Autotrophic and heterotrophic food sources of copepods in the Scheldt estuary as traced by stable C and N isotopes

Estuaries draining densely populated watersheds experience significant anthropogenic pressure and sustain large autotrophic and heterotrophic production owing to an increased input of nutrients and organic matter. Polluted estuaries are often net heterotrophic systems. Our objective was to study the relative contributionof autotrophic and heterotrophic food webs in sustaining the high productivity of pelagic estuarine ecosystems along the estuarine gradient of the Scheldt estuary. We concentrated on the nature of the primary food sources of calanoid and cyclopoid copepods and on the organic substrates supporting heterotrophic production. An extensive study of the monthly variation of delta15N and delta 13C of suspended matter and copepods over a period of two year showed that variations in the relative contribution and isotopic signature of phytoplankton were probably the main factors controlling the seasonal and spatial variation in the delta15N and delta13C signature of suspended organic matter. Comparisons between the seasonal delta 15N and delta13C patterns of suspended matter and copepods showed that the nature of the primary food sources differed between calanoid and cyclopoid copepods and between freshwater and oligohaline-mesohaline reaches. In the oligohaline and mesohaline reaches, calanoid and cyclopoid copepods were mainly supported by autotrophic and heterotrophic biomass, respectively, whereby the latter probably thrived on dissolved organic matter derived from local phytoplankton. In the freshwater section, cyclopoid copepods dominated the copepod community. The important discrepancy between delta15N of cyclopoid copepods and the mode led delta15N of phytoplankton in the freshwater reaches eliminated local phytoplankton or heterotrophs thriving on dissolved organic matter derived from local phytoplankton as a possible food source. This situation was explained via a scenario where bacteria thriving on phytoplankton detritus imported from the tributaries formed the main food source of local cyclopoid copepods. Our observations highlighted a very different ecosystem functioning for the freshwater part compared to the oligo- and mesohaline waters of the estuary proper. In this work, we constructed an isotopic baseline for future studies on the diet and trophic level of planktivorous fish by calculating the annual mean delta15N and delta13C of copepods. However, the highly variable nature of the annual mean delta15N hampers the use of stable N isotopes as a tool to study fish migration between freshwater and marine reaches. Nevertheless, mesohaline stations showed sufficiently distinct delta15N signatures to allow tracing of migration between this habitat and the freshwater reaches

d¹³C and d¹⁵N composition of suspended particular organic matter (SPOM) and zooplankton in the eutrophic Scheldt estuary (Belgium) (poster)

d13C and d15N were used to study food sources of zooplankton in the Scheldt Estuary. The SPOM carried by the river is a mixture of antropogenic and terrestrial detritus with associated bacteria and locally produced autotrophic organic matter. Samples of SPOM and zooplankton were taken monthly from June 1999 to April 2000 at four stations located along a salinity gradient (0-14 ppt). We investigated the relative importance of detritus, heterotrophic and autotrophic organic matter in the diet of zooplankton.Both d15NSPOM and d13CSPOM varied seasonally, paralleling the change in biomass of autotrophic organisms. Generally, phytoplankton bloom periods were characterised by high d15NSPOM (maximum +12.9‰) and low d13CSPOM values (minimum -31.1‰). Winter d15NSPOM and d13CSPOM values were characteristic for pure antropogenic/terrestrial detritus (d15N = +2.5‰; d13C = -26.8‰). The summer increase of d15NSPOM was attributed to autotrophic consumption of NH4+ strongly enriched in 15N due to intense nitrification. However, the seasonal pattern between the stations differed reflecting differences in the timing, duration and intensity of the phytoplankton bloom. d13Czooplankton and d15Nzooplankton co-varied with d13CSPOM and d15NSPOM. The high deviation from the original d13CSPOM and d15NSPOM suggested selective feeding on specific components of the SPOM

Migration of juvenile herring (<i>Clupea harengus</i>) and sprat (<i>Sprattus sprattus</i>) between the North Sea and the Schelde estuary proved by stable C en N isotopes

The temporal changes in abundance of juvenile herring (Clupea harengus, Linnaeus, 1758) and sprat [(Sprattus sprattus, Linnaeus, 1758)] in the Schelde Estuary were examined by means of stable isotopes. Juvenile herring and sprat typically overwinter in the estuary. Herring exhibits a second, smaller density peak in the summer. A similar density peak for sprat is not observed. The temporal use of the estuary by clupeoid fish has previously been attributed to seasonal migrations of juveniles between the North Sea and the estuary. Using stable isotopes of carbon and nitrogen we have tried to elucidate these migration patterns. Herring and sprat were sampled between May 2000 and April 2001. Samples were taken every month in the cooling water of the Doel Nuclear Power Plant (in the brackish part of the estuary) and the Borssele Nuclear Power Plant (at the mouth of the estuary). Using cluster analysis on the d13C and d15N values of individual muscle tissue, fish which recently immigrated from the North Sea (marine group with typical marine isotope values) could be distinguished from individuals which had resided in the estuary (estuarine group with typical estuarine isotope values). The analysis showed that herring and sprat had very similar migration dynamics in the Schelde Estuary, characterized by immigration and emigration almost throughout the year and an intensive migration activity during the winter. Net upstream immigration (i.e. the majority of fish enter the estuary) started in September and peaked in November. During December immigration remained high but had already decreased, which probably explains lower fish densities recorded at Doel. Although the density of herring and sprat further declined in February and March, net seaward emigration sensu strictu (i.e. the majority of the fish leave the estuary) was not demonstrated using the stable isotope technique. During the winter larger proportions of individuals with a typical marine isotope signature at Doel were not only associated with migration but also with a slower tissue turnover rate

Nitrification in a highly polluted European estuary (the Scheldt estuary) and consequences on changing the natural isotopic ratio of ¹⁵N in particulate organic matter (poster)

The Scheldt Estuary is a highly polluted macrotidal estuary draining one of the most densely populated areas in the world (425 inhab/km²). The present nitrogen load to the estuary is approximately of 50 kT N / year from which about 20% is under the form of ammonium. This ammonium is almost completely nitrified in the estuary, even in winter conditions, when low temperatures are known to limit the activity of nitrifying bacteria. This is probably linked to the fact that water residence times are very long (75 days on the average) so that even with reduced nitrification rate depletion of ammonium is still possible. The oxidation of ammonium to nitrate by nitrifying bacteria results in an enrichment of N-NH4+ in its heavy isotope 15N. Indeed, the heavy isotope 15N is discriminated against the light 14N isotope during the oxidation process. Measurements of the delta15N in particulate organic matter and copepods show that the heavy 15N isotope of ammonium is most probably incorporated in the entire microbial food web. This indicates that, at least for some periods of the year, the microbial food web of the estuary is based on micro-organisms (phyto- and bacterioplankton) finding their N requirements by assimilating NH4+ rather than any other inorganic or organic N source

Search for composite and exotic fermions at LEP 2

A search for unstable heavy fermions with the DELPHI detector at LEP is reported. Sequential and non-canonical leptons, as well as excited leptons and quarks, are considered. The data analysed correspond to an integrated luminosity of about 48 pb^{-1} at an e^+e^- centre-of-mass energy of 183 GeV and about 20 pb^{-1} equally shared between the centre-of-mass energies of 172 GeV and 161 GeV. The search for pair-produced new leptons establishes 95% confidence level mass limits in the region between 70 GeV/c^2 and 90 GeV/c^2, depending on the channel. The search for singly produced excited leptons and quarks establishes upper limits on the ratio of the coupling of the excited fermio

Search for lightest neutralino and stau pair production in light gravitino scenarios with stau NLSP

Promptly decaying lightest neutralinos and long-lived staus are searched for in the context of light gravitino scenarios. It is assumed that the stau is the next to lightest supersymmetric particle (NLSP) and that the lightest neutralino is the next to NLSP (NNLSP). Data collected with the Delphi detector at centre-of-mass energies from 161 to 183 \GeV are analysed. No evidence of the production of these particles is found. Hence, lower mass limits for both kinds of particles are set at 95% C.L.. The mass of gaugino-like neutralinos is found to be greater than 71.5 GeV/c^2. In the search for long-lived stau, masses less than 70.0 to 77.5 \GeVcc are excluded for gravitino masses from 10 to 150 \eVcc . Combining this search with the searches for stable heavy leptons and Minimal Supersymmetric Standard Model staus a lower limit of 68.5 \GeVcc may be set for the stau mas

Measurement of inclusive π0\pi^{0} production in hadronic Z0Z^{0} decays

An analysis is presented of inclusive \pi^0 production in Z^0 decays measured with the DELPHI detector. At low energies, \pi^0 decays are reconstructed by \linebreak using pairs of converted photons and combinations of converted photons and photons reconstructed in the barrel electromagnetic calorimeter (HPC). At high energies (up to x_p = 2 \cdot p_{\pi}/\sqrt{s} = 0.75) the excellent granularity of the HPC is exploited to search for two-photon substructures in single showers. The inclusive differential cross section is measured as a function of energy for {q\overline q} and {b \bar b} events. The number of \pi^0's per hadronic Z^0 event is N(\pi^0)/ Z_{had}^0 = 9.2 \pm 0.2 \mbox{(stat)} \pm 1.0 \mbox{(syst)} and for {b \bar b}~events the number of \pi^0's is {\mathrm N(\pi^0)/ b \overline b} = 10.1 \pm 0.4 \mbox{(stat)} \pm 1.1 \mbox{(syst)} . The ratio of the number of \pi^0's in b \overline b events to hadronic Z^0 events is less affected by the systematic errors and is found to be 1.09 \pm 0.05 \pm 0.01. The measured \pi^0 cross sections are compared with the predictions of different parton shower models. For hadronic events, the peak position in the \mathrm \xi_p = \ln(1/x_p) distribution is \xi_p^{\star} = 3.90^{+0.24}_{-0.14}. The average number of \pi^0's from the decay of primary \mathrm B hadrons is found to be {\mathrm N} (B \rightarrow \pi^0 \, X)/\mbox{B hadron} = 2.78 \pm 0.15 \mbox{(stat)} \pm 0.60 \mbox{(syst)}

Search for new phenomena using single photon events in the DELPHI detector at LEP

Data are presented on the reaction \epem~\into~\gamma + no other detected particle at center-of-mass energies, \sqs = 89.48 GeV, 91.26 GeV and 93.08 GeV. The cross section for this reaction is related directly to the number of light neutrino generations which couple to the \zz boson, and to several other phenomena such as excited neutrinos, the production of an invisible `X' particle, a possible magnetic moment of the tau neutrino, and neutral monojets. Based on the observed number of single photon events, the number of light neutrinos which couple to the \zz is measured to be N_\nu = 3.15 \pm 0.34. No evidence is found for anomalous production of energetic single photons, and upper limits at the 95\% confidence level are determined for excited neutrino production (BR < 4-9 \times 10^{-6}), production of an invisible `X' particle (\sigma < 0.1 pb), and the magnetic moment of the tau neutrino (< 5.2 \times 10^{-6} \mu_B). No event with the topology of a neutral monojet is found, and this corresponds to the limit \sigma < 0.044/\epsilon pb at the 95\% confidence level, where \epsilon is the unknown overall monojet detection efficiency