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Benthic Associations of the Shallow Hard Bottoms Off Terra Nova Bay

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Benthic Associations of the Shallow Hard Bottoms Off Terra Nova Bay Antarctic Science 6 (4): 449-462 (1994) Benthic associations of the shallow hard bottoms off Terra Nova Bay, Ross Sea: zonation, biomass and population structure MARIA CRlSTlNA GAMBI, MAURlZlO LORENTI, GlOVANNl F. RUSSO and MARIA BEATRICE SClPlONE Stazione Zoologica “Anton Dohrn” di Napoli, Laboratorio di EcoIogia del Benthos, 80077Zschia (Napoli),Italy Abstract: Quantitative and semi-quantitative samples of phyto- and zoobenthic organisms were collected by SCUBA diving at five stations along a depth transect from 0.5-16 m on the shallow hard bottoms off Terra Nova Bay, Ross Sea, Antarctica. The benthic associations were dominated by two macroalgal species (Iridueu cordutu and Phyllophora antarctica) and by few animal taxa (mainly polychaetes, molluscs and peracarid crustaceans), Distribution at the community and species levels revealed a well-defined zonation pattern as a function of depth, governed mainly by sea ice scouring and melting. Zonation of vagile fauna was also affected by the effects of covering and architecture of the two dominant macroalgae. Species richness and diversity were higher in the Phyllophora-associated community, where habitat complexity and sheltering were higher. The highest faunal abundance (over 82 000 ind.m”) and biomass (macroalgae and fauna wet weight 2392 g rn-2)were recorded at 2 m depth in association with thehidaea covering, where the harsher environmental conditions select a few taxa. The biomass values, even if underestimates of the whole community standing crop, are among the highest recorded in shallow austral biotopes. An autoecological and demographic analysis of the most abundant animal species revealed for some species (e.g. Laevilitorina antarctica and Paramoera walkeri) a quite complex population structure with up to three size classes, including juveniles. In some species, the cohort of juveniles showed a well- defined depth preference probably related to sheltering by the macroalgae. As a whole, the species analyzed showed various andcontrasting reproductive strategies, despite the fact that the environmental conditions along the transect were relatively similar and quite selective. Received 20 September 1993, accepted 20 May 1994 Key words, benthic comunities, zonation, biomass, hard bottoms, ppulation struc’we, Ross Sea, Antarctica Introduction physical factors, with species adapted to high levels of Benthic community zonation and organization patterns in the disturbances (due mainly to sea ice scouring and melting: Antarctic and subantarctic marine environments are well Dayton et al. 1970, Gambi et al. 1992). A few quantitative known, although most of the studies are concentrated in studies have identified the relationship between primary relatively deephabitatswite 1984,Dayton1990). By contrast, production and density of benthic populations (Dayton & Oliver quantitative investigations providing density and biomass 1977, Barry & Dayton 1988) but estimates of benthic biomass estimates arerelatively scarce, especiallyin theupper continental are lacking for the whole Ross Sea. This information is also shelf areas (Muhelenhardt-Siege1 1988, Beckley & Branch necessary to elucidate food availability and energy flow for other 1992). The widespread belief in the high densities and standing marine ecosystems related to the Ross Sea. crops of benthic communities along the Antarctic continental The aim of this research, carried out in the frameworkof the shelf, has not yet been adequately investigated. Such data are Italian“NationalProgrammeofAntarcticResearch’7(P.N.R.A.), of particular importance in the Ross Sea sector, where austral was to describe the zonation, density, and biomass patterns of seas reach the highest latitudes, the production of summer the benthic associations of the shallow hard bottoms off Terra phytoplankton populations is relatively high (Smith et al. 1990, Nova Bay at the community and population levels. These Saggiomo et al. 1992), and important gradients in biogenic and assemblages experience high levels of physical disturbance due lithogenic material sedimentation and accumulation occur mainly to sea ice scouring and melting (Di Geronimo et al. (De Master et al. 1992). 1992). They are characterized mainly by rhodophycean In the Ross Sea coastalzones, benthic communityorganization macroalgae dominatingthe first 30-35 m depth (Cormaci et al. shows,more clearly than in other Antarctic areas, two contrasting 1990, 1992), and by associated vagile invertebrates, mainly structural patterns. On one hand there exist assemblages with represented by polychaetes, molluscs and peracarid crustaceans high diversity and structural and functional complexity, (Gambi & Mazzella 1991). Benthic macroalgae represent an controlled by biological factors (Dayton et ul. 1974, Oliver & important source of energy for the associated grazers (Dhargakar Sla+,-,ry1985, Battershill 1990’, with species having long life- et al. 1988) and, as plant detritus, also for deeper faunal spans, low growth and turnover rates, and high sensitivities to assemblages (Reichardt 1987),being advected and deposited by physical disturbance (Dayton 1989). In contrast, other ice drift and water movement. Thevagilefauna associatedwith assemblages are oligospecific, poorly structured, controlled by these macroalgae may be an important trophic link between 449 Downloaded from https:/www.cambridge.org/core. IP address: 47.88.87.18, on 17 Jan 2017 at 18:42:05, subject to the Cambridge Core terms of use, available at https:/www.cambridge.org/core/terms. https://doi.org/10.1017/S0954102094000696 450 M.C. GAMBI etal. 0.5 m depth (St. 1) epilythic Cyanophytes and Diatoms 2 m depth (St. 2) facies of lridaea cordata 6 rn depth (St. 3) mixed populations 74'40' S of lridaea 8 Phyllophora ~~'~2m depth (St. 4) facies of ' ' Phyllophora antarctica I YYY 16 m depth (St. 5) ;.'h-1Phyllophora a Corallinacea ____ .-. Cyanophytes-Diatoms :?::-: lridaea cordata $' p Phyllophora antarctica y ty Corallinacea o Fig. 2. Sampling design for the hard bottom benthic populations along a rocky cliff. For each station the characteristic algal populations are indicated. Rhodophyta, Phyllophora anfarcfica A. & E.S. Gepp; stations 4 and 5 were characterizedby large coverage of Phyllophora and by a higher occurrence of encrusting Corallinacea (mainly Chlathromorphum lemoineanum Mendoza & Cabioch). At each station two kinds of samples were collected by scraping the Fig. 1. Map of Terra Nova Bay (Ross Sea, Antarctica) with the substrate: a quantitative sample over a surface of 1600 cm2(40 location of the Italian station (circle) and of the sampling site (arrow). x 40 cm) (indicated as samples A), and a semi-quantitative sample over a surface of about 1250cmz (25 x 50 cm) (indicated primary producers and higher consumers, such as demersal as samples C). For station 1only the quantitative sample (1A) predatory fish (Vacchi ef al. 1994). was collected, while for station 2 an additional quantitative sample was taken (samples 2B) in an areanot coveredbylridaea in order to assess the effect of algal cover regardless of depth. Materials and methods The biologicalmaterialwas fixedin4%neutralizedformalin. The studied area is located within Terra Nova Bay (74" 42'S, For the macroalgae, biomass was measured as wet weight in 164" 46' E) (Fig. 1). The coastline of the Bay is characterized formalin. Vagile animals were separated into taxa and counted. mostly by rocky cliffs, with occasional "beaches" formed by The zonation pattern was analyzed by means of the large boulders (Simeoni etal. 1989). The tidal range is 1.5-2 m CorrespondanceAnalysis (C.A.) (Benzecri 1973). Such analysis (Stocchino & Lusetti 1988). Pack ice, about 2-2.5 m thick, was performed usingrow dataof abundanceof all taxacollected. covers the sea surface for 9-10 months ayear. The sampling site The C.A. technique allows the ordination of both station- and was a large, emergent rock located close to the coastline 5 km species-points in the same factorial space. Diversity (Shannon- south of the Italian station, below the land meteorological Weaver index H') and Evenness (J) were measured in each station named 'Camp0 Icaro' (Fig. 1). Phyto- and zoobenthic sample. The biomass and the population structure of the most organisms were collected, during the austral summer 1989-90, important taxa of the vagile fauna associated with the macroalgae, by SCUBA diving along a depth transect at 0.5 m MLLW in term of abundance and functional role, were evaluated. The (station l),2m (station 2), 6m (station 3), 12m (station 4) and selected taxa were: two morphotypes of Harmothoe spinosa 16 m (station 5) depths (Fig. 2). These depths were selected on (polychaetes); Laevilitorina anfarcfica, Onoba gelida, the basis of different algal associations described previously 0. furquefi and Powellisetia deserfa (molluscs); Munna (Cormaci ef al. 1990, Di Geronimo et al. 1992). Station 1was anfarctica (isopod crustaceans); Nofofanais dimorphus (tanaid characterized by bare rocky substrate colonized only by crustaceans); Puramoera walkeri (amphipod crustaceans). cyanophytes and benthic diatoms, station 2 was dominated by Biomass, in many studies along the Antarctic and subantarctic the Rhodophytalridaea cordafa (Turner) Bory, while station 3 coasts, is generally reported only as wet weight because of the was characterized by a mixture of Iridaea and another taxonomic importance of the material collected (Muhlehardt- Downloaded from https:/www.cambridge.org/core. IP address: 47.88.87.18, on 17 Jan 2017 at 18:42:05, subject
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