Benthic Associations of the Shallow Hard Bottoms Off Terra Nova Bay

Home , Phyllophora antarctica

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

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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-

Siege1 1988). In our study, both wet and dry weights were weights for these selected species are reported in Table 11. measured by an indirect, non-destructive method, in order to The zonation of the faunal associations was described by the preserve this interesting biological materialfor further taxonomic Correspondance Analysis (C.A.).The analysis performed with and functional studies. For each species a group of specimens, all samples gave an ordination model where stations were representing the largest size range possible, was selected. For ordered along the first factor (Fl) according to depth. Astrong each specimen morphological parameters (e.g. length, width), discontinuity occurred between station lA, characterized only as well as wet weight (formolized or in alcohol) and dry weight by P. walkeri, and all the other stations, which were ordered in (48 h at 80°C) were measured. For molluscs, both fresh and dry quite dense clusters along the second factor (F2). This pattern weight was measured after digestion of the shells with 2M suggested theprimaryinfluence ofmarineice (depthdependent) phosphoric acid. The parameter that was best correlated with on the algal and faunal distribution. The C.A. was therefore both wet and dry weights was used for each species to measure repeated excluding station 1A from the analysis. For facility of the preserved specimens found in each sample and to obtain the reading of this latter ordination model (Fig. 3), we indicate the size-frequencyhistograms. In the case of high abundances (over position of station 1A with respect to the previous model, and 300 individuals), a randomized sub-sample, representing at only the species that had the highest relative contributions least 10% of the total abundance, was measured. Total biomass (scores) to both Factors (F1 and F2) were numbered (see legend was estimated for each species in each sample from the size- of Fig. 3). A discontinuity, along the first factor (Fl), separated frequency histograms. Histograms were smoothed by a mobile samples of station 2 from those of stations 3,4 and 5. This factor averaging process to better evaluate possible polymodal can be interpreted as the type of algal covering that characterized population pattern. The different size classes were then the different stations. Samples of stations 2 and3were relatively recognized by resolution of the polymodal distribution into more scattered with respect to those of stations 4 and 5, most normal components, according to the method of Bhattacharya probably due to the higher algal patchiness that characterize the (1967), using the ELEFAN PC programme. It is often difficult shallower stations. Most of the species were associated with the to distinguishdiscrete size classes from size frequencyhistograms, deeper stations (Fig. 3); a few species, such as P. wulkeri (no. 9 and many statistical and graphical methods do not allow such in Fig. 3), L. antarctica (no. 3) and M. anturctica (no. 7) were a discrimination,unless large number of specimens are measured associated mainly with station 2. orsize-classesarewellseparated(Grantetul, 1987). Additionally, The number of animal species along the transect showed a size classes do not always correspond to age-classes (cohorts) trend very similar to that of the Diversity (H’) and Evenness (J) particularly for slow growing, polar marine invertebrates. indices (Fig. 4a), withminima at the shallowest samples 1Aand However, for most of the small-sized species studied here, size- 2B, characterized by the absence of macroalgal covering and frequency analysis revealed well-defined size classes, that seem where only the amphipod P. walkeri was abundant. Maximum to correspond to cohorts or age-classes accordingto the literature community diversity was observed in the deepest stations 4 data available on the life history of the species. and 5, characterized by the dominance of Phyllophoru. Mean number of individuals showed a peak in samples 2A and 2C mainly due to L. anturctica with over 70 000 ind.mz (Fig. 4b). Results The trend of the total biomass is quite consistent with that of the abundance (Fig. 4b). There was relatively high variability Community level analysis among replicates (e.g., samples 3A and 3C with 167 and In addition to the dominant rhodophytes Iridaeu cordata and 1205 gmT2,respectively, Table 111) probably related to the Phyllophoru anturctica, 18 other algal species were collected patchiness of the macroalgal covering. Mean biomass values, (Cormaci et ul. 1992), most of them as epiphytes on the however, showed a progressive decrease with depth (Fig. 4b). rhodophyte species. The benthic microflora was represented by Higher values, up to 2392 gm” observed at station 2, were due many genera of epilithic and epiphytic diatoms belonging both to the presence of Iriduea with large, laminar thalli and to mainly to the genera Achnanthes, Cocconeis, Fragilariu, the abundance of relatively large species such as P. wulkeri and Nitzschia, Lycmophoru and Triceratium (Gambi & Mazzella M. antarctica. On the contrary,in deeper samples,the dominance 1991). A total of 37 macrozoobenthic taxa were classified at of Phyllophora, a smaller macroalga, and of small-sized different taxonomic levels within the following groups: crustaceans, such asN dimorphus, appear to be responsible for polychaetes (14 taxa), molluscs (8), crustacean amphipods (6), the decrease of the mean biomass, up to amaximum of 829 gm2 isopods (6), tanaids (l), echinoderms (3) (Table I). Together (Table 111). with these taxa, various meiobenthic taxa (mainly nematodes, Mean abundance and biomass values recorded at each depth crustacean ostracods and harpacticoid copepods), and sessile and those recorded in other coastal Antarctic and subantarctic epiphyte groups (mainly bryozoansand hydroids)were observed. areas are given in Table IV. Although comparisons of such data The nine species selected for biomass estimates and population are often difficult due to different depth ranges, techniques of structure analysis accounted for 91% of the total abundance of sampling, and to the different faunistic groups considered, our the vagile fauna. Correlation coefficients of the allometric dataareamongst the highestrecordedinthe Antarcticcontinental parameters and dry and wet weights, and between wet and dry areas.

Table I. List of the taxa found in the samples and their abundance (number of individuals m-2)

Samples 1A 2A 2B 2c 3A 3c 4A 4c 5A 5C Depth (m) 0.5 2 2 2 6 6 12 12 16 16

ANNELIDA POLYCHAETA Harmothoe spinosa form A Kinberg 262 225 181 264 81 288 Harmothoe spinosa form B Kinberg 412 218 337 504 25 504 Harmothoe cf monroi (Uschakov) - - - - - 8 Harmothoe sp. 18 - - - - - Eunoe anderssoni (Bergstrom) 12 37 25 8 - 8 Pionosyllis cf comosa Gravier 131 106 593 848 75 616 Autolym sp. - - - 16 - - Kefersteinia fauveli (Averincev) 6 12 87 - - 16 Ophryotrocha claparedii (Studer) 18 - - - - - Capitella sp. - 6 - - - 8 Terebellidae gen sp. - - - 8 - 56 Helicosiphon biscoensis Gravier 6 - 50 - 12 224 Serpulidae gen. sp. - - 6 - 6 32 Spirorbidae gen. sp. - - 6 - 6 480 MOLLUSCA GASTROPODA Laevilitorina antarctica (Smith) 70625 462 2856 6640 106 4560 Onoba rurqueti (Lamy) 50 193 181 136 87 640 Onoba gelida (Smith) 56 306 150 360 93 448 Powellisetia deserta (Smith) 200 1618 1075 1192 193 1968 Margarites refulgens (Smith) - - 18 152 31 120 Margarites crebrilirulata (Smith) - - - - - 8 Cuthona georgiana (Pfeiffer) - - - - - 8 MOLLUSCA BIVALVIA Hochstetteria sublaevis (Pelseneer) 75 318 868 80 6 16 CRUSTACEA TANAIDACEA Nototanais dimorphus (Beddard) 5000 5000 6250 10568 2543 2640 CRUSTACEA ISOPODA Munna antarctica Pfeffer 1268 662 456 608 81 256 Munna sp. juveniles 418 556 193 320 12 16 Paramunna rostrata (Hodgson) 137 206 181 64 - 16 Austrosignum sp. 1200 1381 175 0216 50 - Austrofirius furcatus Hodgson - 25 75 - 6 - CRUSTACEA AMPHIPODA Paramoera walkeri (Stebbing) 2043 3118 18 32 6 - Amphilochidae gen. sp. - - 12 - - - Leptocheirus sp. - - 81 8 - - Oradarea spp. 75 400 356 416 125 272 Lysianassidae gen. sp. 18 - - - - - Amphipoda gen. sp. - 343 512 1152 25 1240 ECHINODERMATA Sterechinus neumayeri (Meissner) - - - - 25 - Odontasrer validus (Koehler) - - - 8 - 16 Diplasterias brucei (Koehler) - - - - - 8 Tctal number of species 20 20 25 22 21 27 Number of individuals m-* 82025 15181 14812 23600 3581 14472 Diversity (H) 0.65 2.15 1.35 2.03 1.71 1.32 2.18 Evenness (J) 0.21 0.72 0.52 0.63 0.56 0.44 0.67

Autoecological analysis and demography of the relevant soft and hardsubstrates(Hartmann 1964,Knox 1977).H spinosa animal taxa presents great morphological variability (Hartmann 1964, Desbruyires 1976), and the morphotypes found in our samples Polychaetes: Harmothoe spinosa (forms A and B) Kinberg. (forms A and B) differ in many characters (Gambi unpublished I3 spinosaisoneofthemostcommonandwidespreadpolychaetes data). The allometric analysis revealed that formA was slightly of the austral seas, frequent in shallow and deep biotopes from larger than form B. Despite the relatively low abundances of

Table 11. Correlation coefficients of size parameters and both fresh and dry weights of the selected species of the vagile fauna.

Relationship: size parameter a b C n r

size and fresh weight y = ax + bx2+ cx3 Harmothoe spinosa (A) width 6th seg. -12.242 11.101 11.562 24 0.98'' H. spinosa (form B) width 6th seg. -7.435 12.985 12.617 21 0.99'1 Laevilitorina antarctica shell height -0.123 0.174 0.032 48 0.93** Onoba turqueti width opening -0.073 0.647 0.028 21 0.58' Onoba gelida shell height 0.494 -0.240 0.071 25 0.80" Powellisetia deserta shell height 0.012 -0.031 0.075 25 0.90'. Nototanais dimorphus length -0.194 0.162 -0.020 44 0.89** Munna antarctica thorax length -0.190 0.437 0.025 41 0.90" Paramoera walkeri length -0.845 0.222 0.004 22 0.90"

size and dry weight y = ax + bx2 + cx3 Harmothoe spinosa (A) width 6th seg. -0.722 0.386 2.140 24 0.98** H. spinosa (form B) width 6th seg. -1.380 2.821 1.532 21 0.99** Laevilitorina antarctica shell height 0.028 -0.004 0.014 48 0.99'' Onoba turqueti shell height 0.355 -0.266 0.060 21 0.88** Onoba gelida shell height 0.022 -0.002 0.013 25 0.97'' Powellisetia deserta shell height -0.053 0.070 -0.001 25 0.96" Nototanais dimorphus length -0.041 0.037 -0.002 44 0.901' Munna antarctica thorax length 0.116 -0.038 0.028 41 0.85'' Paramoera walkeri length -0.233 0.086 -0.003 22 0.91**

fresh weight and dry weight y = a + bx Harmothoe spinosa (A) 0.100 0.155 0.99** H. spinosa (form B) 0.197 0.146 0.99'' Laevilitorina antarctica 0.031 0.214 0.93'' Onoba turqueti 0.145 0.069 0.36 ns Onoba gelida 0.004 0.282 0.82** Powellisetia deserta 0.050 0.206 0.85** Notoranais dimorphus 0.016 0.402 0.92** Munna antarctica 0.015 0.184 0.87'' Paramoera walkeri 0.470 0.136 0.95'' ** = significant ath0.01; * = significant at P>O.O5; ns = not significant. both morphotypes, their relatively large sizes accounted for the Molluscs: the molluscs were represented by eight species high biomass values. Some authors (Amaud 1974, Averincev (six prosobranch gastropods, 1opistobranch gastropod and 1 1977) indicate H. spinosa as a carnivore, preying mainly on bivalve). The most abundant species found, and considered small crustaceans and polychaetes. The relationship between below, are all very common in western Antarctic areas abundance and biomass values of this species seems consistent (Ponder 1983, Dell 1990) and all belong to the superfamily with its trophic role (Gambi & D'Agostino 1994). Averincev Truncatelloidea (Ponder 1988), characterized by small sized (1977) in the Davis Sea found continuous reproduction, with up mesograzers. to four sub-generations in a year: two main generations in the Laevilitorina antarctica (Smith) early summer (December-January) and in July-August, two sub-generations in March and in October. The life span was two In the Ross SeaL. anfarcticahas been reported from 15400 m years and the growth rate was relatively high, reaching a body depth. However, according to Dell (1990) the deep records are width of 6.5 mm after one year. In our samples the size- probably based on dead shells and the species does not live frequency analysis of both H. spinosa morphotypes revealed an deeper than 45 m. In our samples, L. anfarcficawas present at unimodalpattern at all depths. The mean body width observed all depths, but was particularly abundant at the shallowest (0.7 mm) suggested juvenile specimens, while the few larger stations (2-6 m depth). L. anfarcficabreeds once a year in late specimens collected did not exceed 2.5 mm in width. The spring-early summer, and produces many juveniles (Dell 1972) juveniles may represent the cohort born in the late spring hatching directly from eggs (Picken 1979). The life span is not (October) or the overlapping of the two last sub-generations. well known but has been estimated at 2-3 years (Picken 1979). The abundance of larger specimens was probably biased by The size-frequencyanalysis of our samples showed apolymodal small sample size and the few replicates made at each depth. pattern at station 2, with three well defined groups that included juveniles (Fig. Sa). These latter are responsible for the high density recorded at this depth (over 70 000 ind.m2) where egg-

a) 2 1A Diversity (H') Evenness (J)

0.6

0.4

0.2

0 1A 28 2A 2c 3A 3c 4A 4c 5A 5c Samples 9 0Diversity Es\m Evenness

3A 15 2C 28

14 b) abundance (ind. m-' x lo3) total biomass (g m-*) . 12 . 1 11 3 6 2A FI 3 5 4A 2 16 5A 4c 11500 .4 1 13 5C 10 1000

18 500

" "n 1A 28 2A 2C 3A 3C 4A 4C 5A 5C 17 Samples

Fig. 3. Ordination model of the Correspondence Analysis (C.A.) u abundance biomass according to the first two Factors (F1 = 51.7%; F2 = 18.0%). Species points: 1= Harmothoe spinosa (form A), 2= Harmothoe spinosa (form B), 3 = Laevilitorinu anturctica, Fig. 4. a. Trend of the Diversity (H') and Evenness (J) values of 4 = Onoba turqueti, 5 = Onoba gelida, 6 = Powellisetia the vagile fauna along the depth transect. b. Trend of faunal deserta, 7 = Munna antarctica, 8= Nototanais dimorphus, abundance (ind.m3 and biomass (g m"). (*abundance for 2.4 9= Paramoera walkeri, 10= Pionosyllis cf comosa, was 82000). 11= Hochstetteria sublaevis, 12= Oradarea spp., 13= Amphipoda gen sp., 14= Paramunna rostratu, 15= Austrosignum sp., 16= Kefersteinia fauveli, P. deserta was more frequent in the deep stations, and the 17= Helicosiphon biscoensis, 18= Margarites refulgens. population analysis revealed only one size class in station 2, and two classes, with the occurrence of juveniles, in deeper stations capsules with developing embryos were also observed. In the (Fig. 5). These size classes may represent two cohorts, with deeper stations only the twolarger size-classes occurred. If each juveniles corresponding to the spring-summer generation and of the size-classes recognized represent a cohort, the largest adults to the autumn-winter one. specimens collected at Terra Nova Bay were at least two years old. Onoba gelida (Smith) 0. gelida is a common species from 4-870 m in the Ross Sea Powelliseta deserta (Smith) (Dell, 1990), and from €450 m in the Davis Sea (Egorova This speciesin the Ross Sea has been recorded from 18m to over 1978). Golikov (1970 cited in Egorova 1978) found that in the 800m depth (Dell 1990). In the Davis Sea, density ofP. deserta Davis Sea depth distribution and density of 0. gelida changed increased with increasing depth, to a maximum of 340 ind.m" according to season and depth, reaching a maximum of (Egorova 1978), and showed two sub-generations in a year, and 480 ind.m-*at 32 m depth. Golikov also observed four sub- a life span that varies, according to the natal season, from 9-14 generations in a year and a life span that varied according to the months (Golikov 1970 cited in Egorova 1978). In our samples natal season, from 7-9 months. In our samples 0. gelida was

Table 111. Biomass values (g m-* wet weight) of the macroalgae and of the selected species of the vagile fauna in the different samples.

1A 2.4 2B 2c 3A 3c 4A 4c 5A 5c Macroalgae Rhodophyta Iridaea cordaia - 1845.60 - 2363.28 59.75 1080.48 - - 13.00 - Phyllophora aniarciica - 149.37 - - 100.00 113.36 469.06 767.52 150.25 808.56

Algal biomass g m-z - 1994.90 - 2363.28 159.75 1193.84 469.06 767.52 163.25 808.56

Vagile fauna Harmoihoe spinosa (A) - 4.89 - 5.22 1.07 0.22 2.70 3.94 1.13 4.00 H. spinosa (form B) - 3.15 3.02 1.58 0.55 0.55 1.07 1.55 4.03 5.88 Laevilitorina aniarciica 0.06 55.89 3.48 4.43 0.68 8.99 3.23 7.53 0.12 5.23 Onoba iurqueii - 0.23 - - 0.86 0.10 0.79 0.59 0.40 2.92 Onoba gelida - 0.03 - - 0.24 0.05 0.11 0.28 0.07 0.33 Powelliseiia deseria - 0.13 - - 0.77 0.17 0.46 0.51 0.08 0.85 Noiotanais dimorphus - 1.25 - 0.03 1.23 0.88 1.08 1.83 0.33 0.34 Munna aniarciica - 3.17 - 7.64 1.84 0.87 1.16 1.55 0.27 0.85 Paramoera walkeri 94.41 27.66 14.43 9.85 0.01 0.001 - - - -

Fauna) fresh weight (g m-*) 94.47 96.44 20.93 28.78 7.27 11.86 10.64 17.80 6.44 20.44 Faunal dry weight (g m-*) 17.61 19.34 3.83 5.11 1.75 2.71 2.26 3.94 1.10 3.50 Total biomass fresh weight (g m-3 94.47 2091.34 20.93 2392.06 167.02 1205.70 479.70 785.32 169.70 829.00

abundant from 6 m depth and density increased slightly to a an evident sexual dimorphism (Sieg & Wagele 1990), is maximum of 448 ind.m-2at 16m (Table I). The size-frequency tubicolous, and exhibits feeding habits that range from herbivory analysis of our population revealed a bimodal pattern that can to scavenging and predation (Olivier & Slattery 1985). In the be produced by the overlapping of different size classes. These Ross Sea region it has been collected from 4-140 m depth, even are difficult to recognize by the analysis due to the relatively low though the preferred depths are 5-19 m (Sieg 1983). In our number of individuals measured and to the short generation samples N. dimorphus was the most abundant taxon among the time of this species. peracarid crustaceans and it was mainly common in association withP. antarctica, at the deeper stations. The preference of this Onoba turqueti (Lamy) species for seaweed habitats is well documented (Amar & Roman 1973-74), but it has also been reported soft-bottoms 0. turqueti is quite common in the Ross Sea where, according in (Oliver & Slattery 1985). The population analysis revealed at to Dell (1990) is distributed from 18-350 m depth. In the Davis all depths a quite complex structure, with a polymodal pattern Sea its distribution and density changed with the season, as for composed of three relatively well-separated size classes 0. gelida, with higher densities (up to 176ind.m.2) at shallower corresponding to juveniles, females and males (Fig. 6a). This depth (4 m) in summer (Egorova 1978). The life cycle showed pattern of size distribution is consistent with the reproductive two sub-generations per year: one in autumn-winter and a cycle of the species that shows a proterogynic sexual inversion second one in spring-summer, the latter composed of larger and during growth (Marinovic 1987, cited in Pearse et al., 1991). more numerous individuals. The life span varies according to The sex-ratio changed along with depth, with males more the natal season between 8 and 13 months (Golikov 1970 cited frequent at shallower depths. Thissuggests that activemigration in Egorova 1978). In our samples 0. turqueti was abundant may occur in males, which generally showed higher motility from 6 m depth, and showed amaximum density of 640 ind.mq2. than females (Shiino 1970). The population analysis revealed at all depths the occurrence of only one size class composed by relatively large specimens Isopods: Munna antarctica Pfeffer (mean shell length 2.5 mm) that may correspond to the cohort born in autum-winter. M. antarctica is aquitecommon isopodinhtarcticwaters, and has been found from 2-300 m depth (Nordenstam 1933, Amar Peracarid Crustaceans: the peracarid crustaceans were & Roman 1973-74). In Adilie Land the species was found to represented by three main groups: tanaids (one taxon), be quite abundant in association with the brown macroalga isopods (four species) and amphipods (six taxa). Phyllogigas grandifolius (Amar & Roman 1973-74). It has an evident sexual dimorphism (Nordenstam 1933), but the life Tanaids: Nototanais dimorphus (Beddard) cycle is unknown. In our samples M. antarctica occurred at all N. dimorphus appears to be the most frequent and widespread depths but reached the highest densities at 2 m in association tanaid of the Antarctic waters (Sieg 1986). The species shows with I. corduta. Males and females had the same dimensions

Table IV. Mean abundance (number of individuals m3and biomass (wet weight, g m2)values of macrobenthos communities in various Antarctic and subantarctic areas.

Location Depth Bottom Abundance Biomass Source (m) range or mean fresh weight ind.m” range or mean g m-2

Lutzow-Holm Bay 1.5-8 hard & mixed - 46-109 Nakajima et al. 1982 (Prince Olaf Coast) Signy Island (S. Orkney Is.) 5-25 soft & mixed - 166-4200 White & Robins 1972 Signy Island 3-35 soft & mixed - 1000-4000 Everson & White 1969 Signy Island 3-35 soft & mixed - 307-789 Hardy 1972 soft 11054 35 Muhlenhardt-Siege11988 Signy Island 127-320 ,, South Shetland Islands 60-850 8742 132 South Shetland Island 46-59 sofi 6720-17960 202-2 17 Mills & Hessler 1974 104 aaso 86 I Greenwich Is. S. Shetland Is. 35-355 soft 6028 164-180 Gallardo & Castillo 1969 Elephant Island 100-400 soft 11951 52 Muhlenhardt-Siege11988 King George Island (S. 15-30 mixed 1141-36213 63-2200 Jazdzewski et al. 1986 Shetland I.) 60-250 soft 889-2a34 153-2464 ,I South Antarctic Peninsula 175-500 soft 360 20 Muhlenhardt-Siege1 1988 Anvers Island 30 sofi 7629 - Lowry 1975 Anvers Island 5-75 soft 18412 - Richardson & Hedgpeth 1977 South Georgia 4-12 hard - 174 Platt 1980 Sabrina coast (Davis Sea) 200 soft - 1363 Ushakov 1963 300 ,, - 183-483 r, Haswell Island (Davis Sea) 2-10 hard - 20-25 Andriashev 1968 14-28 !, - 1000-2000 9, 35-40 - 2000+ , 45-50 ! - 6000 t Haswell Island (Davis Sea) 2-5 hard - 20-25 Gruzov et al. 1967 5-25 - 450 20-30 - 1000 30-40 - 3000 I 40-50 1 - 3000 I 10-40 soft - 600 , Indian Ocean Sector 100-500 soft - 450-500 Belayev 1964 Marion Island 5 hard - 760-3991 Beckley & Branch 1992 10 - 865-1323 ,( 15 - 728-1118 9 Kerguelen Islands 17-181 soft 434-2554 400-500 Desbruytres & Guille 1973 Kerguelen Ridge 100-200 soft - 92.9 Rubinshteyn!, 1991 200-500 43.4 Mc Murdo Sound East 20 soft iia712-1555573 - Dayton & Oliver 1977 (Ross Sea) eutrophic 30 ! 145781 - 1 Mc Murdo Sound West 30 soft 2184-45294 - (Ross Sea) oligotrophic 40 10036 - 500 1960 - I Ttrra Nova Bay (Ross Sea) 0.5 hard 6300 94.4 present paper 2 32261 2241 t, 6 13498 686 12 19206 632 ,I 16 , 9026 500

and the population analysis revealed at all depths one size class P. walkeri is a common circum-Antarctic species andrepresents for both sexes (Fig. 6b). The juveniles, although present (see one of the most abundant amphipods of the Antarctic waters Table I), were not included in the size-frequency analysis (Sagar 1980). The species is also an organism typical of the because their actual abundance and structure may be biased due “fast-ice” community, feeding mainly on the sympagic diatoms to the fact that the young of M. antarctica are pooled with (Gruzov 1977), and it is found from the “beach level” to 310 m juveniles of other Munna species. The sex-ratio observed was depth (Sagar 1980). However, inoursamplesP. walkrishowed c. 1:l and remained quite constant at all depths. high densities only in the shallowest stations (stations 1, 2 and 3), and disappeared below 12 m depth (Table I). The life Amphipods: Paramoera walkeri (Stebbing) cycle, reproductive biology and growth pattern ofP. walken’ are well known in different austral coastal areas (Rakusa-

Laevilitorina antarctica Powellisetia deserta a) St. 2 (2m depth) St. 2 (2m depth) no individuals o individuals 16 N- 116 N - 32 l4 n 12

10 8 rlh 6

0 05 09 13 17 21 25 29 33 37 41 03 07 11 15 19 23 27 31 shell height (rnrn) shell height (rnrn)

St. 3 (6rn depth) St. 3 (6m depth)

no individuals no individuals 16 50 N- 119 N - 263 12l4 1 m 40 1 30 4

05 09 13 17 21 25 29 33 37 41 shell height (mm) shell height (rnrn)

St. 4 (12m depth) St. 4 (12m depth)

no individuals 3o ~ICindividuals 20, I N - 16s

?3 27 31 shell height (rnm) shell height (mm) !d

St. 5 (16m depth) St. 5 d6m depth)

no individuals no individuals 25 N - 10

i 4 05 41 03 07 11 15 19 23 27 31 shell height (mm) Shell height (rnrn)

Fig. 5. Size-frequency histograms at different stations of a. Laevilitorina antarctica, b. Powellisetia deserta.

Suszczewski 1972, Sagar 1980,Sainte Marie 1991);this species in late spring-early summer and show a quite rapid growth breeds seasonally, but after the first or even the second year of coincidentwiththe summerphytoplanktonbloom(Sagar1980). life; eggs are incubated in the female brood pouch for about 4-5 BothRakusa-Suszczewski(1972) at Alasheyev Bight andSagar months, from May-October, and the young stages are released (1980) at Cape Bird (Ross Sea) observed upto threeage-classes

Nototanala dimorphus Munna antarctia (adults) a) St. 2 (2m depth) 6) St. 2 (2m depth) no individuals no individuals 20 40 I N -200 N - 178 30

0 09 13 17 21 25 29 33 37 41 11 15 19 23 27 31 35 39 length (mm) thorax length (mm)

St. 3 (6m depth) St. 3 (6m depth)

no individuals no individuals N - 209 20

13 0 17 21 33 25 28 37 41 11 15 19 23 27 31 35 39 I length (mm) thorax length (mmi

St. 4 d2m depth) St. 4 (12m depth)

no individuals no individuals 20 , I 20 7 15 n 15 10 10

5 5

0 0 09 13 17 21 25 29 33 37 41 11 15 19 23 27 31 35 38 lenght (mm) thorax lenght (mm)

St. 5 (16m depth) St. 5 (16m depth) 2o ili) individuals no mdlvlduals 3 n N - 166

0 09 13 17 21 25 29 33 37 41 11 15 19 23 27 31 35 length (mrn)

Fig. 6. Size-frequency histograms at different stations. a. Nototanais dimorphus. b. Munna antarctica.

in their P. walkeri populations, corresponding to juveniles, polymodal pattern, composed of two groups of adults and one second-summer and third-summer adults. The size frequency group of juveniles (Fig. 7); the group of juveniles was the only analysis of our population revealed a quite complex structure. one occurring at the deeper stations (stations3and4). Thissize- Specimens at station 1(0.5 m depth) showed a restricted range class spectrum fits quite well with the age-class structure of sizes, while those at station 2 (2 m depth) exhibited a recorded by the previous cited authors. The juveniles and the

Paramoera walkerr'

St. 1 (0.5m depth) St. 3 (6m depth) no. individuals no individuals 30 20

N- 92 25 - N= 92

15 1 20 -

15 - 10

10 -

5 5 --

0- 0 32 52 72 92 112 132 152 12 32 52 72 92 112 132 152 body length (rnm) body length (rnrn)

St. 2 (2m depth) St. 4 (12m depth) no. individuals /13 individuals 10 ,

N= 91 N- 6 1

h 2

0 12 32 i 52 72 92 112 132 152 1i body length (rnrn) body length (mrn) Fig. 7. Size-frequency histograms of Paramoera walkeri at different stations.

Discussion smaller group of adults (probably second-summeradults) were The benthic associations of the shallow hard bottoms off Terra quite well defined and also showed size dimensions similar to Nova Bay are composed of few species, but at relatively high those recorded for the population studied by Sagar (1980). The densities, consistent with the relatively low values of diversity larger size group (probably third-summer individuals)was less (H'). These peculiar populations have not previously been defined and had a maximum length of 16.2 mm, while that described for the Ross Sea and differ from other oligospecific observed by Sagar (1980) at Cape Bird reached 18.5mm. Sagar benthic communities observed on the hard bottoms of this area observed at Cape Bird slightly higher dimensions of the three (Daytonetal. 1970,1974),even though arhodophytemacroalgal age-classes and longer life span than those of the Alasheyev covering occurrs in Mc Murdo Sound (Ross Island) (Brown & Bight population studied by Rakusa-Suszczewski(1972), and Keogh 1990). In other Antarctic and subantarctic zones, the correlated this with the lower temperatures occurring at higher presence of large perennial brown algae, absent in the Ross Sea latitudes. ThedifferentdepthdistributionofcohortsofP. walkeri (Cormaci et al. 1992), allows the development of much more at Terra Nova Bay can be related to the different motility of the complex and diversified communities (Amaud 1974). various classes and to different physiological and behavioural The few structuring macroalgal species found in the studied responses to the environmental stress. At station 1, in fact, biotopes and theassociatedvagilefaunashowedaclearzonation characterizedby moreselectivehabitatconditions, both juveniles with depth, and this seems controlled primarily by pack ice and larger (likely older) specimens were very scarce. dynamics. Ice disturbance at shallow depths in Antarcticcoastal zones exerts a strong selective pressure on the organisms, and

the effects on benthic community structure have been well quite abundant in these biotopes (Di Geronimo & Rosso 1992), documented (Dearbom 1963, Dayton et al. 1970, Bellisio et al. were not considered in this analysis. These megafaunal species 1972, Nakajima et al. 1982, Zamorano 1983). However, the may contribute greatly to the overall standing crop but they differencesindepthdistributionand biomass ofthetwo dominant occurred rarely in our small samples. Our biomass values Rhodophyta are probably related not only to physical disturbance, greatly exceed those recorded in the Davis Sea areas (Ushakov but also to different eco-physiological responses to the light 1963, Gruzov el al. 1967, Andriashev 1968), but were lower regime (Brown & Keogh 1990), and to complex biological than those observed in some subantarctic zones were the algal interactions, such as the feeding activities of mesograzer populations are characterized by large brown macroalgae, such herbivores (Gambi et al. 1992). as Desmarestia, Macrocystis and Phyllogigas (White &Robins The diversity of the vagile fauna increased with increasing 1972, Beckley & Branch 1992). depth and as a function of the algal covering. The highest At the population level, literature on the reproductive biology number of species and diversity, observedin associationwith the of someofthe animalspecies studied areconsistentwithour data Phyllophora belt, are probably related both to the less stressful on population structure and in particular with the occurrence of environment where this alga develops, and to the shelter and the juvenile stages in summer for H. spinosa (Averincev 1979, higher number of micro-habitats that it provides for small, L. antarctica (Picken 1979) P. deserta (Egorova 1978) and mobile animals. Phyllophora, with branched, slender and P. walkeri (Sagar 1980). Some species also show different fringed thalli, has amore complex morphology and architecture, reproductive strategies, according to the scheme proposed by than the simple-bladedlridaea. A similar pattern was observed White (1984) for Antarctic poekilotherms. The gastropod at the Vestfold Hills in the zoobenthos associated with the L. antarctica exhibits seasonal reproduction with rapid gonad macrophytes,where the maximum of diversity and biomasswas maturation, and reproductive events each summer. Other recorded in the Phyllophora facies (Dhargalkar et al. 1988). gastropods, such as 0. gelida and 0. turqueti, as well as the The maximum density and biomass observed at station 2 could polychaete H. spinosa show non-seasonal reproduction, with be related partly to majorresource (food) availabilityfor the few juveniles released at various times throughout theyear. Finally, taxa occurring under these stressful environmental conditions. P. walkeri exhibits seasonal reproduction, but with gonad However, differences in faunal density and biomass among development and accumulation of storage material during the depths and variability among replicates, are believed to be first summer, protracted gonad maturation and/or brooding influenced also by the patchiness of the algal covering. This is during winter and release of young stages during the second supported by the comparison between sample 2B, collected in summer. In a recent review on reproductive biology ofhtarctic a non-vegetated spot, and showing considerably lower species marine invertebrates (Pearse etal. 1991) the authors questioned richness, density and biomass values than samples 2A and 2C, most of the widely held reproductive paradigms and described collected at the same depth but in vegetated spots (Fig. 4a & b). different patterns that often occur in the same habitat, and that Macroalgal patchiness, and the factors that affect it (e.g. ice seem to depend mainly on the phylogenetic constraints of the disturbance, substrate morphology etc.) need to be better various faunal groups rather than on the environment. understood. The various and contrasting reproductive patterns observed The observed zoobenthic abundances are among the highest demonstratethat different life strategies are ecologically suitable recorded in austral sea biotopes (Table IV), and are lower only to exploit habitats with high physical disturbance and only short than those observed in the soft-bottom eutrophic community in periods of relatively favorable conditions. the western McMurdo Sound (Dayton & Oliver 1977). In McMurdo Sound, the remarkable east-west and north-south Acknowledgements gradients in the density of benthic populations, have been related to the seasonal (spring-summer) pulse of phytoplankton We wish to thank Dr L. Mazzella and Dr M.C. Buia (Stazione production and to its vertical and horizontal fluxes (Dayton & Zoologica di Napoli) for the data on micro- and macroalgae, Oliver 1977, Barry &Dayton 1988). Data available for coastal respectively. We also appreciated the contribution of Dr T. phytoplankton populations off Terra Nova Bay revealed the D’Agostino to the project, as part of his doctoral thesis, and of occurrence of a strong bloom during the austral summer Dr. G. Kraemer for editorial assistance. Thanks are due to the 1989-90 (Innamorati etal. 1991), rapid removal of nutrients in “guide-incursori” of the Italian Navy of the 5th Italianhtarctic the upper water column (Catalan0 et al. 1994) and remarkably Expedition and to Dr G. Casazza for their thoughful assistance high values of primary production (up to 1119 mg C m-* d-I; during SCUBA diving sampling operations. The paper was Saggiomo etal. 1992). These findings support theview that the greatly improved by the critical reading and useful comments of coastal area at Terra Nova Bay is highly productive. Faunal Profs Paul K. Dayton and Andrew Clarke, and an anonymous density data are also supported by biomass values which were referee. among the highest recorded in Antarctic and subantarctic areas at comparable depth. In addition, the values measured probably underestimate community biomass because large echinoderms, such as Odontaster validus and Sterechinus neumayeri that are

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