Permanent Thermal and Chemical Stratification in a Restored Urban Meromictic Lake

Renata Tandyrak,Michał Łopata,Jolanta Katarzyna Grochowska,Renata Augustyniak

doi:10.3390/w13212979

Abstract

Meromictic lakes are unique aquatic ecosystems that occur extremely rarely. The phenomenon of meromixis can result from both natural and anthropogenic factors. The aim of this study was to analyse thermal and chemical stratification in a small, deep (6 ha, H max = 24.5 m) lake. The evaluated lake had a typical summer thermal profile with a shallow epilimnion, a sharp thermocline, and a distinct monimolimnion layer in the hypolimnion, which was also maintained during circulation. The lake had a clinograde oxygen profile, with an oxygen deficit in the metalimnion and permanent anoxic conditions in the deeper layers, including during circulation. A redox zone was identified during summer stagnation. The monimolimnion formed a thermally isolated layer at a depth of around 15 m, and the chemocline was situated above the monimolimnion. In the chemocline, the EC gradient ranged from 61 to 77 μS·cm−1 per meter of depth in the summer and from 90 to 130 μS·cm−1 per meter of depth during circulation. EC was significantly correlated with Ca2+ concentration (r2 = 0.549). Chemical stratification, particularly with regard to organic matter distribution, was observed in the chemocline. The monimolimnion severely limited nutrient internal loading.

Highlights

The intensity of water mass circulation is an indicator of the lake mixing regime and constitutes a basis for the limnological classification of lakes
The chemocline was identified based on the distribution of electrolytic conductivity values in the vertical profile
The present study is a part of a long-term research project that has been conducted in Lake Starodworskie since 1955, i.e., for more than 60 years

Summary

Introduction

The intensity of water mass circulation is an indicator of the lake mixing regime and constitutes a basis for the limnological classification of lakes. Most lakes in temperate northern regions are holomictic and dimictic water bodies [1]. Water circulation is naturally limited in small lakes that are deeply embedded in the ground. Many of these lakes are shielded from the wind by tall trees or buildings in the immediate vicinity. Meromictic lakes are sometimes surrounded by elevated landforms These factors are important for small lakes with an average axis length of less 1000 m, because the wind, which is slowed down by the obstacles above, regains its original speed at a distance equivalent to 16-times the obstacles’ height [15]

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