Biogeochemical reworking of sewage discharged in the Scheldt Estuary (Belgium) influences nitrogen and carbon isotopic composition of the aquatic food chain D e Brabandere L., R. Fisseha, a n d F. Dehairs Vrije Universiteit Brussel, Laboratory for Analytical Chemistry Pleinlaan 2, 8-1 0 5 0 Brussel E-mail: [email protected] Stable C and N isotope ratios were used to study food web characteristics of the brackish part of the Scheldt Estuary. The river Scheldt is a highly eutrophic system due to heavy antropogenic disturbance as indicated by high organic matter and ammonium loading. By means of stable isotope analysis, an attempt was made to detect trophic relationships of the aquatic food chain. Therefore, baseline C and N isotopic ratios were investigated monthly from May 1999 to March 2000 by measuring the isotopic composition of suspended particulate organic matter (SPOM).Also the 615N and 613C composition of species representing different trophic levels of the subtidal food web (copepods, mysids and fish) and the intertidal food web (endobenthos, shrimps and crabs) were investigated. 613CSpoM and 615NSpoM values varied seasonally with 613C values ranging from -23.7 (January 2000) to 28.0%0 (June 1999) and 6I5N values ranging from +2.3%0 (January 2000) to +12.9%0 (June 1999). For copepods, 613C and 615N values varied from -30.7 to -26.4%0 and from +1 1.1 to +25.5%0 respectively. 613C and 615N values of copepods exhibited a seasonal variation and 615N co-varied with 615NS,ow 613C values of two dominant mysid species varied between -29.0 and -25.5%0, 615N values varied between 11.8 and +25.0%0. Fish 613C values displayed a wide range, from -26.4 to -34.3%0 and 615N values from 15.7 to +33.5%0: depending on the trophic level. In the intertidal food web, 1 7.5 and +26.8%0 macrocrustacea isotopic compositions ranged between -29.0 and -1 8.1 %o and for 613C and 615N respectively, while endobenthic species had 613C values between -21.4 and -24.4%0 and 615N values between 19.2 and +22%o. + + + + The 615N values were high compared to other estuarine environments. According to Hansson et al. (1 997), such a situation reflects a high degree of pollution. However, in the present case, 15N-enrichment in the food chain is not the result of isotope composition of the waste source. Indeed, 615N of sewage SPOM from the Brussels sewer system, which is discharged untreated into the Scheldt via the Zenne and Rupel tributaries, does not exceed +3%0 (Fisseha, 2000), while 615N of riparian vegetation averaged 1 .5%0 (Mariotti et al., 1984). Therefore, the 15N enrichment in the Scheldt trophic system is probably the result of organisms feeding on organic matter which has become enriched during river transport to the North Sea. Intense biogeochemical reworking of the N pools (bacterial mineralisation, followed by nitrification and denitrification) are the probable processes enriching 15N in the nutrient pool (NH4+) utilised by bacteria decomposing detritus (Caraco et al., 1998) and by phytoplankton (Mariotti et al., 1984). These biogeochemical processes are probably subject to seasonal variability as indicated by seasonal variability of 615N signal in SPOM and copepods. Similarly to the pelagic food chain, 15N enrichment of the intertidal food chain due to the incorporation of enriched DIN by phytobenthos has been mentioned by Riera et al., (2000). + References Caraco N.F., G. Lampman, J.J. Cole, K.E. Limburg, M.L. Pace, and D. Fischer. 1998. Microbial assimilation of DIN in a nitrogen rich estuary: implication for food quality and isotope studies. Marine Ecology Progress Series 167:59-7 1 . I I Fisseha R. 2000. Use of stable isotopes as a tracer of domestic sewage pollution in the Woluwe BrookWoluwe sewer collector system. Master Degree in Environmental Science and Technology, Vrije Universiteit Brussel, 8 0 pp. Hansson S., J.E. Hobbie, R. Elmgren, U. Larsson, B. Fry, and S. Johansson. 1997. The stable nitrogen isotope ratio as a marker of food-web interactions and fish migration. Ecology 78(7):2249-2257, Mariotti A., C. Lancelot, and G. Billen. 1984. Natural isotopic composition of nitrogen as a tracer of origin for suspended organic matter in the Scheldt Estuary. Geochimica et Cosmochimica Acta 48:549-555. Riera P., L.J. Stal, and J. Nieuwenhuize. 2000. Heavy S15N in intertidal benthic algae and invertebrates in the Scheldt Estuary (The Netherlands): Effect of river nitrogen inputs. Estuarine, Coastal and Shelf Science 51 :365-372.
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