BENTHIC MARINE HABITATS IN ANTARCTICA
Andrew Clarke
British An tarctic Survey, High Cross, Madillgley Road. Cambridge CBJ OET. UK
Benthic habitats in Antarctica differ from those in other parts of the world in several important characteris lies. Most of the Southern Ocean overlies the abyssal plain, where the sediments are primarily siliceous. Icc-rafted debris provides isolated patches of hard substratum but otherwise little is known of the biology of the deep-sea in Antarctica. Shallow WOller habitats are heavily influenced by icc, with typical intcrtidaJ habi tats being almost devo id of life. Continental shelves are unusually deep arou nd Antarc ti ca and the sediments are predominantly glacial-marine. Antarctica lacks typical flu vial habitats such as rivers. eSlu aries and has very few intertidal mudfl ats, and away from the immediate sublittoral the habitats suffer less physical and biological disturbance than the conti nental shelves of the Arctic.
1. INTRODUCTION contrast betwecn the view of a pristine structured benthic community provided by an ROY and an unsoned pile of Antarctica poses severe challenges for the benthic ecolo dead or damaged specimens provided by a bottom trawl is gist. Most of the Southern Ocean overlies the abyssal plain, striking indeed. It must be recogni zed, however. that iden the continental shelves are unusually deep, and access is tification of organisms on photographs or video film st ill impeded by floating ice she lves or vast areas of seasonal may require access to specimens collected by more conven pack ice. Despite these difficulties there is a long and proud tional means. The im(Xlrtance of new techniques in Antarctic history of benthic investigation in the Antarctic and we benthic biology has been emphasi zed by ArnlZ el al. [1994), know a great deal more about the fauna, at least in shallow but these new techniques bring thei r own challenges. Blil/i water, than is often realized. valli and Dearbom [1967] pioneered the use of underwater The early benthic work largely took place during expedi photogmphic techniques in Southern Ocean benthic research tions concerned primarily with geographic exploration or in their study of deep water communities in the Ross Sea. important physical observations such as geomagne tism or It is only since the work of German benthic ecologists work meteorology. Nevertheless the early collectors were remark ing in the Weddell Sea that significant progress has been ably thorough and Daylon [1990) laments that these pioneers made in obtaining quantitativc ecological data from photo will never gain the recognition they deserve. Headland [1 9- graphic or video techniques [Barthel el al., 1991; GIIII el al., 89) provides a comprehen sive list of polar exploration (up 1991 ; GIIII alld Piepenberg, 1991; E/<11/1 alld GIIII, 1991 ; dating an earlier, and largely unpublished, compilation by GIIII el al., 1994]. Roberts), and valuable historical reviews of Antarctic bi ology are those of Fogg [19941 and EI-Sayed [this volume) . 1.1. Some DejinitiJJns Dell 11 972), While 11 984), Daylon [1990) and ArnlZ el al. 11994) have all reviewed the hi story of benthic marine The definition of Antarctica for a benthic ecologist is not biology in the Southern Ocean. Unti l relatively recently all straightforward, as Dell [1972) has emphasized. Particular benthic work in Antarctica has had to rely on remote. and difficulties surround the islands of the Scotia are, South usually destructive, sampling techniques. Although SCUBA Georgia, and those islands which lie within the Polar Frontal tech niques have been used since the late 19505 [NeilS/lilt. Zone. The definition proposed by Dell [1972) has gained 1959, 1964) and have become increasingly important in the general acceptance, and will be followed here. The Ant last ten 10 fifteen years (Figure I). safety considerations limit arctic is defined as including the islands of the Scotia arc these investigations to the shallowest of depths. The history (South Orkney Islands, South Sandwich Islands and South of SCUBA in Antarctic marine biology has been summa Georgia) and Bouvet0ya, but not Marion Island , Macquaric rized by While lin press) and Da)'1011 [19901. Island. iles Crozet or iles Kerguelen; these are viewed as The advent of sophi sticated remotely-operated vehicles sub-polar habitats and thereby excluded. The outer edge of (ROYs) marks a technical advance which looks likely to the Southern Ocean is laken to be the Polar Frontal Zone. change the face of Antarctic benthic marine biology; the The Southern Ocean thus encompasses roughly 10% of the
Published in 1996 by the American Geophysical Union. 123 124 ECOLOGICAL RESEARCH WEST OF THE PENINSULA
700 sediments are siliceous rather than the carbonates typical of lower latitudes. and there is a strong influence of glacial 600 processes. Close to the Antarctic continent the sediments contain an abundant silt fraction comprised of rock flour with coarse 500 poorly-sorted debris. and containing little calcite or biogenic Q) '"> material. These types of sediment were termed glaciaJ '0 400 marine by the Deutsche Sildpolar-Expedition [Philippi. 19- 0 lOJ. and they form a wide circumpolar band around Antarc iii .c 300 tica (Figure 3). Goodell et al. [1973J have proposed a more E :J rigorous definition of glacial-marine sediments and have Z 200 distinguished four more or less concentric zones distinguish ed on textural grounds. In general there is a decrease in the 100 proportion of coarse material with increasing distance from the eontinen~ and the outermost of the four zones proposed o \-UJOlJl>d.I corresponds to the pelagic clays of the abyssal plain. The northernmost limit of glacial-marine sediments is related to 1960 1965 1970 1975 1980 1985 1990 1995 the surface O°C isothenn, which influences the rate of ice Year berg melting. The distribution of these sediments also depends upon the location of sites of iceberg calving. the Fig. I. The number of SCUBA dives undenaken each year at preferred paths of these icebergs and storm tracks. Signy Island by British Antarctic Survey divers. The solid bar In Antarctica icebergs are the major route for ice-rafted shows dives undenaken through winter fast-icc. The increasing sediment transport. Unlike the Arctic. sea-ice is of relatively reliance on SCUBA tectmiques for work in Antarctica is clear. little importance for moving terrigenous material. largely because the extensive development of ice-shelves essentially world's oceans and is contiguous with the Pacific, Atlantic precludes the capture of sediments by sea-ice. Ice-shelves and Indian Oceans. also greatly reduce the importance of riverine and aeolian (wind-driven) input to the marine system of Antarctica com 2. LARGE SCALE DISTRIBUTION OF BENTHIC pared with the Arctic. Lisitzin [1972] has estimated thai ice HABITATS bergs in Antarctica transport between 35 and 50 x 10' tons of sediment each year, more recently Knox [1994J estimated 2.1. The Deep Sea a figure of 0.5 x 10' tons per year. Beyond the limits of significant ice-rafted input, the gla Even a casual glance at a bathymetric of the Southern cial-marine sediments merge gradually into biogenic oozes. Ocean reveals vast areas of seabed at abyssal depths (Figure The overall character of a biogenic ooze is controlled by the 2). In contras~ a careful sean:h of the literature on Antanotic balance between four main processes: supply of biogenic benthic marine biology will fail to uncover much data on material from the surface layer. dissolution of that material these areas; we know almost nothing of the biology of the during flux 10 the seabed. dilution by non-biogenic material deep sea in Antarctica. Most of what we have learned has and diagenetic alteration after deposition. The low tempera come as the by-product of geological and geophysical sam tures of surface waters in the Southern Oeean mean thai pling programs. the most notable exception to this being the coccolithophorids are absent, and primary production is International Weddell Sea Oceanographic Expedition in 1968 dominated by diatoms. Since dilution by non-biogenic mate and 1969 [Rankin et al .• 1969J. rial is almost non-existent, and the low temperature and Many of the early expeditions to Antanotica took samples depth of the abyssal plaia tend to induce dissolution of car of the seabed [Pirie. 1913; Wiist. 1933; DOl/glas and Camp· bonates. the biogenic oozes of the Southern Ocean are bell-Smith. 1930J. and 142 samples were taken by RRS almost exclusively siliceous. The boundary between sili Discovery and RRS William Scoresby as part of the Dis ceous oozes and the carbonate oozes fonned in wanner sub covery Investigations [Neaverson. 1934]. Further data have polar surface waters is dictated largely by the position of the been obtained by more recent geological and geophysical Polar Frontal Zone. and previous positions of the Polar Front expeditions and are discussed by Domack and McClennen can be determined from the switch between siliceous and [this volumeJ. carbonate sediments in cores. Like the abyssal plains elsewhere. those around Antarc The lack of substantial riverine or aeolian input means tica are composed primarily of soft sediments. They differ that rates of abyssal sediment accumulation around Antarc from sediments in most other deep-sea areas in two ways: tica can be very slow. often less than 0.01 m per 10' years the low temperatures of the surface waters mean that these [Osmond et al .• 1971J, High rates of silica accumulation CLARKE: BENTHIC HABITATS 125
". ·0 . , " -,...! \ > \ . ~ l A-'" T 11\ ReT L C 'A '1 0 , ' ~ , , . ( . ' .". "
." ,
~ . ~ " • .. ,
Fig. 2. A map showing the major bathymetric conlours around Antarctica. The depth scale is in eight shades of grey, and covers the range from (}"IOOO m (palest) 10 7·80CXl m (darkest). Note the extensive areas of l1eep water within the Southern Ocean. Reproduced from Goonell et al. [197 31. have been reported on the continental shelf of the Ross Sea. the sediment trap si le; they also found significant differences and it has been speculated that deposition in the Ross Sea as in the peak nu x rale between ycars. HOlljo [1990J and Do a whole may be equivalent to the total ri verine input of si li· mack Qllll McClellllen [this volume] provide a review of the ca 10 the world's ocean [Ledford-Hoffman et ai .. 1986 ). TIle prese nt glacial marine scdimemation in Antarctica. Southern Ocean dominates the global oceani c si lica cycle. Large icc-rafted boulders (drop-stones) arc important in and it has been estimated th at over 75 % of the current providing iso lated patches of hard substratum on the ot her accumulation of biogenic sili ca in marine sediments lakes wise sofl abyssal plain of the Southern Ocean. The concen place sOUlh of the Polar Frontal Zone [Ledford-Hoffmall et tration of these drup-slOnes is likely (0 decline with distance (Ii .. 1986; JOlles et {Ii., 1990J. In the north-central Weddell from the ca lvi ng ice-front and thei r distribution wi ll innu Sea Fischer er al. [1988J found thaI vertical silica nu x in cncc the colon iza ti on dynamics of some enc ru sting (axa in creased rapidly once open water conditions had arri ved ove r the Southern Ocean. Unfortu nately very little is known of 126 ECOLOGICAL RESEARCH WEST OF THE PENINSULA
Fig. 3. A map showing the main sediment types around Antarctica. Note the distribution of the major sedi ments in broad swathes around Antarctica Closest to the continent are submarine tills and glacial marine sediments; outside this (and also dominating the western South Atlantic) is a narrow band of clay-silt (dark grey), and ,ullOunding thi' a broad band of ,ili""u, COlt: (pale gray). Reproduced from Goodell et 01., [19- 73).
either the distribution or fauna of these drop-stones. of the Southern Ocean abyssal plain is, however, critical to One habitat that was unknown before the mid-1970s and our understanding of the population dynamics of many taxa which is currently the subject of intense investigation is that and of the history of the high latitude marine fauna. of hydrolhennal vent fields [Gage and Tyler, 1991]. The tec tonic history of the Southern Ocean suggests that hydro 2.2. The Conlinenlal Shelves thermal vent fields may well exist there (for example in the eastern Scotia Sea), but as yet none are known. The continental shelves around Antarctica are unusual in The deep-sea floor thus remains as the largest single ben being deep, typically as much as 800 m in places. This is thic habitat in Antarctica, but the least studied. Knowledge primarily the result of the isostatic depression of the conti- CLARKE: BENTHIC HABITATS 127 nent as a whole resulting from the mass of the polar ice cap. 3. SEDIMENTATION, HYDROGRAPHY AND ICE In the past the ice-shelves and glaciers have extended further towards the edge of the continental shelves than they do 3,/, SedimenlDlWn Rates on Continental Shelves now. In some cases the ice extended right to the edge of the shelf, presumably obliterating all life. What is not yet clear Sedimentation rates have been estimated at two sites on is whether there was a time when all shelves were so cov the Antarctic continental shelf. Dunbar el al. [1989) found ered simultaneously; this would have resulted in the com that vertical nux rates varied substantially between the east plete loss of all continental shelf faunas (see discussion in em and western sides of McMurdo Sound, and these differ Clarke and Crame, 1989, 1992). ences could be related to oceanographic patterns. This The extension of glaciers and ice shelves has eroded deep applied to measurements of diatoms, biogenic silica and canyons in some areas of continental shelves, and also lithogenic material [Dunbar el al., 1985, 1989; Dunbar and resulted in a tendency for the inner shelves in some areas to Leventer, 1986). These studies also showed that resuspension be deeper than further out at the shelf-break. Unlike most and lateral advection were important processes in the benthic continental shelves, including those in the Arctic, those environment, transporting biogenic material from shallow surrounding Antarctica do not receive any substantial nuvial sites to deeper basins on the continental shelf. These proc (riverine) input; there are few mudflats and essentially no esses can also move macroalgal debris from shallower wa large riverine deltas in Antarctica. Antarctic continental ters, where production takes place, to deeper waters. Such shelves do, however, receive large amounts of ice-rafted macroalgal debris can form an important contribution to debris and also a substantial input of sediment carried by deep-water carbon flux and incorporation into sediments glacial meltwater. [Liebezeil and von Bodungen, 1987; Reichardl, 1987). It is now clear that the rale of icc-rafted sedimentation in Significant vertical nuxes of faecal pellets associated with polar regions is primarily controlled by four factors: krill grazing can dominate sediment trap samples in Brans o The rate of continental erosion field Strait [von Bodungen el ai., 1988; Wefer el al., 1982, o The thermal structure of glaciers and ice-shelves 1988), whereas on other occasions the nux may be domi (since this influences their erosional and melting be nated by diatom resting spores [Karl el al., 1991 ; Levenler, havior) 1991) or amorphous debris [Schnack, 1985). These results o The size of ice-shelves indicate an important role for zooplankton grazing in medi o Seawater temperature ating the nux of diatoms to the seabed and also demonstrate The balance between these factors is not yet fully under significant spatial and temporal variation in flux rates [Barh stood and is likely to vary from place to place. Thus an ice mann el al., 1991a, 1991b; Karl el al., 1991). shelf with a frozen base would usually result in slower melt ing of icebergs and hence deposition of sediment further 3.2, Hydrography offshore, whereas icebergs calved from an ice-shelf with a liquid base would deposit their sedimentary load very close The vertical flux of organic matter from surface waters to . to shore. Where such ice-shelves are floating the bulk of the seabed links benthic habitats to processes in the over deposition would take place beneath the ice-shelf itself. lying water column. Critical to this pelagic-benthic coupling Spatial variation in deposition will also renect the dynamics is the local hydrography [Grebmeier and Barry, 1991); un of iceberg calving and transport. fortunately there have been few studies of hydrography of In general, sediments on the continental shelf closest to direct relevance to benthic habitats in Antarctica. Two key the continent consist of undifferentiated glacial till, gravels, exceptions, however, have been oceanographic studies at two sands and biogenic deposits. The tills are largely unaffected high latitude sites, McMurdo Sound [Barry, 1988; Barry and by marine processes and occupy the inner third of the conti Dayloll, 1988) and Ellis Fjord [Gallagher alld Bl/rlOlI, 19- nen tal shelves; the larger fragments are angular, faceted and 88), and at King George Island on the Antarctic Peninsula striated. This sediment type (facies) forms the innermost [Kloser el al., 1994). lOne of the four-fold classification of Goodell el al. [1973). These studies have shown that there are complex inter Furt her out towards the edge of the continental shelf are the actions between wind forcing, circulation and meltwater. glacial-marine sand-silts of the outer continental she lf, and Such interactions can impact benthic assemblages through Ihe sediments of the outer continental slope. resuspension of sediment, nux of glacial material or the Little detailed work appears to have been undertaken on advection of particulate food [Kl6ser el al., 1994). In Mc bottom sediments in the area of the Antarctic Peninsula. It Murdo Sound there are striking differences in the benthic is to be presumed that the present depositional regime is af standing crop biomass between either side of the fjord. fected by the relatively low incidence of calving glaciers and which can be related to larger scale patterns of current flow the relatively high incidence of meltwater and aeolain depo [Daylon alld Oliver, 1977; Barry, 1988; Barry alld Daylon, sition from exposed ground, compared with continental 1988]. Benthic assemblages dependent upon the advection shelves close to the ice-bound continent. of primary production are rich and diverse where currents 128 ECOLOGICAL RESEARCH WEST OF THE PENINSULA
Key
Mud
•G" . Sand G~ :J Gravel mg Boulder ~..•. Boulder & Gravel !film Boulder & Mud
Fig. 4. A map of the main bottom sediment types in Borge Bay, Signy Island. From work by British Ant arctic Survey scientists between 1970 and 1982: map compiled by N.S. Gilbert.
bring phytoplankton from the Ross Sea. In contrns~ currents areas of the inner Weddell Sea shelf by ROY have. how from beneath the Ross Ice Shelf advect very little particulate ever, revealed extensive areas of undisturbed complex as material and the associated benthic communities are sparse. semblages. This indicates tha~ in direct contrast to much of the Arctic. the benthic communities of the deeper continental 3.3. Disturbance By Ice shelves around Antarctica are subject to only infrequent physical disturbance by ice-rafted debris. Much of the conti In shallower waters close to land, the benthic fauna can be nental shelf is too deep for direct impact by all but the subject to iceberg impact. The grounding of a large berg can largest icebergs, and the Antarctic marine ecosystem also cause extensive gouging of the seafloor. and it will eradicate lacks the biological agents of disturbance. such as benthic all benthic life in the area of impact. Surveys of the deeper feeding whales. walruses. large crabs and flat-fish. so char- CLARKE: BENTHIC HABITATS 129 acteristic of the Arctic marine system [Oliver and Slattery, cies have a contagious (overdispersed) distribution, which in 1985; Dayton , 1990). tum carries through to the distribution of their epifauna In contrast to the Arctic, there appear to have been few [Picken, 1979]. Although these examples come from the quantitative studies of the incidence of iceberg gouging. South Orkney Islands, they illustrate general principles Several anecdotal studies by SCUBA dtvers of Iceberg which will apply throughout the maritime Antarctic. scours have, however, indicated the extensive damage to both habitat structure and benthic communities [Shabica, 19- 72; Kauffman, 1974]. Sintilar studies at Borge Bay, Signy 4.2. Intertidal Habilals Island have revealed extensive initial damage, but fairly rapid recolonization by meiofauna and some mobile macro Perhaps the most striking feature of the Antarctic marine fauna (L.S. Peck and S. Varthofe, personal commUnICatIOn, ecosystem for a visitor familiar with temperate or tropical 1995). regions is the almost complete absence of an intertidal flora and fauna. This is the result of the direct impact of ice: in 4. FINE SCALE DISTRIBUTION OF BENTHIC winter the entire habitat is encased in solid ice, and in sum HABITATS: THE NEARSHORE ENVIRONMENT mer those areas which do melt out are subject to severe scouring by brash ice, the impact of small bergs, and inter Two important features distinguish the shallow nearshore ntitlent freezing. Tidal pools do exist in some areas, and marine system around Antarctica from those elsewhere in these can be flushed at each tidal cycle beneath an overlying the world. The first is the impact of ice, and the second is cover of snow [Hedgpeth, 1971]. The overall impact of ice the lack of typical fluvial habitats such as estuaries, delias or in the intertidal zone is, however, to lintit the fauna to a few mudflats. mobile forms which ntigrate from deeper water in summer (for example, gastropods such as the Antarctic limpet Nocel 4.1. Shalww Subtidal Nearshore Habilals la concinna, the nemertean Tetrastemmis sp., and many am phipods) and a small number of species which live in crev With the exception of the absence of typical riverine habi ices or under boulders; these can include planarians, poly tats, any SCUBA diver enlering the shallow waters around chaetes, gastropods, bivalves, amphipods and hydroids. The Antarctica would be confronted with the range of substrata flora of the intertidal is dominated by calcareous algae and familiar from elsewhere. These include smooth hard sub microbial assemblages which colonize the habitat afresh each strata, boulder fields, cobbles, pebbles, and soft substrata season once the ice has melted, although small thalli of red ranging from coarse sands to fine silts and muds (Figure 4). algae can be found in sheltered crevices. Sort substrata may show sintilar grain-size distributions to Although the impact of ice is most immediately obvious those from lower latitudes [Gilbert, 199Ia], but for the in the intertidal area, this impact is felt throughout the shal coarser sediments the lack of riverine transport and the input low water marine ecosystem in Antarctica. It is perhaps the from glacial melt or ice·rafted debris means that the grains single most important physical variable influencing the ecol are often more angular and less polished than at lower lati ogy of the benthic flora and fauna in these habitats. tudes. It is likely that this, combined with the generally low sorting, will affect their packing and pore size distribution 4.3. The Influence of lee with consequent impacts on diffusion processes, microbial pathways and the ecology of the meiofauna. Apart from The influence of ice is aU-pervasive in the nearshore anecdotal and unpublished reports, however, relatively liule marine ecosystem of Antarctica, and its affects can be both is known of this aspect of nearshore marine ecology in Ant direct and indirect. In shallow waters ice has a major direct arctica. impact by scouring, which results in the local eradication of Marine habitats exhibit spatial heterogenity on small scales the benthic fauna with a frequency that is related 10 depth. similar to that familiar from terrestrial habitats. Unfortu As a result there is a distinct zonation to the benthic assem nately because of difficulty of access. there have been few blages found in shallow water assemblages around Antarc studies of such small-scale heterogeneity, despite its tica. Zonation is a well-described feature of all intertidal undoubted importance for population dynamics and local and immediately subtidal assemblages around the world, faunal diversity. Richardson [1979] has documented the de where it is primarily a response to physical variables or tailed distribution of benthic substratum and macroalgae in processes which vary along the gradient, although the Borge Bay. Signy Island, South Orkneys (Figure 5). This reponse of the biota may be modified by biological inter study cavern a small pan of the area shown in Figure 4 and actions. What makes the polar regions so di stinctive is the shows that heterogeneity is maintained at the smaller scale . dominant role played by ice in structuring this zonation (for There is also marked heterogeneity in the distribution of the further details see Cwrke, this volume). two dominant species of rnacroalgae in the area, Himaflto In the very shallowest waters the impact of ice is largely thallus grandi/olius and Desmarestia anceps. These two spe- by direct scour or winter freezing. In slightly deeper wa- 130 ECOLOGICAL RESEARCH WEST OF THE PENINSULA
Fig. Sa. Distribution of bottom substrata at Borge Bay, Signy Fig. 5b. Distribution of macroalgae in the same nrca as shown in Island, AnIllrCtica. The bottom types are rock (black), boulder (stip Fig. 50. Areas of decaying macroaJgaJ material are outlined and ple), pebble (fine vertical hatching), sand (cross-hatching), and silt clear, areas of macroaIgae (principally Himantothallus grondi/olius (blanlc). The horizontal hatching at the bottom and top r ters, however, the formation of anchor ice is an important light reaching the seabed. This in !Urn influences primary process. Anchor ice forms on the bottom when undercooled production by macroalgae and benthic microalgae [Gilbert, water freezes, often around sessile benthic organisms which 1991b], and hence benthic invertebrate biomass and pro act as nucleating sites. As the anchor ice grows ill sitll, it ductivity [Dayton et al., 1986]. encases nearby organisms, killing them. Eventually the ice lifts off the bottom, clearing the substratum of benthos as it Ackllowledgements. Research on the benthos of Signy Island, does so. Anchor ice fOnTIS down to depths of over 30 m at Antarctica, has been undertaken as pan of the Nearshore Marine ve ry high latitudes such as McMurdo Sound, where long Biology program of the British Antarctic Survey. Grateful thanks are due to the many biologists, assistants and diving officers who term variations in the intensity of its formation, perhaps have kept this program running from 1962 until its closure in 1995. tlriven by wider scale oceanographic processes operating prior 10 a move to Rothera Station. I thank Martin White and two over similar time scales, may have significant impact on the referees for helpful comments on the manuscript. population dynamics of some benthos [Dayton et 01., 1970]. In the maritime Antarctic, anchor ice formation is less fre quent and is generally at shallower depths. 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