The vertical export of carbon and nitrogen caused by zooplankton diel vertical migration

Fieldwork was conducted at three contrasting sites to test the applicability of an in situ technique (ZOOFLUX) for the assessment of the role of zooplankton diel vertical migration (DVM) in the removal of carbon and nitrogen from the surface layer of the ocean to the depths (the active flux). ZOOFLUX relies on the detection of a significant dawn-dusk difference in the carbon and nitrogen weight of migrating individuals (δ). Therefore, its successful application is highly dependent upon the ecology of the migrant species, the level of individual variability in carbon and nitrogen weight (V), and the number of samples that can be collected (n). At site 1 (the Clyde Sea, western Scotland), Calanus finmarchicus and C. helgolandicus exhibited a variety of migration patterns, and did not always conform to the ‘normal’ DVM pattern of up at dusk and down at dawn (NDVM). As a result, δ was variable and V relatively high, while n was relatively low. When δ was non-significant, the probability of making a Type II statistical error (β) was high. In most cases, both the minimum number of samples (nmin), and the minimum diel change occurring in carbon and nitrogen weight (δmin), would have needed to be unrealistically high before ZOOFLUX was applicable to these species. At site 2 (the Sargasso Sea off Bermuda), Pleuromamma xiphias and krill (Thysanopoda aequalis, Euphausia hemigibba, and E. brevis) both performed NDVM. At site 3 (Doubtful Sound, New Zealand), Nyctiphanes australis performed NDVM at the population level, but some individuals remained at depth during the night, and others at the surface during the day. Despite the more uniform pattern of NDVM at both of these sites, the findings were similar to those at site 1:δ was variable, V relatively high, and n relatively low. Again, this meant that (nmin) and (δmin) were often unrealistically high. These findings are discussed in terms of (1) what we can now say about the factors contributing to the active flux, (2) the applicability of the ZOOFLUX technique, and (3) the way forward for future studies. While the ZOOFLUX technique is advocated for future application, it will only prove successful with a prior knowledge of the DVM behaviour of the target species, and the ability to collect interzonal migrants at the critical moments at which they pass both up and down through the pycnocline during n the die