Evaluating the ecological status of mountain streams from a megacity (Mexico City) with diatoms: development and implementation of morphological and eDNA-based methods

In central Mexico, streams flow from mountain protected areas at high elevations, i.e. abive 2,400 m a.s.l., being exposed to different human activities along their course. The main threats of these aquatic ecosystems include the presence hydraulic infrastructure (e.g. canals and dams), construction bridges or roads, fish farming, agriculture, livestock, unregulated tourism irregular settlements, resulting in nutrient enrichment, organic pollution low hydromorphological quality. Diatoms, widely recognized as valuable indicators environmental conditions, offer a unique opportunity assess ecological quality around this megacity, they respond directly sensitively specific physical, chemical biological changes. To conduct assessment, composition diatom communities has traditionally been characterized by morphological methods most recently DNA (eDNA) metabarcoding. However, only few Basin Mexico have morphologically resolution light microscopy (LM), no eDNA-based studies conducted yet. context, our aims are 1) implement eDNA metabarcoding for identification quantification epilithic diatoms seasonal spatial response three peri-urban 2) develop metric monitor streams. For purpose, we a) hydro-morphological conditions related ecosystem quality, b) diversity community structure nine sites per stream (27 sites). headwaters, well middle lower reaches with land-use anthropologic pressures, were investigated over hydrological periods, post-rainy, dry-cold dry-warm. Species was first under LM scanning electron (SEM), is currently done V4 region 18S rRNA gene fragment chloroplast marker rbcL. Our study resulted 520 infrageneric taxa identified SEM, largest ever found freshwater Mexico. From data, clear gradient almost pristine towards heavily polluted states enter city. These changes reflected variations structure, revealed Diatom Ecological Quality Index (DEQI), newly developed evaluate megacity.