Use of15N/14N rations to evaluate the food source of the mysid,Neomysis intermedia Czerniawsky, in a eutrophic lake in Japan

Abstract

The stable isotope ratios of nitrogen were measured in the mysid,Neomysis intermedia, together with various biogenic materials in a eutrophic lake, Lake Kasumigaura, in Japan throughout a year of 1984/85. The mysid, particulate organic matter (POM, mostly phytoplankton), and zooplankton showed a clear seasonal change in δ15N with high values in spring and fall, but the surface bottom mud did not. A year to year variation as well as seasonal change in δ15N was found in the mysid. The annual averages of δ15N of each material collected in 1984/85 are as follows: surface bottom mud, 6.3‰ (range: 5.7–6.9‰); POM, 7.9‰ (5.8–11.8‰); large sized mysid, 11.6‰ (7.7–14.3‰); zooplankton, 12.5‰ (10.0–16.4‰); prawn, 13.2‰ (9.9–15.4‰); goby, 15.1‰ (13.8–16.7‰). The degree of15N enrichment by the mysid was determined as 3.2‰ by the laboratory rearing experiments. The apparent parallel relationship between the POM and the mysid in the temporal patterns of δ15N with about 3‰ difference suggests the POM (mostly phytoplankton) as a possible food source ofN. intermedia in this lake through the year.