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Phytodetritus deposited on the Antarctic shelf and upper slope: its relevance for the benthic system
Journal of Marine Systems 17 Ž1998. 435–444
Phytodetritus deposited on the Antarctic shelf and upper slope: its relevance for the benthic system Julian Gutt ) , Andreas Starmans, Gerhard Dieckmann Alfred-Wegener-Institut fur ¨ Polar- und Meeresforschung, Columbusstraße, D-27568 BremerhaÕen, Germany Received 15 August 1995; accepted 15 June 1996
Abstract Underwater photography was used to ascertain regional, seasonal, interannual or depth dependent variations in the occurrence of phytodetritus on the Antarctic seafloor in order to explain the patchy distribution patterns of the benthos. The information was obtained from an average of 63 photographs taken at each of 76 stations in the Weddell and Lazarev Sea along a 2300 km coastline during four austral summers between 1986 and 1991. In areas where the shelf was broader than 80 km, the sediment showed a significantly higher phytodetritus cover than on the narrower shelf. This can probably be explained by the lower current velocity on the broader shelf. Significantly higher percentages of phytodetritus cover were also found on the seafloor in areas where the megabenthos cover was relatively low. These results indicate that in areas with a low current velocity, organic particles sink relatively fast onto the seafloor where they are available mainly for deposit feeders. The generally more abundant filter feeders are better adapted to a higher current velocity which transports the particles mainly horizontally over longer periods. No significant relationships were found between other physical parameters and the occurrence of phytodetritus. Therefore, the results are also discussed under the aspect of a weak pelagic–benthic coupling, effected by the long-term development of the benthic system. Resume ´ ´ La photographie sous-marine a ete les variations, selon la localite, ´ ´ utilisee ´ pour elucider ´ ´ la saison, l’annee ´ ou la profondeur, de l’occurrence de phytodetritus sur les fonds marins antarctiques, afin d’expliquer la distribution en taches du ´ benthos. L’information a ete ´ ´ obtenues a` partir de photographies prise a` 76 stations Ž63 photographies en moyenne par station. repartie le long d’une ligne de cote ´ ˆ de 2300 km dans les mers de Weddell et de Lazarev, pendant 4 etes ´ ´ austraux entre 1986 et 1991. Dans les zones ou` le plateau continental s’etendait sur plus de 80 km, la couverture du sediment par les ´ ´ phytodetritus etait Ceci s’explique probablement par ´ ´ significativement plus importante que sur les zones a` plateau plus etroit. ´ une plus faible vitesse du courant sur le plateau large. Un pourcentage significativement plus eleve ´ ´ de couverture par les phytodetritus a egalement ete etait relativement faible. Ces ´ ´ ´ ´ note´ dans les secteurs ou` la couverture par le megabenthos ´ ´ resultats indiquent que dans les zones a` faible courant les particules organiques sedimentent relativement vite sur le fond, ou` ´ ´ elles sont disponibles principalement pour les deposivores. Les filtreurs, qui sont generalement plus abondants, sont mieux ´ ´ ´
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Corresponding author. Alfred-Wegener-Institut fur ¨ Polar- und Meeresforschung, Postfach 120161, D-27515 Bremerhaven, Germany. Tel.: q49-471-4831333; Fax: q49-471-4831149; E-mail: [email protected] 0924-7963r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 7 9 6 3 Ž 9 8 . 0 0 0 5 4 - 2
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adaptes ´ a` un courant plus rapide, qui transporte les particules principalement a` l’horizontale pendant de longues periodes. ´ Aucune relation significative n’a ete Les resultats ´ ´ trouvee ´ entre d’autres facteurs physiques et l’occurrence de phytodetritus. ´ ´ sont donc egalement discutes faible et dependant du developpement a` long ´ ´ dans l’optique d’un couplage pelagos–benthos ´ ´ ´ terme du systeme ` benthique. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Phytodetritus; Antarctic shelf; Benthic system
1. Introduction Sedimentation processes in the Antarctic marine environment have received considerable attention during recent years for a number of reasons: they reflect the release of sea-ice algae in the early summer ŽLegendre et al., 1992. and the sedimentation of phytoplankton blooms late in the year ŽBathmann et al., 1991, Gleitz et al., 1994. in combination with the grazing of zooplankton such as krill ŽCadee ´ et al., 1992, Gonzalez, ´ 1992., copepods, and salps ŽBruland and Silver, 1981. which produce locally high amounts of faeces. Qualitative and quantitative results on the fate of organic material may explain the buffering capacity of the Southern Ocean within the global carbon cycle ŽPriddle et al., 1992.. Geologists and climatologists are interested in recent sedimentation in order to explain long-term signals in sediment cores ŽStein, 1991.. For the complex benthic system ŽDayton, 1990. the occurrence of organic particles close to the seafloor or on the sediment surface is very important, because it serves as its primary food and is thereby partly remineralised ŽLochte and Turley, 1988; Pfannkuche, 1993.. In Antarctica, the shelf is relatively deep with an average water depth of 350 m compared with much shallower shelves worldwide ŽPicken, 1985.. As a result of this unique situation, the benthos is approximately five times further away from the euphotic layer, the source of its primary food, than around all other continents. This may generate a different deposition pattern of organic material compared with other shelf regions because coastal currents considerably affect the sedimentation of particles. The benthos of the high Antarctic shelf is roughly characterised by a weak depth zonation, and an exceptionally patchy spatial distribution pattern of species assemblages. The seafloor is mainly covered by epibenthic, sessile, filter-feeders and associated fauna ŽPicken, 1985; Gutt, 1991a. or alternatively with
comparably few vagrant deposit feeders ŽGutt, 1991b; Gutt and Starmans, in press.. The first aim of this study was to explain different sedimentation patterns on the high Antarctic shelf. For this, different physical parameters which were thought to be of general relevance for this phenomenon were determined: Ø position of the stations, to detect a possible geographical gradient, Ø time of the observation, to consider the seasonality, Ø water depth, to describe a possible vertical zonation, Ø distance between the stations and the ice shelf edge and shelf width since these factors may be indicative of different coastal current velocities. For this purpose, we used underwater photography because the deposited phytodetritus was visible as a green layer covering the sediment. So far, most information about the sedimentation of organic particles was available only from a very few locations where sediment traps were moored in the water column ŽSchalk et al., 1993.. Studies similar to those reported here, and undertaken partly by underwaterphotography, have been conducted by Billett et al. Ž1983., Graf Ž1989., Rice et al. Ž1994. showing distinct pulses of the arrival of phytodetritus on the deep-seafloor. The second aim was to relate the patchy distribution of different benthic species assemblages to the different patterns of phytodetritus deposition. This was possible because each image provides information on both the occurrence of the megafauna and phytodetritus. The results should extend our knowledge about a possible pelagic–benthic coupling in the Antarctic ecosystem with special emphasis on a high spatial resolution. Under this aspect two additional questions arise: Do suspension feeders profit more from the observed sedimentation of phytodetritus than deposit feeders? If so, we would expect a
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Fig. 1. Station map. The broken line parallel to the coast was used as abscissa for the analysis on geographical trends ŽFig. 2.. Dots indicate stations with an especially thick layer of phytodetritus.
positive correlation between the phytodetritus cover and the abundance of megafauna, because benthic assemblages with a high percentage cover are generally dominated by suspension feeders. On the other hand, does the occurrence of phytodetritus favour those species assemblages, which are dominated by motile deposit feeders? We assume that in this case, organic particles remain in the water column for only a short period before they reach the bottom and therefore, are mainly available as food for vagrant benthic grazers. We would, thus, expect a negative relationship between the coverage of phytodetritus and the abundance of benthic megafauna.
Ž1988., cruise reports see Hempel Ž1985., Futterer ¨ Arntz et al. Ž1990., Bathmann et al. Ž1992.. We used a 70 mm underwater camera ŽGutt, 1988. pointing vertically downward, triggered by a bottom contact switch at a constant distance from the seafloor. Since at selected stations the trigger weight was visible on the photographs the size of the photographed area could be calculated. It varied from 0.56 to 1.2 m2 Ž"5%. between the expeditions with a median of 0.9 m2 . The optical resolution was be-
2. Material and methods Photographs were taken on four ‘Polarstern’ expeditions ŽANT III, VI, VII, IX. into the Weddell and Lazarev Sea from 1985 to 1992, between 21 January and 13 March. A total of 4781 photos from 76 stations ŽFig. 1. were analysed. At 83% of all stations an average of 71 photos were made, at 11% of the stations the number of photos was below 30. The water depth was between 99 and 1243 m. For
Fig. 2. Amount of phytodetritus related to geographical location of the stations along the coastline. For the geographical position of the abscissa, see Fig. 1.
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tween ; 0.25 mm and ; 0.50 mm depending on the size of the area photographed. The amount of the greenish coloured organic material deposited on the sediment was classified for each station, into six classes: 0%, 1–5%, 6–25%, 26–50%, 51–75%, and 76–100% cover. The middle of each class was used for the data analyses. Areas covered by benthic organisms were not considered. In order to detect geographical trends the stations were aligned along a line parallel to the coast
ŽSouthwestrNortheast gradient. irrespective of their distance from the coast. Other physical criteria considered as possibly responsible for the variation of the deposited organic material were: time of the year, water depth, distance between stations and ice shelf edge, and shelf width in the area of the station. As a first step in the data analysis, these variables were classified into two groups, e.g., shallow and deep or inshore and offshore stations. These groups were compared by a G-test Žanalysis of a contin-
J. Gutt et al.r Journal of Marine Systems 17 (1998) 435–444
Fig. 4. Amount of phytodetritus related to date of observation.
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Fig. 5. Amount of phytodetritus related to water depth.
high and low cover of the megabenthos in relation to high and low phytodetritus cover. gency table, Sachs, 1984. with respect to their percentage cover Ž) 25% and - 25%.. The null hypothesis, that both groups originate from one population, must be rejected if G ) 3.841 Žtwo-tailed; a s 0.05.. Secondly, a correlation matrix between all the physical variables was calculated to determine any interdependence between them. In such a case, one of the variables must be excluded from the further analysis to avoid bias. Thirdly, factor analysis was performed to detect any relationships between the physical variables and the phytodetritus cover. When two or more variables load high, indicated by high loading factors on one of the resulting imaginary factors, this indicates a relationship between these variables. Finally, the G-test was used to compare
3. Results Geographic variation along the SWrNE gradient is shown on Fig. 2. Along the entire coastline the phytodetritus cover was highly variable. However, the proportion of stations with a high cover Ž) 25%. of phytodetritus was 44% in the easterly Lazarev Sea compared with only 12% at the remaining stations. However, if the entire area of investigation is divided into two subareas of similar extent, the difference in the phytodetritus cover between them was not significant Ž G s 2.40.. At most stations, the thickness of the phytodetritus layer was estimated to be less than
Fig. 3. Underwater photographs showing phytodetritus and megabenthos cover on the sediment surface. Each photograph represents an area of approximately 1 m2 . Ža. Extremely high and thick phytodetritus cover Ž; 87.5%. and poor epibenthos Ž; 1% coverage.. Visible X X organisms: Reteporella antarctica Žbryozoan., Pomachocrinus kerguelensis Žcrinoid.. ANT IXr3, station 189, 7086.3 S 5812.4 E, 476 m. Žb. Low phytodetritus cover Ž; 3%. and much epibenthos Ž; 47% coverage.. Visible organisms: Isodictya sp. Žyellow sponge., Monosyringa longispina Žtubes of infaunal sponge., Iophon radiatus Žencrusting sponge on an ophiuroid cf. Ophiurolepis gelida., Tedania tantula Žsmall tube-shaped sponge, lower left., Notocidaris sp. Žpencil sea urchin, upper and lower centre., Touarella sp. Žgorgonarian, centre., several Synoicium addreanum Žspherice, transparent compound ascidians., Reteporella antarctica Žbryozoan, lower left., CamptoX X plites tricornis Žbushy bryozoan, centre., several ophiuroids. ANT VIIr4, station 307, 7186.5 S 11839.6 W, 178 m. Žc. Intermediate phytodetritus cover Ž; 15%., few infaunal and epifaunal benthic organisms Ž; 12% cover.. Visible organisms: Echiuroinea sp. Žbright prostomium visible, with patches of phytodetritus, which are transported to the mouth and with a pycnogonid resting on the prostomium, upper right., terebellid polychaete with a stellate tentacular crown spread out on the sediment, two colonies of bryozoans Ž Cellarinella spp.. with associated tube-shaped yellow demosponges., Synoicium addreanum Žcompound ascidian, lower right.. ANT VIIr4, station 277, X X 71839.4 S 12836.1 W, 399 m. Žd. High phytodetritus cover Ž; 62.5%. and much epifauna Ž; 48% coverage.. Visible organisms: Tetilla sp. Žwhite sponge, bottom., Isodictya sp. Žyellow sponge, centre right., Haliclonidae Želongated sponges., Ekmocucumis steineni Žbrownish X X holothurian., Abyssocucumis liouÕillei Žwhite holothurian., several anthozoans and ophiuroids. ANT IXr3, station 173, 7080.3 S 7811.1 E, 194 m.
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J. Gutt et al.r Journal of Marine Systems 17 (1998) 435–444 Table 1 Correlation matrix of physical variables Depth Date
Date SWrNE gradient Distance between station and ice shelf edge Shelf width Fig. 6. Amount of phytodetritus related to distance between station and ice shelf edge.
1 mm. At the three stations in the Lazarev Sea with the highest cover this layer was 1–2 cm thick ŽFig. 3a.. This was estimated from the known height of some benthic organisms Že.g., the aspidochirote holothurian Bathyplotes rubipunctatus. which was totally covered by phytodetritus so that only its upper surface was recognisable. Temporal variations are shown by Fig. 4. Regarding interannual differences, high values Ž) 50% coverage. did not occur in 1988 and only at one station in 1989. Values ) 25% were also scarce in 1988 Ž7% of the stations. and most frequent in 1991 Ž33% of the stations.. As regards seasonal variations, highest values Žbetween 76 and 100%. were not found before 26 February, the next lowest class of cover Ž51–75%. not before 6 February. In March, we have relatively few data, but three of the six stations showed high values Ž) 25% cover. compared with 17% for the second half of January and February. No significant difference in the percentage cover was
SWrNE Distance between gradient station and ice shelf edge
0.020 0.407 0.333
0.182 0.143 0.387
0.021
0.246 0.772
0.591
detected Ž G s 0.75. between early and late phases of the entire period of investigation, before and after 14 February. Variations according to water depth are shown by Fig. 5. At the shelf stations Ž- 500 m. the ratio between high Ž) 25% coverage. and lower values was 25:75, on the upper slope it was 12:88. However, this difference was not significant Ž G s 1.53.. The three stations with the extremely high coverage and thick layer of phytodetritus Žnos. 189, 211, 212. were situated over a broad range of relatively deep water Ž476, 693, and 880 m.. Fig. 6 shows the effect of the distance between the station and the ice shelf coast. The stations were classified into those lying closer to and further than 30 km from the ice shelf edge. Despite the fact that only 14% of the offshore stations showed high values and 22% of the near-shore stations had a high cover of phytodetritus, the difference in the percentage cover was not significant Ž G s 0.53.. Shelf width effects are illustrated by Fig. 7. In areas with a broad shelf Ž) 80 km. almost 50% Žfive stations out of 11. showed a high phytodetritus cover whereas this ratio was 21:79 on the narrower shelf. This difference was significant Ž G s 4.19.. Table 2 Loading factors resulting from the factor analysis
Fig. 7. Amount of phytodetritus related to shelf width in the area of the station sampled.
Phytodetritus cover Depth Date SWrNE gradient Distance between station and ice shelf edge
Factor 1
Factor 2
Factor 3
y0.046 0.814 y0.081 y0.717 0.735
0.972 0.058 y0.030 0.347 0.022
y0.041 0.178 0.969 0.208 y0.169
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Fig. 8. Amount of phytodetritus related to the cover of megabenthic organisms on the seafloor.
The correlation matrix of the physical variables is presented in Table 1. The only moderately high correlation coefficients were found between the shelf width and the SouthwestrNortheast gradient Ž r s 0.772., as well as between the shelf width and the distance to the ice shelf edge Ž r s 0.591.. The coefficients for all other combinations were below 0.500. Table 2 shows the results of the factor analysis. The variable ‘shelf width’ was excluded from the analysis because several stations were situated below 500 m. This also solves the problem of having to exclude a variable due to interdependence of variables. The variable ‘phytodetritus cover’ was the only one to have a high loading Ž0.972. on factor 2. All other variables loaded high on the first and third factor, indicating that no clear relationship exists between the occurrence of phytodetritus and the other variables. The relationship between phytodetritus and megabenthos are illustrated by Figs. 3a–d and 8.. Of the 23 stations with a high megabenthic cover Ž) 20%., only three stations had a high abundance Ž26–75% cover. of phytodetritus. The highest abundance class was reached only at stations with a low megabenthic cover. However, the difference was not significant Ž G s 0.72..
4. Discussion The temporal and regional pattern of deposited phytodetritus could be explained only by one of the variables Žshelf width. which were considered to generally affect this phenomenon. Apart from the
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above described trends and relationship, our results show that under each condition Žwater depth, shelf width, season, etc.. highest and lowest phytodetritus cover is possible. This applies to the entire area of investigation as well as to spatially restricted regions; it also applies to the total period of investigation in the austral summer, as well as to temporally closely sampled stations. Therefore, it is also unlikely that there is a direct relationship between the high primary production at the marginal ice zone ŽEl-Sayed and Tagushi, 1981. and the occurrence of phytodetritus on seafloor. This result probably reflects the complexity of the processes which affect the sedimentation of organic particles e.g., melting of sea-ice, primary production which is effected by meteorological processes and grazing of zooplankton, variable sinking rates of the organic particles, horizontal transport by the large- and small-scale near-bottom currents Žvon Bodungen et al., 1985; Schnack, 1985; Nothig and von Bodungen, 1989.. ¨ Fischer et al. Ž1988. found an ‘extreme variability’ of sedimentation rates in the northern Weddell Sea measured by sediment traps. However, the particle flux was the ‘smallest yet observed in the world ocean’. Our results from the southeastern coastal area confirmed the high variability, but we found extremely thick layers of phytodetritus at single stations on the shelf and upper continental slope. The velocity of the coastal current, especially close to the bottom, is of overall relevance to the sedimentation process. However, there are no available high resolution data of the current regime along the coast. Therefore, we used the parameters shelf width and distance the between stations and the ice shelf edge because they were considered to reflect indirectly the current velocity. Above the narrow shelf between 708 and 748S the coastal current is most prominent and dives further south in the Halley Bay area on the much broader shelf ŽCarmack and Foster, 1977; Fahrbach et al., 1992.. The occurrence of more phytodetritus on the broader shelf coincides with few megabenthic filter feeders and relatively more deposit feeders. This supports the hypothesis that the deposit feeders profit most from sedimentation events in combination with low current velocity. On the narrower shelf, the locally high concentrations of filter feeders are better adapted to a more continuous horizontal transport of organic particles because in
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this way they remain in suspension and therefore available for a longer period. In addition, an unknown percentage of the organic particles are removed by suspension feeders such as bryozoans, compound ascidians and sponges before reaching the bottom. This might be an additional explanation for the lower amount of phytodetritus in areas where the filter feeders are abundant. The exceptionally thick layer of phytodetritus at the three stations in the Lazarev Sea can be explained by an intensive storm 9 to 22 days prior to our observation. This resulted in the phytoplankton being mixed over the entire water column ŽBathmann, 1992. before the organic matter sank to the bottom. The general increase of deposited organic material on the seafloor in the first half of February agrees partially with results from sediment traps moored in the southeastern Weddell Sea in summer 1988. These showed a first pulse of sedimentation around 28 January ŽBathmann et al., 1991. and we observed the first high values of phytodetritus on the bottom at the beginning of February. No significant relationship between the phytodetritus cover and the megabenthos was found since both low and high amounts of phytodetritus occur on both the narrow and broad shelf. Although the photographic records were obtained over a period of 5 years and at a high spatial resolution, each photograph represents only a momentary observation providing no information about the origin or fate of the phytodetritus. In contrast, the benthic community pattern reflects an integration of environmental conditions over long periods. This is particularly true since important compartment of the benthic fauna, such as several glass sponges ŽDayton, 1978., which structure to a certain degree the entire system ŽGutt and Starmans, in press. are extremely slow growing ŽBrey and Clarke, 1993.. Our data indicate a less distinct pelagic–benthic coupling, for which the continuous change between ice ages and interglacial periods during the past million years plays an important role. During the ice ages the ice shelf settled on the continental shelf and therefore, these areas were not available as habitat for the benthos. If, during these periods, the benthos lived only on the upper continental slope or in locally restricted areas on the shelf such as inner shelf depressions, the food availability must have been much lower than today due to the almost
permanent sea-ice cover ŽCooke and Hays, 1982.. In this case, the benthos, probably with a much lower abundance and reduced numbers of species, must have been adapted to these conditions. Thus, the same species today live under an excess food availability in austral summer. Similar food conditions for the benthos as during the ice ages might today exist several hundred kilometres under the ice shelf e.g., in the Ross Sea where Dayton Žpersonal communication. photographed aggregations of sponges. This hypothesis is also supported by the relatively high number of the few investigated benthic species which are partly or totally uncoupled from the phase of primary production ŽWhite, 1977; Picken, 1980; Gutt, 1991c; Gutt et al., 1992, Barnes and Clarke, 1995.. If a considerable part of the recent high Antarctic benthos lived further north during the ice ages, the present patchy benthic dispersion pattern may be explained less by the poorly predictable food supply than by a still continuing recolonisation of the shelf. The 12,000 years which have elapsed since the last ice age might not suffice to attain a large scale mature benthic community because many species concerned grow slowly and have very short-lived or non-existing pelagic larval stages and some also reproduce by budding. Based on our results, it seems to be most likely that a pelagic–benthic coupling exists, which, however, is weak due to the long-term development of the ecosystem.
Acknowledgements Thanks are due to two unknown referees. Publication no. 1050 of the Alfred Wegener Institute.
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Graf, G., 1989. Benthic–pelagic coupling in a deep-sea benthic community. Nature 342, 437–439. ¨ Gutt, J., 1988. Zur Verbreitung und Okologie der Seegurken ŽHolothuroidea Echinodermata. im Weddellmeer ŽAntarktis.. Ber. Polarforsch. 41, 1–87. Gutt, J., 1991a. Are Weddell Sea holothurians typical representatives of the Antarctic benthos? Meeresforsch. 33, 312–329. Gutt, J., 1991b. On the distribution and ecology of holothurians in the Weddell Sea ŽAntarctica.. Polar Biol. 11, 145–155. Gutt, J., 1991c. Investigations on brood protection in Psolus dubiosus ŽEchinodermata: Holothuroidea. from Antarctica in spring and autumn. Mar. Biol. 111, 281–286. Gutt, J., Starmans, A., in press. Structure and biodiversity of megabenthos in the Weddell and Lazarev Seas ŽAntarctic.: ecological role of physical parameters and biological interactions. Polar Biol. Gutt, J., Gerdes, D., Klages, M., 1992. Seasonality and spatial variability in the reproduction of two Antarctic holothurians ŽEchinodermata.. Polar Biol. 11, 533–544. Hempel, G., 1985. Die Expedition ANTARKTIS III mit FS ‘Polarstern’ 1984r85. Ber. Polarsforsch. 25, 1–209. Legendre, L., Ackley, S.F., Dieckmann, G., Gulliksen, B., Horner, R., Hoshiai, T., Melnikov, I., Reeburgh, W.S., Spindler, M., Sullivan, C.W., 1992. Ecology of Sea Ice Biota: 2. Global Significance. Polar Biol. 12, 429–444. Lochte, K., Turley, C.M., 1988. Bacteria and cyanobacteria associated with phytodetritus in the deep-sea. Nature 333, 67–69. Nothig, E.-M., von Bodungen, B., 1989. Occurrence and vertical ¨ flux of faecal pellets of probably protozoan origin in the southeastern Weddell Sea ŽAntarctica.. Mar. Ecol. Prog. Ser. 56, 281–289. Pfannkuche, O., 1993. Benthic response to the sedimentation of particular organic matter at the BIOTRANS station, 478N, 208W. Deep-Sea Res. 40, 135–149. Picken, G.B., 1980. Reproductive adaptations of Antarctic benthic invertebrates. J. Linn. Soc. 14, 75–76. Picken, G.B., 1985. Benthic research in Antarctica: past, present and future. In: J.S. Gray, M.E. Christiansen ŽEds.., Marine Biology of Polar Regions and Effects of Stress on Marine Organisms. Proceedings of the 18th European Marine Biology Symposium, University of Oslo, Norway, 14–20 August 1983. Wiley, Chichester, p. 167–184. Priddle, J., Smetacek, V., Bathmann, U., 1992. Antarctic marine primary production, biogeochemical carbon cycle and climate change. Phil. Trans. R. Soc. Lond. B 338, 289–297. Rice, A.L., Thurston, M.H., Bett, B.J., 1994. The IOSDL DEEPSEAS programme: introduction and photographic evidence for the presence and absence of a seasonal input of phytodetritus at contrasting abyssal sites in the northeast Atlantic. Deep-Sea Res. 41, 1305–1320. Sachs, L., 1984. Angewandte Statistik. Springer, Berlin. Schalk, P.H., Brey, T., Bathmann, U., Arntz, W., Gerdes, D., Dieckmann, G., Ekau, W., Gradinger, R., Plotz, E., ¨ J., Nothig, ¨ Schnack-Schiel, S.B., Siegel, V., Smetacek, V.S., Van Franeker, J.A., 1993. Towards a conceptual model of the Weddell Sea ecosystem, Antarctica. In: V. Christensen, D.
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Pauly ŽEds.., Trophic Models of Aquatic Ecosystems. ICLARM Conf. Proc., Vol. 26, pp. 323–337. Schnack, S.B., 1985. A note on the sedimentation of particular matter in Antarctic waters during summer. Meeresforschung 30, 306–315. Stein, R., 1991. Accumulation of organic carbon in marine sediments. In: S. Bhattacharji, G.M. Friedmann, H.J. Neugebauer, A. Seilacher ŽEds.., Lecture Notes in Earth Sciences, 34. Springer, Berlin.
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Phytodetritus deposited on the Antarctic shelf and upper slope: its relevance for the benthic system Julian Gutt ) , Andreas Starmans, Gerhard Dieckmann Alfred-Wegener-Institut fur ¨ Polar- und Meeresforschung, Columbusstraße, D-27568 BremerhaÕen, Germany Received 15 August 1995; accepted 15 June 1996
Abstract Underwater photography was used to ascertain regional, seasonal, interannual or depth dependent variations in the occurrence of phytodetritus on the Antarctic seafloor in order to explain the patchy distribution patterns of the benthos. The information was obtained from an average of 63 photographs taken at each of 76 stations in the Weddell and Lazarev Sea along a 2300 km coastline during four austral summers between 1986 and 1991. In areas where the shelf was broader than 80 km, the sediment showed a significantly higher phytodetritus cover than on the narrower shelf. This can probably be explained by the lower current velocity on the broader shelf. Significantly higher percentages of phytodetritus cover were also found on the seafloor in areas where the megabenthos cover was relatively low. These results indicate that in areas with a low current velocity, organic particles sink relatively fast onto the seafloor where they are available mainly for deposit feeders. The generally more abundant filter feeders are better adapted to a higher current velocity which transports the particles mainly horizontally over longer periods. No significant relationships were found between other physical parameters and the occurrence of phytodetritus. Therefore, the results are also discussed under the aspect of a weak pelagic–benthic coupling, effected by the long-term development of the benthic system. Resume ´ ´ La photographie sous-marine a ete les variations, selon la localite, ´ ´ utilisee ´ pour elucider ´ ´ la saison, l’annee ´ ou la profondeur, de l’occurrence de phytodetritus sur les fonds marins antarctiques, afin d’expliquer la distribution en taches du ´ benthos. L’information a ete ´ ´ obtenues a` partir de photographies prise a` 76 stations Ž63 photographies en moyenne par station. repartie le long d’une ligne de cote ´ ˆ de 2300 km dans les mers de Weddell et de Lazarev, pendant 4 etes ´ ´ austraux entre 1986 et 1991. Dans les zones ou` le plateau continental s’etendait sur plus de 80 km, la couverture du sediment par les ´ ´ phytodetritus etait Ceci s’explique probablement par ´ ´ significativement plus importante que sur les zones a` plateau plus etroit. ´ une plus faible vitesse du courant sur le plateau large. Un pourcentage significativement plus eleve ´ ´ de couverture par les phytodetritus a egalement ete etait relativement faible. Ces ´ ´ ´ ´ note´ dans les secteurs ou` la couverture par le megabenthos ´ ´ resultats indiquent que dans les zones a` faible courant les particules organiques sedimentent relativement vite sur le fond, ou` ´ ´ elles sont disponibles principalement pour les deposivores. Les filtreurs, qui sont generalement plus abondants, sont mieux ´ ´ ´
)
Corresponding author. Alfred-Wegener-Institut fur ¨ Polar- und Meeresforschung, Postfach 120161, D-27515 Bremerhaven, Germany. Tel.: q49-471-4831333; Fax: q49-471-4831149; E-mail: [email protected] 0924-7963r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 7 9 6 3 Ž 9 8 . 0 0 0 5 4 - 2
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adaptes ´ a` un courant plus rapide, qui transporte les particules principalement a` l’horizontale pendant de longues periodes. ´ Aucune relation significative n’a ete Les resultats ´ ´ trouvee ´ entre d’autres facteurs physiques et l’occurrence de phytodetritus. ´ ´ sont donc egalement discutes faible et dependant du developpement a` long ´ ´ dans l’optique d’un couplage pelagos–benthos ´ ´ ´ terme du systeme ` benthique. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Phytodetritus; Antarctic shelf; Benthic system
1. Introduction Sedimentation processes in the Antarctic marine environment have received considerable attention during recent years for a number of reasons: they reflect the release of sea-ice algae in the early summer ŽLegendre et al., 1992. and the sedimentation of phytoplankton blooms late in the year ŽBathmann et al., 1991, Gleitz et al., 1994. in combination with the grazing of zooplankton such as krill ŽCadee ´ et al., 1992, Gonzalez, ´ 1992., copepods, and salps ŽBruland and Silver, 1981. which produce locally high amounts of faeces. Qualitative and quantitative results on the fate of organic material may explain the buffering capacity of the Southern Ocean within the global carbon cycle ŽPriddle et al., 1992.. Geologists and climatologists are interested in recent sedimentation in order to explain long-term signals in sediment cores ŽStein, 1991.. For the complex benthic system ŽDayton, 1990. the occurrence of organic particles close to the seafloor or on the sediment surface is very important, because it serves as its primary food and is thereby partly remineralised ŽLochte and Turley, 1988; Pfannkuche, 1993.. In Antarctica, the shelf is relatively deep with an average water depth of 350 m compared with much shallower shelves worldwide ŽPicken, 1985.. As a result of this unique situation, the benthos is approximately five times further away from the euphotic layer, the source of its primary food, than around all other continents. This may generate a different deposition pattern of organic material compared with other shelf regions because coastal currents considerably affect the sedimentation of particles. The benthos of the high Antarctic shelf is roughly characterised by a weak depth zonation, and an exceptionally patchy spatial distribution pattern of species assemblages. The seafloor is mainly covered by epibenthic, sessile, filter-feeders and associated fauna ŽPicken, 1985; Gutt, 1991a. or alternatively with
comparably few vagrant deposit feeders ŽGutt, 1991b; Gutt and Starmans, in press.. The first aim of this study was to explain different sedimentation patterns on the high Antarctic shelf. For this, different physical parameters which were thought to be of general relevance for this phenomenon were determined: Ø position of the stations, to detect a possible geographical gradient, Ø time of the observation, to consider the seasonality, Ø water depth, to describe a possible vertical zonation, Ø distance between the stations and the ice shelf edge and shelf width since these factors may be indicative of different coastal current velocities. For this purpose, we used underwater photography because the deposited phytodetritus was visible as a green layer covering the sediment. So far, most information about the sedimentation of organic particles was available only from a very few locations where sediment traps were moored in the water column ŽSchalk et al., 1993.. Studies similar to those reported here, and undertaken partly by underwaterphotography, have been conducted by Billett et al. Ž1983., Graf Ž1989., Rice et al. Ž1994. showing distinct pulses of the arrival of phytodetritus on the deep-seafloor. The second aim was to relate the patchy distribution of different benthic species assemblages to the different patterns of phytodetritus deposition. This was possible because each image provides information on both the occurrence of the megafauna and phytodetritus. The results should extend our knowledge about a possible pelagic–benthic coupling in the Antarctic ecosystem with special emphasis on a high spatial resolution. Under this aspect two additional questions arise: Do suspension feeders profit more from the observed sedimentation of phytodetritus than deposit feeders? If so, we would expect a
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Fig. 1. Station map. The broken line parallel to the coast was used as abscissa for the analysis on geographical trends ŽFig. 2.. Dots indicate stations with an especially thick layer of phytodetritus.
positive correlation between the phytodetritus cover and the abundance of megafauna, because benthic assemblages with a high percentage cover are generally dominated by suspension feeders. On the other hand, does the occurrence of phytodetritus favour those species assemblages, which are dominated by motile deposit feeders? We assume that in this case, organic particles remain in the water column for only a short period before they reach the bottom and therefore, are mainly available as food for vagrant benthic grazers. We would, thus, expect a negative relationship between the coverage of phytodetritus and the abundance of benthic megafauna.
Ž1988., cruise reports see Hempel Ž1985., Futterer ¨ Arntz et al. Ž1990., Bathmann et al. Ž1992.. We used a 70 mm underwater camera ŽGutt, 1988. pointing vertically downward, triggered by a bottom contact switch at a constant distance from the seafloor. Since at selected stations the trigger weight was visible on the photographs the size of the photographed area could be calculated. It varied from 0.56 to 1.2 m2 Ž"5%. between the expeditions with a median of 0.9 m2 . The optical resolution was be-
2. Material and methods Photographs were taken on four ‘Polarstern’ expeditions ŽANT III, VI, VII, IX. into the Weddell and Lazarev Sea from 1985 to 1992, between 21 January and 13 March. A total of 4781 photos from 76 stations ŽFig. 1. were analysed. At 83% of all stations an average of 71 photos were made, at 11% of the stations the number of photos was below 30. The water depth was between 99 and 1243 m. For
Fig. 2. Amount of phytodetritus related to geographical location of the stations along the coastline. For the geographical position of the abscissa, see Fig. 1.
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tween ; 0.25 mm and ; 0.50 mm depending on the size of the area photographed. The amount of the greenish coloured organic material deposited on the sediment was classified for each station, into six classes: 0%, 1–5%, 6–25%, 26–50%, 51–75%, and 76–100% cover. The middle of each class was used for the data analyses. Areas covered by benthic organisms were not considered. In order to detect geographical trends the stations were aligned along a line parallel to the coast
ŽSouthwestrNortheast gradient. irrespective of their distance from the coast. Other physical criteria considered as possibly responsible for the variation of the deposited organic material were: time of the year, water depth, distance between stations and ice shelf edge, and shelf width in the area of the station. As a first step in the data analysis, these variables were classified into two groups, e.g., shallow and deep or inshore and offshore stations. These groups were compared by a G-test Žanalysis of a contin-
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Fig. 4. Amount of phytodetritus related to date of observation.
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Fig. 5. Amount of phytodetritus related to water depth.
high and low cover of the megabenthos in relation to high and low phytodetritus cover. gency table, Sachs, 1984. with respect to their percentage cover Ž) 25% and - 25%.. The null hypothesis, that both groups originate from one population, must be rejected if G ) 3.841 Žtwo-tailed; a s 0.05.. Secondly, a correlation matrix between all the physical variables was calculated to determine any interdependence between them. In such a case, one of the variables must be excluded from the further analysis to avoid bias. Thirdly, factor analysis was performed to detect any relationships between the physical variables and the phytodetritus cover. When two or more variables load high, indicated by high loading factors on one of the resulting imaginary factors, this indicates a relationship between these variables. Finally, the G-test was used to compare
3. Results Geographic variation along the SWrNE gradient is shown on Fig. 2. Along the entire coastline the phytodetritus cover was highly variable. However, the proportion of stations with a high cover Ž) 25%. of phytodetritus was 44% in the easterly Lazarev Sea compared with only 12% at the remaining stations. However, if the entire area of investigation is divided into two subareas of similar extent, the difference in the phytodetritus cover between them was not significant Ž G s 2.40.. At most stations, the thickness of the phytodetritus layer was estimated to be less than
Fig. 3. Underwater photographs showing phytodetritus and megabenthos cover on the sediment surface. Each photograph represents an area of approximately 1 m2 . Ža. Extremely high and thick phytodetritus cover Ž; 87.5%. and poor epibenthos Ž; 1% coverage.. Visible X X organisms: Reteporella antarctica Žbryozoan., Pomachocrinus kerguelensis Žcrinoid.. ANT IXr3, station 189, 7086.3 S 5812.4 E, 476 m. Žb. Low phytodetritus cover Ž; 3%. and much epibenthos Ž; 47% coverage.. Visible organisms: Isodictya sp. Žyellow sponge., Monosyringa longispina Žtubes of infaunal sponge., Iophon radiatus Žencrusting sponge on an ophiuroid cf. Ophiurolepis gelida., Tedania tantula Žsmall tube-shaped sponge, lower left., Notocidaris sp. Žpencil sea urchin, upper and lower centre., Touarella sp. Žgorgonarian, centre., several Synoicium addreanum Žspherice, transparent compound ascidians., Reteporella antarctica Žbryozoan, lower left., CamptoX X plites tricornis Žbushy bryozoan, centre., several ophiuroids. ANT VIIr4, station 307, 7186.5 S 11839.6 W, 178 m. Žc. Intermediate phytodetritus cover Ž; 15%., few infaunal and epifaunal benthic organisms Ž; 12% cover.. Visible organisms: Echiuroinea sp. Žbright prostomium visible, with patches of phytodetritus, which are transported to the mouth and with a pycnogonid resting on the prostomium, upper right., terebellid polychaete with a stellate tentacular crown spread out on the sediment, two colonies of bryozoans Ž Cellarinella spp.. with associated tube-shaped yellow demosponges., Synoicium addreanum Žcompound ascidian, lower right.. ANT VIIr4, station 277, X X 71839.4 S 12836.1 W, 399 m. Žd. High phytodetritus cover Ž; 62.5%. and much epifauna Ž; 48% coverage.. Visible organisms: Tetilla sp. Žwhite sponge, bottom., Isodictya sp. Žyellow sponge, centre right., Haliclonidae Želongated sponges., Ekmocucumis steineni Žbrownish X X holothurian., Abyssocucumis liouÕillei Žwhite holothurian., several anthozoans and ophiuroids. ANT IXr3, station 173, 7080.3 S 7811.1 E, 194 m.
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Date SWrNE gradient Distance between station and ice shelf edge Shelf width Fig. 6. Amount of phytodetritus related to distance between station and ice shelf edge.
1 mm. At the three stations in the Lazarev Sea with the highest cover this layer was 1–2 cm thick ŽFig. 3a.. This was estimated from the known height of some benthic organisms Že.g., the aspidochirote holothurian Bathyplotes rubipunctatus. which was totally covered by phytodetritus so that only its upper surface was recognisable. Temporal variations are shown by Fig. 4. Regarding interannual differences, high values Ž) 50% coverage. did not occur in 1988 and only at one station in 1989. Values ) 25% were also scarce in 1988 Ž7% of the stations. and most frequent in 1991 Ž33% of the stations.. As regards seasonal variations, highest values Žbetween 76 and 100%. were not found before 26 February, the next lowest class of cover Ž51–75%. not before 6 February. In March, we have relatively few data, but three of the six stations showed high values Ž) 25% cover. compared with 17% for the second half of January and February. No significant difference in the percentage cover was
SWrNE Distance between gradient station and ice shelf edge
0.020 0.407 0.333
0.182 0.143 0.387
0.021
0.246 0.772
0.591
detected Ž G s 0.75. between early and late phases of the entire period of investigation, before and after 14 February. Variations according to water depth are shown by Fig. 5. At the shelf stations Ž- 500 m. the ratio between high Ž) 25% coverage. and lower values was 25:75, on the upper slope it was 12:88. However, this difference was not significant Ž G s 1.53.. The three stations with the extremely high coverage and thick layer of phytodetritus Žnos. 189, 211, 212. were situated over a broad range of relatively deep water Ž476, 693, and 880 m.. Fig. 6 shows the effect of the distance between the station and the ice shelf coast. The stations were classified into those lying closer to and further than 30 km from the ice shelf edge. Despite the fact that only 14% of the offshore stations showed high values and 22% of the near-shore stations had a high cover of phytodetritus, the difference in the percentage cover was not significant Ž G s 0.53.. Shelf width effects are illustrated by Fig. 7. In areas with a broad shelf Ž) 80 km. almost 50% Žfive stations out of 11. showed a high phytodetritus cover whereas this ratio was 21:79 on the narrower shelf. This difference was significant Ž G s 4.19.. Table 2 Loading factors resulting from the factor analysis
Fig. 7. Amount of phytodetritus related to shelf width in the area of the station sampled.
Phytodetritus cover Depth Date SWrNE gradient Distance between station and ice shelf edge
Factor 1
Factor 2
Factor 3
y0.046 0.814 y0.081 y0.717 0.735
0.972 0.058 y0.030 0.347 0.022
y0.041 0.178 0.969 0.208 y0.169
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Fig. 8. Amount of phytodetritus related to the cover of megabenthic organisms on the seafloor.
The correlation matrix of the physical variables is presented in Table 1. The only moderately high correlation coefficients were found between the shelf width and the SouthwestrNortheast gradient Ž r s 0.772., as well as between the shelf width and the distance to the ice shelf edge Ž r s 0.591.. The coefficients for all other combinations were below 0.500. Table 2 shows the results of the factor analysis. The variable ‘shelf width’ was excluded from the analysis because several stations were situated below 500 m. This also solves the problem of having to exclude a variable due to interdependence of variables. The variable ‘phytodetritus cover’ was the only one to have a high loading Ž0.972. on factor 2. All other variables loaded high on the first and third factor, indicating that no clear relationship exists between the occurrence of phytodetritus and the other variables. The relationship between phytodetritus and megabenthos are illustrated by Figs. 3a–d and 8.. Of the 23 stations with a high megabenthic cover Ž) 20%., only three stations had a high abundance Ž26–75% cover. of phytodetritus. The highest abundance class was reached only at stations with a low megabenthic cover. However, the difference was not significant Ž G s 0.72..
4. Discussion The temporal and regional pattern of deposited phytodetritus could be explained only by one of the variables Žshelf width. which were considered to generally affect this phenomenon. Apart from the
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above described trends and relationship, our results show that under each condition Žwater depth, shelf width, season, etc.. highest and lowest phytodetritus cover is possible. This applies to the entire area of investigation as well as to spatially restricted regions; it also applies to the total period of investigation in the austral summer, as well as to temporally closely sampled stations. Therefore, it is also unlikely that there is a direct relationship between the high primary production at the marginal ice zone ŽEl-Sayed and Tagushi, 1981. and the occurrence of phytodetritus on seafloor. This result probably reflects the complexity of the processes which affect the sedimentation of organic particles e.g., melting of sea-ice, primary production which is effected by meteorological processes and grazing of zooplankton, variable sinking rates of the organic particles, horizontal transport by the large- and small-scale near-bottom currents Žvon Bodungen et al., 1985; Schnack, 1985; Nothig and von Bodungen, 1989.. ¨ Fischer et al. Ž1988. found an ‘extreme variability’ of sedimentation rates in the northern Weddell Sea measured by sediment traps. However, the particle flux was the ‘smallest yet observed in the world ocean’. Our results from the southeastern coastal area confirmed the high variability, but we found extremely thick layers of phytodetritus at single stations on the shelf and upper continental slope. The velocity of the coastal current, especially close to the bottom, is of overall relevance to the sedimentation process. However, there are no available high resolution data of the current regime along the coast. Therefore, we used the parameters shelf width and distance the between stations and the ice shelf edge because they were considered to reflect indirectly the current velocity. Above the narrow shelf between 708 and 748S the coastal current is most prominent and dives further south in the Halley Bay area on the much broader shelf ŽCarmack and Foster, 1977; Fahrbach et al., 1992.. The occurrence of more phytodetritus on the broader shelf coincides with few megabenthic filter feeders and relatively more deposit feeders. This supports the hypothesis that the deposit feeders profit most from sedimentation events in combination with low current velocity. On the narrower shelf, the locally high concentrations of filter feeders are better adapted to a more continuous horizontal transport of organic particles because in
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this way they remain in suspension and therefore available for a longer period. In addition, an unknown percentage of the organic particles are removed by suspension feeders such as bryozoans, compound ascidians and sponges before reaching the bottom. This might be an additional explanation for the lower amount of phytodetritus in areas where the filter feeders are abundant. The exceptionally thick layer of phytodetritus at the three stations in the Lazarev Sea can be explained by an intensive storm 9 to 22 days prior to our observation. This resulted in the phytoplankton being mixed over the entire water column ŽBathmann, 1992. before the organic matter sank to the bottom. The general increase of deposited organic material on the seafloor in the first half of February agrees partially with results from sediment traps moored in the southeastern Weddell Sea in summer 1988. These showed a first pulse of sedimentation around 28 January ŽBathmann et al., 1991. and we observed the first high values of phytodetritus on the bottom at the beginning of February. No significant relationship between the phytodetritus cover and the megabenthos was found since both low and high amounts of phytodetritus occur on both the narrow and broad shelf. Although the photographic records were obtained over a period of 5 years and at a high spatial resolution, each photograph represents only a momentary observation providing no information about the origin or fate of the phytodetritus. In contrast, the benthic community pattern reflects an integration of environmental conditions over long periods. This is particularly true since important compartment of the benthic fauna, such as several glass sponges ŽDayton, 1978., which structure to a certain degree the entire system ŽGutt and Starmans, in press. are extremely slow growing ŽBrey and Clarke, 1993.. Our data indicate a less distinct pelagic–benthic coupling, for which the continuous change between ice ages and interglacial periods during the past million years plays an important role. During the ice ages the ice shelf settled on the continental shelf and therefore, these areas were not available as habitat for the benthos. If, during these periods, the benthos lived only on the upper continental slope or in locally restricted areas on the shelf such as inner shelf depressions, the food availability must have been much lower than today due to the almost
permanent sea-ice cover ŽCooke and Hays, 1982.. In this case, the benthos, probably with a much lower abundance and reduced numbers of species, must have been adapted to these conditions. Thus, the same species today live under an excess food availability in austral summer. Similar food conditions for the benthos as during the ice ages might today exist several hundred kilometres under the ice shelf e.g., in the Ross Sea where Dayton Žpersonal communication. photographed aggregations of sponges. This hypothesis is also supported by the relatively high number of the few investigated benthic species which are partly or totally uncoupled from the phase of primary production ŽWhite, 1977; Picken, 1980; Gutt, 1991c; Gutt et al., 1992, Barnes and Clarke, 1995.. If a considerable part of the recent high Antarctic benthos lived further north during the ice ages, the present patchy benthic dispersion pattern may be explained less by the poorly predictable food supply than by a still continuing recolonisation of the shelf. The 12,000 years which have elapsed since the last ice age might not suffice to attain a large scale mature benthic community because many species concerned grow slowly and have very short-lived or non-existing pelagic larval stages and some also reproduce by budding. Based on our results, it seems to be most likely that a pelagic–benthic coupling exists, which, however, is weak due to the long-term development of the ecosystem.
Acknowledgements Thanks are due to two unknown referees. Publication no. 1050 of the Alfred Wegener Institute.
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