Dissolved inorganic nitrogen uptake kinetics and δ15N of Zostera marina L. (eelgrass) in a coastal lagoon with oyster aquaculture and upwelling influence

Differences in dissolved inorganic nitrogen (DIN) availability can lead to intraspecific physiological differences in the DIN incorporation capacities of seagrasses. The objective of this study was to evaluate these physiological differences in Zostera marina (eelgrass) exposed to anthropogenic (oyster aquaculture) and natural (upwelling events) inputs of DIN in San Quintín Bay (Baja California, México). To this end, shoots from meadows in different sites within the bay (from the bay mouth to the inner parts of the bay), and differently exposed to nitrogen effluents from oyster aquaculture and upwelling events, were incubated with 15N-labeled tracers (15NH4+ and 15NO3−) under laboratory controlled conditions. DIN uptake kinetics and δ15N isotopic signals were assessed in months without upwelling influence (February) and with recurrent upwelling events (June). Results showed that eelgrass is highly efficient in acquiring DIN (mostly NH4+ by leaves and roots) from the water column and sediment pore-water to support their vegetative productivity. However, noticeable intraspecific differences in uptake kinetics strategies were observed. In both months, eelgrass shoots exposed to oyster aquaculture showed higher capacities (maximum uptake rate, Vmax, and/or affinity coefficient, α) to acquire NH4+ compared to shoots from other meadows. This suggests that these plants are efficient in exploiting a valuable source of NH4+ from oyster excretion or biodeposit remineralization. Shoots growing in the bay mouth which are the most exposed to upwelled waters, exhibited relatively low leaf Vmax and Km for NO3− in June. These properties allow these plants to restrict the uptake of NO3− from upwelling events, probably to avoid the potential toxicity resulting from its uptake in excess. Intraspecific differences in δ15N were also evident, probably related to the isotopic signal of the DIN source and the relative incorporation of NH4+ and NO3− by leaves and roots. Generally, higher values of δ15N were found in leaves and rhizomes of plants exposed to oyster aquaculture than those growing in the bay mouth. This indicated the incorporation of more 15N-enriched DIN (mainly NH4+) by the shoots from the meadow near oysters, possibly resulting from biogeochemical processes enhanced by oyster biodeposition. The lower δ15N in plants growing in the bay mouth likely resulted from the higher uptake of more isotopically-depleted NO3− from oceanic waters relative to the total DIN incorporation.