Watershed Effects on Chemical Properties of Sediment and Primary Consumption in Estuarine Tidal Flats: Importance of Watershed Size and Food Selectivity by Macrobenthos

Particulate 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 POM_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 POM_R, indicating their high food selectivity. The species-specific, low- or high-food selectivity was likely linked with deposit-feeding 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.