Brittle star fauna (Echinodermata: Ophiuroidea) of the arctic northwestern Barents sea: composition,abundance, biomass and spatial distribution

 Epibenthic brittle star assemblages were investigated on the northwestern Barents Sea shelf between 81° and 77°N in July 1991. At 9 drift stations in water depths between 80 and 360 m, series of 35–71 photographs, each depicting about 1 m² of the seabed, were taken along transects of about 150- to 300-m length to assess abundances and spatial distribution patterns of adult brittle stars (disc diameter ≥1 mm). Biomass values were derived by combining abundances with size-weight relationships and size frequencies established using specimens from trawl catches. Six brittle star species were identified on the seabed images. Ophiocten sericeum was the most abundant species on shallow shelf banks (≤100 m). Up to 2,800 individuals were counted on a single photograph; median abundances per station ranged from 32 to 524 ind.m^-2 and biomass from 0.3 to 5.0 g ash-free dry weight (AFDW) m^-2. The spatial distribution along the transects (i.e. on the 100-m scale) was, however, extremely patchy. Disc diameters of O. sericeum ranged between 1.6 mm and 15.4 mm. In deeper shelf habitats (>150 m), O. sericeum was rare or absent, and Ophiacantha bidentata dominated the brittle star fauna with median densities and biomasses of 2–49 ind.m^-2 and 0.07–1.9 g AFDW m^-2, respectively. Its disc diameters ranged from 2.9 to 14.4 mm. The other species (Ophiura sarsi, Ophiopholis aculeata, Ophioscolex glacialis, Ophiopleura borealis) occurred in distinctly lower numbers. Our findings provide further evidence that brittle stars dominate epibenthic communities on Arctic shelves and locally reach very high abundances. Dense beds of Ophiocten sericeum seem to be a general phenomenon on high-Arctic shallow shelf banks. Received: 30 March 1995/Accepted: 30 June 1995     

黄河口邻近海域浮游动物群落时空变化特征

利用2012年12月—2013年9月4个季度的现场调查资料研究了黄河口邻近海域浮游动物群落的时空分布特征。研究表明,黄河口邻近海域共鉴定出浮游动物70种,包括浮游幼虫19类。浮游动物优势种主要由夜光虫(Noctiluca scintillans)、小拟哲水蚤(Paracalanus parvus)、双刺纺锤水蚤(Acartia bifilosa)、拟长腹剑水蚤(Oithona similis)、强额拟哲水蚤(Paracalanus crassirostris)、近缘大眼剑水蚤(Corycaeus affinis)、强壮箭虫(Sagitta crassa)、双壳类幼体(Bivalvia larvae)、多毛类幼体(Polychaeta larvae)等种类。黄河口邻近海域浮游动物丰度夏季最高(60620个/m~3),春季(31228个/m~3)和秋季(21540个/m~3)次之,冬季最低(7594个/m~3)。不同季节浮游动物丰度的空间分布具有差异性,春季浮游动物丰度呈现出从近岸到外海降低的趋势;夏季浮游动物形成两个高丰度区,分别位于河口邻近海区和河口东部海区;秋季和冬季浮游动物丰度高值区均位于河口东部海区。浮游动物生物多样性指数均呈现从河口到外海升高的趋势,低值区位于黄河口入海口附近海区。相关性分析显示,黄河口邻近海域浮游动物丰度与海水温度显著正相关(r=0.212,P0.05),表明温度为影响黄河口邻近海域浮游动物丰度变化的主要因素。     

Crustacean assemblage and environmental characteristics of a man-made solar saltwork in southern France,with emphasis on anostracan (Branchiopoda) population dynamics

Physical and chemical variables, anostracan populations (Artemia parthenogenetica and Branchinella spinosa) and other biota were studied during 1996–1997 in a Camargue saltern (max. depth 1 m). The taxonomic composition and density of macroinvertebrates were investigated twice monthly, based on benthic substrate and water column samples. Fauna was composed of three groups in terms of numerical importance. The benthic macroinvertebrates were represented only by nematodes (< 50 ind. m^–2 to > 500 ind. m^–2 in November–December and May respectively). The zooplankton was dominated by crustaceans, one cladoceran, Moina salina (ranging from 670 to 2350 ind. m^–2 in spring), two anostracans, Artemia parthenogenetica (< 50 ind. m^–2 in autumn), and Branchinella spinosa (max. 190 ind. m^–2 in December to absent in April), and two copepods, Cletocamptus retrogressus (max. density 2000 ind. m^–2 in November), and Eurytemora velox (max. density 650 ind. m^–2 in February–March). Insects (Chironomidae, Culicidae) were rare, with mean densities < 1 ind. m^–2. The phenology of each crustacean population is discussed in relation to physical and chemical water variables. Salinity appeared to be of greatest importance regulating the population abundance.     

The genus Sterechinm (Echinodermata: Echinoidea) on the Weddell Sea shelf and slope (Antarctica): distribution,abundance and biomass

Summary Two species of the echinoid genus Sterechinus were documented from 92 trawl stations and 55 photographic stations in the eastern and southern Weddell Sea between 100 and 1200 m water depth. We found two species occuring along the whole shelf and slope, S. neumayeri being more abundant above 450 m water depth and S. antarcticus dominating the deeper regions. The size-frequency distributions of both species indicate differences in growth, mortality and longevity. First estimates of abundance and biomass of S. neumayeri and S. antarcticus are 0.085 ind/m² & 0.005 gAFDW/m² and 0.022 ind/m² & 0.005 gAFDW/m², respectively.AWI Publication No. 392     

Population dynamics of alien gammarid species in the River Odra estuary

During the period of 1988–2000, three alien gammarid species, i.e. Gammarus tigrinus Sexton, 1939, Pontogammarus robustoides G. O. Sars, 1894, and Dikerogammarus haemobaphes (Eichwald, 1841) were recorded in the Odra estuary. This study, the first record of G. tigrinus and P. robustoides in Poland, focuses on the distribution, abundance, and biomass as well as on population structure of the three new species found in the coastal zone of the estuary. G. tigrinus and P. robustoides were first sampled in the Szczecin Lagoon (mid-part of the estuary) in 1988. The densities of both species were high from the time they were first recorded and exceeded 10 000 ind. m^–2, while the biomass was 25 and 140 g m^–2, respectively. G.tigrinus occurred at higher densities in northern and eastern parts of the Lagoon, while P. robustoides was particularly abundant in the south-eastern part, most heavily affected by River Odra discharge. In 1999, the third gammarid, D. haemobaphes, was recorded in the southern part of the estuary. Both in 1999 and 2000, the species was found in qualitative samples (dredge collections of Dreissena polymorpha) taken from the western branch of the river. As estimated from quantitative samples, densities and biomass in the eastern branch were rather low and did not exceed 150 ind. m^–2 and 2.7 g m^–2, respectively. The Szczecin Lagoon population of G. tigrinus produces at least two generations within a year: a summer one and an overwintering one. The latter is dominated by older and larger specimens. The species may reproduce in the Lagoon from April until November.     

Vertical Distribution of Zooplankton in a Strongly Stratified Hypertrophic Lake

The vertical and temporal distribution of metazooplankton in the small hypertrophic, strongly stratified, temperate Lake Verevi (Estonia) was studied during 1998–2001. The zooplankton of Lake Verevi is characteristic of hypertrophic lakes, with a small number of dominant species, rotifers being the main ones, and juveniles prevailing among copepods. In 1999–2001, the average abundance of metazooplankton in the lake was 1570 × 10³ ind m⁻³; in the epilimnion 2320 × 10³ ind m⁻³, in the metalimnion 2178 × 10³ ind m⁻³, and in the hypolimnion 237 × 10³ ind m⁻³. The average biomass of metazooplankton was 1.75 g m⁻³; in the epi-, meta- and hypolimnion, accordingly, 2.16, 2.85 and 0.26 g m⁻³. The highest abundances – 19,136 × 10³ ind m⁻³ and 12,008 × 10³ ind m⁻³ – were registered in the lower half of the metalimnion in 24 May and 5 June 2001, respectively. Rotifer Keratella cochlearis f. typica (Gosse, 1851) was the dominating species in abundance. In biomass, Asplanchna priodonta Gosse, 1850, among the rotifers, and Eudiaptomus graciloides (Lilljeborg, 1888), among the copepods, dominated. According to the data from 2000–2001, the abundance and biomass of both copepods and rotifers were highest in spring. Zooplankton was scarce in the hypolimnion, and no peaks were observed there. During the summers of 1998 and 1999, when thermal stratification was particularly strong, zooplankton was the most abundant in the upper half of the metalimnion, and a distinct peak of biomass occurred in the second fourth of the metalimnion. Probably, the main factors affecting the vertical distribution of zooplankton in L. Verevi are fish, Chaoborus larvae, and chemocline, while food, like phytoplankton, composition and abundance may affect more the seasonal development of zooplankton.     

The benthos off King George Island (South Shetland Islands,Antarctica): further evidence for a lack of a latitudinal biomass cline in the Southern Ocean

The benthic fauna off King George Island (South Shetland Islands, Antarctica) was investigated during "Polarstern" expedition ANT XV/3 in March 1998. Samples were taken along two cross-shelf/slope transects both north (Drake Passage) and south of the island (Bransfield Strait, off Potter Cove) at water depths ranging from 130 m to 2,000 m. For a quantitative inventory, a multibox corer was used at nine stations to collect mostly infaunal macrobenthos; at seven stations, seabed photography was employed concomitantly to survey the epibenthic megafauna. Macrofauna abundances ranged from 730 ind. m^–2 at 2,000 m to >14,000 ind. m^–2 at 100 m; biomass values varied between about 50 g wet mass m^–2 (6 g ash-free dry mass m^–2) at 2,000 m and about 950 g wet mass m^–2 (about 90 g ash-free dry mass m^–2) at 200 m. Densities were dominated everywhere by polychaetes, followed by bivalves, crustaceans and ophiuroids; in terms of biomass, krill and holothurians surpassed polychaetes at some stations. No significant differences between the northern and the southern transects in total abundance and biomass were obvious. Megafauna abundances were clearly higher south of King George Island, totalling about 110–150 ind. m^–2 on the shelf (235–330 m) and about 50 ind. m^–2 at the continental slope (750 m), whereas along the northern transect they reached values of only 21–31 ind. m^–2 on the shelf (130–430 m) and decreased at the continental slope (950 m) to about 5 ind. m^–2. A brittle star, Ophionotus victoriae, strongly dominated the southern-shelf epibenthos, with relative abundances of 70–95% and a biomass of about 40–80 g wet mass m^–2 (about 4–7 g ash-free dry mass m^–2), but was numerically less important at the slope (5%) where ammotheid pycnogonids prevailed (80%). Macro- and megabenthos distribution patterns were characterized by a pronounced shelf-slope gradient – in standing stock as well as in faunistic composition – but this resemblance was statistically not significant. This finding indicates that the spatial distributions of macrobenthos and megabenthos are primarily determined by a depth-dependent factor, most probably food supply, but apparently respond differently to secondary driving forces, possibly seabed features. Our results provide further evidence for the notion that there is no distinct latitudinal gradient in benthic abundance and biomass in the Southern Ocean between the South American Magellan region and high-Antarctic waters of the Weddell Sea.     

Living conditions, abundance and biomass of under-ice fauna in the Storfjord area (western Barents Sea, Arctic) in late winter (March 2003)

The under-ice habitat and fauna were studied during a typical winter situation at three stations in the western Barents Sea. Dense pack ice (7–10/10) prevailed and ice thickness ranged over <0.1–1.6 m covered by <0.1–0.6 m of snow. Air temperatures ranged between –1.8 and –27.5°C. The ice undersides were level, white and smooth. Temperature and salinity profiles in the under-ice water (0–5 m depth) were not stratified (T=–1.9 to –2.0°C and S=34.2–34.7). Concentrations of inorganic nutrients were high and concentrations of algal pigments were very low (0.02 g chlorophyll a l^–1), indicating the state of biological winter. Contents of particulate organic carbon and nitrogen ranged over 84.2–241.3 and 5.3–16.4 g l^–1, respectively, the C/N ratio over 11.2–15.5 pointing to the dominance of detritus in the under-ice water. Abundances of amphipods at the ice underside were lower than in other seasons: 0–1.8 ind. m^–2 for Apherusa glacialis, 0–0.7 ind. m^–2 for Onisimus spp., and 0–0.8 ind. m^–2 for Gammarus wilkitzkii. A total of 22 metazoan taxa were found in the under-ice water, with copepods as the most diverse and numerous group. Total abundances ranged over 181–2,487 ind. m^–3 (biomass: 70–2,439 g C m^–3), showing lower values than in spring, summer and autumn. The dominant species was the calanoid copepod Pseudocalanus minutus (34–1,485 ind. m^–3), contributing 19–65% to total abundances, followed by copepod nauplii (85–548 ind. m^–3) and the cyclopoid copepod Oithona similis (44–262 ind. m^–3). Sympagic (ice-associated) organisms occurred only rarely in the under-ice water layer.     

Life histories of the copepods Pseudocalanus minutus, P. acuspes (Calanoida) and Oithona similis (Cyclopoida) in the Arctic Kongsfjorden (Svalbard)

The year-round variation in abundance and stage-specific (vertical) distribution of Pseudocalanus minutus and Oithona similis was studied in the Arctic Kongsfjorden, Svalbard. Maxima of vertically integrated abundance were found in November with 111,297 ind m⁻² for P. minutus and 704,633 ind m⁻² for O. similis. Minimum abundances comprised 1,088 ind m⁻² and 4,483 ind m⁻² in June for P. minutus and O. similis, respectively. The congener P. acuspes only occurred in low numbers (15–213 ind m⁻²), and successful reproduction was debatable. Reproduction of P. minutus took place in May/June, and stage distribution revealed a 1-year life cycle with copepodids CIII, CIV, and CV as the overwintering stages. Oithona similis exhibited two main reproductive peaks in June and August/September, respectively. Moreover, it reproduced more or less continuously throughout the whole year with all stages occurring during the entire sampling period, suggesting two generations per year. Both species migrated towards greater depth in November, but O. similis preferred to stay longer in the upper 100 m as compared to Pseudocalanus. The reproduction of the two species in Kongsfjorden seemed to be linked to phytoplankton dynamics.     

Macrozoobenthos of Lake Peipsi-Pihkva: taxonomical composition, abundance, biomass, and their relations to some ecological parameters

The large but shallow (3,558 km², up to 15.3 m deep) lake is eutrophic, with Chironomus plumosus and Potamothrix hammoniensis as dominating macroinvertebrates in the profundal. The extensive well-aerated sublittoral with sandy bottom sediments has a mesotrophic appearance and supports a diverse fauna with several oxyphilous species, including a very abundant population of Dreissena polymorpha. The phytophilous fauna is limited to small sheltered areas only. The average abundance of the small animals of macrozoobenthos (without big molluscs) was 2,617 ind. m^–2, their biomass 12.34 g m^–2 (corresponding to 52.2 kJ m^–2) in 1964–1994. The same figures for big molluscs (mostly Dreissena) were 304 ind. m^–2 and 238 g m^–2 in 1964–1994, and even 864 ind. m^–2 and 687 g m^–2 in 1985–1988, at the time of their special mapping. The sublittoral zone revealed the lowest biomass of small animals but the highest biomass of big molluscs. The southern, shallower lake regions, more enriched with nutrients and better protected from wind, revealed a significantly higher biomass of small macrozoobenthos in the near-shore zone than the cleaner and open northern part, while no positive effect of enrichment was observed neither in the biomass of profundal zoobenthos nor in that of big molluscs. The production of the small macrozoobenthos was calculated as 111 and 53 kJ m^–2 during two annual cycles in Lake Peipsi s. s., the most productive period being the autumn overturn. Lake Peipsi-Pihkva has the highest abundance and biomass of macrozoobenthos among the large lakes of North Europe.     

Impact of long-term alternations of discharge and spate on the chironomid community in the lowland Widawka River (Central Poland)

Spatial and temporal distribution, abundance and production of macroinvertebrate communities were estimated over two years in a fifth-order section of the Widawka River. Discharge of this river has been increased artificially by coal mine water inputs. Additionally, during the second year, one of the highest discharges of the current 20-year period was recorded. Chironomidae were co-dominant in macrobenthos, both in a straight reach (WIA) and in a meandering site (WIB). More mosaic habitats resulted in higher densities of midges, reaching 6215 ind.m^–2 in year 1 and 1141 ind.m^–2 in year 2 at WIA, while at WIB 896 densities were ind.m^–2 and 257 ind.m^–2, respectively. Flooding affected the distribution and abundance of the chironomid assemblages. Recolonization by psammophilous Polypedilum began after the various microhabitats were buried with sand. Chironomid production was estimated on a species-specific basis for the dominant taxa. In year 1 (mean annual water temperature 10.0° C) chironomid production was 12.4 g dry wt m^–2 yr^–1 1 at WIA and 1.9 g dry wt m^–2 yr^–1 at WIB. These values sharply decreased in year 2 (mean annual water temperature 9.8° C) reaching 1.9 g dry wt m^–2 yr^–1 at WIA and 0.4 g dry wt m^–2 yr^–1 at WIB, as effects of the high spate.     

Abundance and spatial distribution of Artemia salina in Lake Abert,Oregon

In 1981–1982 Abert Lake had an area of 200 km², with a mean depth of 2.5 m and a total dissolved salt concentration of 82 gl^-1. The spatial distribution and abundance of the brine shrimp, Artemia salina, were monitored for 1981–1982. In 1981 during the midsummer months, with sampling primarily in the eastern side of Abert Lake, brine shrimp populations showed peak densities of 2–4 ind. 1^-1. In 1982, peak abundance occurred in early July and ranged between 5–8 ind. 1^-1. Lakewide estimates of brine shrimp derived from 14 collecting stations and assuming a uniform distribution over the lake resulted in estimates of lakewide abundance of 3 × 10¹¹ adults with a total biomass of 7 × 10⁶ kg. The major algal species in the lake was a benthic filamentous green alga (Ctenocladus circinnatus).     

Spatial distribution of Pleuragramma antarcticum (Pisces: Nototheniidae) near the Filchner- and Larsen ice shelves (Weddell sea/antarctica)

Summary In the zooplankton collected during three German Antarctic expeditions to the southern Weddell Sea in 1979/80, 1980/81 and 1983, the post-larvae of the nototheniid fish Pleuragramma antarcticum were found in 71–94% of the samples and represented 85–98% in numbers of all fish caught. In 1983, the abundance of post-larvae was up to 88 ind./m² (corresponding to 3 ind./m³). Highest concentrations were generally found over the continental slope and innershelf depressions. More than 70% of the post-larvae were caught in the upper 50–100 m water layer, in the well stratified Summer Water of -1.3 to -0.5°C. The Summer Water is shifted towards the outer edge of the shelf by Ekman transport and accumulation of post-larvae in the slope front and eddies can be explained by this drift. Older Pleuragramma antarcticum of age 1 resemble in their vertical distribution the juveniles and adults, 40–60% of which were caught on the shallower parts of the shelf in cold Ice Shelf Water of -1.8 to -2.1°C in depths below 200 m. Mean abundances of the yearclasses varied by a factor of 16 in age 0 fish and by factor of 10 in age 1. The size of a yearclass may be related to the varying appearance and persistence of an ice free coastal polynya.     

Zooplankton distribution in four ponds of different salinity: a seasonal study in the solar salterns of Sfax (Tunisia)

The seasonal distribution of metazooplankton and large-sized ciliates was studied in four ponds of different salinity in the solar salterns of Sfax (Tunisia). Total zooplankton abundance varied from 1 × 10³ to 4.7 × 10⁶ ind m^–3. Salinity had a negative effect on the abundance of copepods and rotifers which were absent in the pond with the highest salt concentration (180) in which the number of taxa was low and Artemia or the ciliate Fabrea largely dominated the zooplankton community. Temperature and the presence of Dunaliella salina as prey appeared as key factors in controlling the abundance of Artemia, while organic detritus appeared as important in the diet of Fabrea. Change in zooplankton species composition along the hypersaline gradient (40–90) was primarily related to salinity. However, our data suggest the importance of both the abundance and composition of food in the spatial and temporal variations of some zooplankton species.     

The drift of zooplankton and microzoobenthos in the river Strandaelva,western Norway

The drift of zooplankton (rotifers, cladocerans, cyclopoid copepods) and microscopical zoobenthos (mainly bdelloid rotifers and small chironomid larvae) was investigated by filtering samples of river water. The number of drifting benthic rotifers varied between 1 000 and 6 000 ind. m^–3 in the lake inlet, and between 30 and 500 ind. m^–3 in the lake outlet, without any seasonal trend. The number of drifting insect larvae was approx. equal in the lake inlet and outlet, with a maximum in summer (250–300 ind. m^–3) and minimum in winter (ca. 10 ind. m^–3). Increasing water flow resulted in an increasing number of drifting zoobenthos. Downstream from the lake, the number of drifting benthic rotifers was increasing from approx. 300 ind. m^–3 in the outlet to 6 500 ind. m^–3 3.4 km downstream, while the number of insect larvae was ca. 100 ind. m^–3 in the outlet and leveled off at approx. 300 ind. m^–3 after 200 m. The number of drifting zooplankton in the lake outlet varied between 20 and 2 000 ind. m^–3 for crustaceans, and between 300 and 20 000 ind. m^–3 for rotifers, both with a maximum in late summer/autumn and a minimum in winter. The number of drifting zooplankton decreased by some 45% in the first 200 m from the lake outlet, but some zooplankton was still found in the drift 3.4 km downstream. The largest species was removed first from the drift. The diurnal variation in the number of drifting zooplankton in lake outlets appear to be related to the vertical migration in the lake, i.e. the largest number drifting when most animals are in the upper water layers.Contribution from the Voss Project, University of OsloContribution from the Voss Project, University of Oslo     

Summer Weddell sea microplankton: Assemblage structure,distribution and abundance,with special emphasis on the Tintinnina

Summary Silicoflagellates, large thecate dinoflagellates and tintinnids were counted and measured in screened (26 m pore-size) pump-samples retrieved from a depth of ca. 9 m in February-March 1987 along a two-leg north-south transect in the Weddell Sea (approx. 62°S to 78°S). Fourteen tintinnid taxa were identified and their individual abundances and biomasses were estimated. Highest biomass, in terms of organic carbon, was recorded for the Tintinnina, which averaged 4.001 mg C m^-3, with mean cell-numbers of 161 ind. l^-1 (maximum: 859 ind. l^-1); followed by the dinoflagellates (1.018 mg C m^-3, mean: 192 ind. l^-1; max.: 1176 ind. l^-1); and the silicoflagellates (0.391 mg C m^-3, mean: 467 ind. l^-1, max.: 3123 ind. l^-1). Conspicuous abundance and settling volume peaks were recorded at some distance off the edge of the ice-pack (at approx. 69°S to 72°S), and in the ice-covered area south of 74–75°S. This pattern was paralleled by changes in the specific makeup of tintinnid assemblages: in the ice-covered southern area Cymatocylis drygalskii and Laackmanniella prolongata were dominant, while in ice-free waters north of 73°S Codonellopsis gaussi, Cd. glacialis and Cymatocylis affinis/convallaria were the main components of the fauna. Overall tintinnid abundances were higher than those reported for many (but not all) extrapolar areas, and the average size of the species present was considerably larger than elsewhere; tintinnid carbon figures were therefore very high, oscillating around 2 mg C m^-3 in the northern ice-free area, and 20 mg C m^-3 in the southern ice-covered zone.     

Oligochaete response to changes in water flow in the Dokka Delta,Lake Randsfjorden (Norway), caused by hydroelectric power development

The results reported here are part of a joint study on possible ecological effects in the delta area of Lake Randsfjorden, Southern Norway, caused by a reduction in water flow as a result of hydroelectric power development.From 1988 to 1990 the mean daily water flow decreased sharply and the total load of suspended matter was greatly reduced. During the same period there was an increase in oligochaete abundance from 6,096 ind. m^–2 in 1988 to 100,051 ind. m^–2 in 1989 and 154,345 ind. m^–2 in 1990. Piguetiella blanci was the dominant species in 1988 making up 48.5% of the total community. In 1989, Limnodrilus hoffmeisteri, Nais variabilis/Nais communis and enchytraeids were the most abundant oligochaetes, making up 29.3%, 26.1% and 28.2%, respectively of the total number. Reduced water flow from 1988 to 1989 and 1990 caused reduced disturbance of the bottom sediments and is considered the main reason for the change in species composition and abundances.     

In-situ investigations on the echinodermAsterias rubens as a predator of soft-bottom communities in the western Baltic Sea

In-situ investigations on the life of the common sea star (Asterias rubens L.) were carried out in 1976, employing the Underwater Laboratory Helgoland in Lübeck Bay (Western Baltic Sea). The abundance ofA. rubens amounted to 2–31 m^–2 on sediment (fine sand), and to 324–809 m^–2 on mobile algal carpets drifting over the bottom. Actual population parameters (abundance, size class distribution) are influenced by both substrate quality and drifting. Stomach investigations revealed prey-size selectivity: Small sea stars feed mainly on the snailHydrobia ulvae when living on the sediment, but on mussel brood(Mytilus edulis) in the phytal. The principal food items of larger sea stars are the sand-dwelling clamMacoma baltica and the phytal-living isopodIdotea baltica respectively.A. rubens is very adaptive to the food available; the diversity of its diet corresponds to the species diversity found in its environment. A change of biotope during active or passive migrations causes switching. The sea star is able to catch motile animals and to dig out infaunal clams. It exhibits a diurnal feeding pattern related to light periodicity; the activity decreases at night. The average frequency of feeding is highly dependent on predator body size; it declines with growth. In-situ experiments indicate an exponential relationship between the feeding duration uponM. baltica and the quotient of clam size to logarithm of sea-star size. An approach is made toward a rough estimate of macrofauna consumption byA. rubens on sediment. The sea star seems to be an important predator and thus a competitor of demersal fishes on soft bottoms of the western Baltic Sea.     

Distribution and production of chironomids (Diptera: Chironomidae) in a shallow, alkaline lake (Neusiedler See, Austria)

From July 1990 to July 1991 the benthic community of the open water zone of Neusiedler See, one of the largest shallow lakes in central Europe, was studied with special reference to the chironomids. Only 16 spp. of chironomids inhabited the sediment of the open water zone. The numerically dominant species were Tanypus punctipennis, Procladius cf. choreus, Microchironomus tener and Cladotanytarsus gr. mancus. Most invertebrates showed a distinct horizontal distribution. Species richness and abundance were highest on muddy and organically rich substrates near the reed belt. Chironomid densities in this area reached 54,000 ind m^–2 and biomass was 2.0 g dw m^–2. The two tanypod species accounted for more than 90% of the standing stock of the macrozoobenthos near the reed belt. The sediment of the open lake and of the eastern part of Neusiedler See was composed of compact clay and sand as a result of the erosion of fine material due to strong waves and currents. Individual densities in these areas were much lower. Production of the numerically dominant species T. punctipennis was estimated using the increment-summation method, whereas production of the remaining species was estimated using an empirically derived multiple regression. Mean annual production of chironomids exceeded 6 g dw m^–2 yr^–1 near the reed belt, but it reached only 0.55 g dw m^–2 yr^–1 in the open lake. These values are rather low compared with other lakes and can be explained by unfavourable sediment conditions due to wave action and by physiological stress due to the water chemistry.     

On the distribution patterns and population dynamics of sublittoral and profundal oligochaeta fauna from Lake Banyoles (Catalonia,NE Spain)

Oligochaeta assemblages from Lake Banyoles were composed of one species of Naididae and eight species of Tubificidae, Potamothrix heuscheri (8–1350 ind. m^–2) being the most abundant and widely distributed species, followed by Limnodrilus hoffmeisteri (25–858 ind. m^–2) and Psammoryctides barbatus (12–792 ind. m^–2). The remaining species were Potamothrix hammoniensis, Potamothrix bavaricus, Limnodrilus claparedeanus, Branchiura sowerbyi, Aulodrilus pigueti and Dero digitata, all of which occurred at lower densities and with restricted distributions.Regardless of sampling depth, maximum densities of oligochaetes were found in winter (up to 5,142 ind. m^–2 in December) and secondarily in June. Immature Tubificidae with hair setae and mature P. heuscheri were present and dominant throughout the year. Cestode parasites were recorded infesting L. hoffmeisteri and P. heuscheri populations.Oligochaeta densities increased with depth, but low oxygen concentrations (less than 1 mg l^–1) during a long period (4 months) in some basins of the lake acted as a key factor in reducing the density of worms and the species richness to one species, Potamothrix heuscheri.