Phytoplankton assemblage at equilibrium in large and deep subalpine lakes: a case study from Lago Maggiore (N. Italy)

In 1996, we studied the phytoplankton seasonal succession in Lago Maggiore (N. Italy) through weekly sampling. Such a frequency enabled us to evaluate the changes of the phytoplankton assemblage in the light of the equilibrium and non-equilibrium theories. The distinct phases of changing of the species composition were identified separating the samples by means of cluster analysis and non-metric multidimensional scaling (NMDS). We recognised well distinct phytoplankton associations, whose seasonal succession followed a clear cyclic path throughout the year, with spring and summer phases respectively characterised by a rapid turn-over of the assemblages and by a relative stability. Moreover, we observed an increase of species number and Shannon-Wiener diversity during the spring, followed by a summer decline of the diversity in spite of an unchanged species number. Because of the dominance of the same few species for about two months during summer, coupled with small fluctuations of the total biomass, we could identify the summer assemblage as a steady state assemblage. The aim of the present contribution, although describing the whole seasonal succession, is to draw the attention towards the species composition at the steady state, taking into account the functional properties of the species involved. The possible role of the metalimnetic niche in selecting a particular summer assemblage in deep and large lakes will be discussed.     

The roles of predation,competition, and exploitation in the trophic dynamics of a warmwater stream: a model synthesis,analysis, and application

We developed a trophic dynamic model of key populations and processes in the New River, West Virginia, to identify the mechanisms most responsible for maintaining food web structure. Key populations were represented by thirteen model components and were aquatic insects; age-1 and age-2 crayfish (three species); age-1 and age-2 hellgrammites (Corydalus cornutus larvae); non-game fishes; age-0, age-1, and adult smallmouth bass (Micropterus dolomieu); age-0, age-1, and adult rock bass (Ambloplites rupestris); and age-0, age-1 to age-3, and adult flathead catfish (Pylodictis olivaris). In this system, crayfish and hellgrammites are harvested to provide bait for the recreational fishery that extensively exploits the three predatory fish species. Predation and intraspecific regulation were represented with nonlinear algorithms, and linear terms represented fishery harvests. Interspecific competition among components occurred through predation on shared prey. Error analysis of the model suggested that predation was the most important mechanism in maintaining system structure (the disposition of biomass among system components). Further, the trophic relation between each component and its prey accounted for 34–64% of the variability in food web structure, whereas predation on each component explained 1–24% of food web structure variability. Therefore, so-called ‘bottom-up’ effects were more influential than ‘top-down’ effects. Interspecific competition and intraspecific regulation had secondary roles in maintaining New River food web structure, although intraspecific regulation was most important to aquatic insects, which were not predatory in our model. Both forms of competition are probably tempered by extensive predation and exploitation in the New River system. Exploitation was a secondary structuring agent to adult smallmouth bass, which experience a high rate of harvest in the New River.     

Predicting diet, trophic level and palaeoecology from bone stable isotope analysis: a comparative study of five red deer populations

C and N stable isotope ratios of red deer (Cervus elaphus) bone collagen (154 individuals) from five modern populations occupying geographically different habitats are reported. No significant difference was observed between deer occupying forested and non-forested environments subject to similar climatic conditions suggesting a simple “canopy effect” is not observed. Mean population δ¹³C is negatively correlated with temperature whereas mean population δ¹⁵N is positively correlated with temperature. A weak but significant positive correlation was observed between deer age and collagen δ¹³C values from the Isle of Rum population (Scotland). The amount of intra-population isotope variability is not consistent among populations; thus significant numbers of individuals from each species are required for modern food web studies, for palaeodietary baseline data, and for palaeoecological studies.     

Plant responses to simulated warming and drought: a comparative study of functional plasticity between congeneric mid and high elevation species

Aims Effects of climate change, especially changes in temperatures and precipitation patterns, are particularly pronounced in alpine regions. In response, plants may exhibit phenotypic plasticity in key functional traits allowing short-term adjustment to novel conditions. However, little is known about the degree of phenotypic plasticity of high elevation species relative to mid elevation congeners.Methods We transplanted 14 herbaceous perennial species from high elevation into two common gardens (1050 and 2000 m.a.s.l.) in the Swiss Alps, and we examined plastic responses in key functional traits to changes in temperature and soil water availability. This design was replicated with 14 congeneric species from mid elevation to assess if the degree of phenotypic plasticity differs between mid and high elevation species. Survival was assessed across two growing seasons, while aboveground biomass and specific leaf area (SLA) were measured after the first growing season, and biomass allocation to belowground and reproductive structures after the second. Moreover, a phenotypic plasticity index was calculated for the functional traits to compare the degree of plasticity between mid and high elevation species.Important findings Aboveground biomass was higher in mid elevation species relative to high elevation congeners in all treatments, yet decreased for both with elevation and drought. Similarly, SLA decreased with elevation and drought. Root mass fraction (RMF) was generally higher in high elevation species, and decreased with drought at the lower site. Drought increased the allocation to reproductive structures, especially when plants were grown at their elevation of origin. Interestingly, no difference was found in the degree of phenotypic plasticity averaged across mid and high elevation species for any of the studied functional traits. These results indicate that phenotypic plasticity in the focal traits did not depend on the elevation of origin of the species. Plasticity was not related to environmental heterogeneity, nor constrained by selective pressures at high elevation. However, both species groups showed a remarkable capacity for short-term acclimation to a prospective climate through rapid adjustments in key functional traits.     

Survival after a diagnosis of testicular germ cell cancers in Germany and the United States, 2002–2006: A high resolution study by histology and age

Introduction: The aim of this study was to provide detailed age-specific (5-year age groups) and histology-specific (histologic subtypes of seminoma and nonseminoma) relative survival estimates of testicular germ cell cancer patients in Germany and the United States (U.S.) for the years 2002–2006 and to compare these estimates between countries. Methods: We pooled data from 11 cancer registries of Germany and used data from the U.S. (SEER-13 database) including 11,508 and 10,774 newly diagnosed cases (1997–2006) in Germany and the U.S., respectively. We estimated 5-year relative survival (5-year-RS) by histology and age based on period analysis. Results: 5-year-RS for testicular germ cell tumors was 96.7% and 96.3% in Germany and the U.S., respectively. 5-Year-RS for spermatocytic seminoma was close to 100% in both countries. 5-Year-RS for nonseminoma was lower than for classical seminoma in Germany (93.3% versus 97.6%) and the U.S. (91.0% versus 98.2%). Among nonseminomas, choriocarcinomas provided the lowest 5-year-RS in both countries (Germany 80.1%, U.S. 79.6%). Age-specific 5-year-RS for seminoma showed only little variation by age. 5-Year-RS for nonseminomas tended to be lower at higher ages, especially for malignant teratoma. Discussion: This is the first study that provides up-to-date survival estimates for testicular cancer by histology and age in Germany and the U.S. Survival after a diagnosis of testicular cancer is very comparable between Germany and the U.S. 5-Year-RS for spermatocytic seminoma was close to 100% and the lowest 5-year-RS occurred among choriocarcinoma. Higher age at diagnosis is associated with a poorer prognosis among nonseminoma patients.     

Lake Erie Microcystis: Relationship between microcystin production, dynamics of genotypes and environmental parameters in a large lake

Cyanobacteria of genus Microcystis sp. have been commonly found in Lake Erie waters during recent summer seasons. In an effort to elucidate relationships between microcystin production, genotypic composition of Microcystis community and environmental parameters in a large lake ecosystem, we collected DNA samples and environmental data during a three-year (2003–2005) survey within Lake Erie and used the data to perform a series of correlation analyses. Cyanobacteria and Microcystis genotypes were quantified using quantitative real-time PCR (qPCR). Our data show that Microcystis in Lake Erie forms up to 42% of all cyanobacteria, and that Microcystis exists as a mixed population of potentially toxic and (primarily) non-toxic genotypes. In the entire lake, the total abundance of Microcystis as well as the abundance of microcystin-producing Microcystis is strongly correlated with the abundance of cyanobacteria suggesting that Microcystis is a significant component of the cyanobacterial community in Lake Erie during summer seasons. The proportion of total Microcystis of all cyanobacteria was strongly linked to the microcystin concentrations, while the percentage of microcystin-producing genotypes within Microcystis population showed no correlation with microcystin concentrations. Correlation analysis indicated that increasing total phosphorus concentrations correlate strongly with increasing microcystin concentrations as well as with the total abundance of Microcystis and microcystin-producing Microcystis.     

Fast response of freshwater consumers to a new trophic resource: Predation on the recently introduced Asian bivalve Limnoperna fortunei in the lower Paraná river,South America

Abstract Comparison of Limnoperna fortunei numbers and biomass in screened (5, 15 and 40 mm) and unscreened cages deployed for 18 months in the lower Paraná delta indicates that predators harvest 26–79% (numbers), or 20–85% (biomass) of the mussel population. Predation impact decreases with mussel size. On average, 6 kg of whole live mussel × m⁻² × year⁻¹ (0.36 g of dry mussel tissue × m⁻² × day⁻¹) were eliminated from the unscreened cages. Cages with 15 and 40 mm screens lost between 1 and 2 kg × m⁻² × year⁻¹. Aquatic mammals, birds, and especially fish, are probably the main consumers of large mussels. Small L. fortunei are most probably eaten by fish and also by several invertebrates, including crustaceans, leeches and gastropods. It is suggested that L. fortunei intercepts a significant fraction of the organic carbon that the Paraná‐Uruguay rivers flush into the ocean, locally boosting numbers of benthophagous animals, deposit feeders and, indirectly, higher level predators. Our results indicate that only 15 years after its first introduction in South America this invasive species is very actively consumed by local predators, but predatory suppression of the mussel seems very unlikely. Comparisons with the effects reported for the zebra mussel (Dreissena polymorpha) in Europe and North America suggest that L. fortunei is consumed more actively and that its negative impact on the local fauna is more restricted. These differences are attributed to the fact that while D. polymorpha feeds chiefly on plankton, a limited resource, L. fortunei feeds on detrital particulate organic matter, whose supply in these large South American rivers largely exceeds consumption.     

Experimental study of trophic cascade effect of silver carp (Hypophthalmichthys molitrixon) in a subtropical lake, Lake Donghu: on plankton community and underlying mechanisms of changes of crustacean community

An enclosure experiment was carried out to test trophic cascade effect of filter-feeding fish on the ecosystem: growth of crustacean zooplankton, and possible mechanism of changes of crustacean community structure. Four fish biomass levels were set as follows: 0, 116, 176 and 316 g m^-2, and lake water (containing ca. 190 g m^-2 of filter-feeding fishes) was comparatively monitored. Nutrient levels were high in all treatments during the experiment. Lowest algal biomass were measured in fishless treatment. Algal biomass decreased during days 21–56 as a function of fish biomass in treatments of low (LF), medium (MF) and high (HF) fish biomass. Crustaceans biomass decreased with increasing fish biomass. Small-bodied cladocerans, Moina micrura, Diaphanosoma brachyurum and Scapholeberis kingii survived when fish biomass was high whilst, large-bodied cladocerans Daphnia spp. and the cyclopoids Theromcyclops taihokuensis, T. brevifuratus, Mescyclops notius and Cyclops vicinus were abundant only in NF enclosures. Evasive calanoid Sinodiaptomus sarsi was significantly enhanced in LF, but decreased significantly with further increase of fish biomass. Demographic data indicated that M. micrura was well developed in all treatments. Our study indicates that algal biomass might be controlled by silver carp biomass in eutrophic environment. Changes of crustacean community are probably affected by the age of the first generation of species. Species with short generation time were dominant and species with long generation time survived less with high fish biomass. Evasive calanoids hardly developed in treatments with high fish biomass because of the (bottle neck) effect of nauplii. Species abundance were positively related to fish predation avoidance. Other than direct predation, zooplankton might also be suppressed by filter-feeding fish via competition.     

Factors affecting the seasonality and distribution of cyanobacteria and chlorophytes: a case study from the large lakes south of the Alps,with special reference to Lake Garda

Chlorophytes and cyanobacteria are among the most typical algal groups, contributing to the aesthetic appearance and quality of the epilimnetic waters of the deep (251–410 m) and large (6.5–49×10⁹ m³) lakes located on the southern edge of the Alps (from oligo-mesotrophy to meso-eutrophy: Maggiore, Garda, Como, Iseo and Lugano). The results obtained from monthly surveys carried out in the largest of these lakes (Garda) have been reported in detail. The thermal stability of the water column and silica depletion were the main factors responsible for the decline of the great spring diatoms. The successive growth of Mougeotia sp. was favoured by its lower sinking velocity and resistance to the increasing grazing pressure. During summer, the maximum stability of the water column, with high vertical nutrient concentration gradients, determined a major algal differentiation with a typical increase, among others, of Chlorococcales at the surface and metalimnetic stratification of various Oscillatoriales. The development of oligotrophic blooms, caused by a rapid thickening at the surface of Anabaena in the eastern, sheltered basin, was also discussed in light of the trophic characterisation of Lake Garda. From autumn to spring, the decreasing light, the increasing mixing depth and nutrient availability favoured a progressive dominance of vertical homogeneous populations of Planktothrix (autumn) and colonial diatoms. The same functional groups of chlorophytes and cyanobacteria have been recognised in the studies published so far on the phytoplankton of deep southern subalpine lakes. In this respect, their morphometric and physical properties appear to constitute a sort of standardising factor, reducing the range of possible dominants. With increasing TP concentrations and biomass, filaments of Planktothrix and Planktolyngbya, along with the Chlorococcales, became important. The dominance of Mougeotia (one of the most characteristic features of these large lakes) appears restricted to a medium trophic range, whereas the distribution of the Chroococcales and other filaments ascribed to Pseudanabaena and/or Limnothrix is more irregular. The conspicuous presence of Aphanizomenon in Lake Lugano is typical and characteristic of a more productive lake. Among the Nostocales, a clear interpretation of the Anabaena blooms in lakes Garda and Iseo is complicated by the peculiar behaviour of filaments concentrating at the surface, which is apparently restricted, within the medium trophic range, to stable water columns.     

Soil animal responses to moisture availability are largely scale,not ecosystem dependent: insight from a cross‐site study

Climate change will result in reduced soil water availability in much of the world either due to changes in precipitation or increased temperature and evapotranspiration. How communities of mites and nematodes may respond to changes in moisture availability is not well known, yet these organisms play important roles in decomposition and nutrient cycling processes. We determined how communities of these organisms respond to changes in moisture availability and whether common patterns occur along fine‐scale gradients of soil moisture within four individual ecosystem types (mesic, xeric and arid grasslands and a polar desert) located in the western United States and Antarctica, as well as across a cross‐ecosystem moisture gradient (CEMG) of all four ecosystems considered together. An elevation transect of three sampling plots was monitored within each ecosystem and soil samples were collected from these plots and from existing experimental precipitation manipulations within each ecosystem once in fall of 2009 and three times each in 2010 and 2011. Mites and nematodes were sorted to trophic groups and analyzed to determine community responses to changes in soil moisture availability. We found that while both mites and nematodes increased with available soil moisture across the CEMG, within individual ecosystems, increases in soil moisture resulted in decreases to nematode communities at all but the arid grassland ecosystem; mites showed no responses at any ecosystem. In addition, we found changes in proportional abundances of mite and nematode trophic groups as soil moisture increased within individual ecosystems, which may result in shifts within soil food webs with important consequences for ecosystem functioning. We suggest that communities of soil animals at local scales may respond predictably to changes in moisture availability regardless of ecosystem type but that additional factors, such as climate variability, vegetation composition, and soil properties may influence this relationship over larger scales.     

Vegetation history and lake-level changes from the Younger Dryas to the present in Eastern Pyrenees (France): pollen, plant macrofossils and lithostratigraphy from Lake Racou (2000 m a.s.l.)

A multi-proxy palaeoecological investigation including pollen, plant macrofossil, radiocarbon and sedimentological analyses, was performed on a small mountain lake in the Eastern Pyrenees. This has allowed the reconstruction of: (1) the vegetation history of the area based on five pollen diagrams and eight AMS¹⁴C dates and (2) the past lake-level changes, based on plant macrofossil, lithological and pollen analysis of two stratigraphical transects correlated by pollen analysis. The palaeolake may have appeared before the Younger Dryas; the lake-level was low and the vegetation dominated by cold steppic grasslands. The lake-level rose to its highest level during the Holocene in the Middle Atlantic (at ca. 5060±45 b.p.). Postglacial forests (Quercetum mixtum and Abieto-Fagetum) developed progressively in the lower part of the valley, while dense Pinus uncinata forests rapidly invaded the surroundings of the mire and remained the dominant local vegetation until present. The observed lowering of the lake levels during the Late Atlantic and the Subboreal (from 5060 ± B.P. to 3590±40 b.p.) was related to the overgrowth of the mire. The first obvious indications of anthropogenic disturbances of the vegetation are recorded at the Atlantic/Subboreal boundary as a reduction in the forest component, which has accelerated during the last two millennia.     

基于高分遥感影像的滇西北村域景观格局演变——以大理市低丘缓坡山地开发区为例

人类的开发活动是造成土地覆盖和景观格局变化的主要原因.村域尺度上高强度的人类开发活动对土地覆盖及景观格局演变的影响规律研究尚不多见.本研究采用2009年的GeoEye-1数据和2014年的WorldView 3数据,利用ArcGIS和ENVI,基于面向对象和人机交互的方法解译影像,应用土地利用转移矩阵和景观指数定量研究大理市海东镇低丘缓坡山区改造过程中的土地覆盖变化和景观格局演变.结果表明: 2009年主要土地覆盖类型是林地、水田和旱地,占总面积的82.8%,2014年林地、推平未建地和水田占总面积的70.9%;研究期间,土地利用变化主要由林地、水田和旱地向推平未建地、建设用地转移,尤其是2014年推平未建地面积达531.57 hm²,其中,来自林地、旱地和水田的面积分别占42.8%、21.7%和14.2%.景观空间格局演变表现为斑块数量和密度增加,平均斑块面积变小;边缘指数和形状指数增加,斑块形状更加复杂;斑块破碎化,整体构成更加多样化.     

Seasonal variation in primary production of a large high altitude tropical lake (Lake Tana,Ethiopia): effects of nutrient availability and water transparency

Primary production rates, chlorophyll and phytoplankton biovolume were measured monthly from April 2003 to November 2004 in Lake Tana, a large tropical lake in the highlands of Ethiopia. The lake is characterised by low nutrient concentrations, and a low water transparency due to high silt load of the inflowing rivers during the rainy seasons (May–November) and daily resuspension of sediments in the inshore zone. The mean chlorophyll-a concentrations varied seasonally and ranged from 2.6 mg m⁻³ to 8.5 mg m⁻³ (mean: 4.5 mg m⁻³) in the offshore zone. Primary production was measured using the light–dark bottles technique. We incubated only at three depths, i.e. 0.6, 1.2 and 1.8 m. Therefore, we may have missed a substantial part of the depth production profile and probably also frequently missed P _max. Gross primary production in the openwater averaged 2.43 g O₂ m⁻² d⁻¹ and ranged between 0.03 g O₂ m⁻² d⁻¹ and 10.2 g O₂ m⁻² d⁻¹; production was significantly higher in the inshore zone. The highest production rates were observed in the post-rainy season (Oct–Nov), which coincided with a bloom of Microcystis and higher chlorophyll levels. This seasonal high production is probably caused by a relatively high nutrient availability in combination with favourable light conditions. The gross primary production rates of L. Tana are among the lowest compared with other tropical lakes. This will be partly the result of our underestimation of gross primary production by often missing P _max. Another cause is the oligotrophic nature of the lake in combination with its relatively low water transparency. The gross primary production per unit chlorophyll in the openwater zone was in the same range as in 30 other tropical lakes and reservoirs. The higher primary production in the inshore zone is probably the result of the daily water column mixing (Z _mix ≥ Z _t) in this area, enhancing nutrient recycling. A large proportion of the annual primary production is realised in one of the four seasons only. This productive post-rainy season is relatively short (2 months) and therefore efficiency of transfer of matter between the first and second trophic level of the Lake ecosystem will be poor.     

Epidemiological,clinical, and public health response characteristics of a large outbreak of diphtheria among the Rohingya population in Cox’s Bazar,Bangladesh, 2017 to 2019: A retrospective study

BackgroundUnrest in Myanmar in August 2017 resulted in the movement of over 700,000 Rohingya refugees to overcrowded camps in Cox’s Bazar, Bangladesh. A large outbreak of diphtheria subsequently began in this population.Methods and findingsData were collected during mass vaccination campaigns (MVCs), contact tracing activities, and from 9 Diphtheria Treatment Centers (DTCs) operated by national and international organizations. These data were used to describe the epidemiological and clinical features and the control measures to prevent transmission, during the first 2 years of the outbreak. Between November 10, 2017 and November 9, 2019, 7,064 cases were reported: 285 (4.0%) laboratory-confirmed, 3,610 (51.1%) probable, and 3,169 (44.9%) suspected cases. The crude attack rate was 51.5 cases per 10,000 person-years, and epidemic doubling time was 4.4 days (95% confidence interval [CI] 4.2–4.7) during the exponential growth phase. The median age was 10 years (range 0–85), and 3,126 (44.3%) were male. The typical symptoms were sore throat (93.5%), fever (86.0%), pseudomembrane (34.7%), and gross cervical lymphadenopathy (GCL; 30.6%). Diphtheria antitoxin (DAT) was administered to 1,062 (89.0%) out of 1,193 eligible patients, with adverse reactions following among 229 (21.6%). There were 45 deaths (case fatality ratio [CFR] 0.6%). Household contacts for 5,702 (80.7%) of 7,064 cases were successfully traced. A total of 41,452 contacts were identified, of whom 40,364 (97.4%) consented to begin chemoprophylaxis; adherence was 55.0% (N = 22,218) at 3-day follow-up. Unvaccinated household contacts were vaccinated with 3 doses (with 4-week interval), while a booster dose was administered if the primary vaccination schedule had been completed. The proportion of contacts vaccinated was 64.7% overall. Three MVC rounds were conducted, with administrative coverage varying between 88.5% and 110.4%. Pentavalent vaccine was administered to those aged 6 weeks to 6 years, while tetanus and diphtheria (Td) vaccine was administered to those aged 7 years and older. Lack of adequate diagnostic capacity to confirm cases was the main limitation, with a majority of cases unconfirmed and the proportion of true diphtheria cases unknown.ConclusionsTo our knowledge, this is the largest reported diphtheria outbreak in refugee settings. We observed that high population density, poor living conditions, and fast growth rate were associated with explosive expansion of the outbreak during the initial exponential growth phase. Three rounds of mass vaccinations targeting those aged 6 weeks to 14 years were associated with only modestly reduced transmission, and additional public health measures were necessary to end the outbreak. This outbreak has a long-lasting tail, with Rt oscillating at around 1 for an extended period. An adequate global DAT stockpile needs to be maintained. All populations must have access to health services and routine vaccination, and this access must be maintained during humanitarian crises.

Jonathan Polonsky and colleagues report on a diphtheria outbreak among Rohingya people in Cox’s Bazar, Bangladesh during 2017-19.     

Effects of clonal variation on growth, metabolism, and productivity in response to trophic factor stimulation: a study of Chinese hamster ovary cells producing a recombinant monoclonal antibody

The growth, metabolism, and productivity of five Chinese hamster ovary (CHO) clones were explored in response to stimulation with insulin (5 mg/L) and LONG^®R³IGF-I (20 μg/L or 100 μg/L). All five clones were derived from the same parental CHO cell line (DG44) and produced the same recombinant monoclonal antibody, with varying specific productivities. There was no uniform response among the clones to stimulation with the different trophic factors. One of the high productivity clones (clone D) exhibited significantly better growth in response to LONG^®R³IGF-I; whereas the other clones showed equivalent or slightly better growth in the presence of insulin. Three out of the five clones had higher specific productivities in the presence of insulin (although not statistically significant); one was invariant, and the final clone exhibited slightly higher specific productivity in the presence of LONG^®R³IGF-I. Total product titers exhibited moderate variation between culture conditions, again with neither trophic factor being clearly superior. Overall product titers were affected by variations in both integrated viable cell density and specific productivity. Nutrient uptake and metabolite generation patterns varied strongly between clones and much less with culture conditions. These results point to the need for careful clonal analysis when selecting clones, particularly for platform processes where media and culture conditions are predetermined.

Electronic supplementary material

The online version of this article (doi:10.1007/s10616-011-9388-z) contains supplementary material, which is available to authorized users.     

Modeling density dependence and climatic disturbances in caribou: a case study from the Bathurst Island complex, Canadian High Arctic

Peary caribou Rangifer tarandus pearyi is the northernmost subspecies of Rangifer in North America and endemic to the Canadian High Arctic. Because of severe population declines following years of unfavorable winter weather with ice coating on the ground or thicker snow cover, it is believed that density-independent disturbance events are the primary driver for Peary caribou population dynamics. However, it is unclear to what extent density dependence may affect population dynamics of this species. Here, we test for different levels of density dependence in a stochastic, single-stage population model, based on available empirical information for the Bathurst Island complex (BIC) population in the Canadian High Arctic. We compare predicted densities with observed densities during 1961–2001 under various assumptions of the strength of density dependence. On the basis of our model, we found that scenarios with no or very low density dependence led to population densities far above observed densities. For average observed disturbance regimes, a carrying capacity of 0.1 caribou km⁻² generated an average caribou density similar to that estimated for the BIC population over the past four decades. With our model we also tested the potential effects of climate change-related increases in the probability and severity of disturbance years, that is unusually poor winter conditions. On the basis of our simulation results, we found that, in particular, potential increases in disturbance severity (as opposed to disturbance frequency) may pose a considerable threat to the persistence of this species.     

Temperature-induced changes in fatty acid dynamics of the intertidal grazer Platychelipus littoralis (Crustacea,Copepoda, Harpacticoida): Insights from a short-term feeding experiment

Dietary lipids, and in particular the essential fatty acids (EFA), EPA (20:5ω3) and DHA (22:6ω3), guarantee the well-being of animals and are recognized for their potential bottom-up control on animal populations. They are introduced in marine ecosystems through primary producers and when grazed upon, they are consumed, incorporated or modified by first-level consumers. As the availability of EFA in the ecosystem is affected by ambient temperature, the predicted rise in ocean temperature might alter the availability of these EFA at the basis of marine food webs. Despite the FA bioconversion capacity of certain benthic copepod species, their lipid (FA) response to varying temperatures is understudied. Therefore, the temperate, intertidal copepod Platychelipus littoralis was offered a mono and mixed diatom diet at 4, 15 °C (normal range) and at 24 °C (elevated temperature) to investigate the combined effects of temperature and resource availability on its FA content and composition. P. littoralis showed a flexible thermal acclimation response. Cold exposure increased the degree of FA unsaturation and the EPA%, and induced a shift towards shorter chain FA in the copepod’s membranes. Furthermore, a mixed diet reduced the impact of heat stress on the copepod’s membrane FA composition. Temperature affected the trophic transfer of EPA and DHA differently. While dietary resources could fully compensate for the temperature effects on total lipid and EPA content in the copepods, no such counterweigh was observed for the DHA dynamics. Heat stress lowered the DHA concentration in copepods regardless of the resources available and this implies negative effects for higher trophic levels.