The freshwater-saltwater transition zones of upper estuaries often contain high concentrations of particulate organic matter (POM) that could potentially support their productive food webs. Our objectives were to define the carbon and nitrogen isotopic characteristics of POM across the estuarine transition zone (ETZ) of the St. Lawrence River, Canada, and to determine the availability of this material to primary consumers. The delta(13)C of seston from upstream freshwater samples (-26.3 +/- 0.9parts per thousand) indicated a large contribution of terrestrially derived organic material, whereas downstream sites had higher values (-24.8 +/- 0.5parts per thousand) attributable to dilution with marine phytoplankton. The ETZ turbidity maximum had more negative values (-27.0 +/- 1.6parts per thousand), especially the <5-mum fraction (-30.0 +/- 1.3parts per thousand) that contributed 56-86% of the POM. The delta(13)C of the dominant consumer species in the ETZ (Bosmina longirostris, Keratella sp., Eurytemora affinis, and adult Dreissena polymorpha) averaged -21.0 +/- 0.9%o, well above the seston values. The delta(15)N of the three animals of lowest trophic position averaged 7.3 +/- 0.8parts per thousand, less than 2parts per thousand above the seston delta(15)N (5.7 +/- 0.2parts per thousand for all sites). Our results imply that the bulk POM is largely detrital and unavailable to the ETZ food web and that the primary consumers feed selectively on phytoplankton despite its low contribution (<10%) to total POM. A cross-system comparison of isotopic data shows that the large enrichment of C-13 in consumers relative to POM is unusual, underscoring the distinctive character of turbid, upper estuarine ecosystems.
