Comparing different methods of calculating volume-weighted hypolimnetic oxygen (VWHO) in lakes

Abstract.

In the limnological literature there is no standard method for calculating hypolimnetic volumes using temperature profile data. Consequently, calculations of hypolimnetic dissolved oxygen concentrations differ based on the method chosen. This study examined the differences in calculations of volume-weighted hypolimnetic oxygen (VWHO) concentrations based on different methodologies and profile sampling resolution. Comparisons of VWHO values indicated that differences among methods were greatest in smaller lakes with less hypolimnetic volume. VWHO calculation methodology based on 1 m resolution for profile sampling, where the hypolimnion is defined as the strata below the lower limit of the thermocline (temperature change < 1°C m⁻¹), produced lower VWHO values compared to other methodologies using coarser (2m) sample resolution and hypolimnetic volume determination based on visually inspecting inflection points of a temperature profile. Differences in calculated VWHO were surprisingly large, highlighting the need for researchers to standardize methodology and sampling resolution used in VWHO calculations. VWHO calculation using a 1 m sampling resolution that defines the hypolimnion as the lower limit of the thermocline is the preferred methodology when VWHO is used as a water quality parameter in assessing the habitat of lake biota sensitive to low VWHO.