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The deep-sea floor has long been considered a 'food desert' but recent observations suggest that episodic inputs of relatively fresh organic matter (phytodetritus) occur and that benthic processing of this material may be rapid. Although the responses of the total community in terms of oxygen consumption and of some individual benthic groups have been identified, the quantitative role of the different groups in the short-term response remains largely unknown. We examined the short-term response in major benthic compartments in an in situ experiment in the NE Atlantic (2170 m water depth) using 13 C-enriched diatoms as a tracer of labile carbon. Within 35 h, 6 mg C m -2 was processed by the benthos, with the majority of the processed carbon recorded as respiration (45%). Among the fauna retained on a 300 µm sieve, Foraminifera were rapid consumers which, together with Bacteria, accounted for 50% of the processing. Therefore, although Bacteria dominate long-term carbon mineralization (as suggested by their general dominance in the benthic biomass), some faunal components, in this case Foraminifera, may play a central role in the rapid initial processing of fresh organic carbon in deep-sea sediments.

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In this study, we show that natural phototrophic populations can be probed individually for their in situ ␦ 13 C signature by linking fluorescence-activated cell sorting and isotope-ratio mass spectrometry (IRMS) using in-line pyrolytic methylation. This novel methodology greatly improved the resolution in discriminating and tracing the differential carbon (C) pathways at the base of the pelagic food web in the cyanobacteria-dominated Lake Loosdrecht (The Netherlands). Our analysis revealed the co-occurrence of phytoplankton taxa differing by 6-10‰ in ␦ 13 C. Predominant micro-and mesozooplankton species reflected this difference as the result of preferential grazing and/ or selective digestion. Flow cytometric (FCM) retrieval of phytoplankton ␦ 13C signatures, applied in conjunction with 13 C-carbonate labeling, also enabled an assessment of in situ population-specific growth rates. Diatoms and green algae exhibited up to ninefold higher growth rates than those for cyanobacterial species. The coexistence of phytoplankton populations widely differing in ␦ 13 C, standing stock, and turnover time has important implications for the interpretation of C transfer in pelagic food webs. Our approach disclosed a disproportional impact on trophic cascades by numerically minor phototrophs that otherwise would have gone unnoticed. Despite the abundance of cyanobacterial-derived C, the zooplankton largely rely on eukaryotic algae for growth. Rotifers take a central position in passing on this algal C to the cyclopoid copepod populations in the lake. The bosminid-dominated cladoceran population uses both the cyanobacterial-and algal-derived C in approximately equal shares.

1. Thermally assisted hydrolysis and methylation of cellular lipids, by means of Curiepoint pyrolysis of intact whole cells in the presence of a quaternary ammonium hydroxide reagent, provided analytical access (pyrolysis-gas chromatography; Py-GC) to the very small amounts of algal carbon delivered by fluorescence-activated cell sorting. Based on differences in pigment composition, population-specific in situ fatty acid profiles could be obtained of the major taxa present in the phytoplankton of Lake Loosdrecht (The Netherlands). 2. By combining Py-GC and compound-specific isotope-ratio mass spectrometry (Py-GC-IRMS) the in situ carbon isotopic signatures could be established of the fatty acid profiles retrieved by flow cytometry. Colonial phytoplankton not amenable to cell sorting and zooplankton specimens were also isotopically characterised with this technique by subjecting handpicked samples to pyrolytic methylation. In this way proxies could be obtained in great detail for isotopic end-members delineating important carbon sources and sinks in the pelagic food web of Lake Loosdrecht. 3. These analyses suggested a significant isotopic heterogeneity among major representatives of the phytoplankton in Lake Loosdrecht. This heterogeneity was also reflected in the isotopic composition of the zooplankton, implying the occurrence of preferential grazing. A differential labelling of the phytoplankton using 13 C-CO 2 in a laboratory confinement, and subsequent monitoring of label transfer to the zooplankton, corroborated selective feeding in some rotifer species. The large-bodied rotifer Asplanchna, previously thought to be predaceous, apparently mainly fed on algae rather than small rotifers, whereas Euchlanis dilatata actively selected filamentous cyanobacteria. Flow cytometric cell sorting in concert with Py-GC-IRMS offers new possibilities in carbon isotope-based food web studies.

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The novel phosphatase substrate, ELF-97 phosphate, yields intensely green fluorescent precipitates of ELF-97 alcohol (ELFA) upon enzymatic dephosphorylation, and thereby traces phosphatase activity back to its producer. In this study, we show that ELFA fluorescence is a useful tool in flow cytometric analysis of natural phytoplankton populations. Presence of endogenous fluorescent pigments allowed flow cytometric distinction of clusters in the phytoplankton community in Lake Loosdrecht (The Netherlands): Eukaryotes (diatoms and green algae), chlorophyll a and b containing but phycobilin-less cyanobacteria (Prochlorothrix hollandica), and phycocyanin-containing cyanobacteria (predominantly Limnothrix sp.). Several, but not all tested cyanobacteria showed ELFA fluorescence. The dominant Limnothrix sp. possesses a derepressible phosphatase, whereas the second most abundant strain, P. hollandica, did not have phosphatase activity. Within both natural and cultured populations of Limnothrix sp. we found discernible levels of ELFA fluorescence, indicating the presence of subpopulations with different physiological characteristics.

Abstract. The relative importance of potential carbon sources in the littoral of Lake Gooimeer, a lake in the centre of the Netherlands, was studied using a combination of '3C/1zC-ratio analysis and carbohydrate composition analysis. The littoral is covered on the land side by a 80 m wide Phragmites australis bed. Potential carbon sources were macrophyte litter, seston and benthic algae. Samples of potential carbon sources, sediments and benthic macrofauna from inside and outside the bed were analyzed for their t3C/t2C-ratio and some for their carbohydrate composition. Results indicate that inside the bed, macrophyte litter was the main source of carbon for both the sediment organic matter and the benthic macrofauna, and that algal material was of minor importance. Outside the bed, production by benthic algae was the main carbon source, with seston as a second source. No macrophyte derived material could be detected outside the reed bed.Abbreviations: DOC (Dissolved Organic Carbon), SOM (Sediment Organic Matter)