JPR Advance Access originally published online on May 14, 2004
Journal of Plankton Research 2004 26(9):1039-1048; doi:10.1093/plankt/fbh094
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Journal of Plankton Research Vol. 26 No. 9 © Oxford University Press 2004; all rights reserved
Trophic diversity within the planktonic food web of the Elbe Estuary determined on isolated individual species by 13C analysis
Institute for Hydrobiology and Fishery Science, University of Hamburg, D-22765 Hamburg, Zeiseweg 9 and 1 Institute for Biogeochemistry and Marine Chemistry, D-20146 Hamburg, Bundesstrasse 55, Germany
* Corresponding Author: kerner{at}uni-hamburg.de
Received December 19, 2003; accepted in principle April 7, 2004; accepted for publication May 4, 2004; published online May 14, 2004
The uptake of organic substrates by heterotrophic planktonic organisms was studied along the freshwater Elbe Estuary in May, July and October 2000 using 13C analysis of individually isolated dominant species of copepoda, cladocera, rotifera and ciliata. Non-sedimenting suspended particulate matter (SPMns) was separated from sedimenting matter and further analysed for the chemical composition of its different size fractions in order to estimate substrate availability. Particles <5 µm accounted for 
15% of total SPMns [40 mg dry weight (DW) L–1] and contained C:N ratios indicating a predominance of living matter (i.e. mass C:N of phytoplankton). All species under study exhibited a high capacity for selective feeding with little variation in the diet along the whole freshwater profile. Picoplankton of 0.2–1.2 µm formed mainly by bacteria had a 
13C value of –26
in May and July and –29 in October, similar to the 13C of the dissolved organic carbon (DOC). Bacteria did not fractionate isotopically and did not preferentially consume specific subunits of their substrate, i.e. they exhibited no trophic shift. It appears that phytoplankton exudates were a minor component to total DOC in this estuary. Phytoplankton was the exclusive food for all phagotrophic organisms under study in July, thus only one trophic level was exhibited. During May and October the grazers under study used different substrates, resulting in a planktonic food web of three trophic levels formed by different species of the taxonomic groups under study. The results indicate a conditioned behaviour with regard to substrate selection allowing the grazers to produce high abundances even when particles were abundant and competition for phytoplankton was high.