Benthic habitats are defined by communities inhabiting the ocean floor. One of the major factors influencing community structure and function in marine ecosystems is physical disturbance. Environmental conditions of the near-shore marine benthic habitats in Antarctica differ fundamentally from those in the rest of the world’s oceans (Thatje et al., 2005). One of the main physical disturbances affecting the near-shore benthic communities on the Antarctic continental shelf is the impact of ice, particularly ice berg scouring or ploughing (Barnes & Conlan, 2007).
Antarctic benthic biota: benthos
The benthos inhabiting the near-shore benthic communities of the Antarctic are predominantly mobile organisms, capable of relocating to deeper zones during winter when the impact of ice is devastating on such communities (Nonato et al., 2000). This is a common pattern for the Antarctic near-shore waters. In general, the harsh conditions in the shallow waters of the Antarctic shoreline enable a low-diversity community dominated by these mobile organisms such as amphipods and isopods. Intermediate disturbances at some locations may enable a community of high benthic diversity on the shelf (Thatje et al., 2005).
Antarctic benthos must continuously recolonize, after escaping major disturbance and must be able to cope with seasonally dark and clear waters (Barnes & Conlan, 2007). Therefore, sessile forms such as sponges, ascidians and anemones are not present in the nearshore benthic communities, as they are subject to freezing in the winter (Nonato et al., 2000). Sessile forms of benthos are commonly found in more stable conditions at deeper zones, where the impact of the action of icebergs is significantly less. Benthic invertebrates with pelagic larval development are the ﬁrst to recolonize habitats that are physically destroyed by grounded icebergs at small and intermediate geographical scales in Antarctica.
In Antarctica, benthic macroalgae significantly contribute to the coastal food web, both directly and as detritus. Species richness tends to decrease with increasing latitude in the Southern Ocean, which may be due to disturbance intensity and temperature (Smale & Barnes, 2008).
The benthic fauna of the Antarctic continental shelf lacks active durophagous predators such crabs, lobsters and many fish (Clarke et al., 2004).
Impact of Ice
The impact of ice disturbance on benthic communities is significant, across very different spatial and temporal scales (Barnes & Conlan, 2007). Ice may completely destroy the seabed and benthic organisms have to recolonize local scourings and continental shelves repeatedly (Nonato et al., 2000).
There are four main forms of ice action influencing near-shore benthic communities: the ice foot, anchor ice, fast ice and ice scour. Benthos are affected by these forms in very different temporal and spatial ways depending on factors such as depth, latitude and the substratum profile (Nonato et al., 2000)
The ice foot
The winter ice foot is a narrow fringe of ice attached to the coast. The formation of an ice foot occurs when seawater or fast ice meets the cold rocky shore and freezes during winter (Smale & Barnes, ____). As it freezes, the layer of ice extends out to sea and covers the intertidal and shallow subtidal zones. The ice foot holds benthic communities in the shallow intertidal and subtidal zone at very early sucessional stages (Smale & Barnes, 2008).
Anchor ice occurs with the formation of platelet ice on the seabed attaching to substratum including stones, ropes, stakes and biota. In some geographical regions, anchor ice is important as intermediate disturbance acting on communities. Comparatively, Anchor ice may rip benthos from the seafloor (Barnes & Conlan, 2007).
During winter, sea surfaces form fast ice. Where fast ice meets the coast, the fast ice forms an ice foot along the shoreline. This severely affects intertidal communities and cryptic species (Barnes & Conlan, 2007).
Ice berg scouring or ploughing is caused by an iceberg running aground on the shelf (Thatje et al., 2005). Ice scour may severely impact benthic communities by mortality, modification of abundance and diversity patterns and changes in community structure (Barnes & Conlan, 2007). If benthic recovery rate is as slow as predicted, then ice scour is estimated to be among the 5 most significant disturbances to ecosystems in the world (Gutt & Starmans, 2001).
The most frequent and severe disturbance to Antarctic nearshore benthic communities is the impact of ice; however, other environmental conditions have an influence on the structure and functioning of these communities. Sedimentation has the potential to smother benthos.
Sedimentation processes in Antarctica include direct input into coastal waters due to glacier action, slumping and deposition of large dropstones. Slumping occurs when unstable sediments slide down the seafloor and can potentially cause high mortality as a result of smothering. Deposition of dropstones occurs on the underside of icebergs as a result of pieces of rock freezing into glacial ice (Smale & Barnes, 2008).
Wave action in some coastal regions of the Antarctic also influences the benthic habitats in those regions. The nearshore benthic communities of the Scotia Arc Islands and the northern tip of the Antarctic Peninsula are partly influenced and structured by wave action. In comparison, areas of seasonal sea ice for most of the year are protected from wave action (Smale & Barnes, 2008).
Other variables also affect benthic communities in the Antarctic such as light, depth, grain size, nutrients and salinity (Cunningham & McMinn, 2004). Also, Antarctic benthos is influenced by predation, competition and recruitment as well as abiotic factors such as substrate, currents, food supply, sedimentation, wave action and ice (Teixido et al., 2002).
Near-shore species exhibit low percentage cover, diversity, ages and a high proportion of pioneer species (Brown et al., 2004). The species must be able to move from shallow areas and also be able to recolonize benthic habitats.
Physical disturbance by ice in near-shore shallow benthic habitats has the potential to remove or translocate macro-benthos (Peck et al., 1999). The consequences of the impacts of ice to Antarctic benthos differ with the type of substratum, the species involved and the frequency and intensity of impacts (Brown et al., 2004). There are three main mechanisms for the recolonization of iceberg impacted sites on shallow soft sediment: (1) locomotion, whereby the benthic communities in shallow water are predominantly those that are motile; (2) dispersal by water currents and (3) larval recolonization (Peck et al., 1999).
Communities in the inter-tidal zone closest to shore experience extreme disturbance, while the communities in the deeper zones undergo much less extreme ice impacts. Intermediate disturbance occurs between the highly impacted shallow sites and infrequently disturbed deep water, and this intermediate disturbance may increase species diversity and abundance.
Near-shore benthic communities in the Southern Ocean exist around all the coastlines of the Antarctic and Sub-Antarctic islands. The benthic region starts at the shore line (intertidal or eulittoral zone) and extends downward along the surface of the continental shelf out to sea. The continental shelf may extend from 0 to 1000 m in depth. The Antarctic continental shelf is large, deep (500-1000 m) and distinguished by extreme seasonality in sea-ice cover and primary production (Smith et al., 2006). The structure and function of near-shore benthic habitats around the Antarctic vary at different geographical scales, depending on the influence of certain disturbance in the region. Impacts occur at different spatial and temporal scales. Figure 1 illustrates some of the ice impacts on benthic habitats during inter-glacial and glacial periods (Thatje et al., 2005).
Figure 1a. illustrates the environmental conditions on the Antarctic continental margins during an inter-glacial period and their influences on the near shore benthic communities (Thatje et al., 2005).
Figure 1b. illustrates the environmental conditions on the Antarctic continental margins during a glacial period and their influences on the near shore benthic communities (Thatje et al., 2005)
The Southern Ocean coastlines are among the most highly physically disturbed sites in the world, and are likely to become even more impacted. Global warming will lead to more ice loading and falling of ice sheets. The structure and function of marine benthic habitats will, and have been, affected by rapid warming in parts of the Antarctic (Smale & Barnes, 2008).
Most importantly, increased marine disturbance such as glaciers retreating, ice loading and falling of ice sheets as a consequence of rapid warming is likely to have the greatest affect on benthic communities in coastal waters (Smale & Barnes, 2008). Ice shelves on the Antarctic Peninsula have changed rapidly, due to warming, causing retreat on both sides of the peninsula (Convey et al., 2009).
Some of the effects of warming and the consequent effect on benthic communities include increased ice loading, resulting from ice shelf collapse (increases ice scour), increased coastal sedimentation associated with ice melt (smothering benthos), benthic response to changes in pelagic system and ocean acidification, which impacts both pelagic and benthic communities (Convey et al., 2009)
Changes in the frequency of ice scouring as a result of these effects of climate change will severely influence benthic communities; however, will not affect the deep benthic communities (Smale & Barnes, 2008).
Near-shore benthic communities are particularly at risk from the environmental impacts of permanent research stations around the Antarctic coastline (Stark, 2000). Contamination of sediments with hydrocarbons and heavy metals is a widespread threat for the benthos. Waste disposal and sewage outfalls near stations also present threats to these communities, in particular.
The Antarctic environment has always had to cope with harsh conditions, with massive disturbance and recolonization associated with glacial cycles. Since the fragmentation of Gondwana, of which East and West Antarctica were separate parts, Antarctica has become increasingly isolated and cooler. The shallower continental shelf had to be repeatedly recolonized during the periods of ice retreat (Barnes & Conlan, 2007).
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