained by the least squares regression technique. The results indicated that NH4 and pH have the most significant effect on DO. The greater amount of DO on the lake surface, i.e., wind effect, and its effect on both nitrification and ammonification processes ensure the absence of hypoxia in surface layers. Therefore, in water column monitoring programs, measuring NH4 and pH would be sufficient. Our results also demonstrated that the potential of hypoxia increases if the amount of NH4 increases in the lower layers of the water column despite releasing DO with nitrate. Strong relations between DO, as the main factor of hypoxia, with NH4 and pH, gives us the opportunity to predict the hypoxia potential and its level. Using time delay between nitrification and ammonification processes, one may predict the changes in the amount of oxygen from the changes in nitrogen.