How will cold-water corals fare in a Changing Ocean?
17 September 2012
Out of sight in the deep oceans, cold-water corals are
facing yet another threat. As the chemistry of the oceans changes
due to increasing CO2 in the atmosphere, these coral
ecosystems, home to thousands of other species, may be unable to
continue growing or even survive. The Changing Oceans Expedition
set out to the North Atlantic during the summer to find out what
the future holds for these amazing animals.
Ocean acidification, often referred to as the ‘the other
CO2 problem’, is arguably the biggest threat facing
marine calcifying organisms today. The amount of CO2 in
the atmosphere has increased exponentially since the industrial
revolution, with much of this CO2 dissolving into the
oceans. The problem with this is that dissolving more
CO2 into the sea decreases its pH, produces more
hydrogen ions and hence increases it acidity. Currently, the pH of
the sea is about 8.1, but increasing atmospheric CO2
levels are predicted to potentially drop this by about 0.3 pH units
by 2100.Whilst it may not sound dramatic, this small drop in pH can
have potentially huge implications for the vast number of marine
calcifying organisms, such as calcareous algae, shell producing
animals, and coral. This is because increasing concentrations of
dissolved CO2 in the oceans decrease the carbonate
saturation of the water, which means there are less carbonate ions
available for corals to make their calcium carbonate skeletons.
In May
2012, the RRS James Cook set sail for the North Atlantic, with an
international team of scientists onboard as part of the Changing
Oceans Expedition. The mission was to examine the potential impact
of ocean acidification and warming on cold-water coral reefs and
the associated reef-creatures. Hidden in the depths of all the
world’s oceans, cold-water corals form vast mounds and reefs, with
the complex 3D structures supporting thousands of species,
including commercial fish species. Although they grow much slower
than their tropical counterparts, the calcium carbonate skeletons
that cold-water corals create can form extensive reefs which
persist even after the animal itself has died. In fact, the
Lophelia pertusa reefs off Norway which cover
approximately 2,000km2, are larger than tropical reefs
in the Seychelles, Belize or Mozambique.
In the
4 weeks at sea, a range of cold-water coral sites were visited,
from the ‘shallow’ reefs of Mingulay in the Outer Hebrides,
dominated by Lophelia pertusa, to the reefs on the
Logachev mounds, spectacular with both Lophelia pertusa
and Madrepora oculata at nearly 1000m depth. Little is
known about both species of corals and the ecosystems they form,
because of their inaccessibility – you can’t just dive down and
explore the reefs like you can in the tropics. Before exploring the
reefs and collecting samples could begin,the location of the reefs
had to be identified. Once at each site, advanced acoustic
techniques, such as multibeam and sidescan sonar systems, were
used to dynamically image the seabed and to identify possible
coral reefs that form mounds growing up from the seafloor. The
Remotely Operated Vehicle (ROV) Holland I was then deployed to
examine these ecosystems in greater detail.
During each ROV dive, the excitement is the lab was
palpable, as high definition images of the spectacular reefs
beneath were beamed back from cameras to the ship. Fish darted
across the screen as the robot ran along its transect, and unusual
sponges and crabs came into view, causing a buzz in the lab. But it
wasn’t all about watching for the team of scientists; short-term
experiments were undertaken to examine the effect of ocean
warming and acidification on the growth and overall health of the
corals. Along with longer term experiments underway at Heriot-Watt
University, this will help to determine whether corals can adapt to
such changes, or whether it will be impossible for them to survive.
Using coral samples carefully collected with the robotic arms of
the ROV, Lophelia pertusa and Madrepora oculata were
maintained in specially designed tanks for the duration
of the cruise. The temperature and CO2 levels in these
‘mini-oceans’ were manipulated to mimic future conditions, so a 3°C
increase and a near-doubling of atmospheric CO2.
Respiration and growth rates of these corals were then
measured over a 3 week period, using an optode system and
radioisotopes, respectively. Concurrent experiments looked at
changes in other aspects of the coral’s biology in response to
their changing environment, including protein expression, microbial
communities and DNA:RNA ratios as a proxy for health. Together,
these parameters will allow us better prediction of how corals
will respond to global climate change.
Alongside
the ROV campaign, a host of other activities took place out at sea.
One such activity was the CTD and SAPS deployment. CTD stands for
conductivity, temperature and depth; the parameters that the
instrument measures at the bottom of the ocean. Attached to the CTD
frame was also a SAPS (Stand Alone Pumping System), which is a big
pump attached to a filter rig with a delayed timer. SAPS was
used to look at the amount of particulate organic carbon
(coral food) that is reaching the reefs. The pump switches on when
it is at the bottom of the ocean, and records how much water flows
through the filters. Following laboratory analysis of how much
carbon is on the filters, how much carbon the corals have access to
can be calculated. This information, combined with surveys of the
reef and CTD data, can help improve understanding of why the
corals live where they do, and what any future changes in climate
and currents may have on these ecosystems.
Four weeks at sea passed by in a flash, and a wealth of information
was collected by all on board. Now back in dry land, it’s time to
process samples, extract data and try to understand what the future
holds for these cold-water creatures in a changing ocean.