Developing nutrient criteria for the Grijalva basin, Mexico

Abstract

Excess nutrient inputs are a major cause of aquatic ecosystem impairment worldwide. Increased total phosphorus (TP) and total nitrogen (TN) concentrations can lead to eutrophication affecting ecosystem functioning and environmental services provided by streams and rivers. Establishing numeric nutrient criteria is a strategy to reduce nutrient inputs into freshwater ecosystems. Our objective was to estimate nutrient concentrations that could be used as guides to establish nutrient criteria for TP and TN in the Grijalva basin, Mexico. We applied the frequency distribution method to water quality monitoring data for subregions (upper, middle, and lower Grijalva) and for the whole basin, considering two stream size categories. Nutrients were also compared among subregions, land uses, and stream sizes. Agriculture and urban areas showed higher nutrient concentrations than other land uses, probably due to the use of fertilizers and inputs of domestic and industrial wastewater. Higher nutrient concentrations were found in the middle Grijalva and in low-order streams. Nutrient concentrations at the 75th percentile for the reference sites were higher than those obtained at the 5th, 16.7th, and 25th percentiles for the general nutrient data, probably due to the high level of human disturbance in the Grijalva basin. Nutrient concentrations at the 25th percentile are probably too high to protect the aquatic ecosystems in the basin, while concentrations at the 5th percentile can be too restrictive for the basin. Based on our results, nutrient concentrations at the 16.7th percentile are proposed as a first approximation for nutrient criteria to protect river systems in the Grijalva basin.

Transport of nutrients into the southern Gulf of Mexico by the Grijalva–Usumacinta rivers

“…The lower Grijalva has experienced extensive land use change in the last decades, losing a large proportion of its forest cover, particularly in Tabasco, where 95% of forested land was lost by 1990 due to conversion into pasture and intensive agriculture (Palomeque‐De la Cruz et al, 2017; Sandoval‐Rivera et al, 2022). In addition, the Grijalva Basin has twice the population of the Usumacinta Basin and, the lower Grijalva concentrates more than 50% of the total basin population, suggesting greater pressure on aquatic ecosystems (Barrientos‐Pérez et al, 2022; Cruz‐Paz et al, 2018; Lázaro‐Vázquez et al, 2018). In contrast, the Usumacinta Basin has 33% percent of its area protected, 50% percent is minimally disturbed, and most parts of the basin (92%) show low population density (<5 people km −1 ) (Cruz‐Paz et al, 2018).…”

“…These comparisons indicate that despite the watersheds of both rivers have experienced the effects of human pressures, concentrations have not reached the levels of other rivers in the region. However, increased in nitrate‐N concentrations since 2000 has been observed in the lower Grijalva (Lázaro‐Vázquez et al, 2018), and high nutrient levels have been reported associated to urban centers in the Middle Grijalva (Barrientos‐Pérez et al, 2022), that indicate that population growth and demand for food are impacting nutrient concentrations in the basin. Chlorophyll levels at Site 5 on the Grijalva during the dry season could indicate, in combination with TP and TN concentrations, eutrophic conditions, while the Usumacinta at Site 7 can be considered in mesotrophic state (Dodds, 2006; Smith et al, 1999).…”

“…For example, phosphorus removal to meet effluent limits of 1 or 2 mg L −1 contributed to decrease P concentrations in other river systems (Bowes et al, 2010; Neal et al, 2010). Unfortunately, effluent nutrient limits in Mexico are high (15 mg L −1 for TP and 25 mg L −1 for TN) compared to other countries and thus need revision to set acceptable nutrient standards to maintain the health of aquatic ecosystems (Barrientos‐Pérez et al, 2022).…”

“…In contrast to the Usumacinta River, four hydroelectric dams have severely altered the Grijalva flow regime (Alcérreca‐Huerta et al, 2019; Lázaro‐Vázquez et al, 2018). Moreover, capital cities of the Mexican states of Chiapas (Tuxtla Gutiérrez, with 604 150 inhabitants) and Tabasco (Villahermosa, with 833 907 inhabitants including the metropolitan area) are located in the Grijalva Basin, contributing to nutrient pollution through the discharge of urban and industrial wastewater (Barrientos‐Pérez et al, 2022; Lázaro‐Vázquez et al, 2018).…”

“…for TP and 25 mg L À1 for TN) compared to other countries and thus need revision to set acceptable nutrient standards to maintain the health of aquatic ecosystems (Barrientos-Pérez et al, 2022).…”