Herein we provide a framework for evaluating the spatiotemporal variation of nitrate and ammonium and their relationships with environmental and anthropogenic variables at different scales. Bimonthly samples were collected from 28 sites in the Duero River, Mexico, from May to December 2013. The river flow changed and lost connectivity during the dry season because more water was diverted for irrigation. Four sites had the highest nitrate and ammonium values (7.6 and 22.1mgL–1 respectively), which were related to direct waste water discharge. The remaining 24 sites were analysed using a multivariate approach. Using hierarchical cluster analysis, seasonal trends were identified with two groupings during July and five in December. Sites were grouped into those with constant discharge and nitrates affected by springs and those with poor water quality (high concentrations of ammonium). Non-metric multidimensional scaling simultaneously revealed the variation in time and space, organised sites into an environmental ammonium gradient and differentiated between seasons according to nitrate levels. Regression tree analysis established a relationship between nutrients and independent variables. At the landscape level, the agricultural area affected nitrate (75%) and urban area affected ammonium (45%); at the basin level, road density influenced both parameters (10.68km per 25km2).

Subtropical rivers in developing countries often lack adequate monitoring, which makes it difficult to comprehensively determine their water quality when faced with different anthropic impacts. There are no proper protocols in the regulations to incorporate indicators and adapt them to different biogeographic regions, limiting the potential success of conservation and restoration of river ecosystems. This study proposes implementing macroinvertebrates as bioindicators of water quality in river ecosystems, and modifying the calibration of the widely used Biomonitoring Working Party (BMWP) index for its adaptation in a subtropical river. The Duero River, Mexico, was used as an example in this study. Data were explored with multivariate statistics, and the water quality and habitat values were averaged to obtain the families’ bioindication values and the index categories. The BMWP adequately described a deterioration gradient from the origin to the river mouth (from fair to extremely polluted), with some intermediate recovery points related to the presence of springs. Its performance was compared with other biological indices and exhibited a positive relationship with all of them. In addition, how BMWP changed over time was analyzed by examining previous samples, and highlighted increased river deterioration over time. A calibrated BMWP will allow for long-term monitoring at a low cost.

A methodology is proposed aimed at defining an integral index of water quality in surface waters that incorporates the information for five variables currently used to independently measure the condition of water in the Cupatítzio River, Michoacán. The variables considered were the current water quality index used by CONAGUA, the concentration of metals, biodiversity as assessed through the BMWP index, microbiological values for Escherichia coli, and the level of toxicity. The index was applied at 17 sites along the Cupatitzio riverbed in the dry season of 2017. Each variable was assigned a rank, which was standardized to a scale of 1–10 and subsequently multiplied by a weight (W) that numerically represented the degree of importance and influence that each factor had in terms of pollution. These factors depended on the anthropic condition of the area, with a value of 5 indicating the method with the most significant impact and 1 the least. The integral index of water quality (IIWQ) was calculated as the arithmetic sum of each factor considered, generating a single value. It had intervals of 15 points minimum to 150 maximum. Five water quality levels were proposed: excellent, good, fair, bad, and very bad. The results showed that, of the 17 sites studied, the majority (ten) were in the fair quality category, ranging from 69 to 95 points; six were in the good category (96 to 122 points); and only one was in the bad category (42–68 points). With the application of this methodology incorporating the information for the five variables already described, it was possible to assess the water quality conditions in the Cupatitzio River as adequate and the water as suitable for its uses in the different socioeconomic activities for which it is destined.