Kinuko Ito

Contributions of salmon-derived nitrogen to riparian vegetation in the northwest Pacific region

AbstractWe examined the relationship between the annual escapement of salmon and the δ15N of willow (Salix spp.) leaves to evaluate the contribution of marine-derived nutrients (MDN) to riparian vegetation around the Pacific Northwest and Northeast regions. Foliar δ15N values ranged from −3.42° to 4.65°. The value increased with increasing density of carcasses up to 500 fish/km and 1500 fish/km. δ15N values were variable at carcass densities below 500 fish/km. Possible factors affecting the fluctuation of δ15N at reference sites are: (1) denitrification; (2) the presence of N2-fixing trees, such as alder; and (3) agricultural runoff. δ15N values at the sites with carcass densities over 500 fish/km were consistently high, while a value of δ15N below zero was observed at only one site (Rusha River; δ15N = −1.87°). At this site, most adult pink salmon returned to limited locations near the estuary because steeper channel gradients acted as a migration barrier, resulting in the negative δ15N value. Neverthele...

Effects of the Great East Japan Tsunami on Fish Populations and Ecosystem Recovery. The Natori River; Northeastern Japan

The tsunami following the Great East Japan Earthquake in March 2011 resulted in significant ground subsidence and deposition of rubble and mud in the Natori River, near the city of Sendai (Miyagi Prefecture), damaging its brackish water ecosystem and fishing grounds. There was a direct impact in the form of annihilation of animal and plant life and disturbance of the habitats throughout. Also, a wedge of seawater ran far upstream, and ground subsidence changed the pattern of tidal flow in the river. Brackish water ecosystems such as that near the mouth of the Natori River are important as nurseries for juvenile fishes and as a fishing ground for bivalves such as clams. The populations of both of these kinds of organism declined drastically as result of the tsunami. The catch per unit effort of ayu fish (sweetfish; Plecoglossus altivelis altivelis) in 2011 was the lowest recorded for the past 5 years, and the population hatching date composition showed a marked absence of early-hatched individuals. In contrast, the residual upstream ayu fish population seems to have grown successfully and reproduced despite the effects of the tsunami: 1 year after the tsunami occurred, the downstream ayu fish population had recovered to the same level as before the event. However, the population of the brackish-water clam, Corbicula japonica, only showed recovery 2 years after the disaster as its habitat has drastically shifted due to movement of the brackish water zone about 1 km upstream. The studies reported here show that the impact of the earthquake and tsunami on pelagic fish and benthic bivalves seems to have been quite different, as in the former recovery was rapid, while in the latter it took much longer. Many other fish species also returned to normal levels within a year, such as stone flounder (Kareius bicoloratus), goby (Acanthogobius lactipes), icefish (Salangichthys microdon) and black porgy (Acanthopagrus schlegelii). The food web structure appears to be slightly different from past years, but the results show that, in general, fish communities are able to recover rapidly from disturbances even as drastic as an unusually large tsunami.

The tiny-leaved orchid Cephalanthera subaphylla obtains most of its carbon via mycoheterotrophy

The evolution of mycoheterotrophy has been accompanied by extreme reductions in plant leaf size and photosynthetic capacity. Partially mycoheterotrophic plants, which obtain carbon from both photosynthesis and their mycorrhizal fungi, include species with leaves of normal size and others that are tiny-leaved. Thus, plant species may lose their leaves in a gradual process of size reduction rather than through a single step mutation. Little is known about how the degree of mycoheterotrophy changes during reductions in leaf size. We compared the degree of mycoheterotrophy among five Japanese Cephalanthera species, four with leaves of normal size (Cephalanthera falcata, Cephalanthera erecta, Cephalanthera longibracteata and Cephalanthera longifolia), one with tiny leaves (Cephalanthera subaphylla), and one albino form of C. falcata (as reference specimens for fully mycoheterotrophic plants). The levels of mycoheterotrophy were determined by stable isotope natural abundance analysis. All Cephalanthera species were relatively enriched in 13C and 15N in comparison with surrounding autotrophic plants. Cephalanthera subaphylla was strongly enriched in 13C and 15N to levels similar to the albinos. Species with leaves of normal size were significantly less enriched in 13C than C. subaphylla and the albinos. Thus, C. subaphylla was strongly mycoheterotrophic, obtaining most of its carbon from mycorrhizal fungi even though it has tiny leaves; species with leaves of normal size were partially mycoheterotrophic. Hence, during the evolutionary pathway to full mycoheterotrophy, some plant species appear to have gained strong mycoheterotrophic abilities before completely losing foliage leaves.

Influence of isotope fractionation on the nitrogen isotope composition of the brown macroalga Undaria pinnatifida : Isotope fractionation in macroalga

Although the stable nitrogen isotope ratio (δ15N value) of macroalgae is used to indicate sources of dissolved inorganic nitrogen in coastal marine environments, little is known about the effects of isotope fractionation on the δ15N value. We compared the δ15N values of Undaria pinnatifida cultivated at an inner bay and an exposed site (sites A and B, respectively) in Shizugawa Bay on the northeastern Pacific coast of Japan. Values for algal growth indicators, including thallus length, area, and weight, were higher at site B than at site A. The δ15N value of U. pinnatifida was significantly lower at site B (4.7 ± 1.0‰; mean ± standard deviation) than at site A (6.7 ± 0.4‰). This difference in the δ15N value of U. pinnatifida could not be explained by the δ15N value of seawater NO3 (source isotopic signature), which was similar at the two sites (7.5 ± 0.3‰). The pooled data from the two sites indicated that the δ15N value of U. pinnatifida decreased with increasing thallus size. The results suggest that the δ15N value of U. pinnatifida is influenced by isotope fractionation.