Effects of the early stage of decomposition on change in carbon and nitrogen isotopes in Sphagnum litter

Abstract

Isotope and elemental composition of carbon (C) and nitrogen (N) as well as its mass loss were measured for Sphagnum fuscum litter after one and two years of incubation in three different soil zones defined by the position of water table in a pristine Sphagnum-dominated peatland on the coast of western Canada. Mass losses were greater for the first year than for the second year, and the greatest loss was found in the oxic zone closest to the peatland surface. Early stage of decomposition clearly affected isotope signatures in Sphagnum litter. Litter δ¹³C values significantly decreased after the first year of incubation. The depletion of ¹³C content during the first year might be related to the loss of more isotopically enriched soluble constituents coupled with the large mass loss. Litter δ¹⁵N values significantly increased after the first year of incubation in spite of the large mass loss. Litters incubated in the oxic zone had the greatest mass loss and ¹⁵N enrichment, suggesting that the enrichment was the result of interactions with soil microbes and preferential loss of lighter N. Conversely, litters incubated in the anoxic zone had smaller mass loss and the amount of N significantly increased, suggesting that the incorporation of bacterial biomass might also contribute to the ¹⁵N enrichment. The ¹⁵N enrichment trend continued in the second year, but the change was not significant as the first year. Increases in the δ¹⁵N values with depth in the near surface Sphagnum peat core suggests that the enrichment trend of litter ¹⁵N abundance with age is likely to continue for much longer periods than observed over the two-year period of this study.     

Vertical, lateral and longitudinal movement of zooplankton in a large river

1. The spatial distribution and movement patterns of zooplankton in large rivers are little known compared with those in lake environments. We conducted a series of studies in the Ohio River (U.S.A.) during the low flow period to assess diel vertical (DVM), longitudinal and lateral movement of crustacean zooplankton. 2. The dominant large zooplankter, the copepod Eurytemora affinis, showed a consistent vertical migration pattern of daytime ascent and night‐time descent during all sampling periods – the reverse of the most common migratory pattern of zooplankton in lakes. The cladoceran Bosmina migrated in a similar way in two of the three sampling periods. Surveys taken longitudinally in the river showed similar trends for both taxa. 3. During the lateral surveys, E. affinis was significantly more abundant in the shallow littoral zone during the night than in the daytime. The combination of vertical and lateral movement patterns along with the diel distribution of zooplanktivorous fish suggest that these movements are a predator‐avoidance mechanism. 4. Sampling programmes in large rivers should consider that larger zooplankton such as E. affinis may not be randomly distributed in the river channel and behaviours such as diel vertical migration may be just as evident in river habitats as in lakes.     

Variations and controls of nitrogen stable isotopes in particulate organic matter of lakes

Nitrogen stable isotope (δ¹⁵N) data of particulate organic matter (POM) from the literature were analyzed to provide an understanding of the variations and controls of δ¹⁵N_POM in lakes at the global scale. The δ¹⁵N_POM variability characterized by seasonal mean, minimum, maximum, and amplitude (defined as δ¹⁵N_POM maximum − δ¹⁵N_POM minimum) from 36 lakes with seasonal data did not change systematically with latitude, but was significantly lower in small lakes than in large lakes. The seasonal mean δ¹⁵N_POM increased from oligotrophic lakes to eutrophic lakes despite large variations that are attributed to the occurrences of nitrogen fixation across the trophic gradient and the differences in δ¹⁵N of dissolved inorganic nitrogen (DIN) in individual lakes. Seasonal mean δ¹⁵N_POM was significantly correlated with DIN concentration and δ¹⁵N_DIN in two subsets of lakes. Seasonal minimum δ¹⁵N_POM in individual lakes is influenced by nitrogen fixation and δ¹⁵N_DIN while seasonal maximum δ¹⁵N_POM is influenced by lake trophic state and δ¹⁵N_DIN. As a result of the dominance of non-living POM in the unproductive surface waters, seasonal δ¹⁵N_POM amplitude was small (mean = 4.2‰) in oligotrophic lakes of all latitudes. On the other hand, seasonal δ¹⁵N_POM amplitude in eutrophic lakes was large (mean = 10.3‰), and increased from low to high latitudes, suggesting that the seasonal variability of δ¹⁵N in the phytoplankton-dominated POM pool was elevated by the greater spans of solar radiation and thermal regimes at high latitudes. The δ¹⁵N_POM from 42 lakes with no seasonal data revealed no consistent patterns along latitude, lake area, and trophic gradients, and a greater than 2‰ depletion compared to the lakes with seasonal data. Along with the large seasonal variability of δ¹⁵N_POM within lakes, these results provide insightful information on sampling design for the studies of food web baseline in lakes. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Vertical migration, hatching rates, and distribution of egg stages in freshwater zooplankton

In shallow, thermally stratifield lakes, the developing eggs carried by pelagic crustacean zooplankton are exposed to regularly alternating higher and lower temperatures during daily vertical migration. Rate summation techniques for determining the duration of egg developiment suggest that (1) the distribution of egg stages in the population will not change because of the alternating temperatures, and (2) the majority of young animals will not be hatched or released in the warmer, upper waters of the lake. This latter result is probably positively adaptive, but it may be brought about entirely mechanistically by the more rapid development in warmer waters.     

The possibility of predator avoidance by Lake Michigan zooplankton

Low densities of Diaptomus ashlandi, Diacyclops thomasi, and Daphnia galeata mendotae were measured at depths where Mysis relicta formed nighttime aggregations. Calculations suggest that mysid predation can not account for the rarity of prey animals at these depths, which further suggests that the prey avoided the mysids. Unlike D. galeata mendotae, Daphnia pulicaria was common in mysid aggregations. The somewhat larger size of D. pulicaria may reduce its vulnerability to mysid predation, and consequently may explain the vertical distribution differences between the two congeners. Vertical distributions of Limnocalanus macrurus and copepod nauplii showed no obvious relationships to the mysid distributions. These were the only two taxa with distributions that were correlated with chlorophyll a concentrations. All crustacean taxa were rare in the epilimnion at night when sonar recorded a dense fish school.     

渔山列岛浮游动物的群落结构特征分析

2011年10月—2012年7月对渔山列岛浮游动物的种类组成、优势种、群落结构、丰度和生物量进行了调查,分析了浮游动物丰度、多样性指数及其与环境因子的相关性。调查共鉴定浮游动物8类45种,其中原生动物类14种,占31．1％;桡足类次之13种,占28．9％。主要优势种有中华哲水蚤（Calanussinicus）、根状拟铃虫（Tintin—nopsisradix）和桡足类幼虫（Copepoditenauplinua）。多样性指数（H＇）7月（3．324）最高,10月（2．988）最低。结合多样性指数及丰富度指数可见,调查期间渔山列岛浮游动物的生物多样性7月最高,物种较丰富。相关性分析表明,浮游动物群落分布与营养盐、水温、盐度等环境因子存在一定的相关性。     

Predation risk and the structure of freshwater zooplankton communities

Summary Many predators inflict substantial mortality on their prey. The prey respond to these selective pressures with changes in their spatial and temporal overlap with the predator (density risk responses), or with changes in their vulnerability to the predator (prey vulnerability responses). Here we develop a simple predation model that permits quantification of the basic response types of the prey in nature. We then test the hypothesis that prey response will be proportional to the intensity of the predation mortality relative to all other sources of mortality and decreased natality acting on the prey. A significant regression relationship is obtained for the prey vulnerability response but not for any of the density risk responses. The individual response values and regression statistics are used to interpret the relative importance of the different response types and to assess the predator's influence on prey community structure.Supported by Lehigh University Environmental Studies Center     

Spatial structure and ecological efficiency in the summer zooplankton of a glacial lake

In June–July of 1993 and 1995, pelagic zooplankton dynamics of aglacial lake was investigated. In those years weather conditions diffferedsubstantially. This resulted not only in a great difference in the watersurface temperature (in 1993, <17.8± 0.47 °C; in 1995, 20.4± 0.33 °C), but also in an increase in the depth of metalimnionzone in 1995 in comparison with 1993. The ecological efficiency(K_e, the ratio of secondary to primary consumer productions,was 0.28 ± 0.031 in 1993 and 0.21± 0.034 in 1995. The higherK_e was accompanied by lowering of spatial structurization,primary and secondary consumer production. This revised version was published online in July 2006 with corrections to the Cover Date.     

Turnover rates of nitrogen stable isotopes in the salt marsh mummichog, Fundulus heteroclitus, following a laboratory diet switch

Nitrogen stable isotopes are frequently used in ecological studies to estimate trophic position and determine movement patterns. Knowledge of tissue-specific turnover and nitrogen discrimination for the study organisms is important for accurate interpretation of isotopic data. We measured δ¹⁵ N turnover in liver and muscle tissue in juvenile mummichogs, Fundulus heteroclitus, following a laboratory diet switch. Liver tissue turned over significantly faster than muscle tissue suggesting the potential for a multiple tissue stable isotope approach to study movement and trophic position over different time scales; metabolism contributed significantly to isotopic turnover for both liver and muscle. Nitrogen diet-tissue discrimination was estimated at between 0.0 and 1.2‰ for liver and –1.0 and 0.2‰ for muscle. This is the first experiment to demonstrate a significant variation in δ¹⁵ N turnover between liver and muscle tissues in a fish species.     

Trophic structure of macro- and meso-invertebrates in Japanese coniferous forest: Carbon and nitrogen stable isotopes analyses

Few studies have been published on the feeding ecology of Japanese soil fauna based on stable isotope analysis. Therefore, the present work aims to use this technique for studying the trophic structure of Japanese soil fauna at two coniferous forests. Significant differences were observed between investigated sites (Arahama and Gamo) in genus richness and abundance, while for Shannon diversity indexes the difference was non-significant. The isotopic signatures (δ¹³C and δ¹⁵N) of the invertebrates collected at Arahama ranged from 0.3 to 6.3‰ for δ¹⁵N and from −27.3 to −23.3‰ for δ¹³C. At Gamo, invertebrates δ¹³C values ranged from −26.1 to −23.5‰ and δ¹⁵N values ranged from 1.6 to 6.8‰. At both sites, invertebrates formed two distinct groups on the basis of combined C and N stable isotope ratios. The locations of these groups related to δ¹³C values. The less enriched group (δ¹³C < −25‰) and the more enriched one (δ¹³C > −25‰). The range of δ¹⁵N for the present animals exceeded two trophic levels. While, the gradual ¹⁵N enrichment within the invertebrates species may indicate the dominance of omnivory in soil food webs. The differences between sites in δ¹⁵N confirm the importance of studying the trophic structure of soil fauna locally.     

Omnivory,vertical food‐web structure and system productivity: stable isotope analysis of freshwater planktonic food webs

1. The energetic hypothesis proposes that the vertical structure of food webs should increase in height with increasing system productivity. I measured the trophic positions and extent of trophic separation between the invertebrate planktivores Mysis relicta and Chaoborus spp. and their putative zooplankton prey along a gradient of lake productivity with the use of stable nitrogen isotopes. 2. In lakes of low productivity, these planktivores were found to be herbivorous, becoming omnivorous at intermediate lake productivities, and only able to be truly zooplanktivorous as lakes approached mesotrophy. A subsequent secondary analysis of literature data revealed that the strength of top‐down trophic cascades among these organisms increased with lake productivity as reflected by relationships between the abundance of planktivores and that of phytoplankton. 3. Increased omnivory under conditions of low productivity, effectively shortening the vertical structure of food webs as predicted by the energetic hypothesis, may produce increased community stability.     

Vertical distributions of zooplankton and population dynamics of Daphnia in a meromictic lake

Vertical distributions of zooplankton were studied in relation to profiles of temperature, oxygen and chlorophyll a in Roi Lake, a small meromictic lake in central Alberta. Zooplankton were distributed fairly evenly through the oxygenated part of the water column in early summer, but a gradual descent of several species became evident in June. The vertical distribution of chlorophyll was dominated by a huge peak at the 8- to 9-m-deep chemocline. the location of a plate of photosynthetic sulfur bacteria. Ambient concentration of chlorophyll was a poor predictor of the numbers of zooplankton and the fecundity of Daphnia pulicaria at different depths, and per capita birth rates of Daphnia were usually highest in the surface waters. The reproductively disadvantageous restriction of daphnids to deep water by late summer and their catastrophic decline in the face of high ambient concentrations of chlorophyll suggest that factors other than temperature and food supply are important in influencing the dynamics and distribution of zooplankton in this lake.     

Use of stable isotopes to quantify nitrogen, potassium and magnesium dynamics in young Scots pine (Pinus sylvestris)

Two-yr-old Scots pine ( Pinus sylvestris ) seedlings were grown in sand culture for 1 yr with a generous supply of a balanced nutrient solution. Trees were repotted into clean sand in February 1998 and given either a reduced or adequate nutrient supply containing enriched ¹⁵N, ⁴¹K and ²⁶Mg to label nutrient uptake during spring 1998. Trees doubled their biomass during the experiment. Whole-tree net photosynthesis was reduced by 43% after 95 d in trees that received the lower nutrient supply ( P < 0.001), although differences in biomass between the two treatments were less pronounced. Remobilization contributed 83, 82 and 52% of the N, K and Mg, respectively, used to support growth of new tissues in trees that received reduced nutrient supply. Those receiving the higher nutrient supply still obtained 44–59% of nutrients used for spring growth of new tissues from remobilization. Current nutrient supply had no significant effect on the amount of N or Mg remobilized to new tissues but K remobilization was less in trees that received the lower nutrient supply ( P = 0.025). The importance of remobilization in young trees and problems associated with quantifying internal cycling of nutrients are discussed.     

Preliminary results of an experimental study of the effects of mysid predation on estuarine zooplankton community structure

In a shallow estuarine system near Beaufort, North Carolina, a period of high winter abundance of the mysidsMysidopsis bigelowi andNeomysis americana was associated with a change in zooplankton species composition, from dominance byAcartia tonsa to dominance byCentropages spp. andSaphirella sp. Both mysids feed onA. tonsa at higher rates than the other copepods. Experiments were carried out in 600–1 000 liter enclosures, in which the initial mysid density was manipulated and the effects on the enclosed copepod community were monitored. Mysid predation had a significant effect on copepod densities. The effects of mysid predation on species composition appeared to depend on the relationship between their prey preferences and the dominant copepod species present in the communities. Under conditions favoring dominance byA. tonsa, the preferred prey species, the results suggested that mysid predation may reduce dominance and increase diversity. But when the less preferredCentropages was dominant, mysid predation had no effect on species composition.     

Nonmetric multidimensional scaling analyses of temporal variation in the structure of limnetic zooplankton communities

Nonmetric multidimensional scaling analysis was used to graphically summarize and analyze seasonal changes in the structure of limnetic zooplankton communities in Blelham Tarn, English Lake District and two large experimental enclosures (tubes A and B) therein. Species abundances in weekly samples taken from June–December, 1976 from the three sites were ordinated. Paths through species space describing temporal changes in community structure indicated that the tube B community oscillated around some average composition and was dominated by small zooplankters whereas those from tube A and the tarn changed quite distinctly through the season and had higher predator densities. Examination of other characteristics of the sites indicated that the observed differences in seasonal cycles could be related to variations in the intensity of predation, the quality of food for herbivorous zooplankters and the dynamics of nutrient input. The importance of considering time variation in lake properties, particularly in assessing lake responses to stress and in typing lakes for experimentation or management, is emphasized.     

The influence of breeding colony and sex on mercury,selenium and lead levels and carbon and nitrogen stable isotope signatures in summer and winter feathers of <Emphasis Type="Italic">Calonectris</Emphasis> shearwaters

Contamination in marine foodwebs is nowadays of great environmental concern owing to the increasing levels of pollution in marine ecosystems from different anthropogenic sources. Seabirds can be used as indicators of regional contaminant patterns across large temporal and spatial scales. We analysed Hg, Se and Pb levels as well as stable isotope ratios of C (¹³C/¹²C, δ¹³C) and N (¹⁵N/¹⁴N, δ¹⁵N) in breeding- and winter-season feathers on males and females of two related shearwater species, providing information on spatiotemporal patterns of contaminants as well as the influence of the trophic ecology of these seabirds on contaminant levels. During the breeding season, Se and Pb concentrations were highest at the Cape Verde archipelago, showing no differences among the other colonies or between the sexes. However, Hg levels varied among colonies, being highest in the Mediterranean, probably resulting from the larger emissions and fallout of this pollutant in Europe. Feathers grown during breeding also showed sexual differences in Hg concentrations and δ¹³C. Differences in Hg concentration between sexes are mainly due to egg-laying decontamination in females. In contrast, differences in Hg among colonies are probably related to differences in trophic ecology, as indicated by δ¹³C and δ¹⁵N measurements. Contaminant concentrations in winter-grown feathers did not show any relationship with stable isotope values but were affected by contaminant loads associated with the breeding season. These findings suggest that the interpretation of contaminant levels of migratory species from feathers moulted out of the breeding season should be made with caution because those values could reflect exposures to contaminants acquired during the breeding season. We conclude that factors other than feeding ecology may play an important role in the interpretation of contaminant levels and their annual dynamics at several spatial scales. Consideration of the relevant temporal context provided by isotopic signatures and contaminant concentrations is important in deciphering contaminant information based on various tissues. Xavier Ruiz: deceased 27 April 2008.     

Simulation of nitrogen assimilation by heterotrophic soil microbial biomass

Assimilation of N by heterotrophic soil microbial biomass is associated with decomposition of organic matter in the soil. The form of N assimilated can be either low molecular weight organic N released from the breakdown of organic matter (direct assimilation), or NH⁺₄ and NO⁻₃ from the soil inorganic N pool, into which mineralized organic N is released (mineralization immobilization turnover). The kinetics of C and N turnover in soil is quantifiable by means of computer simulation models. NCSOIL was constructed to represent the two assimilation schemes. The rate of N assimilation depends on the rate of C assimilation and microbial C/N ratio, thereby rendering it independent of the assimilation scheme. However, if any of the N forms is labeled, a different amount of labeled N assimilation will be simulated by the different schemes. Experimental data on inorganic N and ¹⁵N and on organic ¹⁵N dynamics in soils incubated with ¹⁵N added as NH⁺₄ or organic N were compared with data simulated by different model schemes. Direct assimilation could not account for the amount of ¹⁵N assimilated in any of the experimental treatments. The best fit of the model to experimental data was obtained for the mineralization immobilization turnover scheme when both NH⁺₄ and NO⁻₃ were assimilated, in proportion to their concentration in the soil.     

集约化生产对农田土壤碳氮含量及δ13C和δ15N同位素丰度的影响

集约化生产下农田土壤碳、氮含量变化是衡量土壤肥力持久性的重要指标.对常规水稻-蚕豆轮作地、露地蔬菜地、3年塑料大棚地和10年以上塑料大棚地的土壤pH、电导率(EC)、土壤有机碳(SOC)和总氮(TN)含量及δ13C和δ15N同位素丰度进行测定,研究了集约化生产程度对土壤特性的影响.结果表明:与水稻-蚕豆轮作地相比,露地蔬菜地、3年塑料大棚地和10年以上塑料大棚地0 ～20 cm耕层土壤pH分别降低1.1、0.8和0.7,而土壤EC分别是水稻-蚕豆轮作地的4.2、4.9和5.2倍;土壤碳、氮含量随塑料大棚地生产年限的增加总体上呈先增大后减小的趋势.与水稻-蚕豆轮作地相比,10年以上塑料大棚地0～20、20～40、40 ～60、60 ～ 80、80 ～ 100 cm土层的土壤SOC含量分别下降了54％、46％、60％、63％和59％,土壤TN含量分别下降了53％、53％、71％、82％和85％.农田集约化生产程度显著影响土壤SOC、TN含量和δ13C、δ15N丰度,土壤δ13C丰度与SOC含量呈显著负相关.土壤δ13C丰度可作为评价农田土壤碳循环受人为干扰强度的指标.     

Determination of food sources and trophic position in Malaysian tropical highland streams using carbon and nitrogen stable isotopes

Stable isotope analysis has been extensively used as an effective tool in determination of trophic relationship in ecosystems. In freshwater ecosystem, aquatic invertebrates represent main component of a river food web. This study was carried out to determine potential food sources of freshwater organism together with pattern of trophic position along the river food web. In this study, rivers of Belum-Temengor Forest Complex (BTFC) has been selected as sampling site as it is a pristine area that contains high diversity and abundance of organisms and can be a benchmark for other rivers in Malaysia. Stable isotope ratios of carbon (δ¹³C) and nitrogen (δ¹⁵N) were applied to estimate trophic position and food web paradigm. Analysis of stable isotopes based on organic material collected from the study area revealed that the highest δ¹³C value was reported from filamentous algae (? 22.68 ± 0.126⁰/₀₀) and the lowest δ¹³C was in allocthonous leaf packs (? 31.58 ± 0.187⁰/₀₀). Meanwhile the highest δ¹⁵N value was in fish (8.45 ± 0.177⁰/₀₀) and the lowest value of δ¹⁵N was in autochthonous aquatic macrophyte (2.00 ± 1.234⁰/₀₀). Based on the δ¹⁵N results, there are three trophic levels in the study river and it is suggested that the trophic chain begins with organic matter followed by group of insects and ends with fish (organic matter < insects < fish).