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

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 × 103 kg year−1; phosphorus, 3056.9 × 103 kg year−1) than the Grijalva (nitrogen 19974.7 × 103 kg year−1; phosphorus, 1941.8 × 103 kg year−1). 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. The Grijalva–Usumacinta river system is the second freshwater input and an important source of nutrients for the Gulf of Mexico. Higher nutrient and chlorophyll concentrations were measured in the Grijalva River, as result of higher human pressures related to urbanization and agriculture. Seasonal patterns in discharge and nutrient concentration were stronger in the Usumacinta River, evincing the flow regulation in the Grijalva