Nitrogen Uptake and Internal Recycling in Zostera marina Exposed to Oyster Farming: Eelgrass Potential as a Natural Biofilter

Oyster farming in estuaries and coastal lagoons frequently overlaps with the distribution of seagrass meadows, yet there are few studies on how this aquaculture practice affects seagrass physiology. We compared in situ nitrogen uptake and the productivity of Zostera marina shoots growing near off-bottom longlines and at a site not affected by oyster farming in San Quintín Bay, a coastal lagoon in Baja California, Mexico. We used benthic chambers to measure leaf NH₄⁺ uptake capacities by pulse labeling with ¹⁵NH₄⁺ and plant photosynthesis and respiration. The internal 15N resorption/recycling was measured in shoots 2 weeks after incubations. The natural isotopie composition of eelgrass tissues and vegetative descriptors were also examined. Plants growing at the oyster farming site showed a higher leaf NH₄⁺ uptake rate (33.1 mmol NH₄⁺ m⁻² day⁻¹) relative to those not exposed to oyster cultures (25.6 mmol NH₄⁺ m⁻² day⁻¹). We calculated that an eelgrass meadow of 15-16 ha (which represents only about 3-4 % of the subtidal eelgrass meadow cover in the western arm of the lagoon) can potentially incorporate the total amount of NH₄⁺ excreted by oysters (~5.2 × 10⁶ mmol NH₄⁺ day⁻¹). This highlights the potential of eelgrass to act as a natural biofilter for the NH₄⁺ produced by oyster farming. Shoots exposed to oysters were more efficient in re-utilizing the internal ¹⁵N into the growth of new leaf tissues or to translocate it to belowground tissues. Photosynthetic rates were greater in shoots exposed to oysters, which is consistent with higher NH₄⁺ uptake and less negative δ¹³C values. Vegetative production (shoot size, leaf growth) was also higher in these shoots. Aboveground/belowground biomass ratio was lower in eelgrass beds not directly influenced by oyster farms, likely related to the higher investment in belowground biomass to incorporate sedimentary nutrients.