Vertical distribution of subtropical epiplanktonic copepods

The goal of our project is to increase our knowledge about the co-existence of the numerous small copepod species in the subtropical and tropical open oceans by combining experimental and field studies. In this paper we present the vertical distribution of the copepod assemblages collected in the upper 100 m of the water column during a diel cycle at an oceanic site off Bermuda in summer 2000. Our quantitative analysis ranged from the gross features of the community to order, genus, species and sex, ending with the population structure for one representative calanoid (Mecynocera clausii). Total abundance of copepods sampled with both 63 and 200 mum meshes increased from near the surface to 100 m depth. Of the four orders, the Calanoida and Cyclopoida increased slightly with depth, whereas the Poecilostomatoida hardly occurred in the upper 30 m and then increased by more than one order of magnitude towards 100 m depth. Harpacticoida showed even abundance through the vertical column during the night. Some abundant genera/species displayed hardly any or small differences in abundance over the upper 100 m (e.g. Calocalanus, Oithona); that distribution also showed that genera with entirely different motion and feeding behaviour can occupy the same vertical environment. A second group appeared to avoid the upper mixed layer and upper thermocline almost completely (e.g. Oncaea, Mecynocera clausii, Clausocalanus pergens, Eucalanus hyalinus, Haloptilus longicornis). A third limited group seemed to prefer the upper mixed layer (e.g. Clausocalanus furcatus). Some genera were characterized by a paucity of males whereas others had as many males as females. Our results showed that quantifying the distribution and abundance of genera, species and their copepodid stages seems to be the preferable approach, leading towards an understanding of the functioning of the epipelagial copepod community. We hypothesize that a combination of variables, including feeding behaviour, temperature preference and also predation, could determine the ranges of vertical distribution, which would be flexible because of the effect of each of the variables. It is reasoned that predation was of minor impact in these oligotrophic waters because of the very low abundance of carnivorous species.