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 (POMR) 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 POMR input. We studied 20 estuarine tidal flats of the Pacific Northwest coast of North America, with watershed areas ranging from 7 to 8000 km(2). We used carbon-stable isotope (delta C-13) to test the hypothesis that the nutritional contribution of POMR to macrobenthos is proportional to relative abundances of POMR in tidal flat sediments. The predominant origin of total POM (TPOM) in tidal flat sediments generally shifted from marine-origin POM (POMM) to POMR as watershed area increased; however, terrestrial-origin POMR 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 POMR in POMR-predominant sediments. These species were considered to have low food selectivity; however, the relative nutritional contribution of POMR to these macrobenthos was still lower than the fraction of POMR in sediment TPOM. Other species disproportionately utilized POMM and/or benthic microalgae regardless of the relative abundance of POMR, 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 POMR in an estuary, and food selectivity of estuarine species are key factors controlling the tightness of linkage between watersheds and estuarine food webs.