Chironomidae as indicators of water pollution in Pesquería River (México)

Despite their potential as indicators of water quality and their key role in river ecosystems, Chironomidae is still poorly studied in Neotropical rivers. This lack of knowledge is especially relevant for rivers subjected to intense human activities, such as many rivers in Mexico. The aim of this investigation is to contribute to the knowledge of the midges of the Pesquería River (Mexico) along its main courses and relate the composition and abundance to river health. Thirty samples were collected during two different periods (August 2015 and February 2016) using a D-frame and kick sampling. Thirty-five taxa were found in total, with four taxa found in more than 50% of the sites and 19 only found once. Midges accounted for more than 50% of the total macroinvertebrate abundance. Chironomus gr. plumosus, Rheotanytarsus spp. and Cricotopus gr. bicinctus were the most abundant species. Collector-gatherers dominated in August (71% of individuals), whereas collector-filterers dominated in February (43,2%). The major factor explaining the midge distribution and abundance is pollution, while the structure of riparian area does not explain much of the midge richness. This is most likely related to the organic pollution coming from untreated or poorly treated sewage in the city of Monterrey and its surroundings. Three main sectors are distinguished along the river: i) the upper part section with higher biodiversity and presence of intolerant taxa; ii) the middle sewage polluted area with the presence of large red midges very tolerant to pollution (Chironomus, Dicrotendipes); iii) the lower section in the agricultural zone where the community is dominated by red, small midges (Rheotanytarsus). Overall, our study shows that Chironomidae can be useful as better indicators of water quality when genera or species levels are used instead of family or subfamily, as is usually found in most papers on river pollution.