## 4 Discussion
### Net Autotrophic Lake  
The amount of organic matter produced and consumed within a lake determines its net metabolic balance. Aquatic systems where GPP exceeds R have positive NEP and are net autotrophic, whereas \begin{table}
\begin{tabular}{c c c c c c c c} \hline  & & **T\({}_{\rm w}\)(\({}^{\circ}\)C)** & **Radiation** & **Wind Speed** & **pH** & **ORP (mV)** & **Chl_a_** & **Turb** \\  & & & **(w\(\cdot\)m\({}^{-2}\))** & **(m\(\cdot\)s\({}^{-1}\))** & & & **(ng\(\cdot\)L\({}^{-1}\))** & **(NTU)** \\ \hline SSC (\(n=294\)) & GPP (g O\(\cdot\)m\({}^{-3}\cdot\)d\({}^{-1}\)) & 0.622 ** & 0.269 * & –0.132 * & –0.133 * & –0.360 ** & 0.032 & 0.112 \\  & R (g O\({}_{2}\cdot\)m\({}^{-3}\cdot\)d\({}^{-1}\)) & 0.553 ** & 0.430 ** & –0.081 & –0.089 & –0.339 ** & 0.022 & 0.122 * \\  & NEP (g O\({}_{2}\cdot\)m\({}^{-3}\cdot\)d\({}^{-1}\)) & 0.294 ** & –0.149 & –0.125 * & –0.113 & –0.139 & 0.026 & 0.014 \\ JS (\(n=92\)) & GPP (g O\({}_{2}\cdot\)m\({}^{-3}\cdot\)d\({}^{-1}\)) & 0.597 ** & 0.299 ** & –0.187 & –0.235 * & –0.180 & 0.522 ** & –0.210 \\  & R (g O\({}_{2}\cdot\)m\({}^{-3}\cdot\)d\({}^{-1}\)) & 0.529 ** & 0.263 * & –0.241 * & –0.217 * & –0.169 & 0.426 ** & –0.196 \\  & NEP (g O\({}_{2}\cdot\)m\({}^{-3}\cdot\)d\({}^{-1}\)) & –0.010 & –0.001 & 0.169 & 0.023 & 0.022 & 0.071 & 0.029 \\ \hline \end{tabular}  
*, ** Correlation is significant at the 0.05, and 0.01 level (2-tailed), respectively. GPP, gross primary production; R, ecosystem respiration; NEP, net ecosystem production; Tw, water temperature; ORP, Oxidation reduction potential; Chl \(a\), chlorophyll-_a_ concentration; Turb, turbidity.  
\end{table} 
Table 1: Relationships between metabolism and environmental factors in Shangshancun (SSC) and Jinshu (JS) buoy stations. aquatic systems where more organic matter is respired than produced have negative NEP and are net heterotrophic [22]. If autotrophic, the ecosystem will produce more organic carbon and export or accumulate organic carbon. On the contrary, if heterotrophic, the ecosystem will be dependent on organic matter inputs from adjacent ecosystems [37]. Some previous studies suggest that aquatic ecosystems are frequently net heterotrophic, with respiration exceeding primary production, indicating that the food web of the ecosystem is supported by external inputs of energy [16, 18, 19, 38]. In present study, it is found that annual mean GPP:R ratio is 1.15, indicating that Lake Taihu acts as a net autotrophic lake (Figure 5). Lake Taihu was hypothesized to have net autotrophy because increasing net autotrophic tends to occur in nutrient-rich and productive aquatic ecosystems [19, 39, 40]. Previous studies have also shown that a net heterotrophic metabolism is most likely in unproductive ecosystem, and highly productive ecosystems tend to be net autotrophic [18]. [PERSON] _et al._ found that Lake Taihu was net heterotrophic [23]. They indicated that the high levels of suspended solids and nutrient concentrations limited underwater light which reduced phytoplankton photosynthesis, and the high temperature was good for plankton repiration [23]. The study site of [PERSON] _et al._ is situated in Meiliang Bay, in northern Lake Taihu (Figure 1), which is one of the most eutrophic bays. Two main rivers (Liangxi River and Zhihugang River) empty large amounts of wastewater from factories and residential areas into the bay [28]. The high nutrient level and temperature make Meiliang Bay display net heterotrophy [23]. In the present study, two buoy stations (SSC and JS) are situated in Gonghu Bay, in northeastern Lake Taihu (also see Figure 1), where half of its area covered by abundant macroplant vegetation [27]. Gonghu Bay is an important source of drinking water for Wuxi City and part of Suzhou City. To improve water quality, water transfers from the Yangtze River to the lake through Wangyu River. [PERSON] _et al._ found that the Yangtze River water transfer have notable positive effects on water quality in Gonghu Bay [27].