We examined how nitrogen-stable isotopic signatures of food web components (basal resources, primary and lower consumers, and omnivores) in rivers change with increasing levels of human population density (HPD) in their watersheds. Samples were collected from 22 rivers flowing in the Lake Biwa basin, Japan. Among three potential resources at the base of food webs (epilithon, benthic and suspended particulate organic matter), the mean isotopic values (d 15 N) of the epilithon (4.5-7.8%) were consistently higher than those of other items (1.9-4.2%) and displayed the most pronounced elevation (by 3.3%) with increasing HPD. The mean d 15 N values of the individual taxa of lower consumers (bivalve, snail and caddisfly) tended to increase with increasing HPD, although the pattern and the extent of the elevation were highly variable among the taxa. These results suggest a taxon-specific feature in the N source (or sources) of lower consumers. Our data suggested that human activities (e.g. nutrient loading) potentially induce changes in the N baselines of river food webs. The major N source of bivalves appeared to be shifted from suspended particulate organic matter to other items with increasing HPD. Trophic levels of goby fish (Rhinogobius sp. OR) and shrimp (Palaemon paucidens), being estimated to be at 2.4-3.8 and 2.1-3.4, respectively, did not differ significantly among rivers with different HPD levels.
The egg size of a landlocked goby, Rhinogobius sp. (the orange form), in the Lake Biwa water system was compared between the fluvial-lacustrine and lacustrine types. The former spawned larger eggs than the latter in the laboratory under the same environmental conditions of water temperature and current. This difference in egg size was also found in the field among six local populations from tributary rivers and lake shore. These results suggested that egg size is largely associated with the life history type that has experienced a different selection regimen, such as the risk of larval starvation, associated with their respective life cycles.
Materials and MethodsLake Biwa is located in central Japan (85.6 m a.s.l.) with hundreds of tributary rivers. Rhinogobius sp. (the orange form) spawns both in the lower reach (~ Ͻ150 m a.s.l.) of tributary rivers and the lake shore. Adult fishes for the laboratory experiment were collected using hand nets from the lower reach of the Ado River (Fig. 1, Station A1) and the west shore of the lake (Station B1) on 20-23 Nov. 2001, when the female ovaries are filled with germ cells up to the perinucleolus stage (the yolkless stage; Hidaka and Takahashi, 1987). The standard length (SL) of collected fishes ranged from ~20 to 60 mm (estimated at 0ϩ-2ϩ age based on their trimodal size distributions) at Station A1, while that at Station B1 was ~20-35 mm (0ϩ age), reflecting the difference in the size distribution in the field (Maruyama et al., 2001a). The fish were brought back alive to the laboratory (34°90Ј N, 135°58Ј E), with water temperature being controlled intermediate between Stations A1 and B1 (19°-23°C in range), and maintained at natural day length. The fish were separated by sex, size, and sampling stations, and kept in 60 ϫ 30 ϫ 30 cm aquaria. The fishes were fed with frozen chironomid larvae once a day to saturation until the reproductive season. In the reproductive season (from 1 June to 14 July 2002), each gravid female was, in turn, moved to a 25 ϫ 15 ϫ 15 cm aquarium with a 10 ϫ 10 cm ceramic tile and a similar-sized sympatric male. Eggs spawned on the undersides of the tiles (22 clutches) were preserved in 90% ethanol. The major (a) and minor (b) axes of 10 eggs from each clutch were measured under a dissecting microscope to the nearest 0.01 mm to calculate
IchthyologicalResearch
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