Regulation of Nitrate Uptake by the Seagrass Zostera marina During Upwelling

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 (1h) 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 Quintin Bay meadows, Baja California, Mexico, differentially exposed to upwelled NO3-. Results suggest that NO3- enrichment stimulated DIC and NO3- uptakein winter, reflecting the close relationship between carbon metabolism and NO3- assimilation. Eelgrass shoots showed reduced NO3- incorporationin 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 4.2mmolNm(-2)day(-1). This represents non-trivial water column NO3- removal compared to the estimated oceanic NO3- supply (similar to 7.1mmolm(-2)day(-1)) during upwelling, highlighting the importance of Z. marina beds in controlling the lagoonal N-budget.