Long‐term limnological changes in the Ecuadorian páramo: Comparing the ecological responses to climate warming of shallow waterbodies versus deep lakes

Abstract

Páramos are high‐altitude ecosystems of grasslands and shrubs that sustain high levels of biodiversity and contain numerous lakes, ponds and wetlands that are a crucial source of water for millions. In the Andes, limnological data are rare from páramos and particularly so from shallow waterbodies that are prominent features of the landscape. Here, we analyse fossil diatom assemblages using dated sediment cores from three shallow lakes in the páramo of southern Ecuador and document their response to recent climate changes. The two shallowest sites were <0.5 m deep and contained nearly identical diatom assemblages, dominated by Achnanthidium minutissimum and other benthic taxa. The deepest study site, at 4 m depth, differed notably from its shallower counterparts in that the dominant taxa were tychoplanktonic Aulacoseira species. All three study sites showed only minor assemblage shifts over the past ˜200 years. This contrasts sharply with paleolimnological data from nearby deep lakes (Zmax > 17 m) that recorded abrupt changes in diatom phytoplankton coincident with the onset of higher temperatures, reduced wind speeds and the onset of thermal stratification in recent decades. In temperate and high‐latitude regions, an overriding factor influencing freshwater ecosystems is the duration and extent of ice cover, which itself is closely linked to climate variables. In contrast, there is no winter ice cover and the growing season is continuous year round in our equatorial Andean sites. Instead, rising temperatures are affecting deep lakes primarily by altering the physical structure of the water column resulting in greater periods of thermal stratification, which in turn drives changes in biota and other lake processes. However, this mechanism of change only affects deep lakes because shallow waterbodies are easily mixed by wind. Our data demonstrate the differential response of large, deep lakes in the páramo compared to the relative complacency of changes in shallow waterbodies, which is in marked contrast to similar sites affected by seasonal ice cover.

Mountain lakes: Eyes on global environmental change

“…not thermally stratify have remained relatively unchanged in diatom assemblages, as predicted (Giles et al, 2018).…”

Ecological resilience in tropical Andean lakes: A paleolimnological perspective

“…Benthic Nitzschia spp. in Lake Piñan were associated with regional cropland changes, possibly indicating nutrient increases linked to the alteration of the páramo landscape (Giles et al 2018). In Lake Fondodocha, benthic species indicative of nutrient enrichment ( Sellaphora laevissima and Navicula cryptotenella ) increased with cropland area.…”

“…We propose that lake-specific differences in water depth and catchment characteristics may have modulated the response of diatom communities, producing the absence of synchrony. In the Ecuadorian páramo, shallow lakes (< 8 m depth), such as Fondococha, are strongly influenced by energy transfer from the catchment (e.g., organic matter, floods, land use) (Giles et al 2018), whereas deeper lakes (e.g., Piñan, albeit cored in the shallow platform) respond more to local physical and chemical conditions in the water column (e.g., nutrients, thermal stratification) (Luethje 2020). We confirmed that Andean lake diatom communities are influenced by both local and regional factors (in agreement with Benito et al 2019); thus, the observed asynchrony is likely due to water chemistry differences mediated by lake-specific flushing (Leavitt et al 2009).…”

Identifying temporal and spatial patterns of diatom community change in the tropical Andes over the last c. 150 years

“…() proposed that the increase in D. stelligera and T. flocculosa and associated decrease in A. alpigena in recent sedimentary records have led deep lakes (>17 m) of Ecuador to cross climate‐driven ecological thresholds. In contrast, nearby shallow lakes (<3 m) recorded minor diatom community shifts over time (Giles, Michelutti, Grooms, & Smol, ). Our analyses suggest that major diatom assemblage change is not widespread across the region.…”