Pre-winter distribution and habitat characteristics of polar cod (Boreogadus saida) in southeastern Beaufort Sea

Polar cod was shown to form dense under-ice winter aggregations at depth in the Amundsen Gulf (southeastern Beaufort Sea). In this paper, we verify the premises of the aggregation mechanism by determining the distribution and habitat characteristics of polar cod prior to the formation of winter aggregations. Multifrequency split-beam acoustic data collected in October–November 2003 revealed that polar cod split into two distinct layers. Age-0 polar cod formed an epipelagic layer between 0 and ~60 m depth without any clear large-scale biomass trend. In contrast, adult polar cod tended to distribute into an offshore mesopelagic layer between ~200 and 400 m that shoaled into a denser (1–37 g m^?2) benthopelagic layer on sloping bottoms (between 150 and 600-m isobaths) along the Mackenzie shelf and into the Amundsen Gulf basin. Concentrations peaked in the Amundsen Gulf where estimated total biomass reached ~250 kt. Both age-0 and adult polar cod distributed in the warmer waters (>?1.4 °C). We hypothesise that polar cod concentration over slopes is governed by the combined actions of (1) local currents concentrating both depth-keeping zooplankton and polar cod at the shelf-break and basin slopes and (2) trophic association with these predictable topographically trapped aggregations of zooplankton prey. During freeze-up, these slope concentrations of polar cod are thought to constitute the main source of the observed dense under-ice winter aggregations. The hypothesis of active short-distance displacements combined with prevailing mean currents is retained as the likely aggregation mechanism.     

From polar night to midnight sun: photoperiod, seal predation, and the diel vertical migrations of polar cod (Boreogadus saida) under landfast ice in the Arctic Ocean

The winter/spring vertical distributions of polar cod, copepods, and ringed seal were monitored at a 230-m station in ice-covered Franklin Bay. In daytime, polar cod of all sizes (7–95 g) formed a dense aggregation in the deep inverse thermocline (160–230 m, −1.0 to 0°C). From December (polar night) to April (18-h daylight), small polar cod <25 g migrated into the isothermal cold intermediate layer (90–150 m, −1.4°C) at night to avoid visual predation by shallow-diving immature seals. By contrast, large polar cod (25–95 g), with large livers, remained below 180 m at all times, presumably to minimize predation by deep-diving mature seals. The diel vertical migration (DVM) of small polar cod was precisely synchronized with the light/dark cycle and its duration tracked the seasonal lengthening of the photoperiod. The DVM stopped in May coincident with the midnight sun and increased schooling and feeding. We propose that foraging interference and a limited prey supply in the deep aggregation drove the upward re-distribution of small polar cod at night. The bioluminescent copepod Metridia longa could have provided the light needed by polar cod to feed on copepods in the deep aphotic layers.     

Studies of the effect of environmental factors on the rotifer predator–prey system in freshwater

The aim of this work is to evaluate the effect of environmental factors: temperature and photoperiod on the zooplankton predator–prey system. Rotifers, an important and cosmopolitan group of zooplankton in freshwater, were used in our study. We investigated the effect of temperature (20, 23, and 30°C) and of photoperiod (L:D = 12:0 and 0:12) on the predatory rotifer Asplanchna brightwelli consuming rotifer Brachionus calyciflorus as prey. Under A. brightwelli predation, populations of B. calyciflorus prey were consumed more slowly at 20 ± 1 and 30 ± 1°C as compared to 23 ± 1°C. Prey consumption by A. brightwelli increased from 0.63 ± 0.09 ind. predator⁻¹ at 20°C to a peak of 1.22 ± 0.12 ind. predator⁻¹ at 23°C, then decreased significantly to 0.93 ± 0.14 ind. predator⁻¹ at 30 ± 1°C. In addition, predation responded to temperature changing sensitively and rapidly. Statistical analysis showed that the prey consumption were significant different under altered temperature periods during 12 h. Photoperiod also significantly influenced the rate of A. brighwelli predation. B. calyciflorus suffered less predation in darkness than in light. The rate of prey consumption in light (1.06 ind. predator⁻¹) was twice the average of that in darkness (0.51 ind. predator⁻¹). Furthermore, predation rate varied under changing photoperiod but predators moved back into the light did not resume their original consumption rate. Our results demonstrate that whether the predation in rotifer successfully or not is strongly influenced by temperature and photoperiod.     

Distribution and diet of 0-group cod (<Emphasis Type="Italic">Gadus morhua</Emphasis>) and haddock (<Emphasis Type="Italic">Melanogrammus aeglefinus</Emphasis>) in the Barents Sea in relation to food availability and temperature

Distribution of 0-group cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) in August–September 2005 and 2006 was mainly restricted to the Atlantic waters of the western and central areas of the Barents Sea. The main distribution of 0-group fish overlapped largely with areas of high biomass (>7 gm⁻² dry weight) of zooplankton. The copepod Calanus finmarchicus and krill Thysanoessa inermis, which are dominant zooplankton species in both Atlantic and boreal waters of the Barents Sea, were the main prey of 0-group cod and haddock. The main distribution, feeding areas and prey of 0-group cod and haddock overlapped, implying that competition for food may occur between the two species. However, though their diet coincided to a certain degree, haddock seems to prefer smaller and less mobile prey, such as Limacina and appendicularians. As 0-group fish increased in size, there seems to be a shift in diet, from small copepods and towards larger prey such as krill and fish. Overall, a largely pelagic feeding behaviour of 0-group cod and haddock was evident from this study.     

Sub-lethal levels of waterborne petroleum may depress routine metabolism in polar cod Boreogadus saida (Lepechin, 1774)

Petroleum-related activities in Arctic waters are rapidly increasing parallel to the ongoing thinning of the Arctic sea ice. As part of a series of studies on petroleum-induced stress in polar cod Boreogadus saida, we tested the effects of acute (~60 min) and chronic (4 weeks) exposure to the water soluble fraction (WSF) of petroleum on whole body metabolism inferred from measurements of oxygen consumption rates. The exposure of polar cod to WSF leads to a statistically significant depression in routine metabolism in the order Control (0.260 mg O₂ g fish⁻¹ h⁻¹; N = 6) > Chronic (0.191 mg O₂ g fish⁻¹ h⁻¹; N = 6) > Acute (0.110 mg O₂ g fish⁻¹ h⁻¹; N = 2), decoupling of routine metabolism and body mass but possibly also to a partial metabolic compensation after 4 weeks of exposure. The results are reviewed in context with similar studies on Antarctic and non-polar fishes.     

Major and Trace Elements in Zooplankton from the Northern Gulf of California During Summer

We report the distribution of major and trace element concentrations in epipelagic zooplankton collected in the Northern Gulf of California in August 2003. The Bray–Curtis index defined three element assemblages in zooplankton: (1) major metals, which included only two elements, Na (3.6–17.0%) and Ca (1.0–4.8%). Na had its highest concentrations in the shallow tidally mixed Upper Gulf, where high salinity, temperature, and zooplankton biomass (dominated by copepods) prevailed. Ca showed its highest concentrations south of Ballenas Channel, characterized by tidal mixing and convergence-induced upwelling, indicated by low sea-surface temperature, salinity, and zooplankton biomass; (2) Six biological essential elements, like Fe (80–9,100 mg kg⁻¹) and Zn (20–2,570 mg kg⁻¹), were detected in high concentrations in zooplankton collected near Guaymas Basin, which had high surface temperature and chlorophyll a concentrations. (3) Metals of terrigenous origin, such as Sc (0.01–1.4 mg kg⁻¹) and Th (0.03–2.3 mg kg⁻¹), and redox-sensitive metals, like Co (3–23.8 mg kg⁻¹); this was the assemblage with the largest number of elements (15). Both types of elements of assemblage 3 had maximum concentrations in the cyclonic eddy that dominates the summer circulation in the Northern region. We concluded that sediment resuspension by tidal mixing in the Upper Gulf, upwelling south of Ballenas Channel, and the cyclonic eddy were key oceanographic features that affected the element concentrations of epipelagic zooplankton in the Northern Gulf of California. Oceanographic mechanisms such as these may contribute to element incorporation in marine organisms in other seas.     

Abundant dissolved genetic material in Arctic sea ice Part II: Viral dynamics during autumn freeze-up

Viruses play a significant role in nutrient cycling within the world’s oceans and are important agents of horizontal gene transfer, but little is know about their entrainment into sea ice or their temporal dynamics once entrained. Nilas, grease ice, pancake ice, first-year sea ice floes up to 78 cm in thickness, and under-ice seawater were sampled widely across Amundsen Gulf (ca. 71^° N, 125^° W71^\circ \hbox{N}, 125^\circ \hbox{W}) for concentrations of viruses and bacteria. Here, we report exceptionally high virus-to-bacteria ratios in seawater (45–340) and sea ice (93–2,820) during the autumn freeze-up. Virus concentrations ranged from 4.8 to 27 × 10⁶  ml⁻¹ in seawater and, scaled to brine volume, 5.5 to 170 × 10⁷ ml⁻¹ in sea ice. Large enrichment indices indicated processes of active entrainment from source seawater, or viral production within the ice, which was observed in 2 of 3 bottle incubations of sea ice brine at a temperature (-7^°C-7^\circ\hbox{C}) and salinity ( 110 \permille110 \permille) approximating that in situ. Median predicted virus-to-bacteria contact rates (relative to underlying seawater) were greatest in the top of thick sea ice (66–78 cm: 130×) and lowest in the bottom of medium-thickness ice (33–37 cm: 23×). The great abundance of viruses and more frequent interactions between bacteria and viruses predicted in sea ice relative to underlying seawater suggest that sea ice may be a hot spot for virally mediated horizontal gene transfer in the polar marine environment.     

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.     

Photosynthetic characteristics and physiological plasticity of an Aphanizomenon flos-aquae (Cyanobacteria, Nostocaceae) winter bloom in a deep oligo-mesotrophic lake (Lake Stechlin, Germany)

In winter of 2009/2010, Aphanizomenon flos-aquae bloomed in the ice and snow covered oligo-mesotrophic Lake Stechlin, Germany. The photosynthesis of the natural population was measured at eight temperatures in the range of 2–35°C, at nine different irradiance levels in the range of 0–1,320 μmol m⁻² s⁻¹ PAR at each applied temperature. The photoadaptation parameter (I _k) and the maximum photosynthetic rate (P _max) correlated positively with the temperature between 2 and 30°C, and there was a remarkable drop in both parameters at 35°C. The low I _k at low temperatures enabled the active photosynthesis of overwintering populations at low irradiance levels under ice and snow cover. The optimum of the photosynthesis was above 20°C at irradiances above 150 μmol m⁻² s⁻¹. At lower irradiance levels (7.5–30 μmol m⁻² s⁻¹), the photosynthesis was the most intensive in the temperature range of 2–5°C. The interaction between light and temperature allowed the proliferation of A. flos-aquae in Lake Stechlin resulting in winter water bloom in this oligo-mesotrophic lake. The applied 2°C is the lowest experimental temperature ever in the photosynthesis/growth studies of A. flos-aquae, and the results of the P–I and P–T measurements provide novel information about the tolerance and physiological plasticity of this species.     

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     

Biosorption of Ni (II) by Schizosaccharomyces pombe: kinetic and thermodynamic studies

The potential of the dried yeast, wild-type Schizosaccharomyces pombe, to remove Ni(II) ion was investigated in batch mode under varying experimental conditions including pH, temperature, initial metal ion concentration and biosorbent dose. Optimum pH for biosorption was determined as 5.0. The highest equilibrium uptake of Ni(II) on S. pombe, q _e, was obtained at 25 °C as 33.8 mg g⁻¹. It decreased with increasing temperature within a range of 25–50 °C denoting an exothermic behaviour. Increasing initial Ni(II) concentration up to 400 mg L⁻¹ also elevated equilibrium uptake. No more adsorption took place beyond 400 mg L⁻¹. Equilibrium data fitted better to Langmuir model rather than Freundlich model. Sips, Redlich–Peterson, and Kahn isotherm equations modelled the investigated system with a performance not better than Langmuir. Kinetic model evaluations showed that Ni(II) biosorption process followed the pseudo-second order rate model while rate constants decreased with increasing temperature. Gibbs free energy changes (ΔG°) of the system at 25, 30, 35 and 50 °C were found as −1.47E + 4, −1.49E + 4, −1.51E + 4, and −1.58E + 4 J mol⁻¹, respectively. Enthalpy change (ΔH°) was determined as −2.57E + 3 J mol⁻¹ which also supports the observed exothermic behaviour of the biosorption process. Entropy change (ΔS°) had a positive value (40.75 J mol⁻¹ K⁻¹) indicating an increase in randomness during biosorption process. Consequently, S. pombe was found to be a potential low-cost agent for Ni(II) in slightly acidic aqueous medium. In parallel, it has been assumed to act as a separating agent for Ni(II) recovery from its aqueous solution.     

Winter as a nutritional bottleneck for North American porcupines (Erethizon dorsatum)

North American porcupines are distributed across a wide variety of habitats where they consume many different species of plants. Winter is a nutritional bottleneck for northern populations, because porcupines remain active when environmental demands are high and food quality is low. We used captive porcupines to examine physiological responses to low-quality diets at high energy demands during winter at ambient temperatures as low as −39°C. We did not observe an endogenous pattern of body mass gain or loss when porcupines were fed a low nitrogen diet (1.1% dry matter) ad libitum through winter. Dry matter intake declined from 43.6 to 14.6 g kg^−0.75 d⁻¹ even though ambient temperatures declined from −3 to −30°C, which indicates a seasonal decrease in metabolic rate. Porcupines consuming white spruce needles maintained digestive efficiency for energy (61%) and neutral detergent fiber (NDF) (50%). However, low requirements for energy (398 kJ kg^−0.75 d⁻¹) and nitrogen (209 mg kg^−0.75 d⁻¹) minimized the loss of body mass when intakes were low and plant toxins increased urinary losses of energy and nitrogen. Porcupines were also able to tolerate low intakes of sodium, even when dietary potassium loads were high. Porcupines use a flexible strategy to survive winter: low requirements are combined with a high tolerance for dietary imbalances that minimize the use of body stores when demands exceed supply. However, body stores are rapidly restored when conditions allow. Porcupines posses many physiological abilities similar to specialist herbivores, but retain the ability of a generalist to survive extreme conditions by using a variety of foods.     

Satellite tracking of a killer whale (Orcinus orca) in the eastern Canadian Arctic documents ice avoidance and rapid, long-distance movement into the North Atlantic

Killer whales (Orcinus orca) occur in the eastern Canadian Arctic during the open-water season, but their seasonal movements in Arctic waters and overall distribution are poorly understood. During August 2009, satellite transmitters were deployed onto two killer whales in Admiralty Inlet, Baffin Island, Canada. A whale tracked for 90 days remained in Admiralty and Prince Regent Inlets from mid-August until early October, when locations overlapped aggregations of marine mammal prey species. While in Admiralty and Prince Regent Inlets, the whale traveled 96.1 ± 45.3 km day⁻¹ (max 162.6 km day⁻¹) and 120.1 ± 44.5 km day⁻¹ (max 192.7 km day⁻¹), respectively. Increasing ice cover in Prince Regent Inlet in late September and early October was avoided, and the whale left the region prior to heavy ice formation. The whale traveled an average of 159.4 ± 44.8 km day⁻¹ (max 252.0 km day⁻¹) along the east coast of Baffin Island and into the open North Atlantic by mid-November, covering over 5,400 km in approximately one month. This research marks the first time satellite telemetry has been used to study killer whale movements in the eastern Canadian Arctic and documents long-distance movement rarely observed in this species.     

Comparative physiology of Australian quolls (Dasyurus; Marsupialia)

Quolls (Dasyurus) are medium-sized carnivorous dasyurid marsupials. Tiger (3,840 g) and eastern quolls (780 g) are mesic zone species, northern quolls (516 g) are tropical zone, and chuditch (1,385 g) were once widespread through the Australian arid zone. We found that standard physiological variables of these quolls are consistent with allometric expectations for marsupials. Nevertheless, inter-specific patterns amongst the quolls are consistent with their different environments. The lower T _b of northern quolls (34°C) may provide scope for adaptive hyperthermia in the tropics, and they use torpor for energy/water conservation, whereas the larger mesic species (eastern and tiger quolls) do not appear to. Thermolability varied from little in eastern (0.035°C °C⁻¹) and tiger quolls (0.051°C oC⁻¹) to substantial in northern quolls (0.100°C oC⁻¹) and chuditch (0.146°C oC⁻¹), reflecting body mass and environment. Basal metabolic rate was higher for eastern quolls (0.662 ± 0.033 ml O₂ g⁻¹ h⁻¹), presumably reflecting their naturally cool environment. Respiratory ventilation closely matched metabolic demand, except at high ambient temperatures where quolls hyperventilated to facilitate evaporative heat loss; tiger and eastern quolls also salivated. A higher evaporative water loss for eastern quolls (1.43 ± 0.212 mg H₂O g⁻¹ h⁻¹) presumably reflects their more mesic distribution. The point of relative water economy was low for tiger (−1.3°C), eastern (−12.5°C) and northern (+3.3) quolls, and highest for the chuditch (+22.6°C). We suggest that these differences in water economy reflect lower expired air temperatures and hence lower respiratory evaporative water loss for the arid-zone chuditch relative to tropical and mesic quolls.     

Chronic coral consumption by butterflyfishes

Interactions between predators and prey organisms are of fundamental importance to ecological communities. While the ecological impact that grazing predators can have in terrestrial and temperate marine systems are well established, the importance of coral grazers on tropical reefs has rarely been considered. In this study, we estimate the biomass of coral tissue consumed by four prominent species of corallivorous butterflyfishes. Sub-adult butterflyfishes (60–70 mm, 6–11 g) remove between 0.6 and 0.9 g of live coral tissue per day, while larger adults (>110 mm, ~40–50 g) remove between 1.5 and 3 g of coral tissue each day. These individual consumption rates correspond to the population of coral-feeding butterflyfishes at three exposed reef crest habitats at Lizard Island, Great Barrier Reef, consuming between 14.6 g (±2.0) and 19.6 g (±3.9) .200 m⁻² day⁻¹ of coral tissue. When standardised to the biomass of butterflyfishes present, a combined reefwide removal rate of 4.2 g (±1.2) of coral tissue is consumed per 200 m⁻² kg⁻¹ of coral-feeding butterflyfishes. The quantity of coral tissue removed by these predators is considerably larger than previously expected and indicates that coral grazers are likely to play an important role in the transfer of energy fixed by corals to higher consumers. Chronic coral consumption by butterflyfishes is expected to exact a large energetic cost upon prey corals and contribute to an increased rate of coral loss on reefs already threatened by anthropogenic pressure and ongoing climate change.     

The feeding rates ofPleurobrachia (ctenophora) andSagitta (chaetognatha), with notes on the potential seasonal role of planktonic predators in the dynamics of north sea zooplankton communities

The zooplankton off the north-east coast of England has been the subject of a number of studies focusing on its productivity. It has also been shown to be representative of the zooplankton of much of the western North Sea. The community contains a number of predatory species, three of which are widely described as ‘voracious’, the ctenophorePleurobrachia pileus, the chaetognathSagitta elegans and the hyperiid amphipodThemisto compressa (≡ Parathemisto gaudichaudi). This study investigates the role of these planktonic predators in this community, with special reference to the seasonal changes in predation pressure. The functional response ofPleurobrachia pileus feeding onAcartia was determined from laboratory experiments. It was found to be linear at prey densities typical of UK coastal waters, although the linear relationship appeared to break down at low and high prey densities. Feeding rate data forSagitta elegans were obtained from gut content analysis and published laboratory derived estimates of digestion time. Of the 1,789 individuals examined 198 (11.1%) had food remains in the gut. A linear relationship betweenSagitta length and prey size was established and the daily feeding rate ofSagitta elegans was estimated to be 0.4 prey items d⁻¹ ind⁻¹. For comparative purposes, the proportion of the copepod standing stock removed bySagitta elegans, Pleurobrachia pileus andThemisto gaudichaudi was estimated for each month of the year. From this model it was shown thatThemisto applied the most predation pressure, andSagitta elegans applied the least predation pressure of the three planktonic predators considered. The impact ofPleurobrachia will be to a large extent offset due to its peak of seasonal abundance coinciding with the zooplankton peak in the summer.     

An approach for the assessment of risk from chronic radiation to populations of phytoplankton and zooplankton

A conceptual model of the effects of chronic radiation on a population of phytoplankton and zooplankton in an oceanic nutrient layer is presented. The model shows that there are distinct threshold dose rates at which the different plankton populations become unsustainable. These are 10,400 μGy h⁻¹ for phytoplankton and 125 μGy h⁻¹ for zooplankton. Both these values are considerably greater than the current screening values for protection of 10 μGy h⁻¹. The model highlights the effects of predator–prey dynamics in predicting that when the zooplankton is affected by the radiation dose, the phytoplankton population can increase. In addition, the model was altered to replicate the dose rates to the plankton of a previous ERICA Irish Sea assessment (24 μGy h⁻¹ for zooplankton and 430 μGy h⁻¹ to phytoplankton). The results showed only a 10% decrease in the zooplankton population and a 15% increase in the phytoplankton population. Therefore, at this level of dose, the model predicts that although the dose rate exceeds the guideline value, populations are not significantly affected. This result highlights the limitations of a single screening value for different groups of organisms.     

Zooplankton boom and ice amphipod bust below melting sea ice in the Amundsen Gulf, Arctic Canada

Early summer in the Arctic with extensive ice melt and break-up represents a dramatic change for sympagic–pelagic fauna below seasonal sea ice. As part of the International Polar Year-Circumpolar Flaw Lead system study (IPY-CFL), this investigation quantified zooplankton in the meltwater layer below landfast ice and remaining ice fauna below melting ice during June (2008) in Franklin Bay and Darnley Bay, Amundsen Gulf, Canada. The ice was in a state of advanced melt, with fully developed melt ponds. Intense melting resulted in a 0.3- to 0.5-m-thick meltwater layer below the ice, with a strong halocline to the Arctic water below. Zooplankton under the ice, in and below the meltwater layer, was sampled by SCUBA divers. Dense concentrations (max. 1,400 ind. m⁻³) of Calanus glacialis were associated with the meltwater layer, with dominant copepodid stages CIV and CV and high abundance of nauplii. Less abundant species included Pseudocalanus spp., Oithona similis and C. hyperboreus. The copepods were likely feeding on phytoplankton (0.5–2.3 mg Chl-a m⁻³) in the meltwater layer. Ice amphipods were present at low abundance (<10 ind. m⁻²) and wet biomass (<0.2 g m⁻²). Onisimus glacialis and Apherusa glacialis made up 64 and 51% of the total ice faunal abundance in Darnley Bay and Franklin Bay, respectively. During early summer, the autochthonous ice fauna becomes gradually replaced by allochthonous zooplankton, with an abundance boom near the meltwater layer. The ice amphipod bust occurs during late stages of melting and break-up, when their sympagic habitat is diminished then lost.     

Changes in bacterial and ciliate densities with trophic status in Mediterranean shallow lakes

Ciliate and bacterial densities and their link with eutrophication were studied in fourteen shallow lakes in northwest Spain. Total phosphorus (TP) in these lakes varied between 30 μg l⁻¹ and 925 μg l⁻¹ and chlorophyll a concentration (chla) between 0.5 μg l⁻¹ and 107 μg l⁻¹. Bacterial abundance ranged from 1 × 10⁶ to 14 × 10⁶ cells ml⁻¹, while ciliate abundance ranged from 0.6 cells ml⁻¹ to 229 cells ml⁻¹. Lakes were classified into three trophic types from their TP and chla concentrations. Bacterial abundance was significantly correlated with trophic type, as well as with TP and with chla separately, whereas ciliate abundance was only correlated with chla. No significant relationship could be established between bacterial and ciliate abundance across the trophic gradient. A general pattern was observed in the ratios of bacterial abundance to TP and chla concentrations, of decreasing ratios with increases in the nutrient loading. This pattern was not found for ciliates. The dominant zooplankton group in 13 of the 14 lakes studied was Rotifera, which accounted for a mean of 71% of total zooplankton abundance (41% of zooplankton biomass). The positive correlation between bacteria and ciliates with this group, and the absence of any relationship with Cladocera suggest that top down control by cladocerans was weaker in our lakes than previously shown in northern European shallow lakes. Rotifers could be important predators of bacteria in the high-nutrient lakes of our study. Higher slopes of regressions on bacterial abundance towards the hypertrophic range indicate that top-down control was weaker in our lakes than in northern European shallow lakes.