Nitrate leaching from three afforestation chronosequences on former arable land in Denmark

In regions dominated by agricultural activities, nitrogen (N) is recognized as a major pollutant in aquatic environments. In north‐western Europe, afforestation of agricultural land is part of a strategy to improve water quality. In Denmark, former arable land has been afforested during the past 40–50 years. This study evaluated the effect of afforestation of former arable land on nitrate leaching, based on three afforestation chronosequences. Precipitation, canopy throughfall and soil water were collected and soil moisture was monitored at two Danish locations, Vestskoven (nutrient‐rich, medium deposition) and Gejlvang (nutrient‐poor, high deposition). Afforestation was performed using Norway spruce [Picea abies (Karst.) L.] and common oak (Quercus robur L.) at Vestskoven and Norway spruce at Gejlvang. The results suggest that afforestation of former arable land initially leads to lower nitrate leaching than that occurring under the former agricultural land use, and largely below the standard of 50 mg NO⁻₃ L⁻¹ for groundwater to be utilized as drinking water. Nitrate concentrations became almost negligible in forest stands of 5–20 years of age. However, after canopy closure (>20 years) nitrate concentrations below the root zone and nitrate leaching tended to increase. This was attributed to increased N deposition with increasing canopy development and decreased N demand once the most N‐rich biomass compartments had been built up. Nitrate leaching started to increase at a throughfall deposition level of about 10 kg N ha⁻¹ yr⁻¹. Compared with nutrient‐poor sandy soils, nutrient‐rich clayey soils appeared more vulnerable to disturbance of the N cycle and to increased N deposition, leading to N saturation and enhanced nitrate leaching. In approximately the first 35 years after afforestation, nitrate leaching below the root zone was generally higher below oak than below Norway spruce.     

Delimiting species in the hoplonemertean genus Tetrastemma (phylum Nemertea): morphology is not concordant with phylogeny as evidenced from mtDNA sequences

The marine genus Tetrastemma contains monostiliferous hoplonemertean (phylum Nemertea) species which mostly undergo direct development with no free-swimming stages of larvae. The lack of a pelagic phase, and the fact that many benthic species lay eggs attached to the bottom substrate, are obvious restrictions on dispersal and gene flow. Nevertheless, some of the species, for example T. candidum and T. melanocephalum , are described as ubiquitous and are reported from waters all over the world. We studied genetic variation and evolutionary relationships in order to assess whether they are concordant with external characters in samples of nine morphologically distinct forms formally named as different Tetrastemma species, from different geographical localities. We estimated the phylogeny and species network based on 539 base pairs of the mitochondrial protein-coding gene cytochrome oxidase-1 (CO1) for 30 ingroup specimens. From this, we assessed the evolutionary history and phylogenetic relationships between these forms. We conclude that in most cases there was no correspondence between evolutionary lineage and morphotype. Our results thus indicate that morphological species delimitation in nemerteans may be questionable, and that this in turn may have a profound effect upon estimates of species diversity within the phylum.  © 2005 The Linnean Society of London, Biological Journal of the Linnean Society , 2005, 86 , 201–212.     

Non‐lethal predator effects on the performance of a native and an exotic crayfish species

1. I tested the hypothesis that the potential for non‐lethal effects of predators are more important for overall performance of the fast‐growing exotic signal crayfish (Pacifastacus leniusculus Dana) than for the slower growing native noble crayfish (Astacus astacus L.). I further tested if omnivorous crayfish switched to feed on less risky food sources in the presence of predators, a behaviour that could reduce the feeding costs associated with predator avoidance. 2. In a 2 month long outdoor pool experiment, I measured behaviour, survival, cheliped loss, growth, and food consumption in juvenile noble or signal crayfish in pools with either a caged predatory dragonfly larvae (Aeshna sp.), a planktivorous fish that do not feed on crayfish (sunbleak, Leucaspius delineatus Heckel), or predator‐free controls. Crayfish had access to multiple food sources: live zooplankton, detritus and periphyton. Frozen chironomid larvae were also supplied ad libitum outside crayfish refuges, simulating food in a risky habitat. 3. Crayfish were mainly active during hours of darkness, with signal crayfish spending significantly more time outside refuges than noble crayfish. The proportion of crayfish outside refuges varied between crayfish species, time and predator treatment, with signal crayfish spending more time in refuges at night in the presence of fish. 4. Survival in noble crayfish was higher than in signal crayfish, and signal crayfish had a higher frequency of lost chelipeds, indicating a high level of intraspecific interactions. Crayfish survival was not affected by the presence of predators. 5. Gut‐contents analysis and stable isotope values of carbon (δ¹³C) and nitrogen (δ¹⁵N) indicated that the two crayfish species had similar food preferences, and that crayfish received most of their energy from feeding on invertebrates (e.g. chironomid larvae), although detritus was the most frequent food item in their guts. Signal crayfish guts were more full than those of noble crayfish, but signal crayfish in pools with fish contained significantly less food and fewer had consumed chironomids compared with predator‐free controls. Length increase of signal crayfish (35%) was significantly higher than of noble crayfish (20%), but signal crayfish in pools with fish grew less than in control pools. 6. This short‐term study indicates that fish species that do not pose a lethal threat to an organism may indirectly cause reductions in growth by affecting behaviour and feeding. This may occur even though prey are omnivorous and have access to and consume multiple food sources. These non‐lethal effects of predators are expected to be particularly important in exotic crayfish species that show a general response to fish, have high individual growth rates, and when their feeding on the most profitable food source is reduced.     

Long-bone geometry in columnar-limbed animals: allometry of the proboscidean appendicular skeleton

Limb-bone allometry was investigated for 19 species of proboscideans, spanning almost the entire phylogenetic spectrum. More archaic proboscideans ('gompthotheres') have substantially thicker long-bone diaphyses relative to length than elephantids, as has been suggested previously, but contrary to previous suggestions it could not be confirmed that Mammuthus had more massive long-bone diaphyses on average than extant Elephas and Loxodonta . When correcting for phylogeny, the circumference of the limb bones to their length in proboscideans as a group generally scale with negative allometry, becoming stouter with increased length, as would be expected from limb mechanics. Few slopes were, however, statistically significantly negatively allometric. Correcting for phylogeny produced better correlations than traditional regression analyses, in contrast to most other studies where the reverse is the case. Intraspecific analyses of extant Elephas and Loxodonta , in addition to Mammuthus primigenius , Mammut americanum, and Gomphotherium productum , also resulted in negatively allometric regression slopes, frequently conforming to the theory of elastic similarity, as could be expected from the columnar posture of proboscideans. At present the reasons for the more massive limbs of gomphotheres s.l. are not fully understood. © 2007 The Linnean Society of London, Zoological Journal of the Linnean Society , 2007, 149 , 423–436.     

Long‐term ammonium nutrition of Arabidopsis increases the extrachloroplastic NAD(P)H/NAD(P)+ ratio and mitochondrial reactive oxygen species level in leaves but does not impair photosynthetic capacity

Ammonium nutrition has been suggested to be associated with alterations in the oxidation‐reduction state of leaf cells. Herein, we show that ammonium nutrition in Arabidopsis thaliana increases leaf NAD(P)H/NAD(P)⁺ ratio, reactive oxygen species content and accumulation of biomolecules oxidized by free radicals. We used the method of rapid fractionation of protoplasts to analyse which cellular compartments were over‐reduced under ammonium supply and revealed that observed changes in NAD(P)H/NAD(P)⁺ ratio involved only the extrachloroplastic fraction. We also showed that ammonium nutrition changes mitochondrial electron transport chain activity, increasing mitochondrial reactive oxygen species production. Our results indicate that the functional impairment associated with ammonium nutrition is mainly associated with redox reactions outside the chloroplast.     

Niche segregation of coexisting Arctic charr (Salvelinus alpinus) and brown trout (Salmo trutta) constrains food web coupling in subarctic lakes

1. Generalist fish species are recognised as important couplers of benthic and pelagic food‐web compartments in lakes. However, interspecific niche segregation and individual specialisation may limit the potential for generalistic feeding behaviour. 2. We studied summer habitat use, stomach contents and stable isotopic compositions of the generalist feeder Arctic charr coexisting with its common resource competitor brown trout in five subarctic lakes in northern Norway to reveal population‐level and individual‐level niche plasticity. 3. Charr and trout showed partial niche segregation in all five lakes. Charr used all habitat types and a wide variety of invertebrate prey including zooplankton, whereas trout fed mainly on insects in the littoral zone. Hence, charr showed a higher potential to promote habitat and food‐web coupling compared to littoral‐dwelling trout. 4. The level of niche segregation between charr and trout and between pelagic‐caught and littoral‐caught charr depended on the prevailing patterns of interspecific and intraspecific resource competition. The two fish species had partially overlapping trophic niches in one lake where charr numerically dominated the fish community, whereas the most segregated niches occurred in lakes where trout were more abundant. 5. In general, pelagic‐caught charr had substantially narrower dietary and isotopic niches and relied less on littoral carbon sources compared to littoral‐caught conspecifics that included generalist as well as specialised benthivorous and planktivorous individuals. Despite the partially specialised planktivorous niche and thus reduced potential of pelagic‐dwelling charr to promote benthic–pelagic coupling, the isotopic compositions of both charr subpopulations suggested a significant reliance on both littoral and pelagic carbon sources in all five study lakes. 6. Our study demonstrates that both interspecific niche segregation between and individual trophic specialisation within generalist fish species can constrain food‐web coupling and alter energy mobilisation to top consumers in subarctic lakes. Nevertheless, pelagic and littoral habitats and food‐web compartments may still be highly integrated due to the potentially plastic foraging behaviour of top consumers.     

Variability in exchange of CO2 across 12 northern peatland and tundra sites

Many wetland ecosystems such as peatlands and wet tundra hold large amounts of organic carbon (C) in their soils, and are thus important in the terrestrial C cycle. We have synthesized data on the carbon dioxide (CO₂) exchange obtained from eddy covariance measurements from 12 wetland sites, covering 1–7 years at each site, across Europe and North America, ranging from ombrotrophic and minerotrophic peatlands to wet tundra ecosystems, spanning temperate to arctic climate zones. The average summertime net ecosystem exchange of CO₂ (NEE) was highly variable between sites. However, all sites with complete annual datasets, seven in total, acted as annual net sinks for atmospheric CO₂. To evaluate the influence of gross primary production (GPP) and ecosystem respiration (R_eco) on NEE, we first removed the artificial correlation emanating from the method of partitioning NEE into GPP and R_eco. After this correction neither R_eco (P= 0.162) nor GPP (P= 0.110) correlated significantly with NEE on an annual basis. Spatial variation in annual and summertime R_eco was associated with growing season period, air temperature, growing degree days, normalized difference vegetation index and vapour pressure deficit. GPP showed weaker correlations with environmental variables as compared with R_eco, the exception being leaf area index (LAI), which correlated with both GPP and NEE, but not with R_eco. Length of growing season period was found to be the most important variable describing the spatial variation in summertime GPP and R_eco; global warming will thus cause these components to increase. Annual GPP and NEE correlated significantly with LAI and pH, thus, in order to predict wetland C exchange, differences in ecosystem structure such as leaf area and biomass as well as nutritional status must be taken into account.     

Extinction and isolation gradients in metapopulations: the case of the pool frog (Rana lessonae)

Loral extinction along the intrinsic isolation gradient within metapopulations is reviewed with particular reference to a study of the pool frog ( Rana lessonaé ) on the northern periphery of its geographical range. As in the pool frog, many other different tax a show significantly increased extinction probabilities with increased interpopulation distance. Present data imply that the relative impact of demographic and genetic factors in such stochastic extinctions depends on the genetic history of the metapopulation; data also imply that populations fluctuate more greatly in size than predicted from demographic models which have been commonly referred to. By mitigating such fluctuations and inbreeding, and compensating for emigration, immigration undoubtedly 'rescues' local populations from extinction. In this way, and not just in terms of recolonization, connectivity is concluded to be a key to metapopulation persistence. Implications for conservation are also presented.     

Nemertean systematics and phenetic classification: an example from a group of hoplonemerteans

The applicability of numerical methods to the taxonomy of the Phylum Nemertea is assessed for a group of hoplonemertean species. The classifications obtained from various cluster and ordination methods are evaluated with reference to their predictability, i.e. the number of character or character states that can be predicted about species A knowing that it belongs to class a.