Appendices 5-13 - Nautilus Cares - Nautilus Minerals

4.2.4 Distribution
Horizontal
Solwara 1 Offshore June 2008
Hydrobiology Pty Ltd
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The main factors influencing the distribution of species in this zone are oceanographic
structure, currents and primary productivity regime. Water masses of the mesopelagic zone,
particularly the deeper portions of the zone, are relatively uniform at the scale of ocean
basins, presenting a relatively uniform physical environment over very large areas.
However, the upper portion of the mesopelagic zone interacts with the lower portion of the
epipelagic zone and is exposed to oceanographic features such as internal waves that can
result in features such as oxygen gradients, nutriclines, turbulence and other structure,
which may provide some horizontal variability to the mesopelagic zone (see Section 4.1.2).
Mid-water and near-bottom currents, upwelling or eddies induced by benthic features, such
as seamounts or rapid depth changes, has been implicated in the attraction or aggregation of
mesopelagic organisms off the eastern Australian coast (Brewer et al. 2008). On the relatively
featureless seafloor at the Solwara 1 project area, the influence of currents is expected to be
minor and the distribution of food is likely to be the main limiting factor within the
mesopelagic environment. However, oceanographic measurements made at the Solwara 1
site have identified some interesting features in the mesopelagic zone. Relatively high
currents were observed in surface waters, overlaying a region of relatively low current
speeds, overlaying another deeper zone of relatively high current speeds at depths between
140 m and 250 m at various times (B. King, pers. comm.). This observation is consistent with
the presence of long-amplitude internal waves that could be established as a result of deep
water masses interacting with the eastern side of the Bismarck Sea (western side of New
Ireland) or topographically-induced mixing as a result of water movement through St.
Georges Passage, that pushes water through a shallow, narrow passage possible inducing
some upwelling. Such oceanographic processes may explain, for example, the dissolved
oxygen profiles measured at the site (see Section 3) because such processes can disturb
vertical structures in the water column and entrain oxygen and nutrients into deeper layers.
Such processes may also have relevance to the distribution of mesopelagic communities
because these water movements may passively aggregate mesopelagic plankton or attract
mesopelagic nekton to areas of high turbulence or productivity.
In addition, oceanographic instruments deployed at the Solwara 1 site have detected
particulate matter originating from below the ocean surface, falling through the water
column at velocities that are faster than typical for falling particulate organic matter (POM,
otherwise known as ‘marine snow’). One hypothesis put forward to explain this observation
is that the instruments are detecting material originating from black smokers being
transported through the water column, trapped at depth and falling down through the water
column (B. King, pers.comm). If present, these kinds of processes may also be controlling
the horizontal distribution of mesopelagic organisms in the project area, although has not
been demonstrated in other studies. Midwater trawling above vent sites has not identified
significant differences in micronekton populations from trawls at similar depths at non-vent
site (P. Herring, pers. comm.).
POM is the main consistent input of nutrition to the mesopelagic zone and would be
expected to be a driver of distribution and abundance of mesopelagic organisms. As will be
described further in Section 8 on trophic webs, delivery of organic ‘fall-out’ to the
mesopelagic zone is controlled by a number of processes centred in the epipelagic zone.
Marine snow consists of detritus, faeces and carcasses of dead animals, typically colonised
by bacteria (Radchenko 2007). As will be discussed in Section 7 (Bioluminescence), these
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