# Differentiation in Aquatic Metabolism between
## 4 Discussion
### Drivers of Temporal Variation in Metabolism  
Our results showed that the temporal patterns of rates of GPP and R resembled those from other studies where the highest mean monthly values of GPP and R reached in late summer and the lowest in winter months [14; 45; 46]. This seasonal pattern was evidenced by the high dependency found between daily estimates of GPP, water temperature, and PAR. This agrees with the results for other shallow lakes (where seasonal patterns of GPP were driven by the temporal variations in water temperature and irradiance [14; 45; 22; 46]). Besides, high summer temperatures can trigger high algal biomass that contributes to high rates of GPP [14]. However, in this study GPP showed weak correlation with algal biomass (chl-a) while both GPPd and GPPm models included chl-a as a non-significant predictor. This implies that high GPP rates in summer are driven by the growth of aquatic vegetation and periphyto in the littoral. The contribution of aquatic macrophytes in GPP was recently shown by [PERSON] et al. [46] in a study of two eutrophic lakes with similar nutrient concentrations and contrasting submerged plant cover with the lake with abundant submerged plants presenting substantially higher GPP rates than the one with no macrophytes. Previous studies also had shown the significant effect of benthic vegetation in the metabolic processes by comparing vegetated littoral habitats with pelagic open water [18; 19; 47]. PAR and water temperature also were significant predictors for both Rd and Rm models which agrees with previous studies [14; 22; 46]. Regarding the temporal variation of the metabolic balance, it was shown that NEP inclined towards autotrophy during the spring months (April to June), when temperature and irradiance started to increase, but for the rest of the year it remained clearly heterotrophic. Positive monthly averages of NEP were observed only in site S2 from April to June of 2016. Higher negative rates of NEP were calculated during autumn (peak in November 2016 and 2017) when high respiration coincided with low productivity. This finding practically implies that respiration in autumn was predominantly fueled by decomposition of allochthonous organic material originated most likely from organics accumulated in the bottom following the highly productive summer months. This conclusion is further supported by a relatively moderate coupling we found between R and GPP. In general, a strong coupling between R and GPP indicates that respiration of autotrophs and heterotrophs relies on the autochthonous organic compounds that the autotrophs produce. The proportion of the GPP that is inspired is estimated by models as high as approximately 80% and a slope of respiration on GPP between 0.8 and 1.0 usually reflects a high dependency of R on GPP [16]. In contrast, in highly eutrophic lakes the respiration-GPP coupling decreases substantially due to respiration of excessive production accumulated in the system [16; 40]. In our case a slope of 0.77 of R on GPP suggests that respiration is likely fueled by allochthonous organic material, meaning either accumulated organics or inputs from the land-water interface, explaining the persistence of heterotrophy over longer periods. High contribution of allochthonous organic matter combined with favorable abiotic conditions enhance the growth of the heterotrophic community resulting in higher respiration rates than primary production [48]. In our case it is possible that the decayed aquatic plants in late autumn provide an additional source of organic matter that boosts the microbial respiration. Besides increasing respiration, input of allochthonous organic carbon into lake food webs has been shown to constrain productivity [49] by limiting light and nutrient availability further pushing metabolic balance to heterotrophy. It is very likely that the heterotrophic littoral greatly shapes the metabolic balance of the whole ecosystem shifting it towards heterotrophy even in the pelagic habitats.