NITROGEN ISOTOPES IN MACROALGAE ALONG THE COAST OF JAPAN – INSIGHTS INTO ORGANIC COASTAL POLLUTION

Macroalgae attached to substrates are well known to record organic pollution via their nitrogen isotope signatures. Wastewater (from human sewage or animal waste) entering the coastal zone is expected to have high δ¹⁵N values and macroalgae record this elevated δ¹⁵N signature in their tissues. Here we present data from macroalgae collected in 2015, 2016, and 2023 along coastal areas in Japan. We focus on four regions, Tokyo Bay (N=13), Oshima Island offshore from Tokyo (N=19), Sanriku coast in North Eastern Japan (N=7), and Amanohashidate in northern Kyoto Prefecture (N=10). Sites within Tokyo Bay generally had higher values (+12‰ to +16‰), whereas the site just outside the Bay was lower (+5‰). Nitrogen isotope values from the Sanriku coast were variable with values between +5‰ and +14‰ and on average were lower than the sites in Tokyo Bay. While samples from Oshima and Amanohashidate are still being processed, these preliminary results indicate organic nutrient loading along the Japanese coast. The data in this study could provide a baseline to compare with future isotope data of macroalgae to assess the efficiency of upgrades to waste water treatment plants.