4.1. Distribution of Páramo Lakes
The altitudinal distribution of páramo lakes responds to the glacial modeling of the Andes of Colombia. During the last glacial maximum, glaciers reached 4300 m a.s.l. [27
], whereas the formation of lake basins at lower altitudes can be traced back to older cold periods [29
]. These dual erosive glacial phases could be the explanation for the bimodal lake distribution found. The páramo complexes, where glaciers still exist today, are the ones with lakes at the higher altitudes, suggesting a more recent basin formation. These lakes show small and silted basins. Nonetheless, there is no significant general relationship between altitude and lake number or size.
The complexes of Las Hermosas, Sierra Nevada de Santa Marta, and Los Nevados are of greater interest from the conservation perspective because of the higher number of lakes than in other complexes and the large proportion with protection status. In contrast, a small number of lakes are part of protected areas in the complexes of Cruz Verde-Sumapaz, Santurbán, and Pisba.
4.2. Eastern Cordillera Lake Characteristics
Generally, glacier modeling in temperate mountain regions has caused relatively deep basins compared to the lake area, in contrast to ice modeling in subarctic areas [30
]. The average depth of lakes in the Pyrenees is around 15–20 m [31
]. Lakes in the Andean equatorial region tend to show lower average depth, e.g., around 10 m in the Cajas massive in Southern Ecuador [32
]. Existing information indicates that the lake aspect ratio for the Colombian Andes tends to be closer to subarctic lakes than those of the temperate zone [15
]. Our survey confirms this tendency. The reason may be related to an erosive process in a more open high-mountain landscape than in other ranges with narrower valleys.
Unlike most alpine high-mountain watersheds, the páramo is covered by dense shrubby vegetation [33
]. High air humidity throughout most of the year and average temperature values between 3–13 °C [34
] promote extensive marsh vegetation around the lakes, which are limited by an extensive belt of terrestrial–aquatic transition zone [35
]. Consequently, the organic matter load is potentially high, explaining the consistently high TOC values in the lakes studied [36
]. This feature also makes the páramo lakes more similar to tundra lakes than temperate alpine lakes.
The Secchi disk values indicated that the transparency of the studied lakes in the Eastern Cordillera is very low compared to that in the temperate zone [30
] and mountain lakes from Southern Ecuador [37
]. However, despite the low water transparency, the light penetration is sufficient for macrophyte and algal growth at the bottom of most of the lakes due to the shallowness. This fact has major implications for the ecosystem as it increases the lake’s primary productivity [39
The investigated lakes showed pH values mainly in the acidic range, with low buffer capacity because of the low cation concentration [41
]. TOC and pH were not correlated (r
= 0.1, p
= 0.5, n
= 60), indicating a low contribution of organic acids to acidity, which was primarily related to the inorganic ionic balance. All in all, the high diversity in TOC concentration among lakes merits further investigation about the TOC specific nature and the process determining its levels.
Total nitrogen and phosphorus levels were significantly higher than those reported for high-mountain lakes in other areas of the world [42
]. Ammonium accumulation at levels above nitrate is particularly remarkable, which may point to some limitations for nitrification or a continuous flux from deep layers and sediments. This issue, and the nitrogen cycle as a whole, considering atmospheric deposition, watershed loadings, and fixation, deserve further research in these lakes [44
The proportion of PO43−
respective to TP fluctuates strongly but is remarkably high (>20%) in many lakes. This feature and the relatively high nitrogen levels points to a light limitation in some of these lakes. Thus, there is a significant relationship between the ratio PO43−
/TP and the TOC (r
= 0.32, p
= 0.02, n
= 51). In any case, TP is high, indicating mesotrophic to eutrophic conditions, in contrast to the usual oligotrophic character of high-mountain lakes [10
]. On the one hand, the relatively low oxygen values at the bottom may facilitate phosphorus diffusion from recycling in the sediments [45
]. On the other hand, the shallowness of the lakes fosters the relevance of the littoral and bottom primary producers and the influence of thycoplankton in open waters [46
]. Although the characteristics pointed here refer to the páramo lakes of the Eastern Cordillera, there is no reason to expect that they will not apply to the whole set of páramo lakes in Colombia.
4.3. Páramo Lakes in the Context of Global Change
The main factors associated with global change that could affect the páramo lakes are the increase in air temperature, changes in the rainfall regime, changes in land use, and atmospheric transport of pollutants. These processes have been widely documented in remote mountain lakes [48
]. The potential impacts of these processes range from in-lake moderate changes (e.g., water column stability, oxygen demand) to large watershed and external loading modifications (e.g., water level, organic loads, pollutant accumulation), eventually leading to marked changes in communities and food webs (Figure 6
Studies have projected an increase in air temperature of between 3 and 4 °C in the Andes high mountains [51
], resulting in some of these lakes possibly exceeding 20 °C during periods of high radiation. Higher temperatures will accelerate organism metabolism and, consequently, biogeochemical processes. Respiration is more sensitive than photosynthesis to temperature, but higher recycling may result in a higher trophic state of the lakes [52
]. Higher biomass and metabolism will increase oxygen demand [53
], and warmer waters and stronger stratification may limit the supply [54
]; thus, the lakes may become more similar to tropical lakes of lower altitudes [55
]. Oxygen values in hypolimnion already tend to be low in the lakes studied, so an increase in hypoxia will enhance phosphorus mobilization. Lakes that currently have the highest phosphorus values could experience significant water quality deterioration in the coming decades. This is the case of lakes near the main urban centers of Colombia. These systems, due to their proximity to the city of Bogotá, presented a history of high affectation during the last two centuries and, therefore, could have a low capacity to mitigate the impact of the increase in temperature. On the other hand, during the dry season all these effects can be enhanced by substantially increasing the metabolism of the lakes. Since our study is based on a sampling carried out during the dry period, the chemical composition of the lakes in future scenarios, with longer periods of drought, could be similar to the more enriched lakes that were observed. During the rainy season, dilution could reduce the concentration of nutrients, but could increase the contribution of organic matter from the basin.
Climate change may also affect rainfall seasonality, with more extended and more frequent periods of drought expected [1
]. Our results show that the lakes of the Eastern Cordillera are very shallow ecosystems, so that longer periods of drought will have a great impact. Even though there are a high number of small bodies of water, the shallow depth determines that the water storage capacity will be low. Aquatic plant communities, especially those that inhabit the terrestrial–aquatic transition and submerged macrophytes without rapid regeneration, will be quickly affected by these changes [56
]. Most of the lakes studied showed a photic layer that reaches the sediment, so the reduction in depth could increase the role of algal productivity in the deeper lakes, while the shallower lakes could suffer a progressive invasion of littoral plants. More importantly, páramo peatlands may experience periods of high release of organic material to the lakes, enhancing lake metabolism and oxygen demand. The shifting conditions may last for long periods, and human land use in the watersheds may enhance the processes. The long-range transport of pollutants to these lakes, another component of global change, is largely unknown [15
]. The lake watershed protection should reduce undesirable synergies between global change and local land use and provide better stewardship of any unavoidable shift produced by climate transitions [58
4.4. Páramo Lake Conservation
The results of the lake chemical analyses did not show a pattern, from which it could be concluded that a low lake trophic state is related to protection status (Figure 7
). Despite the imbalance in the number of lakes considered between conserved and non-conserved areas, the high variability and the high concentration of TP and TOC in lakes of some National Parks such as Chingaza and Sumapaz suggests that this result shows more the degree of affectation suffered by the different complexes than the current conservation status. The System of Protected Areas in Colombia is relatively recent. Páramo protection is mainly under the jurisdiction of the national park system (33%), and smaller areas are managed by municipal or private regional reserves (12%). The majority of the páramo ecosystems were declared protected in 1977 as part of the National Natural Parks System of Colombia (decree 2811–1974 of the Republic of Colombia, 1975). Many of these areas previously had agricultural, livestock, and mining activities. Currently, most of them still have some type of agricultural management with low impact, such as extensive livestock. Surveying the past is possible with paleolimnological techniques [59
]. The study of lake sediments can evaluate the effectiveness of the watershed conservation status and define reference conditions to manage human land use and climate interaction (e.g., land erosion and siltation of the lakes).
Strategic planning should also consider increasing the protected areas. Páramo ecosystems are a unique natural heritage for biodiversity. Fish introduction is one of the leading food web alterations in high-mountain lakes worldwide [60
], including trout in the Andean tropical lakes [61
]. Protection should reduce trout spreading and other undesirable fish species. Furthermore, if climate change modifies water availability, hydrological interventions (e.g., building dams, diverting streams) could be a pressure that prevention and planning should anticipate. The results presented here constitute a baseline for the definition of the trophic status of the lakes and for their monitoring. The shallow depth and the high content of organic matter and nutrients of most ecosystems suggest that they are ecosystems that require specific actions even in protected areas. The total elimination of livestock, the control of the introduction of trout and the assessment of the real potential of some lakes to provide water to local communities are actions that should be implemented urgently. The regular measurement of the water transparency, TP, TKN and the oxygen concentration in the hypolimnion would allow a response from these ecosystems to these actions during the progression of climate change.