Research Article: Historical δ15N records of Saccharina specimens from oligotrophic waters of Japan Sea (Hokkaido)

Date Published: July 12, 2017

Publisher: Public Library of Science

Author(s): Takanori Kuribayashi, Tsuyoshi Abe, Shigeru Montani, Syuhei Ban.

http://doi.org/10.1371/journal.pone.0180760

Abstract

Historically Saccharina spp. beds occurred along the west coast of Hokkaido, an oligotrophic area, and were commercially exploited. Currently extensive commercial Saccharina spp. beds do not form due to nutrient limitations. Here, we postulate that nutrients assimilated by paleo-Saccharina spp. beds may have been derived from spawning herrings (Clupea pallasii) acting as organisms that formed a vector from their feeding grounds (Okhotsk Sea and Pacific Ocean) to their spawning area (west coast of Hokkaido, Japan Sea). To test this hypothesis we examined stable nitrogen isotope ratios (δ15N) of 100– to 135–year-old Saccharina specimens preserved at the Herbarium (Hokkaido University Museum). δ15N values of the paleo-Saccharina specimens collected from this region were in the range of 10‰, which is significantly higher than the current 3–7‰ in freshly sampled Saccharina spp. This high δ15N indicates that spawning herring (Clupea pallasii) had potentially been a significant source of dissolved inorganic nitrogen (DIN) absorbed by Saccharina, acting as an organism forming a vector for transporting nutrients from eutrophic to oligotrophic coastal ecosystems. Our findings support the hypothesis of so-called “herring-derived nutrients.”

Partial Text

The west coast of the Japan Sea off Hokkaido, Japan is affected by the northeastward-flowing Tsushima Warm Current that is characterized by oligotrophy compared with the other coasts (Okhotsk Sea and Pacific coast) of Hokkaido. This oligotrophic condition is one of the causes behind “isoyake” namely, the recent decrease or disappearance of kelp resulting in the establishment of poorly vegetated areas [1,2]. On the other hand, before the early 20th century, large-scale Saccharina beds had formed [3], and large numbers of herring were caught along the west coast of the Japan Sea off Hokkaido [4]. Because of the high biological productivity, large quantities of nutrients were expected to have been available in this region. However, no scientific data were available to describe the previous levels of nutrients present in this region.

Fig 1 shows δ15N changes (1881–2014) in Saccharina specimens collected from coastal areas around Hokkaido, Japan: (a) The west coast of the Japan Sea off Hokkaido, (b) The coast of the Pacific Ocean off Hokkaido, (c) The coast of the Okhotsk Sea off Hokkaido. The δ15N of 100– to 135– year-old Saccharina samples collected from the west coast of the Japan Sea off Hokkaido (from 1881 to 1920) was significantly higher (mean ± S.D. 10.4 ± 1.1‰, n = 38) compared with that of freshly collected Saccharina (from 1971 to 2014) from the same region (5.3 ± 1.4‰, n = 105) in all Saccharina specimens: Saccharina japonica var. ochotensis, Saccharina japonica var. religiosa, and Saccharina japonica. Interestingly, the δ15N of other species of seaweeds collected from the west coast tended to be similar. In contrast, the δ15N of Saccharina collected from other coasts, i.e., the Pacific Ocean and Okhotsk Sea off Hokkaido, did not show the same trend as the west coast. The δ15N values of the three sea areas were divided into three periods (1881–1920, 1921–1970, and 1971–2014). By Kruskal-Wallis test, a significant difference was shown in the 9 groups (p < 0.01). Moreover, by Steel Dwass test, the δ15N of Saccharina samples collected from the west coast of the Japan Sea off Hokkaido (from 1881 to 1920) were significantly higher (p < 0.01) compared with values of other groups.   Source: http://doi.org/10.1371/journal.pone.0180760