Abstract
This study examined whether river has an influence on primary producers and consumers by comparing un-eutrophicated coastal lagoons with and without direct river influence (AK: Akkeshi-ko, HN: Hichirippu-numa), based on the carbon and nitrogen stable isotope ratios (δ13C, δ15N), for the sustainable use of the shallow coastal area. The C/N ratios, δ13C and δ15N of organic matters in the both lagoons showed that most of the organic matters were not carried from the land, but were produced within the both lagoons. In particular, nutrients concentrations and standing stocks of primary producers showed that the benthic primary producers depending on pore water DIN in HN, and also water column DIN in AK, were dominant, respectively. In addition, δ15N values of primary consumers (clam and oyster) were approximately 1.5‰ higher in AK than HN, while δ13C values were similar in the both lagoons. The enriched δ15N values in AK suggest feeding on epiphytic microalgae depending on water column DIN than epipelic microalgae depending on pore water DIN. These results suggest that river inputs contribute to the development of the benthic ecosystem, which utilizes water column and pore water DIN.
