## 1 Introduction  
The biogeochemical cycles in lake ecosystems are related to metabolic processes. Lake metabolic balances represent the difference between photosynthesis rates (CO\({}_{2}\) consumption) and respiration rates (CO\({}_{2}\) production) [1]. Quantifying metabolism has been an influential effort in aquatic research [2]. Gross primary production (GPP), ecosystem respiration (R), and net ecosystem production (NEP) display large temporal and spatial variations across different aquatic ecosystems [3; 4; 5]. NEP is useful for understanding the role of lake as sources or sinks of atmospheric CO\({}_{2}\) through net heterotrophic or autotrophic annual balances, and can be used to evaluate the trophic state of ecosystems [2, 5, 6]. Previous studies have used measurements of GPP and R to assess the metabolic balance of aquatic ecosystems and evaluate their roles as autotrophic or heterotrophic systems [7, 8]. In these studies, the average time scale is either a season or a year. However, daily measurements of these values now can be obtained by the new, reliable, and accessible technologies [9]. The continuous monitoring is not only beneficial to strengthen the predictions of how changes in environmental factors affect aquatic metabolism, but also an essential aquatic ecology method for understanding the complex changes in the biosphere [2]. The oxygen mass balance method has been applied in many previous studies of lake metabolism [3, 5, 10]. Measurements of diel dissolved oxygen (DO) concentrations are widely used to examine ecosystem primary production and respiration in aquatic systems, particularly in lakes [3, 5, 11; 12; 13]. Quantification and monitoring of diel DO assume that changes in DO concentration in a lake reflect the biological balance which is a result of photosynthetic production and respiratory consumption as well as physical exchange of oxygen between air and water. The diel DO method has advantages such as no bottle and container effects, ease of data collection and measurement of whole-ecosystem metabolism, and the potential of providing estimate of spatial and temporal variability [14]. Therefore, the long-term monitoring of DO dynamics in lakes plays a significant role in quantification of lake metabolism. Previously, autotrophy or metabolic balance has been assumed to occur in most aquatic systems [15]. However, it is now proven that the majority of freshwater systems are heterotrophic [16; 17; 18]. Metabolism is driven by different physical, chemical, and biological forces, such as temperature, irradiance, wind speed, pH, dissolved organic carbon (DOC), inorganic nitrogen, total phosphorus (TP), chlorophyll-\(a\) (Chl \(a\)), and phytoplankton, _etc._[3, 18; 19; 20; 21]. Although recently there have been numerous advances in the study of ecosystem metabolism [2, 22], the majority of this research has been conducted in temperate waters, with little in subtropical and tropical waters [2], and even fewer studies examine eutrophic subtropical shallow lakes, where sunlight can reach the benthal layer and water column productivity is high [23]. Lake Taihu is a eutrophic subtropical lake, and the third largest freshwater lake in China. The watershed is located in the Yangtze River Delta and is the most densely populated (about 1100 inhabitants km\({}^{-2}\)) and economically developed area in China (contributing about 13% of the nation's gross domestic product). Since the 1980s, the lake has become eutrophic due to its multiple uses [24]. Total nitrogen (TN) ranged between 0.38 and 21.93 mg\(\cdot\)L\({}^{-1}\), TP varied from 0.013 to 1.459 mg\(\cdot\)L\({}^{-1}\), DOC had an annual mean concentration of 3.76-8.97 mg\(\cdot\)L\({}^{-1}\), and Chl \(a\) concentration reached a peak of 327.6 \(\mathrm{\SIUnitSymbolMicro g}\cdot\mathrm{L}^{-1}\)[25]. Lake Taihu, with high production in the littoral zone, has simultaneously high nutrient and DOC concentrations, and will fall on the high end of both primary productivity and DOC gradients, while the metabolic balance remains uncertain.