Regulation of Nitrate Uptake by the Seagrass Zostera marina During Upwelling
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Prolonged nitrogen (N) fertilization can impact seagrass survival and productivity; however, the effects of N enrichment pulses (e.g., upwelling or sediment resuspension) remain poorly understood. This study examined the effects of short-term (1 h) pulsing of nitrate (NO3−) enrichment, simulating an upwelling event, on dissolved inorganic carbon (DIC) and NO3− uptake capacities, critical in controlling eelgrass productivity. Zostera marina dominates submerged vegetation in coastal lagoons influenced by upwelling in the California Current system. Laboratory incubations were conducted in winter (non-upwelling) and spring (upwelling) with shoots collected from San Quintín Bay meadows, Baja California, Mexico, differentially exposed to upwelled NO3−. Results suggest that NO3− enrichment stimulated DIC and NO3− uptake in winter, reflecting the close relationship between carbon metabolism and NO3− assimilation. Eelgrass shoots showed reduced NO3− incorporation in spring; neither NO3− uptake nor photosynthesis increased when exposed to high NO3−. Saturation of spring shoots at lower ambient NO3− concentrations may be interpreted as a physiological strategy to restrict metabolically costly NO3− incorporation during upwelling; this regulation of NO3− uptake strongly contrasts to the apparently full exploitation of this nutrient by seaweeds also dominant within the bay, as indicated in previous works. Despite their reduced NO3− uptake, eelgrass meadows near the bay mouth acquire NO3− at rates up to 4.2 mmol N m−2 day−1. This represents non-trivial water column NO3− removal compared to the estimated oceanic NO3− supply (~ 7.1 mmol m−2 day−1) during upwelling, highlighting the importance of Z. marina beds in controlling the lagoonal N-budget.
KeywordsZostera marina Upwelling Pulsed nitrate fertilization DIC uptake N uptake
This research was funded by the National Council for Science and Technology (CONACYT, Mexico, project CB-2010-01-154376 awarded to VFCI). JMSG was supported by a postdoctoral grant from the Mexican Ministry of Public Education (SEP, PROMEP/103.5/13/5009). We are especially grateful to Julieta Hernández, Nevia Alfaro, and Jesús Galarza (IIO-UABC) for their technical support.
We sincerely thank two anonymous reviewers for their comments which helped to improve the final version of this manuscript.
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