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 δ13C and δ15N signatures. δ15N of suspended matter showed large seasonal variation. Minimum values ranged from −0.5‰ in the freshwater zone (spring situation) to +2.3‰ in the mesohaline zone (winter situation). Maximum values (summer situation) ranged from +8.8‰ in the freshwater zone to +12.9‰ in the mesohaline zone. δ13C showed less seasonal variation and ranged overall from −31.1‰ in the freshwater zone to −23.7‰ in the mesohaline zone. During the growth season, decrease of δ13C and increase of δ15N 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. nkton 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, δ15N of zooplankton reached a value as high as +25.5‰, the highest value observed during this study. During spring, present-day δ15N 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 δ15N 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.