Investigations on the Trophie Position o/ a Group o/ Pelagic Organisms (Euphausiacea). I I I. The Group as a Food Source
The position of a group of pelagic organisms among the food webs depends upon (1) its abundance in the biomass;
(2) the factors allowing its utilization by predators; among these, the size distribution of animals and their bathymetric repartition appear to prevail. In the equatorial and southtropical Pacific Ocean, euphausiids represent 8 % of the total macroplanktonie and mieronektonic biomass, i.e., approximately 13 g/1000 m s wet weight (somewhat less in oligotrophie tropical zones, a little more in the richer equatorial belt); Euphausia diomedae accounts for more than 50 % of this biomass. Individuals measuring 9 to 18 mm in total length (4 to 37 mg wet weight) constitute 84 % of the whole biomass of the group; the importance of each species in terms of size groups is discussed. Depth distribution is examined at the specific level. By night, 75 % of the biomass concentrates in the 0 to 160 m water layer (ca. 10 g/1000 m s wet weight; main species Euphausia diomedae), 22 % between t60 and 300 m (3 g/1000m s main species Nematoscelis spp.), and 3% (0.3 g/t000 m s) in deeper layers. During the day, the only species abundant at depths less than 400 m belong to the genera Stylocheiron and Nematoscelis; specimens smaller than 15 mm and 20 mg remain at depths shallower than 200 m, individuals between 15 and 22 mm (20 to 65 mg) appear around 200 to 400 m, and larger animals are restricted to depths below 400 m.