| Zusammenfassung: | Paniculate organic matter transported from rivers to estuaries (POM R ) varies quantitatively and qualitatively across estuaries; however, a lack of comparative studies poses a challenge in general understanding of responses of estuarine food webs to POM R input. We studied 20 estuarine tidal flats of the Pacific Northwest coast of North America, with watershed areas ranging from 7 to 8000 km². We used carbon-stable isotope (δ¹³C) to test the hypothesis that the nutritional contribution of P0M R to macrobenthos is proportional to relative abundances of POM R in tidal flat sediments. The predominant origin of total POM (TPOM) in tidal flat sediments generally shifted from marine-origin POM (POM M ) to POM R as watershed area increased; however, terrestrial-origin POM R with high C/N predominated sediment TPOM even in estuaries with small watershed areas. Some macrobenthos species assimilated POM sources in proportion to sediment TPOM composition, and incorporated POM R in POM R -predominant sediments. These species were considered to have low food selectivity; however, the relative nutritional contribution of POM R to these macrobenthos was still lower than the fraction of POM R in sediment TPOM. Other species disproportionately utilized POM M and/or benthic microalgae regardless of the relative abundance of P0M R , indicating their high food selectivity. The species-specific, low-or highfood selectivity was likely linked with depositfeeding and filter-feeding, respectively. Hence, our hypothesis was supported conditionally. Our findings indicate that watershed area, relative abundance of POM R in an estuary, and food selectivity of estuarine species are key factors controlling the tightness of linkage between watersheds and estuarine food webs.
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