71% of Earth is covered in water, 2.4% of this is stored in glaciers and polar ice caps which form a major part of the cryosphere (Chen et al. 1998). The high latitudes are a unique feature on the planet due to ice being present nearly all year round; this has significant effects on the biogeochemical trends of the oceanic and atmospheric circulations (Rahmstorf 2002). However, the relationships between the physical and biological processes related to ice is limited, this is due to the difficulties and dangers of field data collection. Nevertheless the Arctic and Antarctic are under ongoing studies by both the Natural Environmental Research Council and the British Antarctic Survey (NERC & BAS 2010).
The World Meteorological Organisation (WMO) has classified ice into two major categories; glacier ice and sea ice. Glacier ice is either part of a continental ice cap or glacier, which then collapses at the grounding line or once calved, drift as icebergs. These icebergs can run aground up to depths of 600m. Whereas sea ice is seasonal, multiyear or ridged ice and has depths of approximately 40m, whereas first-year ice is only 0.5-2m thick (Gutt 2001).
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Large expanses of ice that have an interface with a water body produce icebergs, for example, the Arctic ice shelf in winter. Icebergs are calved from the ice cliff and become free floating, and then drift in accordance to the ocean and wind currents until they ground on the seabed(Bigg et al. 1997). Once grounded, the iceberg is scraped, or scoured, along the seabed, this in effect ploughs the seabed thus damaging any benthic organisms and their habitats in the impact zone. The amount of disturbance created varies depending on the frequency, magnitude and the benthic community structure advancement (White & Pickett 1985).
Due to the length of non-glaciated coastline in the Arctic (>140,000 km), sea ice is more influential in the northern hemisphere compared to the southern hemisphere (Gutt 2001). Large expanses of sea ice and icebergs reduce the total size of phytoplankton blooms by blocking incident light from reaching lower levels in the water column thus reducing photosynthesis levels and growth rates (Arrigo et al. 2002).
Ice cliffs and sheets have a seasonal cycle of winter advancements and summer retreats. During the summer retreats, ice breaks off from the main ice cliff forming icebergs; this calving of icebergs is the most efficient method of ice loss, also known as ablation. This can be caused by different processes; the main cause of calving is due to fractures in the ice resulting from englacial shear stresses (Benn et al. 2007). These weaken areas of the ice, which eventually break free from the ice edge. Some of these fractures have water flowing through them, and are then classed as Moulins. The water is formed after an area of surface ice has melted, warming the ice allowing tunnels to form which also help to weaken the ice cliff and thus allow icebergs to break off (Benn et al. 2007). These Moulins and meltwater layers can refreeze (see Figure 1). Another way ice can be calved is by the cliff face being undercut by wave action, the weight of the overhanging ice causes the shelf to shear off (Scambos et al. 2008).
Figure 1; Icebergs calving from the Petermann Glacier, Greenland. The striped pattern that can be seen is caused by re-freezing of melt water layers (Fortier 2008).
Dr. Martin Fortier from the University of Laval, Canada, has been studying the Petermann Glacier in Greenland and its major calving events in both 2001 and 2008 in association with NSERC (Natural Sciences and Engineering Research Council of Canada). In 2001 the Peterman glacier calved roughly 86 kmÂ² from its ice front. The glacier calved a further 27 kmÂ² of ice in July 2008 (although it did not completely leave the boundaries of the Fjord until August). Between then and the end of September 2008 the glacier calved a further 3.6kmÂ² of ice.
There is variation in iceberg calving velocity. In the northern hemisphere, there is a range from 7700m per year in western Greenland to 100m per year near Svalbard (Pelto & Warren 1991). Ice calving velocity changes with the type of fracture found at the ice edge. The fracture type depends on englacial and subglacial velocity gradients. In places of large velocity gradients, deep crevasses caused by large shear englacial and subglacial stresses will cause a failure in the ice formation and will fracture. When the velocity gradients are smaller, the surface crevasses that are produced are less likely to trigger calving (Pelto & Warren 1991). There is also spatial variation in calving velocity due to water depth and temperature. An increase in water depth causes a decrease in calving velocity, whereas an increase in temperature promotes calving (Pelto & Warren 1991, Benn et al. 2007).
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Free-drifting icebergs enrich the marine environment by depositing trapped sediment into the water column as they melt. Iron particles are transported in this way, and thus made available to phytoplankton during summer melt, in the vicinity of the marginal plate zones thus providing phytoplankton excess nutrients to fully bloom (Lancelot et al. 2009). However, large expanses of iceberg coverage reduce the total size of phytoplankton blooms by blocking the incident light from reaching lower levels in the water column thus reducing photosynthesis levels and growth rates (Arrigo et al. 2002). The extensive sea ice cover caused by the iceberg B-15 delayed the plankton bloom in 2000-2001 resulting in a substantial drop in the total annual phytoplankton production estimated for all regions of the Ross Sea (Arrigo et al. 2002). It has also been observed that during the lifespan if an iceberg it will alter shape, due to the melting of the ice in contact with the water column, the alteration of the iceberg shape will move its centre of gravity. This movement will cause the iceberg to tip or overturn to restore its balance in the water. When the iceberg overturns, sediment and nutrients are deposited into the water column (Bigg et al. 1997).
A reduction in phytoplankton blooms would also affect larger organisms such as fish, penguins and marine mammals. There would be a reduction in a major food source; this would have a negative influence on population numbers of species higher in the food chain/web (Arrigo et al. 2002).
Figure 2: The abrasive action of grounding ice masses creates ice gouges into the bedrock
and seafloor sediments. Amundsen Sea off Marie Byrd Land, Antarctica. Map width is
approx. 30 km.(Ryan & Carbotte 2009).
When free-floating ice or icebergs make contact with the seabed, gouge or scour marks can be seen as a result of the erosional process of abrasion, which caused by the keel of the iceberg scraping along the seabed surface (Figure 2). These scour marks can be found throughout the geological record when the ice sheet coverage has extended over hard substrate, the most recent large event causing these marks was during the last glacial maximum (Ruddiman & McIntyre 1981). Grounding can result in the formation of black pool features; these are depressions in the seafloor which fill with hypoxic sulphide-rich water which can result in the death of sessile epifauna and a fatal trap for motile animals. Migration routes and foraging successes might also be affected by newly calved or grounded icebergs that block passageways (Arrigo et al. 2002).
When free floating icebergs come into contact with the seabed there is a disturbance caused which was defined by White & Pickett (1985) as;
"â€¦a discrete event in time that disrupts ecosystem, community, or population structure and changes resources, substrate availability, or the physical environmental"
The amount of disturbance caused is influenced by the main factors outlined in Table 1. There will be a greater disturbance if there is a high frequency of grounding icebergs, for example on the western Eurasian and northeastern American shelf or if the impact of the iceberg is of a high magnitude (Schulson 1999). Glacier termini prevent sessile organisms such as sponges, corals and mussels, Mytilus edulis, from settling due to the constant movement and abrasion caused by ice calving from the ice edge (Gutt 2001). There are records of icebergs have scoured the seafloor at depths 3 - 15m in Canadian Arctic altering the ecosystem functioning to deposit feeders and scavengers when previously there were more predators and suspension feeders (Conlan et al. 1998, Gutt 2001). The algal distribution in McMurdo Sound which opens out into the Ross Sea (Antarctica) is determined by the damage inflicted by grounding ice, which also affects the upper limit of benthic fauna in the same manner.
Species richness (the number of different species in a given area), of both regional and local types showed significant unimodal patterns related to latitude, with a peak in low latitude areas and decreasing towards higher latitudes (Witman et al. 2004).
Spatial distribution, including relationship to geographic, topographic, environmental, and community gradients
Mean number of events per time period. Frequency is often used for probability of disturbance when expressed as a decimal fraction of events per year
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Return interval / or turnover time
The inverse of frequency; mean time between disturbances
Mean time needed to disturb an area equivalent to the study area
A scaled inverse function of variance in the return interval
Area or size
Area disturbed. Expressed as area per event, area per time period, area per event per time period, or total area per disturbance type per time period. Percentage of total area available
Physical force of the event per area per time (e.g., current speed surrounding iceberg)
Impact on the organism, community or ecosystem
Effects on the occurrence of other disturbances (e.g., drought increases fire intensity)
Table 1. Definitions of the factors that affect disturbance levels. Adapted from (White & Pickett 1985)
A variation in the iceberg calving rate can affect the benthic environment. An increased rate of calving and thus grounding of icebergs will prevent complex and climax ecosystems from establishing, due to the frequent damage and sediment disruption caused. This would result in ecosystems that consist of mainly pioneer and motile species. However if there is an increase in grounding icebergs in an area with previously low contact rates, there will be greater damage done to the individuals and structures formed. This would be because the recovery time would be greater (Gutt & Starmans 2001).
Grounding and scour events damage or fatally injure larger organisms such as kelp, sea urchins and bivalves (Conlan et al. 1998). Buccind gastropods and amphipods are found scavenging on remnants of disturbed and destroyed bivalves inside the scours. Scavenging crustaceans and deposit feeding polychaetes were abundant in the shallows outside and within the scours, while predatory amphipods and opportunistic polychaetes burrow into the displaced sediments. This effect of disturbance and recolonisation has also be observed in trawl and dredged tracks (Brylinsky et al. 1994, Currie & Parry 1996).
Ice calving has significant effects on the biology of the oceans, not only on a physical level with direct impacts from grounding icebergs. But also by causing knock on effects of reducing the size of phytoplankton blooms which affect larger organisms and marine mammals by limiting their food source. The ice calving velocity has an effect on the benthic ecosystem, by preventing complex ecosystems establishing, due to the frequent damage and sediment disruption. Ice scouring also appears to favor deposit feeders and scavengers rather than burrowing organisms such as bivalves.
To fully understand grounding and scour on benthic communities, the life history of the organisms must be studied further. However, due to the difficulties and dangers of field data collection, the relationships between the physical and biological processes related to ice is limited.
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