We tracked flows of carbon and nitrogen during an experimental phytoplankton bloom in a natural estuarine assemblage in Randers Fjord, Denmark. We used 13 C-labeled dissolved inorganic carbon to trace the transfer of carbon from phytoplankton to bacteria. Ecosystem development was followed over a period of 9 d through changes in the stocks of inorganic nutrients, pigments, particulate organic carbon and nitrogen, dissolved organic carbon (DOC), and algal and bacterial polar-lipid-derived fatty acids (PLFA). We quantified the incorporation of 13 C in phytoplankton and bacterial biomass by carbon isotope analysis of specific PLFA. A dynamic model based on unbalanced algal growth and balanced growth of bacteria and zooplankton adequately reproduced the observations and provided an integral view of carbon and nitrogen dynamics. There were three phases with distinct carbon and nitrogen dynamics. During the first period, nutrients were replete, an algal bloom was observed, and carbon and nitrogen uptake occurred at a constant ratio. Because there was little algal exudation of DOC, transfer of 13 C from phytoplankton to bacteria was delayed by 1 d, compared with the labeling of phytoplankton. In the second phase, the exhaustion of dissolved inorganic nitrogen resulted in decoupling of carbon and nitrogen flows caused by unbalanced algal growth and the exudation of carbon-rich dissolved organic matter by phytoplankton. During the final, nutrient-depleted phase, carbon and nitrogen cycling were dominated by the microbial loop and there was accumulation of DOC. The main source (60%) of DOC was exudation by phytoplankton growing under nitrogen limitation. Heterotrophic processes were the main source of dissolved organic nitrogen (94%). Most of the carbon exudated by algae was respired by the bacteria and did not pass to higher trophic levels. The dynamic model successfully reproduced the evolution of trophic pathways during the transition from nutrient-replete to -depleted conditions, which indicates that simple models provide a powerful tool to study the response of pelagic ecosystems to external forcings.Understanding the transfer of carbon and nutrients between the environment and autotrophs and heterotrophs is key to furthering our knowledge on biogeochemical cycling and ecosystem functioning and how both relate. Ecologists have long distinguished two trophic pathways in the pelagic environment-the herbivorous or classical food web and the microbial loop-but now acknowledge the existence of a continuum of trophic structures with the herbivorous food 1 Corresponding author (K.vdMeersche@nioo.knaw.nl). AcknowledgmentsWe thank Joop Nieuwenhuize for analytical and logistic support, our Eurotroph colleagues for a stimulating research environment, and two anonymous reviewers for constructive feedback. We thank Wim Vyverman and Luc De Meester for constructive remarks. The modeling part of this research was performed in the frame of a master's thesis at Ghent University (K.V.d.M.).

The invertebrate zooplankton fauna of the Westerschelde (Belgium and The Netherlands) was investigated during 2 yr by means of monthly samples along a salinity gradient. Copepods were usually the most abundant holoplanktonic metazoans except in the freshwater zone where Rotifera were most numerous. The combination of a classification technique and an ordination-regression technique proved to be a valuable tool for the analysis of such an extensive data set. The presence of 4 groups was established, representing spatially distinct populations but with temporally shifting boundaries. Few zooplankton species were truly estuarine in their distribution, but many were derived from nearby coastal waters. This intrusion of marine species started in spring, reaching their most upstream distribution and highest densities in summer-early fall, then declining and retreating from the estuary, disappearing in winter. Fringing this community was a transition group with low densities, but many species. This brackish-water community consisted predominantly of the calanoid copepod Euryternora affinis. It appeared in late fall, spread out seaward to obtain its maximum density and distribution in winter-early spring. Densities then declined and the community was absent by late summer-early fall. The freshwater zone near the port of Antwerp, Belgium, was characterized by a paucity of large zooplankters, despite the high primary production in this zone. This is probably due to the low oxygen availability in this area. A canonical correspondence analysis revealed 2 major environmental axes. The salinity gradient (mainly spatial) explained most of the variance. Strongly correlated with this factor were dissolved oxygen content and secchi disc visibility. The temperature gradient (mainly temporal) was almost perpendicular to the salinity axis, indicating little or no correlation. Of lesser importance was the load of suspended matter, which was highest in the brackish area in autumn-winter. Chlorophyll content of the water was unimportant in explaining community structure. Copepod dry weight was maximal in spring in the brackish part (500 mg m-3); a lower maximum (260 mg mW3) was observed in summer in the marine part of the estuary.

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cold-water coral (cWc) reefs are one of the most diverse and productive ecosystems in the deep sea. especially in periods of seasonally-reduced phytodetritus food supply, their high productivity may depend on the recycling of resources produced on the reef, such as dissolved organic matter (DoM) and bacteria. Here, we demonstrate that abundant suspension feeders Geodia barretti (high-microbialabundance sponge), Mycale lingua (low-microbial-abundance sponge) and Acesta excavata (bivalve) are able to utilize 13 c-enriched (diatom-derived) DoM and bacteria for tissue growth and respiration. While DoM was an important potential resource for all taxa, utilization of bacteria was higher for the sponges as compared to the bivalve, indicating a particle-size differentiation among the investigated suspension feeders. interestingly, all taxa released 13 c-enriched particulate organic carbon, which in turn may feed the detritus pathway on the reef. especially A. excavata produced abundant (pseudo-)fecal droppings. A second stable-isotope tracer experiment revealed that detritivorous ophiuroids utilized these droppings. The high resource flexibility of dominant reef suspension feeders, and the efficient recycling of their waste products by the detritivore community, may provide important pathways to maintain the high productivity on cold-water coral reefs, especially in periods of low external food supply. Cold-water coral (CWC) reefs 1 rank amongst the most metabolically-active ecosystems of the deep sea 2,3. During the spring phytoplankton bloom, the CWC reef community is supported by the export of phytodetritus from the surface ocean 4-6 , but for significant parts of the year, the deep reefs are limited in particulate food sources 5,7. Dissolved organic matter (DOM) could then act as an alternative C source (dissolved organic carbon, i.e. DOC). DOM is permanently present in the deep sea, but typically at low concentrations of <50 μΜ DOC 8. However, mucus production by the CWCs and the metabolic activity of the dense reef epifauna, significantly increases the DOM concentration in the reef water 9-11. The elevated, labile DOM measurably stimulates bacterial abundance and productivity 9,11,12. Utilization of permanently present and enhanced DOM and bacteria within the reef community could retain energy on the reef that otherwise would be lost (Fig. 1a). The faunal community on CWC reefs is characterized by a high abundance of active suspension feeders 13,14. Three abundant species, especially on Norwegian CWC reefs, include the emergent high-microbial-abundance (HMA) sponge Geodia barretti, the emergent low-microbial-abundance (LMA) sponge Mycale lingua, and the bivalve Acesta excavata. Mycale lingua colonizes the live coral framework 15 , G. barretti lives on the dead reef framework or in sponge grounds neighbouring the reefs, and A. excavata forms clusters within grooves or beneath overhangs of the reef framework 15. With their high water processing rates 16,17 , these active suspension feeders may readily access resource...