This paper provides evidence of the effects of urban wastewater discharges on the trophic state and environmental quality of a coastal water body in a semiarid subtropical region in the Gulf of California. The concentrations of dissolved inorganic nutrients and organic matter from urban wastewater primary treatment were estimated. La Salada Cove was the receiving water body and parameters measured during an annual cycle were temperature, salinity, dissolved oxygen, nitrite, nitrate, ammonia, orthophosphate, and chlorophyll a. The effects of sewage inputs were determined by using Trophic State Index (TRIX) and the Arid Zone Coastal Water Quality Index (AZCI). It was observed that urban wastewater of the city of Guaymas provided 1,237 ton N yr−1 and 811 ton P yr−1 and TRIX indicated that the receiving water body showed symptoms of eutrophication from an oligotrophic state to a mesotrophic state; AZCI also indicated that the environmental quality of the water body was poor. The effects of urban wastewater supply with insufficient treatment resulted in symptoms of eutrophication and loss of ecological functions and services of the coastal ecosystem in La Salada Cove.

Nutrient flux, net metabolism, and N 2 fixation and denitrification processes were estimated and compared in two semi-arid subtropical coastal lagoons in the Gulf of California: El Soldado (ES), where no wastewater is discharged, and El Rancho (ER), where shrimp farm effluents are regularly discharged. Biogeochemical processes were evaluated with the LOICZ model. Flushing time was <2 days in both systems. Nutrient fluxes were higher in ER than in ES and both systems acted as nutrient sinks for most of the year. ER showed a larger nutrient flow rate in the summer and autumn due to the input of shrimp-farm effluents. Nitrogen fluxes increased in both ER and ES in the winter in response to the increased nutrient supply from coastal upwellings. ER and ES both showed autotrophic metabolism and N 2 fixation in the spring, autumn, and winter, but heterotrophic metabolism and denitrification in the summer. Denitrification dominated in ER (−2.21 mmol m 2 day −1 ) and values were higher than those estimated in ES (−0.45 mmol m 2 day −1 ). The comparative analysis between ES and ER evidenced the significant changes in its biogeochemical performance caused by the input of anthropogenic nutrients, and can orient the environmental management of coastal lagoons.

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The hydrological characteristics and the rate of organic matter supply from coastal lagoons help to understand their responses to nutrient input by anthropogenic and natural sources. The aim of this study was to determine the hydrologic and trophic status of the El Rancho-Empalme lagoon system in a semiarid region in the Gulf of California, Mexico. This lagoon system consists of two geomorphological subsystems which are affected by nutrient inputs from shrimp farm effluents and coastal upwelling. Sampling was conducted over the course of one year and included measurements of temperature, salinity, and dissolved oxygen, surface water grab samples to analyze nitrite, nitrate, ammonia, orthophosphate, and chlorophyll

Se evaluaron el estado trófico, la dinámica de nutrientes y el metabolismo neto de una laguna costera subtropical ubicada en la costa este del Golfo de California. La laguna Santa María es un sistema receptor de aguas residuales urbanas, camaronícolas y agrícolas. En la laguna y el mar adyacente se realizaron muestreos de agua en verano e invierno para obtener información de temperatura, salinidad, oxígeno disuelto, nutrientes inorgánicos disueltos y clorofila a. El estado trófico se estimó con el índice TRIX, mientras que los flujos de nutrientes y el metabolismo neto se estimaron con el modelo biogeoquímico LOICZ. La laguna exhibió un estado mesotrófico en verano e invierno. Las tasas de renovación del agua estimadas fueron de 22 días en verano y 10 días en invierno. Los flujos de nutrientes mostraron que la laguna fue sumidero en verano (ΔNID = –0.84 mmol/m2/día y ΔFID = –0.12 mmol/m2/día) y fuente en invierno (ΔNID = 0.04 mmol/m2/día y ΔFID = 0.15 mmol/m2/día). En verano, el metabolismo neto del ecosistema (MNE) fue autótrofo (13 mmol C/m2/día) y predominó el proceso de fijación de nitrógeno (+ 1.2 mmol/m2/día). En invierno, el MNE fue heterótrofo (–16.36 mmol/m2/día) y predominó la desnitrificación (–2.3 mmol/m2/día). Se concluye que la laguna Santa María funciona asimilando nutrientes con predominancia de metabolismo autótrofo y fijación de nitrógeno en verano, así como exportando nutrientes con predominancia de metabolismo heterótrofo y desnitrificación en invierno. La laguna mantiene un estado mesotrófico en ambas estaciones del año mostrando capacidad de asimilar, transformar y exportar las cargas de nutrientes sin manifestar procesos adversos de eutrofización.