Growth and reproductive responses of the seagrass Zostera marina to sediment nutrient enrichment

Nutrient loading into coastal sediments is increasing due to anthropogenic activity and climate change. We examined the effects of sediment nutrient enrichment on the growth and reproduction of Zostera marina by adding nitrogen (N) and phosphorus (P) fertilizers into sediments. Areal productivity and shoot density increased by ca. 60% in N and N+P enrichment plots and by ca. 20% in the P enrichment plots. Biomass and shoot height were also higher in the N and N+P enrichment plots than in the P enrichment and control plots. These results suggest that sediment N availability was more important than P availability in stimulating the vegetative growth of Z. marina. The density and morphology of reproductive shoots and seed production increased in only the N enrichment plots. The sediment N enrichment stimulated both the vegetative growth and sexual reproduction, improving the meadow resilience through both sexual and asexual mechanisms. The P enrichment slightly increased only the vegetative growth and might have limited influence on seagrass reproduction. According to these results, the alteration of the sediment nutrient regimes might shift the balance between the vegetative growth and sexual reproduction of Z. marina. These findings may have important implications for the management of seagrass meadows under fluctuations in sediment nutrients caused by anthropogenic activity and climate change.