Macroalgae as a Proxy for Organic Pollution along the Coast of Japan

            Following the Green Revolution, nitrogen availability increased tremendously due to industrialized agriculture and inorganic fertilizer creation. This has led to increased eutrophication in global coastal regions, creating algal blooms and dead zones. In addition to fertilizer runoff, sewage from faulty or aging wastewater treatment plants or leaking septic tanks contribute to increased nitrogen loading to aquatic systems. Tracing the source of nitrogen pollution is the first and most crucial step in mitigating and preventing eutrophication. This study uses stable nitrogen isotopes of macroalgae collected along the coast of Japan to detect sources of nitrogen loading. A total of 49 samples of macroalgae were collected from four regions of Honshu Island: Tokyo Bay in Tokyo Prefecture (N=13), Oshima Island in Ehime Prefecture (N=19), the Northeast Sanriku Coast in Iwate Prefecture (N=7), and the Amanohashidate coastline in Kyoto Prefecture (N=10). Of the samples taken, 35 were analyzed and compared against the population density of their respective prefectures. Samples from Tokyo Bay and the southern portion of the Sanriku Coastline exhibited elevated δ¹⁵N (+12‰ to +16‰), an indication of sewage entering the aquatic environment. δ¹⁵N values of macroalgae decreased with distance to populous city areas, revealed by lower δ¹⁵N values (+5‰) in algae samples collected from Oshima Island and along the northern portion of the Sanriku Coastline. These preliminary results can help identify regions at risk of eutrophication and which are of concern to human health due to wastewater entering the aquatic system along the coastlines of Honshu Island.