Because they live there, their offspring are released locally
and are more likely to find a suitable place to settle.
They can have very high densities in the coastal waters
and competition for space to settle can be fierce as most
spaces are already colonized.
Within the open ocean, it is difficult to track the source
of fouling organisms, so how species get to where they
are found is often very poorly understood. The only solid
structures are transient passing vessels or the surfaces
of larger animals. The hulls and ballast tanks of shipping
vessels passing The Array must therefore be considered
as potential sources of fouling species.
It can be assumed that most of the plastic debris this
project seeks to intercept came from a number of coastal
sources, where it may have collected a fouling community,
and therefore the number of potential species must be
higher than for any one geographic region. Because of the
intense competition for space to settle, it is reasonable
to assume that the plastic will carry fouling organisms.
Indeed, one study found a healthy adult barnacle growing
attached to the tag ring on the leg of a migrating seabird.
Barnacles often grow on oceanic species, such as turtles
and whales. The Array will provide an attractive source
of shelter for these species, where it would otherwise be
unavailable. It is reasonable to assume that The Array will
be approached by a greater number of mobile species
than an equivalent patch of open water. These mobile
species could carry potential fouling recruits to The Array.
Additionally, it has been shown for many fouling species
that biofilms can act as an attracting cue to settlement.
Depending on how The Array is deployed, it may acquire
organisms en route from the coast to its fixed location in
the gyre. If towed it will likely carry its own potential fouling
community with it.
Temperate coastal regions typically display strong seasonality
of recruitment, growth and reproduction of fouling
species. Many locations under the heaviest fouling
pressure are located in tropical and subtropical regions.
Because of their limited seasonality, they tend to have
relatively constant temperatures and seasonal variations
are dominated by differences in precipitation only. As The
Array is initially intended for deployment in a subtropical
region, effects of seasonality can be expected to correlate
well with this.
5.3.2. Likely effects on the Array
Weight
One of the largest challenges presented by fouling on a
floating structure is that of increased biomass resulting
in a heavier weight. The structure will be designed to sit
at an optimal position on the surface. But if the weight is
significantly increased, it could float below the surface
or even sink entirely. The floating barriers have a finite
buoyancy force per length unit, fully determined with the
cross-sectional dimensions of the buoyancy element, as
well as the buoyancy element’s material. Optimal conditions
in both nutrient availability and environmental parameters
can lead to the designer’s worst case scenario,
with a mass of biofouling that can reach a few kilograms
per square meter. Since the conditions in the open ocean
are far from optimal, the weight of the biofouling mass
on a PVC surface will likely not reach more than 0.5-2 kg/
m2 over a period of 180 days as calculated in Guam in
the West Pacific (Rowley, D. M., 1980). However, the worst
case scenario wil be used in the design of The Array to ensure
that the booms will still have enough buoyancy after
an extensive period of colonization. Considering an estimated
excess buoyancy force of 500-1500 kg/m (depending
on floater material and diameter), we deem it unlikely
that the potential increase of weight by biofouling will
significantly alter the behavior of the structure.
Struct ural damage
The amount of structural damage depends on the materials
used for construction. Metallic materials, of which
the platform for example is designed to be manufactured
out of, must be protected from the corrosive properties of
salt water and will require a coating to perform this function.
The open ocean environment is often referred to as
a desert and any solid structure within it may be likened
to an oasis because it offers shelter normally unavailable.
They become fouled by ocean fouling organisms such as:
gooseneck barnacles (Lepas sp.) tunicates, Bryozoans,
but also hydroids, borer and mussels (until about 50
km offshore). The fouling assemblage will attract grazing
species as the microcosm develops. Grazing species
may significantly damage any coatings that protect the
surface. Once the underlying surface is exposed, the
seawater may quickly cause serious structural damage.
Nevertheless, the rate of biofouling varies over time, and
depends on several factors, such as the time at which
accumulation occurs. It is therefore advisable to focus
special attention on the maintenance of coatings of the
collection platform.
Differential fo uling
At least four di