Seiji Yanai

Energy input from a riparian forest into a headwater stream in Hokkaido, Japan

We investigated the amounts and types of organic matter that entered a coastal forest stream in Hokkaido, northern Japan. Organic matter input from the riparian forest was collected using litter traps, and benthic organic matter was collected from the streambed. Plant parts were weighed, and monthly nitrogen and carbon inputs were calculated. Organic matter input was highest in October, accounting for 58 % of the total annual amount, whereas input between May and August comprised only 22 % of the total amount. Frass and various plant parts, such as flowers and green leaves, contributed nitrogen from May to August, and the total input during this period accounted for 39 % of the annual input, which was comparable to the input recorded in October. In May and June, senescent leaves accounted for 30-40 % to the total standing crop although only green leaves entered the stream during this period. To explore the relationship between benthic organic matter and stream macroinvertebrate density, we collected the two dominant summer growing shredder species (Goerodes complicatus [Trichoptera] and Sternomoera rhyaca [Gammaridea]). Densities of S. rhyaca were significantly related to the standing crop of detritus; G. complicatus showed the same trend, although the relationship was not significant. During spring and summer, plant matter that contained high levels of nitrogen contributed to increased nutrient resources in the stream. This organic matter probably affected the growth and development of stream macroinvertebrates when resources were scarce during early summer.

Shredder colonization and decomposition of green and senescent leaves during summer in a headwater stream in northern Japan

We conducted a decomposition experiment using green and senescent maple and alder leaves in a coastal headwater stream in Hokkaido, northern Japan, during June and July 2000. We estimated whether shredder colonization on the leaves and leaf breakdown differed between green and senescent leaves during the experimental period. Late-instar Lepidostoma complicatum (Trichoptera) and Sternomoera rhyaca (Amphipoda) were the predominant shredder taxa among the macroinvertebrates that colonized litterbags. There was no significant difference in shredder colonization between green and senescent leaves although we found a significant difference between maple and alder leaves. The colonization patterns of large individuals of L. complicatum and S. rhyaca differed from those of small individuals. All decomposition coefficients of green and senescent leaves were high. During the experiment, decomposition was significantly faster in maple than in alder leaves, although no significant difference was found between green and senescent leaves. However, the fragmented nitrogen portion was higher in green leaves than in senescent leaves during the experiment. Higher nitrogen release (2–2.5 times more) as particulate organic matter in green than in senescent leaves indicates that green leaves may be a potentially valuable food resource for other macroinvertebrate collector–gatherer species.

Decomposition of leaves in coastal brackish water and their use by the macroinvertebrate Anisogammarus pugettensis (Gammaridea)

The brackish waters along the Hokkaido coast contain a mixture of plant material of both marine and terrestrial origin that provide food and habitat sources for many macroinvertebrates. Field and laboratory experiments were used to investigate the amount of breakdown of seaweed (kelp) and terrestrial oak leaves (green, senescent, and stream-water soaked (‘conditioned’)). In addition, the consumption and growth rates of Anisogammarus pugettensis were compared for the different leaf treatments. The breakdown of kelp was greater than that of terrestrial leaves. Among the three types of oak leaves, the amount of breakdown of green leaves was smallest and the breakdown of conditioned senescent oak leaves was 1.5 times greater than that of senescent leaves. Conditioning of leaves in fresh water enhances leaf decomposition in brackish water. The growth rates of A. pugettensis were similar and fastest when fed seaweed and a seaweed–leaf mixture (0.035 ± 0.009 and 0.043 ± 0.013 mg mg–1 day–1, respectively). The rapid colonisation of oak, despite its low nutritional value, suggests that leaves are important as a habitat, rather than as a food resource. Availability of both seaweed and terrestrial leaves as food resources and habitat would yield benefits for seawater-inhabiting macroinvertebrates.