δ15N and δ13C dynamics of suspended organic matter in freshwater and brackish waters of the Scheldt estuary

Suspended particulate organic matter was sampled monthly between June 1999 and April 2000 in the Scheldt river and estuary to investigate the seasonal and spatial patterns of delta(13)C and delta(15)N signatures. delta(15)N of suspended matter showed large seasonal variation. Minimum values ranged from -0.5parts per thousand in the freshwater zone (spring situation) to +2.3parts per thousand in the mesohaline zone (winter situation). Maximum values (summer situation) ranged from +8.8parts per thousand in the freshwater zone to +12.9parts per thousand in the mesohaline zone. delta(13)C showed less seasonal variation and ranged overall from -31.1parts per thousand in the freshwater zone to -23.7parts per thousand in the mesohaline zone. During the growth season, decrease of delta(13)C and increase of delta(15)N of suspended matter were due to local phytoplanktonic and bacterial biomass. There is strong evidence that the N-15 enrichment of suspended matter during the growth season reflects the N-15 enrichment of the ambient NH4+ pool induced by nitrification and NH4+ uptake.Zooplankton in the mesohaline section of the river was consistently enriched in N-15 relative to suspended matter but followed its seasonal trend. During summer and autumn the isotopic offset between zooplankton and the suspended particulate organic matter was consistent with a pattern of selective feeding on phytoplankton. During summer, delta(15)N of zooplankton reached a value as high as +25.5parts per thousand, the highest value observed during this study. During spring, present-day delta(15)N of suspended matter in the oligohaline and mesohaline section increased compared to the 1970s, probably because today nitrification, which enriches the NH4+ pool in N-15, starts earlier in the season. For summer, the discrepancy between present-day suspended matter delta(15)N values and those observed in the 1970s was even larger, especially in the oligohaline and freshwater reaches, probably as a result of improved O-2 conditions now favouring nitrification. Likewise, the present decreased input of N-15-depleted sewage will enhance N-15 enrichment of suspended matter during the growth season. (C) 2002 Elsevier Science B.V. All rights reserved.