# Differentiation in Aquatic Metabolism between
## 1 Introduction  
Ecosystem metabolism is a fundamental process that integrates the rates of production and consumption of organic matter in a given ecosystem [1]. This practically means that the balance between productivity and consumption of organic compounds defines whether the ecosystem will act as source or sink for atmospheric carbon dioxide. Lakes in particular are considered as hot spots for global carbon cycles [2; 3] because they can act as storage reservoirs and processors of terrestrial carbon [2]. Thus, it is not surprising that there is a growing interest for understanding the drivers of metabolism in lakes as it is of critical importance for describing responses of regional and global carbon fluxes to future environmental changes [3; 4; 5]. Yet, the dynamic nature of physicochemical and biological processes in lakes makes explanation of the variance in metabolic regime an extremely difficult task. Indeed, the variability in lake metabolism is related to numerous factors such as the availability of phosphorus and dissolved carbon in water [6; 7; 8], the lake morphometry [9; 10; 11], hydrodynamic processes and water temperature oscillations [6; 12]. This implies that the role of lakes in the carbon cycling is not always obvious as it not only relies on the response of the ecosystemto dynamic processes, such as eutrophication, but also depends on the geographic context [13]. Consequently, gross primary production and ecosystem respiration may display large temporal and spatial variations within lakes resulting in shifts in metabolic state between autotrophy, that is when net ecosystem production is positive, and heterotrophy, when net ecosystem production is negative [14; 15; 16]  
Estimation of aquatic metabolism using free-water measurements of dissolved oxygen concentration has become increasingly more common as the availability of new, cheap and reliable sensors facilitate high-frequency measurements and continuous monitoring of metabolic processes [4]. This technique is based on the assumption that changes in oxygen concentration reflect the biological balance between photosynthetic production and respiratory consumption as well as the physical exchange of oxygen between air and water. Thus, gross primary production (GPP), ecosystem respiration (R) and the metabolic balance (net ecosystem production, NEP) can be estimated throughout a 24-h period [9]. Once technical improvements in oxygen sensors made possible the continuous monitoring of oxygen concentration and relevant physico-chemical parameters, the diel oxygen technique has been widely used in immunology for assessing the temporal dynamics of aquatic metabolism and analyzing the main drivers [4]. Nevertheless, most studies on aquatic metabolism have been limited in north-temperate systems and concern the pelagic habitats without taking into consideration the metabolic processes that occur in the littoral [17]. Only a few studies have covered both pelagic and littoral habitats [18; 19; 20] and even fewer have taken into account the role of the littoral aquatic vegetation and periphytic communities [21]. In addition, there is a profound lack of studies that have dealt with the dynamics of ecosystem metabolism in shallow warm lakes. Particularly in polymictic shallow lakes, which are susceptible to warming and wind induced mixing effects, aquatic metabolism is expected to display highly dynamic behavior with abrupt changes within short periods (e.g., weeks) and to present larger variability in the littoral than the open water due to the presence of benthic littoral communities and their interaction with the land-water interface (e.g., benthic aquatic vegetation) [22]. The overall aim of this study was to expand our knowledge of spatiotemporal dynamics of metabolism in polymictic shallow lakes with emphasis placed on the role of aquatic vegetation that grows in littoral habitats.