Spatial distribution of bacterivorous nematodes in a Chinese Ecosystem Research Network (CERN) site

Geostatistics was applied to assess the spatial distribution of bacterivorous nematodes in a Chinese Ecosystem Research Network (CERN) site in northeast China. A 42×30 m plot was divided into grids with 7×5 m spacing that included 49 sampling points. Soil samples were collected from depths of 0–10 cm and 10–20 cm. The results showed that Cephalobus and Panagrolaimus were the most dominant bacterivores, which together represented 74 and 82% of total bacterivorous nematodes on average at depths of 0–10 cm and 10–20 cm, respectively. Within-site variability was high for these nematode genera. Geostatistical analysis indicated that a major part of this variability was spatially dependent; variograms suggest that 55–100% of sample population variance was related to spatial autocorrelation over ranges of 9–91 m. The mean numbers of total bacterivores and Panagrolaimus populations were higher at 0–10 cm depth than at 10–20 cm depth. Between the two depths, different distribution patterns were found for Panagrolaimus, whereas similar patterns were found for Cephalobus. The maps obtained by kriging well illustrated horizontal distribution patterns of bacterivorous nematodes at two depths at field scale.     

Spawning of hermatypic corals in Bermuda: a pilot study

This study investigates spawning of 4 hermatypic coral species from the subtropical environment of Bermuda. Laboratory evidence of spawning behaviour is supported by synchronous field observations. Development of scleractinian planulae to postlarval stages is recorded. Diploria strigosa, D. labyrinthiformis, Montastrea annularis and M. cavernosa shed highly buoyant, pigmented eggs (300–440 µm diam.) during July to September 1986. Brief spawning periods, synchronous between conspecific colonies, were recorded for M. annularis (July and August) and M. cavernosa (August) within 1 d of the last quarter of the lunar cycle. In August, there were overlaps amongst the spawning dates of D. strigosa and the Montastrea species. Nocturnal spawning periods differed between M. annularis and M. cavernosa. This constitutes the first evidence from an Atlantic community of overlapping spawning dates amongst several faviid species, and of the accumulation of scleractinian eggs and planulae in surface slicks.     

Spatial patterns and ecology of benthic communities on a high-latitude South Florida (Broward County,USA) reef system

High-latitude reef communities consisting of typical Caribbean fauna of variable composition and density exist on four parallel ridges at varying depths along the Broward County (Florida, USA) coast. Two of these ridges, at 7–13 and 15–30 m in depth, are drowned early Holocene coral reefs of 5 and 7 ky uncorrected radiocarbon age. In this study, community data were collected on each reef using 50-m, line-intercept transects. Using multivariate non-metric statistics, the data show distinct differences in benthic community structure across several spatial gradients. Diversity indices revealed that while all values were low, species diversity (H) was consistent throughout the county, and species richness (d) and evenness (J) increased along a north-south gradient. Scleractinian coral (27 species) cover was low (<6%) in all areas, and Montastrea cavernosa dominated as the major hermatypic scleractinian. Notably absent was the major Caribbean reef-builder Acropora palmata. A rich alcyonacean fauna was present on all studied reefs, and these were typically the most important faunal group determining community structure. Although during the 2-year period 2000–2002 mean water temperatures never fell below 20 °C, scleractinia were small in size.     

Mitotic index and size of symbiotic algae in Caribbean Reef corals

Size and frequency of division were determined for zooxanthellae from nine scleractinian coral species collected in February, 1983 at Discovery Bay, Jamaica, from four depths over a 51 m bathymetric range. Mean diameters of zooxanthellae ranged from 6.4 to 12.6 m. Small zooxanthellae were found in Madracis mirabilis, Eusmilia fastigiata and Dendrogyra cylindrus whereas larger cells were seen in Agaricia agaricites, Porites astreoides, Montastrea cavernosa and Acropora cervicornis. Zooxanthellae division was not phased over a diel cycle. The percentage of zooxanthellae in a paired stage of cytokinesis (mitotic index or MI) was highly variable and ranged from 1.1% to 14.1%. Values measured in E. fastigiata and D. cylindrus were greater than in the other corals. MI was higher in branch tips of A. cervicornis than in branch bases. Daily average MI was negatively correlated with mean cell diameter and for a majority of the coral species increased with habitat depth. MI values were used to estimate specific growth rates and generation times for zooxanthellae in vivo for comparison with other dinoflagellates.     

The diversity,biomass and production of zooplankton in Lake Inarijärvi

About 650 zooplankton samples were collected from Lake Inarijärvi in 1977–1979 from the littoral and pelagial zones of the lake. One hundred and twenty-three zooplankton taxa were found and most of them can be considered euplanktonic.The most important species were Holopedium gibberum, Daphnia cristata, Cyclops spp. and Eudiaptomus spp. Mean pelagial zooplankton biomass was 0.29 g m^–3 in the 0–5 m depth zone, 0.17 g m^–3 in 5–10 m and 0.11 g m^–3 in 10–20 m.The zooplankton biomass at a sandy shore was about 0.09 g m^–3, at a stony shore 0.05 g m^–3 and at a vegetated shore 0.76 g m^–3. About 70% of the whole zooplankton production consisted of crustaceans.The sum of herbivore and carnivore zooplankton production in the pelagial area during the summer was 210–330 kg ha^–1 × 3 months.     

Variation in coral growth rates with depth at Discovery Bay,Jamaica

Growth rates, determined by X-radiographic measurement of skeletal extension, decreased with depth for four of six species of coral examined at Discovery Bay, Jamaica. Growth of Porites astreoides, Montastrea annularis, Colpophyllia natans, and Siderastrea siderea decreased significantly with depth over a 1- to 30-m depth range. In Montastrea cavernosa, the highest growth rate occurred in the middle of the sampled depth range. Agaricia agaricites had no measurable change in growth rate with depth. A compilation of available growth data for Atlantic and Pacific corals shows a strong pattern of highest growth rates a short distance below the surface and a decrease with depth.Operated by Martin Marietta Energy Systems, Inc., under Contract No. DE-AC05-840R21400 with the U.S. Department of Energy. Publication No. 2467, Environmental Sciences Division, ORNL     

Abundance,population dynamics and production of Chironomidae (Diptera) in an ul traoligotrophic lake in South Greenland

Macrozoobenthos of the ultraoligotrophic Lake 95 (61°N, 46°W, 8 ha, z_max=18 m, ) is composed of about 14 taxa dominated by 12 Chironomidae species. Abundance, life cycle, biomass and production were estimated for the six dominant taxa. Abundance declined fromca. 4150 at 2.5 m depth toca. 1400 ind m^–2 at 16 m depth and averagedca. 3200 ind m^–2 on a lakewide basis. By numbers,Heterotrissocladius changi andH. oliveri dominated the average fauna.H. changi was common at the 2.5 m and 5 m depth stations, whereasH. oliveri dominated from 5 m depth downwards. Chironomids showed mainly a 1-yr life cycle, but apparently bothHeterotrissocladius species had two contemporary cohorts with emergence in midsummer and late autumn/early spring, respectively. Average annual ratio was 4.2 and 4.6 forH. oliveri andH. changi, respectively. Annual production varied from 0.3 g ash-free dry weight (AFDW) m^–2 y^–1 at 16 m depth to 1.6 g AFDW m^–2 y^–1 at 2.5 m depthH. changi contributed 45%, fiveMicropsectra spp. 17% andH. oliveri 15% to total average production, which on a lakewide basis wasca. 1.1 g AFDW or 25 kJ m^–2 y^–1. Lake 95 thus belongs at the very low end of measured lake zoobenthic productions, which range from 10 kJ m^–2 y^–1 in Arctic lakes toca. 1600 kJ m^–2 y^–1 in highly eutrophic shallow lakes.     

Vertical distributions and relations of euphausiid populations off Elephant Island,March 1984

Summary Distributional relationships are described for post-larval and larval Euphausia superba and Thysanoessa sp. (probably macrura) and post-larval Euphausia frigida collected in 0–70/80 m and 0–175/200 m depth ranges with a MOCNESS sampler north of Elephant Island (61°S, 55°W) during 17–23 March 1984. Larval E. superba (predominantly calyptopes stage 2 and 3) were rare shallower than 80 m at night. Day catches of post-larval E. suberba were small and night catches were primarily near the top of the thermocline above 50 m depth. Thysanoessa sp. occurred throughout the 0–200 m depth range and was abundant in the upper 80 m both night and day. E. frigida migrated to the upper 80 m at night from deeper day depths. Larval stages of E. superba and bost-larval stages of all three species demonstrated independent and variable vertical distribution patterns both night and day. Changes in E. superba abundance and distributional patterns could to a certain extent be associated with observed environmental changes. An increase in larval and decrease in post-larval E. superba abundances between 0–80 m was associated with an intrusion of cold water at depth. At night, vertically restricted concentrations of post-larval E. superba were associated with shallow mixed layer depths, and a significant vertical separation of developmental stages and size categories was observed only during periods of stratification in the upper 80 m. Fluctuations in the distribution and abundance of Thysanoessa sp. and distribution of E. frigida did not appear to be influenced by physical parameters within the upper 80 m. Within the 0–80 m depth range, the distributions of these two species differed from each other and from E. superba and showed large tow to tow variability that could not be related to physical parameters in the upper water column.     

Growth and age structure of the swan mussel Anodonta cygnea (L.) at different depths in lake Mattsee (Salzburg, Austria)

Unionid clams were collected at 1–2 m, 3–4 m and 6–7 m depth in lake Mattsee, a moderately mesotrophic lake, to investigate the effect of depth on clam growth and age structure. No significant differences in age structure of Anodonta cygnea were found (p=0.65). Three and ten years old clams were present at all depths, but in different percentages. Whereas at 1–2 m 13.3% of the collected clams were <4 years old, this percentage was 4.4% at 6–7 m and 7.1% at 3–4 m. A greater percentage (6.7%) of older mussels (9, 10 years) were collected at 6–7 m than at 1–2 m (2.2%). Growth declined with depth. Total length at a given age of clams at 1–2 m and 3–4 m did not differ (p=0.54), whereas differences were significant between clams at 1–2 m and 6–7 m (p<0.05) as well as between 3–4 m and 6–7 m (p<0.05). The Growth constant k was highest at 1–2 m depth.     

Climatic adaptability of populations ofDiplotaxis erucoides D.C. (Brassicaceae) from Sicily,based on leaf morphology,leaf anatomy and δ13 C studies

The morphological and anatomical variability ofDiplotaxis erucoides populations from Sicily was investigated. Populations growing during the summer months exhibit distinct xeromorphic features. Leaf area is strongly reduced and leaf thickness is increased when compared with winter populations. Cell size, as well as cell arrangement and mesophyll cell surface area differ significantly between summer and winter populations. Leaf thickness is almost three times higher in summer populations andA (cell)/A, i.e. the mesophyll cell surface area per unit leaf area changes from about 16 for winter populations to almost 52 for summer populations. These differences are partly due to differences in intercellular volume and partly due to alterations in mesophyll cell sizes. The organic materal of the summer populations exhibits ¹³C values in the order of –27%. to –28%., while the corresponding values for the winter populations are in the order of –31%. to –33%.. Analysis ofD. erucoides populations from the transition period revealed intermediate ¹³C values. Anatomical variations such as reductions or increases ofA (cells)/A and changes of intercellular volume correlate with the corresponding ¹³C data. The ¹³C data are discussed in conjunction with the differences in leaf anatomy.     

The distribution of Heterotrissocladius oliveri Saether (Diptera: Chironomidae) in Lake Michigan

Fifty one chironomid species were identified from 504 samples collected at depths ranging 8 to 267 m in Lake Michigan, U.S.A. Heterotrissocladius oliveri Saether occurred in 32% of these samples and had an average abundance of 22 m^–2 which was similar to other estimates from the Great Lakes. Maximum average lake-wide density was at 30 to 60 m (41 m^–2). At depths 60 m, H. oliveri was the dominant chironomid species comprising 75% of total Chironomidae. The substrate preference of H. oliveri differed within each depth regime considered: at 30–60 m, 2–3 ; at 60–120 m, 3–5 , 7–9 ; and at 120–180 m, 6–8 . Abundance was notably reduced at all depths in substrates characterized as medium silt (5–6 ). On a lake-wide basis, the distribution pattern suggested H. oliveri was most numerous from 30 to 60 m along the southwestern, eastern, and northern shorelines and at 60–120 m depths along the southern and eastern shorelines. Increased abundance in the South Basin was concurrent with evidence of increased sedimentation at 60 to 100 m. However, in several other areas of the lake, high densities were associated with medium to very fine sands relatively free of silts and clays. This observation suggested occurrence of H. oliveri was minimally affected by sediment type.Widely variable, but generally elevated water temperatures likely prevent H. oliveri from establishing a substantial population density at depths < 30 m. With increased depth, temperature fluctuation is negligible and food is more stable, though the source is variable. Factors limiting abundance of H. oliveri at depths 30 m were related to decreased food supply due to distance from shore, food sources of lower value (clays), and, most importantly, to reproductive replenishment.Although still oligotrophic in nature, high density occurrences in both high and low sedimentation areas of the lake suggest the trophic indicator status of H. oliveri might be broader than previously thought.     

Intraspecific morphological variation with biometry of Distephanus speculum (Silicoflagellata)

Living specimens of Distephanus speculum (Silicoflagellata) were collected in waters of <150 m depth in the Pacific Sector of the Southern Ocean (57°31.6′S, 150°02.9′E), South of Tasman Island, and a biometric study has been conducted. The skeleton of D. speculum has a simple morphology, which can be readily expressed quantitatively. Using a new methodology, repeated measurements were obtained and converted to numerical indices. The circumference of the basal rings was the most useful character for subdivision into groups, and using this parameter we recognized two populations in the 620 studied specimens: (1) 500 individuals were placed in Group A, with a basal ring circumference of 53 to 67 µm, and a difference in length of spine axes from 2 to 6 μm; and (2) 120 individuals were placed in Group B (basal ring circumference 80 to 105 µm, difference in length of spine axes 3 to 6 µm). The circumference of the basal ring in Group B is approximately 10 µm smaller than in Distephanus speculum giganteus [Bukry, D., 1979. Comments on opal phytoliths and stratigraphy of Neogene Silicoflagellates and Coccoliths at Deep Sea Drilling Project Site 397 off Northwest Africa. Initial report of the DSDP, Washington, 49:977–1009.], and Distephanus boliviensis [Bukry, D., 1976a. Silicoflagellate and coccolith stratigraphy, Norwegian–Greenland Sea. Initial Report of DSDP LEG 38. Initial report of the DSDP, Washington, 38:843–855.; Bukry, D., 1976b. Silicoflagellate and coccolith stratigraphy, southeastern Pacific Ocean, Deep Sea Drilling Project Leg 34. Initial report of the DSDP, Washington, 34:715–735].     

Vertical distribution in and isolation of bacteria from Lake Vanda: an Antarctic lake

Vertical distribution of bacteria in Lake Vanda, an Antarctic meromictic lake, was examined by the acridine orange epifluorescence direct count method. Total bacteria were 10⁴–10⁵ cells · ml^–1 in the water at 55 m depth and above, and increased drastically to 10⁷ cells · ml^–1 in the bottom water. Filamentous or long rodshaped bacteria occurred at a high frequency in the upper layers, but in the bottom layers most bacteria were coccoidal or short rods. Mean bacterial cell volume in water of between 10 m and 60 m deep was fairly large compared with common bacterial populations in seawater and lake water. Aerobic heterotrophic bacteria were recovered from the water of a depth of 30 m and above, and were assumed to belong to Caulobacter. Viable heterotrophic bacteria were not recovered from the high salinity deep water by media prepared with the same deep water. Phototrophic purple non-sulphur bacteria were isolated by enrichment cultures from water at 55 m depth.     

Feeding and diel vertical migration cycles of Metridia gerlachei (Giesbrecht) in coastal waters of the Antarctic Peninsula

Diel vertical migration and feeding cycles of adult female Metridia gerlachei in the upper 290 m of a 335-m water column were measured during a total of 65 h in two periods of early summer (Dec 20–21 and Dec 25–26, 1991). Samples collected in eight depth strata by 35 MOCNESS tows (333-m mesh) were analyzed for abundance and mean individual gut pigment content. Most of the copepod population was concentrated in a 50-m depth interval at all times. Feeding began simultaneously with nocturnal ascent from a depth of 200–250 m at 18:00 h (local time), when the relative change in ambient light intensity was greatest. Ingestion rate increased exponentially (k_i = 0.988 h^–1) at double the gut evacuation rate (k_e = 0.488 h^–1) as the population moved upward at 22.3–26.5 m h^–1 through increasing concentrations of particulate chlorophyll-a. Although the bulk of the population did not move to depths shallower than 50 m, and began its downward migration at a rate of 20.8–31.7 mh^–1 in complete darkness, individual females continued to make brief excursions into chlorophyll-rich surface waters (4–8 g l^–1) during the first few hours of population descent. Ingestion rate diminished abruptly by one order of magnitude (k_i = 0.068 h^–1) at dawn ( 0330 h). Within four more hours, the population had reached its daytime depth and gut pigment content remained constant at a minimum value until the next migration cycle. No feeding appeared to take place at depth during the day. Ingestion by M. gerlachei females removed < 4% of daily primary production, with only 20% of this amount being removed from surface waters by active vertical transport.     

The distribution and abundance of krill faecal material and oval pellets in the Scotia and Weddell Seas (Antarctica) and their role in particle flux

Summary The abundance and depth distribution of zooplankton faeces in spring to early summer were investigated along meridional transects (47°W and 49°W) that extended from the Scotia Sea (57°S) across the Weddell-Scotia Confluence and into the Weddell Gyre (62°S). The sea ice edge retreated from 59°30S to 61°S during the study. Faeces were sampled with nets, Niskin bottles and sediment traps and subsequently analysed by light and electron (SEM) microscopy. Krill faecal strings and oval faecal pellets of unknown origin were by far the most important zooplankton faeces and highest concentrations were always found in the Confluence often close to the ice border. Krill faeces were usually more abundant in the uppermost layer (0–50m) where they contributed an average of 130 g dry weight m^–3. There was an exponential decrease with depth, with a minimum of 0.6 g dry weight m^–3 in the 500–1000 m stratum. Oval pellets were more evenly distributed in the upper 1000 m of the water column, with an average of 9 g dry weight m ^–3, although there was a small peak (20 g dry weight m^–3) in the subsurface layer (50–150 m depth). Consecutive collections (day-night) of krill faeces using drifting sediment traps showed that only the larger strings sank from 50 to 150 m depth. Peritrophic membranes appeared to deteriorate during sinking. Diatoms (in particular Nitzschia and Thalassiosira spp.) contributed by far the bulk of material in krill and oval faeces. In samples collected near or under the pack ice, remains of crustaceans in both krill- and oval faeces were also found.Data presented here were collected during the European Polarstern Study (EPOS) sponsored by the European Science Foundation     

Dinophysis blooms in the deep euphotic zone of the Baltic Sea: do they grow in the dark?

In situ growth rates of the toxin-producing dinoflagellate Dinophysis norvegica collected in the central Baltic Sea were estimated during the summers of 1998 and 1999. Flow cytometric measurements of the DNA cell cycle of D. norvegica yielded specific growth rates (μ) ranging between 0.1 and 0.4 per day, with the highest growth rates in stratified populations situated at 15–20 m depth. Carbon uptake rates, measured using ¹⁴C incubations followed by single-cell isolation, at irradiances corresponding to depths of maximum cell abundance were sufficient to sustain growth rates of 0.1–0.2 per day. The reason for D. norvegica accumulation in the thermocline, commonly at 15–20 m depth, is thus enigmatic. Comparison of depth distributions of cells with nutrient profiles suggests that one reason could be to sequester nutrients. Measurements of single-cell nutrient status of D. norvegica, using nuclear microanalysis, revealed severe deficiency of both nitrogen and phosphorus as compared to the Redfield ratio.It is also possible that suitable prey or substrate for mixotrophic feeding is accumulating in the thermocline. The fraction of cells containing digestive vacuoles ranged from 2 to 22% in the studied populations. Infection by the parasitic dinoflagellate Amoebophrya sp. was observed in D. norvegica in all samples analysed. The frequency of infected cells ranged from 1 to 3% of the population as diel averages, ranging from 0.2 to 6% between individual samples. No temporal trends in infection frequency were detected. Estimated loss rates based on observed infection frequencies were 0.5–2% of the D. norvegica population daily, suggesting that these parasites were not a major loss factor for D. norvegica during the periods of study.     

Diversity and spatial pattern of coral communities in the Red Sea upper twilight zone

Summary This is the first study based on numerical analysis of the abundance of 11 scleractinian corals of depths at between 100–210 m in the Red Sea twilight zone. Two distinct coral communities were found: a Leptoseris fragilis community at a depth of 100–130 m (zone 1) and a Dendrophillia horsti community below 130 m (zone 2, 3). Population densities and coral coverage are very low; distribution of individuals is highly clumped. Highest observed densities on 100 m² were 2720 individuals for L. fraglis, 2720 for D. horsti and 2260 for Javania insignis. Calculated coverage rates were maximally 3.6% (L. fragilis), 0.08% (D. horsti) and 0.11% (J. insignis). L. fragilis, the only symbiont bearing coral, was very abundant. It has an unusual depth range for a photosynthesising coral. Coral density is only weakly correlated with hard bottom coverage. Species diversity with an average of 8 species is highest at 120–170 m and decreases in shallower and deeper water. The study depth range is a transient zone for coral distribution. It contains the upper distribution limits of a few deep sea corals and the lower ones of several shallower water species. Ahermatypic corals, collected at 160–170 m depth, were transplanted from their original depth to 159, 118, 70 and 40 m; after one year most species survived transplantation far beyond their upper distributional limits. The symbiotic L. fragilis, collected at 120 m, survived transplantation to deep water (159 m) as well as shallow zones (90, 70 and 40 m). The study demonstrates the feasibility of line-transect methods for coral community studies with a submersible.     

Species richness,composition, and abundance of fish larvae and juveniles inhabiting natural and developed shorelines of a glacial Iowa lake

Synopsis Young-of-the-year fish communities in naturally vegetated sites were compared with those inhabiting nearby sites where lakeshore development (i.e., construction of homes, boat docks, and beaches) reduced nearshore macrophyte species richness and abundance. The study was conducted in a 2266 hectare, glacially formed, eutrophic lake in northwestern Iowa during the summers of 1987 and 1988. Study sites were divided into 3 depth zones, and fishes were collected by seining (0–1 m), plexiglass traps (1–2 m), and a nonclosing Tucker trawl (2–3 m). Species richness and total fish abundance were consistently greater in natural than in developed sites in both nearshore (0–1 m) and intermediate (1–2 m) depth zones, but differed little between natural and developed sites in the offshore (2–3 m) depth zone. Nearly 50% of the species sampled, including yellow perch Perca flavescens and bluegill Lepomis macrochirus, inhabited limnetic areas as larvae before migrating inshore as juveniles. Eighteen of the 20 fish species collected as juveniles were in greater abundance in natural than in developed sites. Smallmouth bass Micropterus dolomieui was the only game species consistently found in equal or greater abundance in developed sites. Within all sites, juvenile fishes were generally most abundant where macrophyte abundance and species richness were greatest. Findings from this study demonstrate the importance of nearshore aquatic vegetation to fishes during their first summer of life. If nearshore vegetation beds of lakes continue to be regarded as a nuisance and indiscriminately removed, important fish nursery habitat will be lost. The short-term result will likely be reduced year-class strength of vegetation-dependent species. More importantly, the long-term effects will be changes in fish community richness and composition which will, in turn, alter the lake's fishery.     

Survival and photosynthetic activity of different Dinophysis acuminata populations in the northern Baltic Sea

This study deals with a recently found phenomenon in the northern Baltic Sea: the occurrence of the dinoflagellate Dinophysis acuminata in the deep water below the thermocline. This was first observed in July 2001 at the station BY 15 in the Gotland Deep, where a sharp and intensive chlorophyll fluorescence signal was encountered at 77 m depth. The fluorescence peak was due to a dinoflagellate community dominated by Dinophysis acuminata (approximately 18 000 cells l⁻¹). The survival of this community was followed in laboratory incubations in low light (20 μE m⁻² s⁻¹) and low temperature (+5 °C). After 5 weeks incubation, 67–84% of the initial cell abundance was lost, while few D. acuminata cells survived up to 24 weeks in the original sample. During the incubation, the fluorescence signal of the cells became fainter and the chloroplasts smaller and aggregated. On two occasions a D. acuminata cell was found attached to a smaller cell by a thin cytoplasm strand, possibly indicating mixotrophic behavior. During the following summer (2002), the photosynthetic efficiency of D. acuminata collected from thermocline layers of few stations and from the nitracline (75–80 m) at one station was studied in photosynthesis irradiance (P–E) incubations. Photosynthetic activity occurred in all populations, with differences in their photosynthetic carbon uptake rates. Photosynthesis of D. acuminata populations was saturated between 250 and 500 μE m⁻² s⁻¹; maximum cell-specific carbon uptake rates (P_m) ranged from 160–925 pg C cell⁻¹ h⁻¹. The P_m-rates in populations originating below the thermocline and in an artificially darkened population were markedly lower than in populations from upper water layers. The varying maximum photosynthetic rates of these populations may reflect their history, e.g. time spent in different light environments.     

Population dynamics and energy budget of Marionina southerni (Cernosvitov) (Enchytraeidae,Oligochaeta) in the shallow littoral of Lake Esrom,Denmark

Marionina southerni (Cernosvitov) was numerically the dominant oligochaete in a shallow, strongly exposed surf zone locality (0–1 m depth) in the mesotrophic Lake Esrom. It comprised 21,500 ind m^–2 or nearly 50% of the total oligochaete community, which otherwise was dominated by Nais spp. and the lumbriculid Stylodrilus heringianus Claparède. M. southerni appears to have a 1-y life cycle. Individual biomass ranged from 3 to 48 µg ash free dry weight. Annual net production at the site was 5.1 kcal m^–2 (size-frequency method) with a P/B ratio of 2.5. This is about 0.5% of the estimated mean zoobenthic production in the littoral zone from 0 to 2 m depth in Lake Esrom. At 5° C M. southerni showed a constant oxygen uptake down to 25% oxygen saturation, but practically no regulatory respiration was found at 20 °C. In the field M. southerni was never found in sediment with less than 60% oxygen saturation. The annual community respiration of M. southerni was 14.2 kcal m^–2, and annual assimilation thus made up 19.3 kcal m^–2 with a net production efficiency of 26.5%.