Ichthyoplankton
Ichthyoplankton, in particular fish eggs are important for
the ecosystem because of their role in the reproduction
cycle of numerous fish species. Therefore it is important
for The Ocean Cleanup Array to not catch these earlystage
organisms. Although absolute certainty about the
interaction between the structure and fish eggs can only
be obtained through field testing. Fluid dynamics simulations
(as used in chapter 3.3) can be used to predict
whether fish eggs would be caught or carried away by the
current passing underneath the barriers. Considering the
specific density of fish eggs is between 1019 and 1023
kg/m3, and their size is between 0.5 mm and 5 mm (with
a mode of 1.5 mm) (Tanaka, Y., 2007), fish eggs should be
carried away by the downward current in front of the barriers.
Worst case scenario
Assuming that all zooplankton that comes into contact
with either the booms or the platform will perish, most
of it will sink to the ocean floor and become available
for pelagic recycling or benthic organisms instead of being
pumped into the platform. So even though they are
themselves killed in the process, nutrient removal from
the food web is limited to the fraction of zooplankton that
is sucked into the slurry pump. As a result, The Ocean
Cleanup Array will likely cause a local shift in deep sea
benthic processes as more planktonic remains fall to the
seabed. Biomass removal by the Array will then be limited
to the fraction of the amount of water that comes into
contact with the Array that is pumped into the platform:
2,400 m³*d-1 / 36.29*108 m³*d-1 = 6.61*10-7 (daily pump
volume / daily water flow through Array)
6.61*10-7 * 1.04*107 kg = 6.91 kg (dry) zooplankton biomass
6.3.5 Reduct ion of zooplankton bycatch by The
Ocean Cleanup Array
The amount of bycatch will be highest during the night
when zooplankton surfaces to graze on phytoplankton
(Lo et al., 2004). To reduce bycatch, platform pumps could
operate 8 hours a day and be switched off at night. This
enables the zooplankton to surface during the night, forage
on phytoplankton and afterwards sink below boom
depth.
Lastly, recent studies on plastic transfer between
trophic levels showed that microplastics can enter the
food chain through ingestion by microzooplankton and
can then be transferred to higher trophic levels (Setälä,
Fleming-Lehtinen, & Lehtiniemi, 2014). Considering these
observations and the likelihood of increased concentrations
of microplastics in the NPSG, zooplankton bycatch
might even serve to remove plastic-laden organisms from
the food web.