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We report the trophic structure of a myctophid assemblage by measuring the isotopic niches of 14 species living in Kerguelen waters, southern Indian Ocean. Most of the species show distinct isotopic niches that differ by at least one of the two niche axes ([[delta].sup.13]C habitat and [[delta].sup.15]N trophic position), indicating trophic partitioning within the assemblage. Strong niche segregation occurs within each of the three most common genera of myctophids (Electrona, Gymnoscopelus, and Protomyctophum), illustrating the different mechanisms (habitat and dietary segregation) that allow coexistence of closely related species. Calculated trophic levels (TLs) of myctophids ranged from 3.3 to 4.2, showing that they are secondary and tertiary consumers in the pelagic ecosystem. The positive relationship between TL and standard length of fish points out a structuring effect of size, with larger species (Gymnoseopelus spp.) occupying a higher trophic position than smaller species (Krefftichthys anderssoni and Protomyctophum spp.). Myctophids occupy an intermediate trophic position between macrozooplanktonic crustaceans and seabirds and marine mammals within the pelagic ecosystem. However, the TLs of large myctophids overlap those of crustacean-eating seabirds [e.g., Eudyptes spp. (crested penguins) and Pachyptila belcheri]. The isotopic niche of myctophids indicates that Aptenodytes patagonieus (king penguin) adults prey upon K. anderssoni when they feed for themselves, thus exemplifying the usefulness of isotopic datasets on potential prey of predators to depict trophic relationships.

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.dsr.2009.05.013 Byline: Jean-Philippe Labat (a)(b), Stephane Gasparini (a)(b), Laure Mousseau (a)(b), Louis Prieur (a)(b), Marc Boutoute (a)(b), Patrick Mayzaud (a)(b) Keywords: Zooplankton; Biomass; Size distribution; Mesoscale eddies; Optical plankton counter; Pelagic environment; Northeast Atlantic Ocean Abstract: We examined the mesoscale distribution of zooplankton populations using a continuous recording system: the optical plankton counter (OPC). Data were collected in the mid-latitude northeast Atlantic inter-gyre region in April and September 2001 during the POMME 2 and POMME 3 cruises. This sector of the North Atlantic system is characterized by subduction phenomena and mesoscale eddies. Estimated mean biomass was 2.88DWgm.sup.-2 in April and 1.64DWgm.sup.-2 in September with populations dominated by small copepods of the genera, Clausocalanus, Paracalanus and Oithona. Day-night changes in vertical distribution appeared to be seasonally variable. During April, absolute concentrations within the upper layer above 50m were higher at night. During September, vertical profiles of relative biomass were quite similar for day and night. Highest depth-integrated biomasses were located mainly on the periphery of anticyclonic eddies, with maxima related to the increase in depth range of vertical distribution. This pattern suggested that maximum biomass was associated with the most dynamic parts of the frontal features. Other zones of high zooplankton biomass were associated with the centers of cyclonic eddies and high fluorescence values. Using a 3D view, we found that zooplankton distribution showed a more complex pattern than in a 2D view with variable vertical distribution. Hence, proper description of the distribution of zooplankton underlines the need to describe this submesoscale with an order of magnitude around 10 nautical miles. Author Affiliation: (a) UPMC Univ Paris 06, UMR 7093, LOV, Observatoire oceanologique, F-06230 Villefranche-sur-Mer, France (b) CNRS, UMR 7093, LOV, Observatoire oceanologique, F-06230 Villefranche-sur-Mer, France Article History: Received 6 February 2008; Revised 12 May 2009; Accepted 22 May 2009