[A-CG51] Coastal Ecosystems-2. Coral reefs, seagrass and macroalgal beds, and mangroves

[ACG51-P01] Visualization of seagrass buffering efficiency to terrestrial impacts on coral reef area using a coupled pelagic-benthic model incorporating CNP isotopes/tracers

*Takashi Nakamura¹, Yuta A. Takagi¹, Masataka Muto², Jorge Luis Suarez Caballero¹, Haruko Kurihara³, Yu Umezawa⁴, Toshihiro Miyajima⁵ (1.School of Environment and Society, Institute of Science Tokyo, 2.Tokyo Institute of Technology (present address: JAXA), 3.University of the Ryukyus, 4.Tokyo University of Agriculture and Technology, 5.AORI, The University of Tokyo)

Keywords:coral reef, seagrass, coupled pelagic-benthic model, numerical simulation, isotope, tracer

Terrestrial loads, such as nutrients and sediment runoff, may negatively impact on coral reef ecosystems (e.g., Fabricius, 2005). Some ecosystems that inhabit between the river mouth and coral community area, such as seagrass meadows and mangroves, have the potential to buffer the terrestrial impact on coral communities in coral reef areas. However, the quantitative evaluation of these buffering efficiencies by seagrasses or mangroves has not been well studied. In this study, we developed a novel integrated pelagic-benthic model incorporating carbon, nitrogen and phosphorus isotopes/tracers, and quantitatively evaluated and visualized the buffering effect by seagrass meadow on the Shiraho coral reef area, east coast in Ishigaki Island.
The proposed model is composed of a pelagic lower-trophic level model and a benthic model. In addition, the model is coupled with a seagrass model. The pelagic model includes some water quality parameters; dissolved oxygen (DO), total alkalinity (TA), dissolved inorganic carbon (DIC), nitrate (NO₃⁻), ammonium (NH₄⁺), phosphate (PO₄³⁻), four functional groups of phytoplankton, one functional group of zooplankton, three functional groups of particulate organic matter (POM) (coarse POM, labile POM, refractory POM) and two functional groups of dissolved organic matter (DOM) (labile DOM and refractory DOM), and two functional groups of particulate inorganic carbon (PIC; such as CaCO₃) (living PIC and dead PIC). The benthic model includes DO, TA, DIC, NO₃⁻, NH₄⁺, PO₄³⁻, coarse POM, labile POM, refractory POM, labile DOM, refractory DOM, dead PIC, MnO₂, Mn²⁺, FeOOH, FeOOH≡PO₄³⁻, Fe²⁺, FeS, FeS₂, SO₄²⁻, H₂S, and S⁰. In the model, aerobic respiration, denitrification, iron reduction, manganese reduction, and sulfate reduction are mainly included in organic matter decomposition processes. All parameters including carbon, nitrogen, and phosphorus elements have carbon 13 isotope, nitrogen 15 isotope, and phosphorus tracer element, respectively, for chasing how the element from the specific sources will be reallocated to the other compartment. The seagrass model incorporates nutrients (NO₃⁻, NH₄⁺, PO₄³⁻) uptake process by seagrass leaves and roots based on incubation experimental results (Takagi et al., in prep.). Therefore, the model can estimate not only the total uptake rate of nutrients but also the uptake rate of specific sources of nutrients, such as river discharge and submarine groundwater discharge. This ecosystem model is coupled with the Coupled Ocean-Atmosphere-Wave-Sediment Transport (COAWST) Modeling System (Warner et al., 2010), and the seagrass vegetation drag module implemented in the COAWST modeling system (Beudin et al., 2016) was also activated to evaluate sediment trapping efficiency and reduction efficiency of sediment resuspension by seagrass.
This integrated model was applied to the Shiraho coral reef area, Ishigaki Island, following the simulation domain settings by Nakamura et al. (2018). Carbon, nitrogen and phosphorus in DIC and nutrients (NO₃⁻, NH₄⁺, PO₄³⁻) supplied from both Todoroki River and submarine groundwater were specifically marked as tracer elements, and the DIC and nutrients with tracers were discharged into the Shiraho computational domain. Red soil was also discharged from Todoroki River.
We set two cases; healthy seagrass meadows (Case 1), and all seagrass removed (Case 2) for assessing the seagrass buffering efficiency of terrestrial impacts by comparing Cases 1 and 2. The simulation results suggested that seagrass meadows effectively trap sediment discharged from the river and reduce sediment resuspension. In addition, it was visualized that nutrients derived from both river water and groundwater were effectively absorbed by seagrass and accumulated in the seagrass body using C, N, and P tracer simulation.

References:
Beudin et al. (2016) doi: 10.1016/j.cageo.2016.12.010
Fabricius (2005) doi: 10.1016/j.marpolbul.2004.11.028
Nakamura et al. (2018) doi: 10.1007/s00338-017-1632-3
Warner et al. (2010) doi: 10.1016/j.ocemod.2010.07.010