## 4 Discussion
### Late Holocene Evolution of Lake Primary Production  
A gradual increase in the TOC and TN for LGSP and LCSP was reflected at 100 BC-AD 150 onwards and reached a maximum during the present-day period (Figure 3a,b). This trend might suggest increased diagenetic processes that, with time, led to the gradual breakdown of organic matter [2; 65]. Nevertheless, this gradual increasing trend is absent in the LCSP record, which is, by contrast, characterised by relatively lower values of TOC and TN, but also BSi and \(\delta^{13}\)C, between 850 and 1050 AD, pointing out very low lake paleoproductivity. These results might suggest moist and cold conditions coinciding with persistent cool La Nina-like conditions between 650 and 950 AD, as reported by [PERSON] and [PERSON] in Laguna San Pedro (38 \({}^{\circ}\)S) [11]. Variations of the C/N ratio and \(\delta^{15}\)N were observed from 100 BC-AD 150 to the present day and particularly, from 450 AD and 520 AD in LCSP and LGSP, respectively. This might be related to an increased organic matter input derived from the watershed area of the lakes, but probably also increased the lake primary production as evidenced by the fossil pigment data. Indeed, fossil pigments were detected in LCSP from 450 AD onwards and were not present prior to this period. The carotenoids identified suggest the presence of cyanobacteria, chlorophyta, bacilliariophyta and dinoflagellates (Figure 4a) in LCSP. In the LGSP, pigments were detected throughout the entire core, albeit at low concentrations prior to 1900 AD. [PERSON] et al. in Laguna Aculeo (34 \({}^{\circ}\)S) [9], reported that around 500-700 AD was characterised by a high frequency of flooding, an event that could have influenced the watersheds, dragging allochthonous matter towards the lakes, leading to changes in C/N, \(\delta^{15}\)N, TOC, TN and primary production during this period. Interestingly, no obvious changes were present in the proxy records coincident and immediately after the Spanish colonisation in the region, around the middle of the XVI century (1536 to 1541 AD). Nevertheless, the highest values of C/N, which indicated a mixture of terrestrial and aquatic organic matter around 1550 AD for both lakes, resulting in higher catchment erosion and an increasing lake terrigenous supply, could be due to the settlements in the watershed. According to historical records described by [PERSON] et al. [77] and [PERSON] [18], the Mapuches lacked intensive productive activities before the Spanish settlements. Accordingly, little impact was generated in these watersheds. Contrarily, in the middle of the XVI century, the Spanish colonisation at the south of the Biobio river caused the introduction of intensive exotic crops and livestock of ungulates (horses, sheep and cattle) [14; 17; 21; 23; 24; 77; 78]. It has been previously suggested that the development of settlements had a moderate impact on the pre-Columbian ecological landscape [19; 78], which could favour the inputs of organic matter to the aquatic systems observed in C/N values as a possible signal of those changes. The most obvious changes in the proxy records occurred in recent decades in both sediment cores. In particular, the C/N ratio and \(\delta^{13}\)C decreased in both lakes, while the total chlorophyll and carotenoid concentrations increased. Combined, this suggests that phytoplankton primary production increased in recent decades. Nevertheless, apart from an increase in Chl-\(b\), no obvious changes were observed in the relative abundance of the other marker pigments, apart from a slight decrease in diatoxanthin. Interestingly, this is more or less coincident with a decrease in BSi, which might suggest that diatoms and/or chrysophytes became less abundant, while chlorophytes became more dominant (higher Chl-\(b\)) in recent decades. However, it must be noted that the sharp rise in total pigments in the most recent samples might reflect diagenesis and the degradation of pigments with time.