The Biological Significance of Iceberg Calving and Grounding

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.

Conclusion

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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