[A-CG23] Coastal Ecosystems - 1. Water Cycle and Land-Ocean Interactions

[ACG23-09] Analysis of the coastal food web and influence of terrestrial input in Toyama Bay using δ¹³C and δ¹⁵N

Chihiro Urasawa¹, *JING ZHANG¹, Osamu Inamaura², Shota Kambayashi¹, Shohei Hattori³, Naohiro Yoshida³ (1.University of Toyama, 2.Uozu Aquarium, 3.Tokyo Institute of Technology)

Keywords:Toyama Bay, Carbon and Nitrogen stable isotope ratio, Food web

Toyama Bay is a semi-enclosed bay facing the Sea of Japan, which has a distinctive coastal environment. It receives a large amount of freshwater (such as river and submarine groundwater discharge (SGD)) and it is also affected by the Japan Sea water. In this study, to evaluate terrestrial effect on coastal organisms in Toyama Bay, we analyzed food web structure and estimate utilization situation of organic matter and nutrient (NO₃^-) derived from land in food web using stable isotope ratios (δ¹³C・δ¹⁵N ; δ¹⁵N・δ¹⁸O in nitrate).
The δ¹³C values of consumers (such as bivalve, crustacea and fish) in the coastal area of Toyama Bay were clearly higher than those in the riverine particulate organic matter (POM). This indicates that consumers do not use terrestrial organic matter as their carbon source. The calculated contribution of benthic microalgae to diet of consumers was about 30 - 60 %, suggesting that ¹³C-enriched benthic microalgae is an important carbon source for the coastal food web in Toyama Bay.
Primary producer in coastal area of Toyama Bay shows much lower δ¹⁵N values than typical primary producer in other coastal area. In addition, δ¹⁵N (NO₃^-) values of coastal area in Toyama Bay are also lower than the δ¹⁵N (NO₃^-) values of deep seawater of Toyama Bay and Japan Sea water. Low δ¹⁵N (NO₃^-) values were also observed in the rivers and SGD around Toyama Bay. Our results suggest that coastal organisms in Toyama Bay are influenced by terrestrial input of low δ¹⁵N (NO₃^-) through the rivers and SGD.