d15N and d13C 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 delta13C and delta15N signatures. delta15N of suspended matter showed large seasonal variation. Minimum values ranged from -0.5 promille in the freshwater zone (spring situation) to +2.3 promille in the mesohaline zone (winter situation). Maximum values (summer situation) ranged from +8.8 promille in the freshwater zone to +12.9 promille in the mesohaline zone. delta13C showed less seasonal variation and ranged overall from -31.1 promille in the freshwater zone to -23.7 promille in the mesohaline zone. During the growth season, decrease of delta13C and increase of delta15N of suspended matter were due to local phytoplanktonic and bacterial biomass. There is strong evidence that the delta15N enrichment of suspended matter during the growth season reflects the delta15N enrichment of the ambient NH4+ pool induced by nitrification and NH4+ uptake. Zooplankton in the mesohaline section of the river was consistently enriched in delta15N 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, delta15N of zooplankton reached a value as high as +25.5 promille, the highest value observed during this study. During spring, present-day delta15N of suspended matter in the oligohaline and mesohaline section increased compared to the 1970s, probably because today nitrification, which enriches the NH4+ pool in delta15N, starts earlier in the season. For summer, the discrepancy between present-day suspended matter delta15N 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 delta15N-depleted sewage will enhance delta15N enrichment of suspended matter during the growth season.
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