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Dinitrogen (N2) fixation rates were determined in the phyllosphere and in sediments with and without the presence of eelgrass (Zostera marina) in San Quintin Bay, an upwelling-influenced coastal lagoon in the NE Pacific. Samples were collected during winter 2015 at 4 sites with a gradient of oceanic influence and contrasting impact from oyster aquaculture. N2 fixation rates were determined with the acetylene reduction assay. Treatments under light and in the dark, and with and without sodium molybdate resulted in similar fixation rates, suggesting that heterotrophic nonsulfate-reducing bacteria made the largest contribution to N2 fixation, while sulfate-reducing bacteria had low fixation activity at most stations. N2-fixation rates in sediments ranged from 7 to 12 micrometers mol m–2 h–1 and were similar to those in other temperate seagrass-dominated estuaries. Winter conditions were likely responsible for small spatial differences in N2 fixation rates throughout the lagoon, even between vegetated and unvegetated sites and among depth sections of sediment cores. During winter, Z. marina growth rates and biomass are low, resulting in low and less variable release of labile organic carbon, which acts as substrate for diazotrophs. The lowest N2 fixation rates were measured at a site where high denitrification rates have been observed, probably reflecting a competition between diazotrophs and denitrifiers. The highest fixation rates were measured at the innermost station where oceanic nitrate is scarce. Epiphytic bacteria contributed ~7% of the total N2 fixation, with rates of <0.5 micrometers mol m–2 h–1. N2 fixation potentially supplied 5–10% of Z. marina N requirements and could supply ~30% of the N loss via denitrification in winter.   Se determinaron las tasas de fijacion de nitrogeno molecular (N2) en la filosfera y en sedimentos con y sin presencia de pasto marino (Zostera marina) en bahia San Quintin, una laguna costera del Pacifico nororiental influenciada por surgencias. Se recolectaron muestras durante el invierno de 2015 en 4 sitios a lo largo de un gradiente de influencia oceanica y con diferente grado de impacto por la ostricultura. Las tasas de fijacion de N2 se determinaron con el metodo de reduccion de acetileno. Las tasas de fijacion en los tratamientos con luz y en oscuridad, y en los tratamientos con y sin molibdato de sodio fueron similares, lo que sugiere que las bacterias heterotrofas no sulfatoreductoras fueron las principales contribuyentes a la fijacion de N2, mientras que las sulfatoreductoras presentaron baja actividad de fijacion en la mayoria de los sitios. Las tasas de fijacion de N2 presentaron un intervalo de 7 a 12 micrometros mol m–2 h–1 y fueron similares a las reportadas para otros estuarios dominados por praderas de pastos marinos en zonas templadas. Las condiciones ambientales de invierno probablemente determinaron las pequenas diferencias en las tasas de fijacion de N2 a lo largo de la laguna, incluso entre sitios con y sin vegetacion o entre las diferentes secciones de los nucleos de sedimento. Durante el invierno, las tasas de crecimiento y las biomasas de Z. marina son bajas, lo que resulta en una liberacion baja y poco variable de carbono organico labil, sustrato importante para los diazotrofos. Las tasas de fijacion de N2 mas bajas se midieron en el sitio donde aparentemente se presentan altas tasas de desnitrificacion, lo que probablemente refleja una competencia entre diazotrofos y desnitrificadores. Las tasas de fijacion mas altas se midieron en la estacion mas interna, donde es escaso el aporte de nitrato oceanico. Las bacterias epifitas contribuyeron alrededor del 7% de la fijacion total de N2, con tasas <0.5 micrometros mol m–2 h–1. La fijacion de N2 potencialmente abastecio entre el 5% y 10% del N requerido por Z. marina y podria abastecer ~30% del N que se pierde via la desnitrificacion en invierno.

Benthic nitrogen fixation in Zostera marina meadows in an upwelling-influenced coastal lagoon