[A-CG53] Coastal Ecosystems - 1. Water Cycle and Land-Ocean Interactions

[ACG53-07] Evaluation of nutrient flux and carbonate system in subtropical shallow seagrass meadows

Kento Suzuki¹, *Yu Umezawa¹, Keisuke Umebayashi¹, Kurihara Haruko², Takashi Nakamura³, Toshihiro Miyajima⁴ (1.Tokyo University of Agriculture and Technology, 2.The University of the Ryukyus, 3.Institute of Science Tokyo, 4.Atmosphere and Ocean Research Institute, the University of Tokyo)

Keywords:coral reefs, seagrass bed, nutrient uptakes, photosynthesis/respiration, calcification/dissolution

Terrestrial loadings with higher nutrient concentrations, and increased seawater temperatures & acidification associated with global climate change, negatively impact coral communities. Against these environmental stresses, seagrass beds are expected to function as buffer vegetation zones due to the metabolic activities of seagrass beds involving elements such as carbon and nitrogen. However, while many studies have focused on the metabolic activities of seagrasses as individual plants, few studies have quantified the buffering capacity of seagrass beds, including the sediment and the other organisms. In this study, we observed changes in nutrients and carbonate systems in seagrass beds based on the chamber method both in the field (Bise coral reefs) and in outdoor flow tanks, using Thalassia hemprichii, a representative seagrass species in tropical and subtropical areas, and discussed the function of seagrass beds in coral reef ecosystems.
Both experimental systems transferred from the seagrass and sandy zones showed nutrient uptake trends in response to nutrient addition at different levels. Especially in the seagrass zone, higher nutrient uptake was also observed at night, compared with the sandy zone. While photosynthesis and calcification dominated during the day in the seagrass zone, dissolution calcareous sediment was dominant at night in both seagrass and sandy zones.
Seagrass beds distributed near the coast of coral reefs may provide suitable habitats for corals and other organisms by absorbing high concentrations of nutrients and re-releasing them at moderate concentrations. In addition, it is expected that growth-friendly environments for calcifying organisms such as corals are provided by seagrass beds due to their functions, that is, 1) CO₂ absorption (increase in pH) by photosynthesis and 2) dissolution of calcium carbonate sediment at reductive sediment in seagrass bed, resulting in releasing high TA (Total Alkalinity) water into the water column.