Lipid‐rich zooplankton subsidise the winter diet of benthivorous Arctic charr (Salvelinus alpinus) in a subarctic lake

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The vertical distribution and abundance of <Emphasis Type="Italic">Gammarus lacustris</Emphasis> in the pelagic zone of the meromictic lakes Shira and Shunet (Khakassia,Russia)

The vertical distribution and abundance of Gammarus lacustris in the pelagic zone of two fishless meromictic lakes, L. Shira and L. Shunet, in Southern Siberia (Russia), was studied with the underwater video recording system and using vertical hauls. In both lakes, during summer stratification, Gammarus was distributed non-homogenously, with a stable peak in the metalimnion. The average depth of Gammarus population in the pelagic zone was significantly correlated with the depth of the thermocline. Gammarus abundances obtained using vertical plankton hauls with net were quite comparable with those obtained from video records. The peak abundance of Gammarus in the pelagic zone of the lakes observed with underwater video amounted up to 400 individuals m⁻², while the peak animal densities in the metalimnion reached 50 ind. m⁻³. The data are compared with previously published abundances of Gammarus in the littoral of Lake Shira. Both littoral and pelagic can be equally important habitats for amphipods in meromictic lakes. The absence of fish in the pelagic zone, high oxygen concentration, low water temperature, increased seston concentration, elevated water density in the metalimnion and the anoxic hypolimnion can be the most probable combination of factors that are responsible for the peak of Gammarus in the metalimnion of these lakes.     

Growth, biomass, and production of two small barbs (Barbus humilis and B. tanapelagius, Cyprinidae) and their role in the food web of Lake Tana (Ethiopia)

Growth, biomass and production of two small barbs (Barbus humilis and Barbus tanapelagius) and their role in the food web of Lake Tana were investigated. From length–frequency distribution of trawl monitoring surveys growth coefficient, Φ′ values were estimated at 3.71–4.17 for B. humilis and 3.70–4.14 for B. tanapelagius, respectively. Values for B. humilis were confirmed in pond experiments. Mean biomass of the small barbs was 13.3 kg fresh wt ha⁻¹, with B. humilis being most abundant in the littoral and sub-littoral zones, whereas B. tanapelagius was most abundant in the sub-littoral and pelagic zones. The two small barbs had a production of 53 kg fresh wt ha⁻¹ year⁻¹. Although their P/B ratios of about 4.0 were relatively high for small cyprinids, both their biomass and production were low in comparison with other small fish taxa in other tropical lakes. Of the zooplankton production only about 29% was consumed by the small barbs. However, they did not utilize calanoid copepods, which were responsible for approximately 57% of the zooplankton production and it is likely that small barb production was food limited during certain periods of the year. Piscivorous labeobarbs consumed about 56% of the small barbs production annually, but additionally, Clarias gariepinus, and many bird species were also preying on them. Therefore, limitation of Barbus production by predation during certain periods in the year cannot be excluded.     

Variable littoral‐pelagic coupling as a food‐web response to seasonal changes in pelagic primary production

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Composition and spatial variability of macroplankton and micronekton within the Antarctic Polar Frontal Zone of the Indian Ocean during austral autumn 1997

Net sampling and continuous acoustic measurements within the Antarctic Polar Frontal Zone (APFZ) and in the vicinity of the Prince Edward Islands were conducted during austral autumn (April/May) 1997 to describe the composition and distribution of macrozooplankton and micronekton, and to investigate their relations to the prevailing oceanographic regime in the area. Two major circulation patterns associated with the Subantarctic (SAF) and Antarctic Polar (APF) Fronts existed in the oceanic environment surrounding the Prince Edward Islands, promoting high cross-frontal mixing both upstream and downstream of the islands. Average abundance and biomass of macroplankton/micronekton in the top 300-m layer were 21 ind. 1000 m⁻³ and 467 mg DW 1000 m⁻³, respectively. Pelagic crustaceans (euphausiids and amphipods), fish, chaetognaths and gelatinous zooplankton dominated numerically and by biomass. Continuous acoustic measurements displayed elevated pelagic biomass at the SAF and APF. Although four groupings of stations were identified using cluster analysis, a single macroplankton/micronekton community was recognized in the top 300-m layer throughout the offshore area of the APFZ. A modification of the APFZ community was observed within the inter-island region. Subantarctic species dominated zooplankton samples throughout the APFZ, although subtropical species were also well represented at stations occupied in the northern region of the APFZ. A biological response reflected in macroplankton community composition, resulting from an extensive cross-frontal mixing, was observed within the APFZ around the Prince Edward Islands. Accepted: 27 November 1999     

Increasing fish production from wetlands at Lake Victoria, Uganda using organically manured seasonal wetland fish ponds

The processes driving primary productivity and its impacts on fish production were investigated in field trials in eight seasonal earthen wetland ponds ‘Fingerponds’ (192 m²) in Uganda between 2003 and 2005. The ponds were stocked by the seasonal flood with predominantly Oreochromis spp. at densities ranging from 0.1 to 0.5 fish m⁻². Chicken manure (521, 833 or 1,563 kg ha⁻¹) was applied fortnightly. Results showed that primary productivity was enhanced with maximum average net primary productivity (±Standard Error) of 11.7 (±2.5) g O₂ m⁻² day⁻¹ at the Gaba site and 8.3 (±1.5) g O₂ m⁻² day⁻¹ at the Walukuba site. Net fish yields were higher in manured ponds with up to 2,670 kg ha⁻¹ yield for a 310 day growth period compared to less than 700 kg ha⁻¹ in unmanured ponds. Fish production was limited mainly by high recruitment, falling water levels, light limitation from high suspended solids and turbidity, and low zooplankton biomass. It was concluded that Fingerponds have a high potential for sustainable fish production and can contribute to the alleviation of protein shortages amongst the riparian communities around Lake Victoria. Production can be enhanced further with improved stock management.     

Seasonal fluctuations in macrophyte cover and water transparency of four brown-water lakes: implications for crustacean zooplankton in littoral and pelagic habitats

The dynamics of crustacean zooplankton in the littoral and pelagic zones of four forest lakes having variable water qualities (colour range 130–340 mg Pt l⁻¹, Secchi depth 70–160 cm) were studied. The biomass of zooplankton was higher in the littoral zone than in the pelagic zone only in the lake having the highest transparency. In the three other lakes, biomass was significantly higher in the pelagic zone than in the littoral zone. In the two lakes with highest transparency, the littoral biomass of cladocerans significantly followed the development of macrophyte vegetation, and cladoceran biomass reached the maximum value at the time of highest macrophyte coverage. In lakes with lowest transparency, littoral zooplankton biomass developed independently of macrophyte density and decreased when macrophyte beds were densest. The seasonal development of the littoral copepod biomass did not follow the development of macrophytes in any of the lakes. The mean size of cladocerans in the pelagic zone decreased with increasing Secchi depth of the lake, whereas in the littoral zone no such phenomenon was detected. Seasonally, when water transparency increased temporarily in two of the lakes, the mean size of cladocerans in the pelagic zone decreased steeply. For copepods, no relationship between water transparency and body size was observed. The results suggested that in humic lakes the importance of the littoral zone as a refuge decreases with decreasing transparency of the water and that low water transparency protects cladocerans from fish predation. All the observed between-lake differences could not be explained by fish predation, but were probably attributed to the presence of chaoborid larvae with variable densities. Feeding efficiency of chaoborids is not affected by visibility and thus they can obscure the relationship between water quality, fish density, and the structure of crustacean zooplankton assemblages. Handling editor: S. I. Dodson     

Mesozooplankton beneath the summer sea ice in McMurdo Sound,Antarctica: abundance,species composition,and DMSP content

The summer Phaeocystis antarctica bloom increases under-ice phytoplankton biomass in McMurdo Sound, Antarctica. The magnitude of mesozooplankton grazing on this bloom is unknown, and determines whether this production is available to the pelagic food web. We measured mesozooplankton abundance and body content of dimethylsulfoniopropionate (DMSP) during the McMurdo Sound austral summer (2006 and 2006–2007). Abundance varied from 20 to 4,500 ind. m⁻³ (biomass 0.02–274.0 mg C m⁻³), with peaks in mid-December and late-January/February. Abundance was higher but total zooplankton biomass lower in our study compared to previous reports. Copepods and the pteropod Limacina helicina dominated the zooplankton in both abundance and biomass. DMSP was detected in all zooplankton groups, with highest concentrations in copepod nauplii and L. helicina (95 and 54 nmol mg⁻¹ body C, respectively). Experiments suggested that L. helicina obtains DMSP by directly grazing on P. antarctica, which often accumulates to high biomass under the summer sea ice in McMurdo Sound.     

Feeding of <Emphasis Type="Italic">Themisto libellula</Emphasis> (Amphipoda Crustacea) on natural copepods assemblages in an Arctic fjord (Kongsfjorden,Svalbard)

Ingestion rates and selectivity of the Arctic pelagic amphipod Themisto libellula were studied experimentally in Kongsfjorden (Svalbard, 78°N) during the summer period. Feeding incubations were conducted on naturally occurring copepod communities at different concentrations ranging from 25 to 250 preys L⁻¹. The ingestion rates increased with food availability from 1.3 to 17.7 preys ind⁻¹ day⁻¹, which corresponded to 0.3–11% of body carbon day⁻¹. Despite the high prey concentration used in the experiments the satiation level was not reached. We suggested that T. libellula is able to take the maximum benefit from dense patches of preys, which represent a good adaptation to the high variability in food supply characteristic of polar environment. Copepodids stage III of Calanus spp. appeared to be the preferred preys of T. libellula. Smaller copepods such as Oithona similis and Pseudocalanus spp., were also selected but only when their relative abundance exceeded 25% of the total prey available. The potential predation impact of T. libellula is discussed in relation to the mesozooplankton small-scale patchiness and predator abundance.     

Age and growth comparisons of Hovsgol grayling (Thymallus nigrescens Dorogostaisky, 1923), Baikal grayling (T. baicalensis Dybowski, 1874), and lenok (Brachymystax lenok Pallas, 1773) in lentic and lotic habitats of Northern Mongolia

Despite concern over the conservation status of many Mongolian salmonids and the importance of their ecological role in Mongolia's aquatic ecosystems, little is known about their basic biology. Hovsgol grayling (Thymallus nigrescens) is endemic to Lake Hovsgol, Mongolia and listed as endangered on the Mongolian Red List. Baikal grayling (T. baicalensis) and lenok (Brachymystax lenok) are found in lakes and rivers throughout the Selenge drainage. A detailed study of the age and growth of these three salmonids was conducted based on 1,682 samples collected from July 2006 to July 2013 in Lake Hovsgol, its outlet the Eg River, and one of the Eg's largest tributaries, the Uur River. Our results suggest that Hovsgol grayling in particular can reach a much older maximum age (17 years in our samples) than previously believed based on aging from scales. Female Hovsgol grayling were heavier at a given length than their male counterparts. Lenok had a greater average length‐at‐age in Lake Hovsgol compared to the rivers and greater weight‐at‐length in the warmer Uur River than in the Eg; female lenok from the rivers had a greater average length‐at‐age than their male counterparts. This study provides critical new information for the management and conservation of these threatened salmonid species in Mongolia.     

The aggregation of polar cod (Boreogadus saida) in the deep Atlantic layer of ice-covered Amundsen Gulf (Beaufort Sea) in winter

During the Circumpolar Flaw Lead System Study (CFL, 2007–2008), large aggregations of polar cod were detected in winter in the Amundsen Gulf (Western Canadian Arctic) using the EK60 echosounder of the CCGS Amundsen research icebreaker. Biomass estimated over 10 months reached a maximum of 0.732 kg m⁻² in February. Aggregations were encountered only in the presence of an ice cover from December to April. The vertical extent of the aggregations was dictated by temperature and zooplankton prey distribution. In winter, polar cod generally occupied the relatively warm deep Atlantic Layer (>0°C), but a fraction of the densest aggregations occasionally followed zooplankton prey up into the cold Pacific Halocline (−1.6 to 0°C). The diel vertical migration of polar cod was precisely synchronized with the seasonally increasing photoperiod. Throughout winter, polar cod aggregations migrated to progressively deeper regions (from 220 to 550 m bottom depths) in response to increasing light intensity, presumably to avoid predation by visual predators such as the ringed seal. Comparing Amundsen Gulf and Franklin Bay indicates that the entrapment of polar cod in embayments during winter is an important mechanism to provide marine mammal predators with dense concentrations of their main prey within their diving range.     

Feeding dynamics and respiration of the bottom-dwelling caridean shrimp Nauticaris marionis Bate, 1888 (Crustacea: Decapoda) in the vicinity of Marion Island (Southern Ocean)

The feeding dynamics and oxygen uptake of the bottom-dwelling caridean shrimp Nauticaris marionis were studied during the April/May 1984, 1996 and 1997 cruises to Marion Island (Prince Edward Islands, Southern Ocean). N. marionis is thought to have an opportunistic feeding mode. Prey composition varied considerably between the years and sites investigated. Overall, benthic (mainly hydrozoans and bottom-dwelling polychaetes) and, at times, pelagic (largely euphausiids and copepods) prey items dominated in the stomachs of N. marionis both by occurrence and by volume. Generally, pelagic prey contributed more to the diets of smaller shrimps, while benthic prey was a more important component in the guts of larger specimens. Wet, dry and ash-free dry weight were determined for specimens used in respiration experiments. The respiration rates of N. marionis females with carapace length 6.6–11.1 mm ranged from 80 to 250 μl O₂ individual⁻¹ · h⁻¹, or from 0.588 to 2.756 μl O₂ · mg⁻¹ dry weight h⁻¹. Regression analyses showed highly significant correlations between oxygen consumption and carapace length for N. marionis. Daily ingestion rates estimated using an in situ gut content analysis technique (4.4% of body dry weight) and an energy budget approach (average 4.7% of body dry weight, range 2.0–7.5%) showed good agreement with each other. Accepted: 29 July 1998     

Relative importance of biotic and abiotic factors affecting bacterial abundance in Lake Biwa: an empirical analysis

To determine the relative importance of factors affecting bacterial abundance in Lake Biwa, correlation and multiple regression analyses were performed with relevant biotic and abiotic variables. Data used in the analyses were collected weekly from April 1997 to June 1998 at a pelagic site in the north basin. The bacterial abundance ranged from 1 to 7 × 10⁶ cells ml⁻¹, and its spatio-temporal pattern was virtually identical to that in previous studies conducted 12–15 years ago. In the surface layer (0–12.5 m), bacterial abundance was significantly correlated with water temperature and with protozoan and metazoan grazers, but not with chlorophyll a and nutrient concentrations. The results suggest that loss factors rather than growth factors are more important in determining bacterial abundance in this lake. However, grazing effects on bacterial abundance differed among zooplankton. Bacterial abundance correlated negatively with phagotrophic nanoflagellates (PNF) and Daphnia, but positively with Eodiaptomus. Thus, PNF and Daphnia act to reduce the bacterial abundance, while Eodiaptomus acts to stimulate. In contrast, these biotic factors did not explain changes in bacterial abundance in the middle (12.5–25 m) and deep (>25 m) layers. Instead, the bacterial abundance in the deep layer was highly correlated with vertical mixing regimes, suggesting that bacterial abundance was directly or indirectly affected by abiotic factors. These results indicate that bacterial abundance in Lake Biwa was regulated by different factors at different depths. Received: February 8, 2000 / Accepted: August 29, 2000     

Trophic importance of the chaetognaths Eukrohnia hamata and Sagitta gazellae in the pelagic system of the Prince Edward Islands (Southern Ocean)

Trophodynamics and predation impact of the 2 dominant chaetognaths Eukrohnia hamata and Sagitta gazellae were investigated at 19 stations in the vicinity of the Prince Edward Islands and at a 24-h station occupied at the sub-Antarctic Front in late summer (April/May) 1996. During the entire investigation, the zooplankton assemblages were numerically dominated by copepods with densities ranging from 21 to 170 ind. m⁻³. Amongst the copepods, Clausocalanus brevipes, Metridia gerlachei and M. lucens dominated accounting for >90% of the total. Generally, chaetognaths were identified as the second most important group composing at times up to 30% (mean = 14.7%) of total zooplankton abundance. Of the two chaetognath species, E.␣hamata was generally numerically dominant. Gut content analysis showed that both chaetognath species are opportunistic predators generally feeding on the most abundant prey, copepods. No feeding patterns were evident during the 24-h station, suggesting that both species feed continuously. The feeding rates of E. hamata ranged from 0 to 0.50 prey ind. day⁻¹ and between 0 and 0.90 prey ind. day⁻¹ for S. gazellae. The maximum total predation impact of E. hamata was equivalent to 5.2% of the copepod standing stock or up to 103% of copepod production per day. For S. gazellae the predation impact was lower, reaching a level of 3.2% of the copepod standing stock or 63% of the daily copepod production. Chaetognaths can, therefore, be regarded as an important pelagic predator of the Prince Edward Islands subsystem. Received: 27 March 1997 / Accepted: 11 September 1997     

Distribution of polar cod and age-0 fish in the U.S. Beaufort Sea

Little is known about species composition, distribution, and abundance of pelagic fish in the U.S. portion of the Beaufort Sea continental shelf and slope. To inventory the community and describe pelagic fish distributions relative to water characteristics, a systematic survey was conducted in August 2008. Acoustics (38 kHz), midwater trawling, and CTD casts were used to sample water depths from 20 to 500 m. Age-1+ polar cod (Boreogadus saida) was the dominant fish species, with peak densities of 155,000 # ha⁻¹ at bottom depths of 100–350 m. Age-0 fish (polar cod, sculpin (Cottidae family), and eelblenny (Lumpenus sp.)), dominated the pelagic biomass at bottom depths between 20 and 75 m, with peak densities of 160,000 # ha⁻¹, but were also found in surface waters at bottom depths >75 m. Age-1+ polar cod were associated with cold, saline waters likely of Chukchi Sea origin and mirrored published foraging distributions for beluga whales (Delphinapterus leucas). Conversely, age-0 fish were found in warm, fresher water, likely of ice melt and/or riverine origin, throughout the study area. This study provides a necessary baseline for the development of Arctic assessment surveys and management plans for polar cod.     

Cultivation of the green alga, Codium fragile (Suringar) Hariot, by artificial seed production in Korea

Codium fragile (Suringar) Hariot is an edible green alga farmed in Korea using seed stock produced from regeneration of isolated utricles and medullary filaments. Experiments were conducted to reveal the optimal conditions for nursery culture and out-growing of C. fragile. Sampling and measurement of underwater irradiance were carried out at farms cultivating C. fragile at Wando, on the southwestern coast of Korea, from October 2004 to August 2005. Growth of erect thalli and underwater irradiance were measured over a range of depths for three culture stages. During the nursery cultivation stage (Stage I), growth rate was greatest at 0.5 m depth (0.055 ± 0.032 mm day⁻¹), where the average midday irradiance over 60 days was 924 ± 32 μmol photons m⁻² s⁻¹. During the pre-main cultivation stage (Stage II), the greatest growth rate occurred at a depth of 2 m (0.113 ± 0.003 mm day⁻¹) with an average irradiance of 248 ± 116 μmol photons m⁻² s⁻¹. For the main cultivation stage (Stage III) of the alga, thalli achieved the greatest increase in biomass at 1 m depth (7.2 ± 1.0 kg fresh wt m⁻¹). These results suggest that optimal growth at each cultivation stages of C. fragile could be controlled by depth of cultivation rope.     

Long-term study of zooplankton in the estuarine system of Ribeira Bay,near a power plant (Rio de Janeiro,Brazil)

The structure of the zooplankton community in an estuary adjacent to the Admiral álvaro Alberto Nuclear Power Plant at Angra dos Reis, state of Rio de Janeiro, Brazil was studied from 2001 through 2005. At that time, the power plant had been operating for 20 years. The results were compared with a previous study in 1991–1993. The zooplankton was sampled 4 times a year, in vertical hauls using a 150 μm net, at two fixed points near the plant’s intake and discharge sites. Temperature, salinity, and chlorophyll a were measured. The water of Ribeira Bay is warm, with salinities typical of Coastal Water and more saline waters. Zooplankton density at the discharge site (Mean: 245,434 ind m⁻³, SD: 335,358 ind m⁻³) was higher than that at the intake site (Mean: 84,634 ind m⁻³, SD: 101,409 ind m⁻³). A total of 121 mesozooplankton taxa were recorded from 2001 to 2005. Copepoda constituted the most common taxon and comprised more than 57% of the total zooplankton, followed by cladocers and gastropod larvae. A seasonal zooplankton cycle was observed only during 2004; in other years, the plankton varied only between years. Overall mesozooplankton abundance at the discharge site was similar to levels reported from the inner zone of this estuary in 1991–1993. Surface temperature was the important factor structuring the zooplankton community at the discharge site. No effect on the mesozooplankton by passage through the condensers could be discerned, and no permanent negative influence on the plankton populations could be detected. Guest editors: U. M. Azeiteiro, I. Jenkinson & M. J. Pereira Plankton Studies     

Biomass Production with Willow and Poplar Short Rotation Coppices on Sensitive Areas—the Impact on Nitrate Leaching and Groundwater Recharge in a Drinking Water Catchment near Hanover,Germany

In a lowland drinking water catchment area, nitrate leaching as well as groundwater recharge (GWR) was investigated in willow and poplar short rotation coppice (SRC) plantations of different ages, soil preparation measures prior to planting and harvesting intervals. Significantly increased nitrate concentrations of 16.6 ± 1.6 mg NO₃-N L⁻¹ were measured in winter/spring 2010 on a poplar site, established in 2009 after deep plowing (90 cm) but then, subsequently decreased strongly to below 2 mg NO₃-N L⁻¹ in spring 2011. The fallow ground reference plot showed nitrate concentrations consistently below 1 mg L⁻¹ and estimated annual seepage output loss was only 1.36 ± 1.1 kg ha⁻¹ a⁻¹. Leaching loss from a neighboring willow plot from 2005 was 14.3 ± 6.6 kg NO₃-N ha⁻¹ during spring 2010 but decreased to 2.0 ± 1.5 kg NO₃-N ha⁻¹ during the subsequent drainage period. A second willow plot, not harvested since its establishment in 1994, showed continuously higher nitrate concentrations (10.2 ± 1.7 NO₃-N L⁻¹), while a neighboring poplar plot, twice harvested since 1994 showed significantly reduced nitrate concentrations. Water balance simulations, referenced by soil water tension and throughfall measurements, showed that at 655 mm annual rainfall, GWR from the reference plot (300 mm a⁻¹) was reduced by 40 % (to 180 mm a⁻¹) on the 2005 willow stand, mainly due to doubled rainfall interception losses. However, transpiration was limited by low soil water storage capacities, which in turn led to a moderate impact on GWR. We conclude that well-managed SRC on sensitive areas can prevent nitrate leaching, while impacts on GWR may be mitigated by management options.     

The viability to a wall shear stress and propagation of <Emphasis Type="Italic">Bifidobacterium longum</Emphasis> in the intensive membrane bioreactor

Bifidobacterium longum grew at 65 L pilot scale of the membrane bioreactor (MBR), externally fitted with ceramic membrane (0.7 m²). Cell mass at the MBR reached 22.18 g L⁻¹ as dry cell weight in 12 h, which is 8.44 times higher than cell mass attained at the vial culture. The growth rate in the vial culture was μ = 0.385 h⁻ and at the batch culture was μ = 1.13 h⁻ in the exponential period and μ = 0.31 h⁻¹ in the stationary period. In the fed-batch mode was μ = 1.102 h⁻¹ for 6 h with inoculation and declined to μ = 0.456 h⁻¹ with feeding of feed medium. The growth rate at the MBR was μ = 0.134 h⁻¹. The number of viable cells was 6.01 × 10¹² cfu L⁻¹ at the batch culture, but increased to 1.15 × 10¹⁴ cfu L⁻¹ at the MBR culture. The specific growth rate of viable cell number (colony-forming units per liter, per hour) improved by 6.01 times from the batch to the MBR culture. The wall shear stress mainly generated by the pump, and the membrane incorporated into the MBR was controlled during the cultivation at the MBR. The viability of B. longum declined to under 10% in the first 2 weeks of the 4-week stability test (40°C) as B. longum was exposed to over wall shear stress 713 Pa, but the viability improved to 30–40% in wall shear stress of 260 Pa or STR culture. The loss in the cell viability can be saved by managing with wall shear stress during the cultivation at the MBR.