The Biological Significance of Iceberg Calving and
Grounding
Introduction
The study of Glacial and Ice
movements and interactions are important as glacial and ice processes can have
an effect on the biology of an area especially when the ice comes into contact with
the seabed or as it interacts with the water column. Two main processes which
can have an effect on the biology of the area these are calving, the process by
which ice is ablated from the edge of a glacier or ice shelf, and grounding,
which is the process that occurs when an iceberg comes into contact with the
seabed and alters its morphology. An iceberg is a large chunk of ice which
drifts into the ocean after being broken off the terminus or edge of a glacier.
An iceberg is formed because the end of the glacier is unstable due to the
forward motion of the glacier. The process of icebergs forming ,where ice breaks
off a glacier, is called calving. The change in the rate of iceberg calving can
have a dramatic effect on the entire ecosystem as it can affect phytoplankton
blooms which in turn affects other trophic levels and therefore the biology of
the entire area. Icebergs are also a key transporter of sediment, minerals and
nutrients. Icebergs can transport a lot of iron into the water column which can
affect plankton blooms and key processes such as Carbon drawdown. Although a small amount of iron can help phytoplankton blooms which helps to capture
carbon dioxide from the atmosphere. However large amounts of iron released into
the water column can hinder the phytoplankton bloom and therefore reduce carbon
drawdown. Grounding is the process in which icebergs make contact with the
seabed this can be due to the iceberg drifting into shallower waters. When the
icebergs make contact with the seabed and continue to move they leave scours
which are characteristic curvilinear features typically 1-2 m in depth and
30-40 metres in width. These marks can reach several hundred metres this is dependent
on how long the iceberg stays in contact with the seabed. This can be anywhere
from a few minutes to several months. Grounding changes the structure of both
the sediments on the seafloor and the keel of the iceberg. The changes to the
structure of the seabed sediments can have a pronounced effect on the biology
of the area. Scouring of the seabed can actually
increase the biodiversity of the area as long as it is not frequent but if the
grounding happens regularly then the fauna does not have time to recolonize.
Grounding can also have a short term biological effect of damaging or killing
large organisms such as sea urchin and kelp.
The aim of this essay is to fully explore the biological
effects and significance of both processes calving and grounding.
Biological Impact of Calving
Calving is the process by which new icebergs are
formed. Icebergs are a product of tide and wave action which creates stress
fractures in the ice cap or glacier at the terminus. The terminus of the iceberg
is the point at which a glacier meets the ocean this part of a glacier or ice
sheet is often unstable because of glacial movements such as basal sliding.
When stress fractures occur and the tides and waves exert yet more pressure on
the glacier a large mass of ice breaks off and floats out to sea.
Calving is a natural process which occurs at a
certain rate at all ice sheets connected the ocean however an increase in the
calving rate or a particularly large iceberg calves off a glacier then this can
disrupt many processes in the ocean. The speed at which icebergs calve can
affect the biology of the water column. The iceberg calving velocity changes
with glacier and the type of fractures and the instability at the glacial
terminus. The type of fractures found at glacial edges depends on the type of
glacier as well as englacial and subglacial velocity
gradients. Iceberg
calving velocity can change because of water depth and temperature. Iceberg
calving can increase as water depth decreased and as temperature increases. (Pelto
& Warren 1991, Benn et al. 2007).
Icebergs can have a biological effect as
they deposit trapped sediment from the terrestrial zone into the water column
as they melt. One of the main minerals transported like this is iron. Iron can
have a particularly significant effect on the biology of an area. Iron is made available
to phytoplankton in excess which then limits phytoplankton bloom which reduces
primary productivity of an area. (Lancelot et al. 2009). (Arrigo et al. 2003).
Icebergs also affect the biology of the water column when they alter in
shape as they melt, due to being in contact with the water, the changes in the shape can cause the centre of gravity of the iceberg to shift resulting in the
iceberg tipping or overturning in the water. This can also deposit sediments
and nutrients in the water which in excess can hinder phytoplankton blooms.
Changes in phytoplankton populations can have dramatic effects higher up the
trophic levels as many organisms rely on phytoplankton. (Bigg et al. 1997).
An increase in the number of icebergs, caused by an
increase of calving velocity, can restrict pack ice from its natural drift
course. This can lead to more pack ice present in an area in the spring or
summer when previously the pack ice would have drifted past that area. (Arrigo et al
2002) If the ice cover is significant the area is less suitable
for phytoplankton growth and the algal growing season is reduced in length. In
some areas, the primary productivity was reduced by more than 40% in an area
where pack ice drift had been disrupted. (Wang, et, al.
2014)
When pack ice drift is
disrupted and the phytoplankton growth is effected the phytoplankton population
can shift to another structure which is slightly more suitable to the new
conditions this can influence the abundance and behaviour of organisms higher
up the trophic levels as well as altering other important biogeochemical
processes such as carbon drawdown. (De Baar, et, al. 1995) Other biological effects of an alteration in
the phytoplankton populations are that in many areas, other organism’s lifecycles
are reliant on the predictability and availability of the food supply in the
spring in summer. Two specific organisms which can be affected by an increase
of pack ice caused by increased iceberg calving are zooplankton and Emperor or
Ade’lie penguins. Some zooplankton such as krill and copepods release their
eggs to coincide with phytoplankton bloom in spring. An increase in iceberg
calving can delay the bloom which can cause several problems either there will
decrease nutrients when the zooplankton eggs hatch or if reproduction is
delayed there will not be enough lipid reserves left from the previous year to
produce eggs. (Hagen, 1999). Larger
organisms such as the Ade’lie penguins time their reproduction so chicks fledge
when there is maximum food available, In early summer so if to the plankton
bloom is delayed it could lead to higher chick mortality if not enough food is
available. Both these organisms are sensitive to environmental disruption
especially temporal shifts in food source availability. (Ainley, D, G. 2002)
Biological
Impact of Grounding
Icebergs can ground this is process by which an
iceberg drifts into shallower areas and the keel of the iceberg makes contact
with the seabed. Once an iceberg has made contact with the
seabed it will continue to drift. This process produces long narrow furrows
called gouges or scours. (Gutt, J. 2001)
These scouring events can kill or damage large
organisms on the seabed. Large organisms which are often affected are kelp sea
urchins and bivalves. Scavenging
organisms then migrate to the furrows in order to feed on the destroyed
bivalves and urchins. Common scavengers are Buccind gastropods and amphipods
crustaceans and deposit feeders are typically found on the edges of the scour marks
and many predatory amphipods and Polychaeta burrow into disrupted sediments. (Texido,
et, al. 2007)
Grounding of icebergs can
also affect the biology of the area. After an area of the seabed has been gouged
all biodiversity is removed. After the iceberg has lifted and drifted away from
the area recolonization can take place. Recolonization of these gouges can
actually increase the biodiversity of the gouged area. (Smith, et, al. 2007) This because different stages of recolonization
have different organisms and species that occupy the area. When the seabed has
been scoured different recolonization stages coexist in one area increasing the
biodiversity. Although this is not always the case as in frequently scoured
areas, the area does not have time to recover as there are slow rates of fauna
growth. This means that in heavily scoured areas the biodiversity is decreased.
(Gutt, et, al. 2001)
Keats, et, al. 1985 also
found that plant biodiversity is increased by ice scouring in the Arctic in
years when the canopy of Alaria esculenta
was removed by ice scouring. The light was no longer a limiting factor as it
was in other years and there was increased biodiversity and additional annual
species were most abundant. In previous years only 1% of light was able to penetrate
through the Alaria canopy so other perennials
are unable to coexist. In undisturbed areas, it was mostly dominated by
predators and suspension feeders but once scouring occurs the area becomes
dominated by a higher proportion of scavengers and deposit feeders
Iceberg scouring can also have
another effect on the biology the creation of ‘black pools’. Black pools are
found in the Canadian Arctic Archipelago. Black pools consist of sediment
depressions caused by ice scouring and the release of brine from sea ice. These
depressions fill with hypoxic, sulphide-rich water and cause mortality of
infauna and sessile epifauna as well as a fatal trap for mobile animals. (Reimnitz, et, al. 1972)
The process of grounding,
scouring and the recolonization creates a patchy pattern on the sea floor of
benthic Epifauna. The patterns of recolonization and scouring are not
predictable meaning the effect of grounding and the biological significance is
not either.
Both iceberg calving and grounding which are natural
glacial processes have a profound effect on the biology of the water column and
the benthic substrate. The production of icebergs in a process call calving can
effect on biology in several ways. The melting of icebergs then they come into
contact with the water deposits sediment and iron which can limit phytoplankton
blooms and affect carbon drawdown. When an iceberg alters in shape they can
overturn and deposit sediments and nutrients into the water column which can
also affect the different organisms. An increase in iceberg calving velocity can disrupt the flow of ocean pack ice this can delay
the phytoplankton bloom, this can affect the biology of the area as many
organisms time their reproduction to coincide with the phytoplankton blooms and
if there is a delay there will be higher juvenile mortality and changes in
biodiversity patterns.
Iceberg
grounding can also affect the biology of an area by creating large furrows
where the iceberg keel makes contact with the seabed. This scouring if not too
frequent can increase the biodiversity of an area as after scouring occurs
recolonization can occur and many different stages of the recolonization stages
can coexist. It creates a patchy pattern on the sea floor with certain
organisms being found in certain places within or outside the scours. However, the process of grounding is unpredictable and so is the patterns of
recolonization, therefore, it is less certain and harder to measure its effects on
the biology of the area. Ice scouring
is a devastating disturbance on the polar benthos and is more significant as
disturbances of the same magnitude in non-polar regions as the fauna and flora
has slower recovery rates. Both these glacial processes have a profound effect
on the biology of the area, these effects can be good ,an increase in
biodiversity from infrequent scouring, or bad such as the delay in the
phytoplankton bloom which can in turn affect the entire food web and organisms
higher up the trophic levels such as Empire Penguins and Zooplankton. The
effect of grounding and calving is important as it can affect the entire
ecosystem from the smallest organisms to larger organisms. However, as the
effects of grounding is harder to see as the majority of the impacts happen on
the sea floor so this is where the majority of any new research needs to be to
fully understand ice-seabed interactions.
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