## 4 Discussion
The annual GPP/R ratio in both lakes was near one, indicating that the main substrate for R is the GPP within the lake; nevertheless, the fact that NEP < 0 determines that this would be insufficient. Possibly, the origin of organic resources inspired during heterotrophy conditions might arise either from the excess of GPP generated when the system is under autotrophic conditions or from the allochthonous material. The allochthonous input of organic matter via streams is an important source of supplementary energy for the community R [14; 16]. Besides, both lakes are located in important agricultural areas where fertilization of crops with nitrogen is a common practice applied from June to July (seedtime) and from September to October (tillering) [65]. In a previous study in LS an increment in GPP and R values was observed after water input [7], and an increase in nutrient concentrations and a decrease in conductivity, leading to changes in plankton composition and biomass [13]. In this study, an increase in GPP and R values was also observed in LS, accompanied with the maximum increase in its volume. Considering that the water comes from the irrigation of crops, this finding suggests that the supply of nutrients, new organic material, and/or plankton organisms from the river would promote GPP and R, respectively. According to [5], the input of DOC and nutrients increases with a high drainage ratio (catchment area to lake surface area). Also, small lakes possess a long shoreline relative to lake volume and receive significant inputs of organic matter [66], increasing R and leading to NEP < 0 conditions [1; 10]. In the same manner, in SG, GPP, and R showed a stronger negative relation with lake volume, but NEP was positively correlated. Nevertheless, SG has a different hydrological function from LS, where greater water input was recorded, but the lake volume did not respond immediately to these variations as in LS. The explanation for this is that SG is a flushing lake and part of the input and production within the lake of DOC, nutrient and organisms are delivered to the sea. Nevertheless, the increases in nutrient concentrations coinciding with the increments in water input and lake volume could explain the positive correlation with NEP values for both lakes. Finally, according to [PERSON]'s stable states theory [31], LS is in a clear state during the whole study period and dominated by the presence of charophytes which prevent the resuspension of sediments [13], whereas SG is in a turbid state during the whole study period and dominated by phytoplankton [36; 37; 67]. It is well known that solar light promotes primary production (GPP) and the later the respiration processes (R) that occur within a lake ecosystem [68]. The fact that the euphotic zone is reduced in SG, unlike LS, could explain the lower GPP rates and explain why it presented a stronger negative relation with lake volume. Meanwhile in LS, GPP was weakly related to lake volume because charophytes (and phytoplankton as well) receive light during the whole year, independently of the significant water level fluctuations. In a study of 25 Danish lakes with alkaline and nutrient-rich catchments and relatively low transparency conditions [5], it was found that the benthic zone was dominated by sediments rich in carbon, with a high demand of oxygen. This suggests the dominance of heterotrophic processes, which is in accordance with the case of SG. Furthermore, according to a study performed in two eutrophic lakes with similar nutrient concentrations, they found that the one with submerged plants presented substantially higher GPP rates than the one that only had phytoplankton and periphyton [69]. In this study, LS presents submerged charophyytes, and the GPP rates were greater than in SG. Furthermore, if salinity conditions are considered, LS is a mesosaline lake. Increased salinity could result in ion toxicity and osmotic stress accompanied by high mortality or lower reproduction and growth rates for salt-sensitive taxa [70; 71].