Nutrient overenrichment in aquatic environments, or eutrophication, is increasingly affecting seagrass habitats around the world, leading to the degradation of seagrass ecosystems and the loss of associated ecosystem services. The resistance of seagrass to nutrient enrichment is believed to be site specific, yet the direct correlations between environmental factors and the resistance of seagrass meadows has not yet been adequately resolved. The response of seagrass beds to long-term multiple-level sediment nutrient enrichment represents a current research gap. This study therefore assessed the in situ long-term influence of four levels of sediment urea fertilizer addition on Zostera muelleri plants using a manipulative experimental design at three sites with diverse sediment characteristics. Specifically, this study investigated the response of porewater nutrient concentration, seagrass morphology indices, and seagrass plant tissue carbon and nitrogen contents under experimental nutrient stress levels. Over a 13-month experiment, sediment nutrient addition substantially increased the ammonium concentrations in the sediment porewater, significantly decreased seagrass morphology indices, and caused a carbon and nitrogen imbalance in plant tissues. Ammonium toxicity was more pronounced in seagrass meadows growing on sandy substrates with lower relative above-ground biomass, which showed low resistance to eutrophication at sediment ammonium concentrations of up to 1 × 104 μM. Plants growing in muddy sand substrates with higher relative above-ground biomass were more resistant to nutrient impacts, however, with a delayed negative response observed in response to higher nutrient concentrations (up to 1 × 105 μM). Our findings suggest that seagrasses in low-nutrient sandy environments show more rapid responses to eutrophication, although seagrasses in both sandy and muddy substrates exhibited declines in response to a long-term elevation of sediment nutrient concentrations. While continuing to assess the relative susceptibility of seagrass habitats across differing environments to inform coastal management, conservation efforts should allocate resources accordingly and prioritize reducing nutrient overenrichment in sensitive seagrass meadows.
|Number of pages
|Aquatic Conservation: Marine and Freshwater Ecosystems
|Published - Jul 25 2019
Bibliographical noteKAUST Repository Item: Exported on 2020-10-01
Acknowledgements: The authors thank Prof. Simon Thrush for assistance with the structuring of the article and suggestions for the statistical analyses of the data. The authors also thank the Manaaki Te Awanui research team (Caine Taipa, Paranihi Lovett, and Vanessa Taikato), the Coastal Marine Field Station of the University of Waikato (David Culliford and Te Puea Dempsey), the National Institute of Water and Atmospheric Research Ltd (Sanjay Wadhwa), Cawthron Institute (Emma Newcombe), the School of Environment (Natalia Abrego), and Leigh Marine Laboratory of the University of Auckland for logistics, field, and laboratory work support. This research was funded in part by the Bay of Plenty Regional Council.