Sediment Oxygen Demand in Cochin backwaters, a tropical estuarine system in the south-west coast of India

Benthic nutrients and oxygen fluxes at the water sediment interface in a pearl farming atoll (Ahe, Tuamotu, French Polynesia)

Benthic exchanges of oxygen and nutrient at the sediment-water interface were investigated under light and dark conditions at 5 selected sites in a sub-tropical atoll. Mean oxygen fluxes were - 1316.5 ± 242.0 μmol m^-2 h^-1 and mean effluxes of oxygen under light conditions were 2231.7 ± 626.4 μmol m^-2 h^-1, presumably due to microphytobenthos present at the sediment-water interface. The consequences of this high related productivity was a systematic consumption of nutrients (DIN, PO₄ and Si(OH)₄) during almost all light incubations, contrasting with the effluxes of nutrients during dark incubations. Our results suggest that the sediments were net autotrophic and the oxygen balance in favor of microbenthic production when compared to community demand. Diurnal rates of gross benthic primary productivity were high (3423 ± 1192 μmol m^-2 h^-1) which emphasize the role of microphytobenthos in maintaining the oxygen reservoir in tropical lagoons.

Estandarización de un método de laboratorio para la medición de la demanda béntica de oxígeno

Actualmente no hay un método estandarizado para medir la Demanda Béntica de Oxígeno (SOD, por sus siglas en inglés Sediment Oxygen Demand), por ello, se propuso en este trabajo estandarizar una metodología para la estimación de este parámetro utilizando sedimentos de distintas características, extraídos con una draga Eckman. Para determinar la SOD en el laboratorio, se realizaron mediciones del oxígeno disuelto cada hora durante cuatro horas, en una adaptación del montaje de laboratorio propuesto por Bowman y Delfino en 1980 [1], donde el reactor utilizado cumple con la relación volumen/área recomendada por estos autores (132 L/m2). Los sedimentos analizados, variaron su granulometría desde 1 a 0.5 mm, con contenidos de materia orgánica desde 0% hasta 54%. Se encontró que el método es preciso, mostrando coeficientes de variación entre el 1.81% y 19.51% para las muestras estudiadas, se comprobó la exactitud para una de ellas con un porcentaje de error de 1.54%; además, se encontró que el límite de detección del método fue de 0.102 g O2/m2*d. Se concluyó también que al aumentar el contenido de materia orgánica en los sedimentos, aumenta el valor de la SOD.

A bench-scale assessment for phosphorus release control of sediment by an oxygen-releasing compound (ORC)

The effects of oxygen-releasing compound (ORC) on the control of phosphorus (P) release as well as the spatial and temporal distribution of P fractions in sediment were studied through a bench-scale test. An ORC with an extended oxygen-releasing capacity was prepared. The results of the oxygen-releasing test showed that the ORC provided a prolonged period of oxygen release with a highly effective oxygen content of 60.6% when compared with powdery CaO2. In the bench-scale test, an ORC dose of 180 g·m(-2) provided a higher inhibition efficiency for P release within 50 days. With the application of the ORC, the dissolved oxygen (DO) concentration and redox potential (ORP) of the overlying water were notably improved, and the dissolved total phosphorus (DTP) was maintained below 0.689 mg·L(-1) compared to 2.906 mg·L(-1) without the ORC treatment. According to the P fractions distribution, the summation of all detectable P fractions in each sediment layer exhibited an enhanced accumulation tendency with the application of ORC. Higher phosphorus retention efficiencies were observed in the second and third layers of sediment from days 10 to 20 with the ORC. Phosphorus was trapped mainly in the form of iron bound P (Fe-P) and organically bound P (O-P) in sediment with the ORC, whereas the effects of the ORC on exchangeable P (EX-P), apatite-associated P (A-P) and detrital P (De-P) in the sediment sample were not significant. The microbial activities of the sediment samples demonstrated that both the dehydrogenase activity (DHA) and alkaline phosphatase activity (APA) in the upper sediment layer increased with the ORC treatment, which indicated that the mineralization of P was accelerated and the microbial biomass was increased. As the accumulation of P suppressed the release of P, the sediment exhibited an increased P retention efficiency with the application of the ORC.