[A-HW27] Biodiversity, nutrients and other materials in ecosystems from headwaters to coasts

[AHW27-10] Assessment of environmental impacts using nitrogen stable isotope ratios of seaweeds and terrestrial groundwater samples from in Sekisei Lagoon, Okinawa

*Nanoka Hirata¹, Takahiro Hosono², Takatomo Ikehara³, Ko Yasumoto⁴, Jun Yasumoto⁵, Mariko Iijima⁶, Ryuichi Shinjo⁷ (1.Kumamoto University, 2.Faculty of Advanced Science and Technology, Kumamoto University, 3.Graduate School of Science and Technology, 4.Kitasato University School of Marine Biosciences, 5.University of the Ryukyus, Faculty of Agriculture , 6.National Institute of Advanced Industrial Science and Technology , 7.Research Institute for Humanity and Nature)

Keywords:Sekisei Lagoon, Nutrients, Groundwater, Stable nitrogen isotope, Seaweed

One of Japan's largest coral reefs, Sekisei Lagoon, which stretches between Ishigaki and Iriomote Islands in the Yaeyama Islands, Okinawa Prefecture, is experiencing a decline in its coral reef ecosystem and an increase in the number of seaweed species. One of the possible causes is nutrient run-off from the land. Opinions differ on the direct causal relationship between nutrient salts and corals, but it is believed that nutrient salt loading promotes algal growth and causes some changes in the ecosystem structure. In order to solve the nutrient runoff problem, it is necessary to clearly demonstrate to what extent and how nutrient runoff from terrestrial areas affects marine areas. However, few studies have focused on the nutrient linkages between land and sea, which are different environments. Therefore, using the nitrogen stable isotope ratio (δ¹⁵N), which is used as a tracer of anthropogenic nitrogen discharge, the effects of nitrogen loading from land to sea were qualitatively analyzed by examining both terrestrial groundwater and marine algae in the sea area.
Groundwater was sampled on four islands around Sekisei Lagoon (Ishigaki, Kuroshima, Taketomi and Kohama) and seaweed was collected from a wide area around the islands. The δ¹⁵N of the groundwater and seaweeds, respectively, was measured using a mass spectrometer (ThermoFisher Scientific, Delta V) and the results compared.
Analysis of the ionic composition and stable isotope ratios of the groundwater showed that the values differed from island to island due to the topography and industry, including areas of mixing with seawater and the Ryukyu limestone unique to Okinawa Prefecture. The δ¹⁵N in groundwater was generally distributed in the range of 7-10‰, and a number of sites with possible denitrification were observed.
The δ¹⁵N of seaweeds was generally within the range of 0-3‰, although there were locally large δ¹⁵N values at some sampling points. The highest δ¹⁵N was found at a site near the drainage outlet of the Taketomi Island shrimp farm, suggesting that the impact of anthropogenic nitrogen loading from the terrestrial area is also expressed in seaweeds near the coastal area. In the eastern part of Kuroshima, where the terrestrial nitrogen load is considered to be high due to groundwater results and the influence of livestock farming, the δ¹⁵N of seaweeds tended to increase at sites close to the coast and decrease as the distance from the shore increased. The results also showed that the δ¹⁵N of seaweeds showed a good correlation with the accumulated phosphorus concentration in the bottom sediment of Sekisei Lagoon.
Finally, by matching the results of both groundwater and seagrass with δ¹⁵N, the impact of anthropogenic and livestock nitrogen loading from land to sea could be quantified and demonstrated. Although various issues remain, such as distance from coastal areas, consideration of areas with high terrestrial-derived loads and the validity of threshold values, the nitrogen stable isotope ratios can provide clues to clarify areas with high nutrient loads from terrestrial areas and to identify areas for improvement in future nature conservation activities.