Temporarily summer-stratified lakes are common: profile data from 436 lakes in lowland Denmark

ABSTRACT

Lakes that undergo temporary stratification in summer do not fit into the classic categorisation of polymictic or dimictic lakes, but how common are they and what are the effects of stratification on the development of anoxia? We used monthly and bimonthly (twice per month) temperature and oxygen profile data from 436 Danish lakes (area range 1–3954 ha, maximum depth range 1.3–45 m) and defined a stratification indicator based on the temperature difference between the upper and lower 2 m of the water column. The stratification indicator had values between −1.6 and 17.7 °C and was significantly and strongly related to lake maximum depth and significantly but less strongly related to lake area. The indicator was highly variable, especially in lakes with maximum depths between 4 and 10 m, where intermediate indicator values suggest one or several mixing events during summer. The dissolved oxygen concentration in summer at the bottom was often <1 mg/L, even when the difference between top and bottom temperature was as low as 0.5–1.0 °C. Temporarily stratifying lakes with frequent mixing events over the summer are probably common in temperate lowland areas but are easily overlooked in routine monitoring programs. Temporary stratification has pronounced implications for the oxygen concentrations and potentially also for the biota and interactions between sediment and water.

KEYWORDS:

• lake area
• lake depth
• mixing
• oxygen
• stratification indicators
• temperature

Acknowledgements

The former Danish counties and The Danish Environmental Protection Agency are acknowledged for many years of careful sampling of Danish lakes. We are grateful to Anne Mette Poulsen and Tinna Christensen for editorial and layout assistance. This study was supported by funding from the Poul Due Jensen Foundation and inspired by the projects in Lake Ormstrup and by the AU Centre for Water Technology (watec.au.dk).

Disclosure statement

No potential conflict of interest was reported by the author(s).

Declaration of interests

The authors report no competing interests.

Additional information

Funding

This work was supported by Poul Due Jensens Fond (Grundfos Foundation): [Grant Number 2020-068]. Thomas Davidson was supported by Danmarks Frie Forskningsfond Natur og Univers project GREENLAKES (No.9040-00195B) and by PONDERFUL - POND Ecosystems for Resilient FUture Landscapes in a changing climate (Grant agreement ID: 869296) H2020 LC-CLA-2018-2019-2020.