d¹³C and d¹⁵N dynamics of particulate 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 d13C and d15N signatures. d15N of suspended matter showed large seasonal variation. Minimum values ranged from -0.5 pro mille in the freshwater zone (spring situation) to + 2.3 pro mille in the mesohaline zone (winter situation). Maximum values (summer situation) ranged from + 8.8 pro mille in the freshwater zone to + 12.9 pro mille in the mesohaline zone. d13C showed less seasonal variation and ranged overall from -31.1 pro mille in the freshwater zone to -23.7 pro mille in the mesohaline zone. During the growth season, decrease of d13C and increase of d15N of suspended matter were due to local phytoplanktonic and bacterial biomass. There is strong evidence that the 15N enrichment of suspended matter during the growth season reflects the 15N enrichment of the ambient NH4 + pool induced by nitrification and NH4+ uptake. Zooplankton in the mesohaline section of the river was consistently enriched in 15N 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, d15N of zooplankton reached a value as high as + 25.5 pro mille, the highest value observed during this study. During spring, present-day d15N of suspended matter in the oligohaline and mesohaline section increased compared to the 1970s, probably because today nitrification, which enriches the NH4+ pool in 15N, starts earlier in the season. For summer, the discrepancy between present-day suspended matter d15N values and those observed in the 1970s was even larger, especially in the oligohaline and freshwater reaches, probably as a result of improved O2 conditions now favouring nitrification. Likewise, the present decreased input of 15N-depleted sewage will enhance 15N enrichment of suspended matter during the growth season.