Convective mixing and high littoral production established systematic errors in the diel oxygen curves of a shallow, eutrophic lake

Abstract

The diel (24‐h) oxygen (O2) curves approach has become a popular method for analyzing gross primary production (GPP) and ecosystem respiration (ER) rates in aquatic systems. Despite the simplicity of this approach, there remain aspects of the calculation and interpretation of diel O2 curves which may skew results, with potentially large implications for estimates of metabolic rates. One common problem in lakes is the occurrence of unexpected changes in O2 concentrations (for instance, increasing overnight O2 concentrations). Such changes have typically been ascribed to the random mixing of pockets of O2. It has thus been suggested that negative GPP or positive ER values should be included in calculations, on the assumption that under‐ and overestimates should occur with equal frequency, and thus cancel each other out. Our data from a shallow, eutrophic lake provided a high share of negative GPP values. We argue that these may have been the result of elevated littoral productivity coupled with convective currents produced by consistent differences in the heating or cooling of littoral and offshore waters. Such phenomena might be common in small, sheltered lakes where the role of mixing by wind is diminished. We conclude that a failure to account for consistent metabolic gradients and periodic convective mixing may lead to a chronic underestimation of metabolic rates in lakes when using the diel O2 curves method.

Under‐ice metabolism in a shallow lake in a cold and arid climate

“…Increased DO in the late evening DO as well as negative GPP and R values may also have resulted from advective transport of DO (cf Staehr et al., ). Brothers, Kazanjian, Köhler, Scharfenberger, and Hilt () also found that littoral production and horizontal advection established systematic errors in the diel DO profiles of a shallow, eutrophic lake, a phenomenon that may explain the observed diurnal asymmetry between DO evolution and PAR in WL.…”

Differential Heating Drives Downslope Flows that Accelerate Mixed‐Layer Warming in Ice‐Covered Waters

“…convection (Farmer, 1975;Jonas et al, 2003;Mironov et al, 2002) and downslope gravity currents, the latter originating from differential heating in the shallows (Kenney, 1996;Kirillin et al, 2015). In lakes, these currents transport heat and biogeochemical tracers between nearshore and the interior basin (Brothers et al, 2017;Encinas Fernández et al, 2016;Farrow, 2004), supporting the biogeochemical connectivity between littoral and pelagic zones. Under-ice gravity currents have been observed, and their transport has been implicated in the basin-scale thermal structure and circulation of polar lakes (Cortés & MacIntyre, 2019;Kirillin et al, 2015).…”

Seasonal and diel variation in greenhouse gas emissions from an urban pond and its major drivers

“…Allochthonous C is known to drive CO 2 emission in small and shallow ponds, owing to their high perimeter to volume ratio (Finlay et al ; Holgerson ; Wilkinson et al ). Other possible factors that may explain the discrepancy between the measured CO 2 emission and estimated internal CO 2 production include inaccurate estimates of oxygen saturation and gas transfer velocity, as well as a possible underestimation of daytime respiration (Del Giorgio and Williams ; Staehr et al ; Brothers et al ). Estimating respiration rates per organism group is a difficult practice as well and comes with several uncertainties and caveats (Del Giorgio and Williams ; Martínez‐García et al ), which was confirmed by the fact that our modeled ER only accounted for roughly half of the oxygen diel‐curve–derived ER.…”