Download Global Ocean Sprawl: A Driver of Jellyfish Blooms

 Global Ocean Sprawl:
A Driver of Jellyfish
Blooms
Lily Munsill
Life 2.0 Research Paper
Bio401/ Senior Seminar
Wheaton College, Norton, Massachusetts,
USA
December 1, 2014
Introduction
The world is on a slimy slope
to global jellyfish ecosystem domination. There is no single cause to
increasing
jellyfish, or Cnidarian, populations, but rather many contributing
anthropogenic factors including eutrophication,
climate change, ocean
acidification, and overfishing. An often-overlooked factor however is global
ocean sprawl, or the
proliferation of artificial structures in the ocean.
Global ocean sprawl is hypothesized to be a driver of jellyfish blooms
in
coastal systems as it provides valuable habitat for jellyfish polyps, the
benthic stage of the jellyfish lifecycle (Duarte
et al 2012). The observed
increase in jellyfish populations in coastal systems has harmful implications
for fisheries and
environmental health, causing potentially irreversible
ecological phase shifts and trophic cascades (Lyman et al
2006). Technology:
Global Ocean Sprawl
Global
ocean sprawl applies to any man-made structure that is in the ocean. Examples
of artificial structures in
the ocean include docks, pipes, ship bottoms,
sunken ships, docks, buoys, offshore wind turbines, harbor infrastructure,
seawalls and breakwaters, piers, aquaculture infrastructure, artificial reefs,
and oil rigs.
Figure 1: Activities, structures, and
industry growth contributing to global ocean sprawl. MT=metric tons, µ=annual
growth
rate. Other numbers show global occurrence. Areas without numbers show unquantifiable structures (cemented shoreline,
breakwaters, recreational boating, buoys and moorings) that account for a large
portion of artificial structures in the ocean.
Additionally, intentional
artificial reefs are listed as being present in at least 34 countries. This
figure depicts the largest
existing contributors to global ocean sprawl and
shows industry growth of marine transport, aquaculture, and offshore wind
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energy which will all continue the further input of artificial structures into
the ocean in the future (figure from Duarte et al.
2012).
Increasing
coastal populations allow for the expansion of coastal industries like shipping
and aquaculture, which
contributes to the global ocean sprawl of artificial
structures. Figure 1 depicts some of the larger contributors to global
ocean
sprawl. There are more than 14,000 commercial harbors globally and 4100
kilometers of channels and inlets,
providing infrastructure for polyp growth,
the benthic stage of the jellyfish lifecycle (Duarte et al 2012). Growing
industries like aquaculture, offshore wind energy, and marine transport are
also large contributors to polyp growth,
growing at rates of 7.5%, 28.3%, and
3.7% per year, respectively (Duarte et al 2012). Artificial structures have
been
introduced into the ocean for as long as human civilization, however,
global ocean sprawl has not been seen as a
problem until recently where the
magnitude of artificial structures in the ocean has been observed to impact
ecosystems
(Duarte et al 2012).
Biology:
Influences on Cnidarian Populations
Cnidarians are some of the
world’s oldest and most resilient organisms. Cnidarian populations are
influenced by a
number of natural and anthropogenic factors including rising
sea temperatures, increased ocean acidity, high salinity,
eutrophication,
overfishing, and natural global oscillations (Condon et al 2012, Quinones et al
2013, Purcell 2012,
Haroldsson et al 2012, Uye 2011, Lyman et al 2010, Richardson et al 2009).
Without
any anthropogenic influence, Cnidarian populations fluctuate cyclically;
observed minima and maxima of
jellyfish populations
supports global jellyfish oscillation cycles with 20 year periods (Condon et al
2012). Jellyfish
abundance also correlates with major physical phenomena, like
El Nino and El Nina periods of the El Nino Southern
Oscillation cycle;
Cnidarian populations increase during the El Nino and El Viejo periods when
climate is warmer, but
decreases are observed with the cooler La Niña period
(Quinones et al 2013).
Cnidarian
populations have been fluctuating as a consequence of anthropogenic influences
before the introduction of
artificial structures in the ocean became
problematic (Brotz et al 2012). In addition to
natural phenomena
eutrophication, climate change, and overfishing have been
leading contributors to increasing populations (Purcell 2007,
Purcell 2012, Haroldsson et al 2012). Phase shifts in pelagic ecosystems have been related to
overfishing and the filling
of an absent niche by jellyfish; an alarming
figure, 100-120 million tonnes of fish is annually removed from global
oceans
(Richardson et al. 2009).
The
low water clarity, high nutrients, and anoxia in eutrophic regions are all
favorable conditions for jellyfish
(Purcell 2012). As nonvisual predators,
jellyfish can outcompete lower trophic level fish as water clarity decreases
with
eutrophication (Haroldsson et al 2012).
Jellyfish can survive in very low oxygen environments, up to <1 mg 02 per
liter, well below the toxic level for fish, around 2 or 3 mg per liter (Purcell
2012).
Climate change
also creates ideal conditions for jellyfish blooms (Purcell 2007). Warming sea
temperature has been
associated with higher rates of asexual reproduction in
some jellyfish like moon jellies, Aurelia labiata,
(Purcell 2007)
and a scyphozoan species Cotylorhiza tuberculata
(Purcell et al. 2012). Some fish species cannot survive the harsher
conditions
associated with climate change, especially ocean acidification, further
enabling jellyfish domination (Purcell
et al 2012). Stronger winds associated
with climate change will be beneficial to dispersal of jellyfish populations,
aiding in the spread of blooms (Lyman et al 2010).
Most
of these anthropogenic factors influence the pelagic phase of the Cnidarian
lifecycle, however, the benthic is
often overlooked. Some classes in the phylum
Cnidaria, like Scyphozoa,
Hydrozoa, and Cubozoa have a benthic stage
in their
lifecycle where a sessile benthic polyp attaches to a substrate, which produce ephyrae, or juvenile jellyfish
medusa (Duarte et al 2012).
Polyps can produce asexually and during each asexual reproductive event, a
single polyp
can create up to 40 juvenile medusa
(Duarte et al 2012). Benthic polyps can remain attached to a substrate for many
years, releasing medusa seasonally or when
environmental conditions are optimal for the jellyfish (Duarte et al 2012).
A
single polyp can produce hundreds or thousands of medusae, and those medusa can go on to produce thousands or
millions of
polyps (Duarte et al 2012). As Cnidarians have characteristics of a fast
growing r-selected population, the
ability of these polyps to produce many medusa allows for exponential growth of jellyfish
populations.
The
Hybrid System: Cnidarian Benthic Polyp Growth on Artificial Structures
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Artificial
structures in the ocean are hypothesized to magnify the intensity of jellyfish
blooms (Duarte et al 2012).
The proliferation of artificial structures in the
ocean has had unintended consequences for jellyfish populations by
providing
valuable substrate for the benthic Cnidarian polyp to attach and release
juvenile medusa, especially in areas
with soft benthic sediments that lack
natural rock substrates (Duarte et al 2012). The introduction of artificial
structures
provides a perfect substrate in regions with less then ideal
substrates for attachment. The magnitude of space that
artificial structures
provide polyps is of concern since polyps are only a few millimeters in length for
most Cnidarian
species, but polyps can be found on artificial structures in
densities from 10,000-100,000 individuals per square meter
(Duarte et al.
2012). Thus seemingly insignificant artificial substrates, even plastic garbage
pollutants like plastic
bottles can contribute greatly to blooms since polyps
have high-density populations.
Blooming
Cnidarian populations have found to be correlated with areas with high
occurrences of artificial
substrates, and when artificial structures are
removed, densities of medusa has declined (Duarte et al. 2012). Artificial
structures, especially in harbors, provide shaded surfaces
which are preferable to polyp growth, and provide shelter to
polyps in
areas where high wave energy would otherwise destroy them. The high level of
artificial structures in ports in
combination with high fishing pressures,
eutrophication, increased pollution, and hypoxia creates perfect environments
for polyp growth in conjunction with the removal of competing species (Duarte
et al. 2012). In this way, artificial
structures provide a nursery habitat for
blooming Cnidarian populations.
Consequences
and Future Predictions
Cnidarian
blooms as an unintended consequence of global ocean sprawl have negative
impacts on ecosystems,
causing ecological phase shifts and shifting towards
Cnidarian dominance in the lower trophic level (Lyman et al
2006).
Additionally, blooms have economic consequences as they impact tourism, fishing
industries, and nuclear power
plants (Richardson et al 2009). However, artificial
structures also have unintended positive consequences as they
provide benthic
substrate for many organisms including corals and oysters, providing a
productive marine habitat in
areas of otherwise poor nutrients and low
biodiversity (Duarte et al 2012).
As
a consequence of increasing sea temperature and ocean acidification, in
combination with the increasing
presence of artificial structures in the ocean,
scientists expect to see future increases in occurrence and intensity of
Cnidarian blooms (Duarte et al 2012, Brotz et al
2012, Richardson et al 2009). Both climate change and the existence
of
artificial structures in the ocean are likely irreversible, so jellyfish
populations may continue to increase globally. While
the expansion of coastal industries can’t be slowed, artificial substrates can
be modified with different
surfaces that polyps cannot attach to, which would
effectively reduce coastal jellyfish blooms (Duarte et al 2012). In
addition, eutrophication induced blooms can be mitigated with better coastal
management and run off systems.
Furthermore, research can be conducted to
reverse existing jellyfish blooms (Gibbons, Richardson 2013). Global
ocean sprawl has unintentionally provided habitat for benthic jellyfish polyps.
This new hybrid technology,
combined with other favorable factors for jellyfish
growth including eutrophication, increasing temperatures, and ocean
acidification is causing coastal populations of jellyfish to increase, which
consequently causes irreversible ecological
phase shifts and trophic cascades.
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