Dam construction and nutrient loading are among the greatest threats to freshwater ecosystems, altering ecological processes and the provisioning of ecosystem services.Temporal change in hydrology and ambient nitrogen and phosphorus concentrations was studied on the Grijalva, a large tropical river in southern Mexico, where four hydroelectric dams operate and where land conversion has impacted the freshwater environment. Temporal changes in discharge and in river chemistry were examined by analysing long-term discharge and nutrient data using the software Indicators of Hydrologic Alteration and Mann-Kendall tests. Furthermore, additional water chemistry samples were collected to examine seasonal nutrient dynamics in the lower Grijalva. Long-term discharge data indicated dam construction has severely altered temporal patterns in discharge and other hydrological characteristics. The lower Grijalva has also experienced increase in nitrate concentrations through time, which may be attributed to the expansion of agricultural and urban areas in the watershed.In contrast, total phosphorus appeared to decline at the sites influenced by dam construction. Lower nutrient concentrations were recorded upstream from the city of Villahermosa, suggesting that inputs from urban areas may have contributed to nutrient loading. Additionally, higher nitrate and total phosphorus concentrations were detected in tributaries draining intensive agricultural and suburban areas. Collectively, the results from the study suggest that dam construction and land conversion in large, tropical watersheds can produce chemical and hydrological changes, which may negatively impact important ecosystem services-such as fisheries and the provisioning of sources of drinking water-and may compromise the integrity of coastal zones.
Nutrient transport by rivers strongly influences ecosystem productivity and hypoxia zone formation in coastal areas. In this study, we estimated the transport of nitrogen and phosphorus by the Grijalva and Usumacinta rivers that combined represent the second largest source of fresh water into the Gulf of Mexico. Water samples were collected monthly within an annual cycle at different sites (mainstem and tributaries) in the Lower Grijalva and Usumacinta basins to encompass the seasonal variation in discharge. Nitrogen (total nitrogen, nitrate-N, ammonium-N, and nitrite-N), phosphorus (total phosphorus, soluble reactive phosphorus), and chlorophyll concentrations were determined, and annual loads estimated using the Beale ratio approach. Nutrient and chlorophyll concentrations were greater in the Grijalva than in the Usumacinta River. Stronger seasonal patterns in nutrient concentrations and discharge were identified in the Usumacinta, evincing the flow regulation experimented by the Grijalva. In the Grijalva, higher nutrient concentrations were measured at the De la Sierra and Pichucalco rivers, compared to the regulated Carrizal. TN:TP ratios were greater during the dry season in both rivers, and higher TN:TP occurred in the Usumacinta than in the Grijalva.The Usumacinta showed greater nutrient loads (nitrogen, 26790.6 Â 10 3 kg year À1 ; phosphorus, 3056.9 Â 10 3 kg year À1 ) than the Grijalva (nitrogen 19974.7 Â 10 3 kg year À1 ; phosphorus, 1941.8 Â 10 3 kg year À1 ). However, the Grijalva River showed higher soluble reactive phosphorus and ammonium-N loads and yields than the Usumacinta, likely explained by greater delivery of urban wastewater in the lower Grijalva Basin. The Grijalva-Usumacinta represents an important source of nitrogen compared to other rivers draining into the Gulf of Mexico, even though more disturbed rivers can exceed the phosphorus load. This study can contribute to the modelling of nutrient delivery into the Gulf of Mexico and to better understand the influence of tropical rivers in the formation of coastal hypoxia.
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