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5. TWILIGHT ZONE CARBON
Mesopelagic fish that live in deep waters undertake a vertical
migration at night to feed on zooplankton in the surface
waters of the ocean. During the day, to avoid predation,
these fish descend back to the ocean’s ‘twilight zone’ at
depths of 200 to 1000 meters, transporting substantial
quantities of organic carbon away from the surface and
ultimately releasing it as faeces, which sink further into the
depths (Figure 2, service 5) (Davison
et al.
2013). Through
this mechanism, carbon is effectively transported below the
upper thermocline, the depth zone in which most carbon
remineralization occurs (Davison
et al.
2013).
Commercial fisheries do not currently target mesopelagic
fish and it has been suggested that these fish undertake net-
avoidance behaviour, which reduces their accidental capture
in current fishing gears (Irigoien
et al.
2014). Twilight
Zone Carbon may be under-valued in current estimates of
oceanic carbon cycling, as recent research suggests that the
total biomass of mesopelagic fish may be between 1,000 to
10,000 megatons; ten times higher than previous estimates
(Irigoien
et al.
2014).
Twilight Zone Carbon, possibly the most intact biological
mechanism of marine vertebrate oceanic carbon cycling
(Irigoien
et al.
2014), appears to provide a direct two-step route
from the ocean surface to the deep sea and sediment, where
carbon can be stored for millennia or longer (Lutz
et al.
2007).
Vertical migration of mesopelagic fish
transports carbon away from surface
waters to depths of 200-1000m