## 4 Discussion  
Rates of ecosystem metabolism obtained in both lakes showed large variations on a daily basis and across seasons during the study period. The general annual balance characterized both shallow lakes as heterotrophic. With respect to other studies that have dealt with ecosystem metabolism, only a few were carried out in shallow saline systems and at low frequency [7; 58; 59]. Moreover, most of them were in the Northern Hemisphere and only for the few months when they were iceless (e.g., [6]) and not over the entire year as in this study. If LS and SG NEP rates are compared with other shallow lakes, most of them showed higher negative [2; 10; 59; 60; 61] or positive NEP values [6; 10; 59; 62]. However, most of the other data referred to periods of only a few months, usually over the summer, when productivity was greatest. In this study the net heterotrophic conditions found contradict the expected NEP \(>\) 0 values according to their nutrient and Chl \(a\) concentrations [4; 10], but again, these studies are based on the analysis of a short period and may differ in comparison with the estimations of ecosystem metabolism based on an entire year. Daily GPP and R were closely linked during the entire study period. This is consistent with the positive correlation founded between GPP and R, and the monthly NEP values near zero during most of the study period. According to [1] this could be explained by the relatively rapid respiration by bacteria and zooplankton of the newly produced organic matter within the lakes. This could be the case in both lakes as high abundances of zooplankton for LS (up to 1300 ind L\({}^{-1}\)), bacteria (up to log\({}_{10}\) 3.89 colony formation units (CFU)), and mesozoplankton (up to 100 ind L\({}^{-1}\)) for SG are cited in previous studies [13; 37; 35]. In other studies, GPP and R were are also coupled and positively correlated as in this case [6; 8; 16]. Nevertheless, it was documented that the presence of high winds or unusual precipitation events could temporarily produce decoupling between these two variables [63; 64; 9]. However, although LS and SG are characterized by the presence of high winds [43], neither of the lakes showed this kind of disruption. Figure 6: Correlation matrix relating the weekly averages for metabolic variables GPP: Gross primary production, R: community respiration, and NEP: net ecosystem production (mmol m\({}^{-3}\) day\({}^{-1}\)) with the environmental variables Temp: water temperature (\({}^{\circ}\)C), Vol: lake volume (m\({}^{3}\) seg\({}^{-1}\)), Cond: electrical conductivity (mS cm\({}^{-1}\)), and wind speed (m s\({}^{-1}\)) for the Sauce Grande lake during the study period. According to the temporal pattern for metabolic rates in both lakes, maximum values for GPP and R were found during warmer months and minimum values towards colder months. This was reflected in the positive correlations of the metabolic rates with water temperature. This is in agreement with the results cited for other shallow lakes where seasonal patterns are the result of variations in light and temperature [2; 6; 7; 16; 61]. As well as the water temperature, most of the processes were influenced by lake volume for both lakes, and wind speed and conductivity for LS. In other studies, wind speed presented a negative relationship with GPP and NEP values associated with turbidity conditions [6]. Nevertheless, in LS, it was positively related to the metabolic rates because, unlike other shallow lakes, LS has charophytes on the bottom that prevent the resuspension of sediments with major wind speed. Finally, the use of lake volume instead of water level, as in other studies [6; 61], resulted in better comparisons between the morphologically different lakes as in this case. SG presented a greater area than LS, but the lake volume was similar and so they were comparable. In this study, the lake volume was negatively correlated with GPP and R, as in other studies, and could be attributed to the dilution of organisms and nutrients within the lake [61].