Funding period: 11/2020 - 11/2024

The correct quantification of the mass and energy exchange between inland waters and the atmosphere is of great importance for both scientific and practical issues. Exact direct measurements are possible, but expensive and technically challenging. Thus, different gradient approaches, such as the ‘turbulent boundary layer’ (TBL) approach, provide the methodological backbone to determine diffusive gas fluxes, energy fluxes and evaporation rates from inland water utilizing easy–to–measure limnologic and atmospheric variables. However, the reliability of such flux approximations significantly depends on (i) the parameterisation of the transfer coefficient and (ii) the representativeness of input data. In order to enhance our capabilities to determine fluxes from inland waters, exchange processes will be intensively studied in this project. In particular, we aim to (A) improve the predictive power of gradient approaches and (B) quantify effects of spatial and short-term variations of meteorological and limnological drivers on flux approximations.
Two long-term and four additional short-term intensive measurement experiments will be performed at Bautzen reservoir in Lusatia (Germany) to observe mass and energy fluxes under different weather and limnic conditions as well as on different scales of space and time. A floating outdoor laboratory equipped with an eddy covariance measurement system and several meteorological, hydro-chemical and hydro-physical sensors will be used for direct continuous measurements of fluxes and variables that are unaffected by land surfaces and are representative for the pure water-atmosphere-interaction. Three additional satellite platforms with a simplified set-up will be utilized to detect the spatial variations of atmospheric and limnic conditions along the fetch. Furthermore, the occurrence and the effects of surface films and micro-stratification in the uppermost water layers will be examined by additional hydro–chemical field and laboratory experiments.
The combination of experimental fieldwork, statistical analyses and model–based investigations as well as the nexus between limnological and micrometeorological researches provide the foundations to better understand the processes that are relevant for the mass and energy exchange on different temporal and spatial scales. Our analyses will focus on the development of novel methods to (a) parameterise atmospheric transfer coefficients, especially under low turbulence conditions, (b) evaluate effects of limnological phenomena such as micro-stratification and surface films, and (c) quantify influences of atmospheric and limnologic heterogeneities on flux estimates. Additionally (d), we will examine atmospheric turbulence structures and develop models for the numerical description of spectra and cospectra of atmospheric variables to improve the correction of damping losses and therefore the fidelity of eddy covariance measurements above water surfaces. 

Laufzeit: 1.1.2021 - 31.12.2023

Durch globalen Wandel verändern sich die Winterbedingungen in Seen rapide. Eine Einschätzung der Folgen ist zum jetzigen Zeitpunkt schwierig, weil sich Limnologen historisch auf die „Vegetationsperiode“ von Frühling bis Herbst konzentriert haben und daher wenig über die Winterökologie bekannt ist. Bis vor kurzem galt der Winter im Allgemeinen als ökologisch ruhend, da das Lichtangebot erst für das Phytoplanktonwachstum ausreichend ist, wenn die Schichtung im Frühjahr einsetzt. Entgegen dieser Annahme gibt es viele Seen, in denen Phytoplankton - insbesondere großzellige Diatomeen (Kieselalgen) - im Spätwinter vor der Schichtung dichte Blüten bilden können. Dieses Phänomen ist in gemäßigten Seen nicht ungewöhnlich, bisher jedoch unterforscht. Es gibt Hinweise darauf, dass diese Blüten einen starken Einfluß auf die Seeökosysteme in den folgenden Jahreszeiten haben, weil sie Nährstoffe binden und die Phytoplanktonbiomasse verringern. In diesem Projekt wollen wir die Ursachen und Wirkung winterlicher Diatomeenblüten untersuchen und charakterisieren.
 

Laufzeit: 1.10.2017 - 30.9.2020

Funding period: 2019 - 2023

Goal: In this project, we investigate the processes that control the abundance and characteristics of gas bubbles in freshwater ecosystems, along with an assessment of their role in transporting gases, dissolved and particulate matter. We distinguish between bubbles generated by air entrainment at the water surface, bubbles nucleating in the pelagic zone due to excess dissolved gas pressure and bubbles formed in aquatic sediments. We hypothesize that these three different types of bubbles have distinct properties.