Carbon dioxide exchange above a Mediterranean C3/C4 grassland during two climatologically contrasting years

Eddy‐covariance measurements of net ecosystem carbon exchange (NEE) were carried out above a grazed Mediterranean C3/C4 grassland in southern Portugal, during two hydrological years, 2004–2005 and 2005–2006, of contrasting rainfall. Here, we examine the seasonal and interannual variation in NEE and its major components, gross primary production (GPP) and ecosystem respiration (R_eco), in terms of the relevant biophysical controls. The first hydrological year was dry, with total precipitation 45% below the long‐term mean (669 mm) and the second was normal, with total precipitation only 12% above the long‐term mean. The drought conditions during the winter and early spring of the dry year limited grass production and the leaf area index (LAI) was very low. Hence, during the peak of the growth period, the maximum daily rate of NEE and the light‐use and water‐use efficiencies were approximately half of those observed in the normal year. In the summer of 2006, the warm‐season C4 grass, Cynodon dactylon L., exerted an evident positive effect on NEE by converting the ecosystem into a carbon sink after strong rain events and extending the carbon sequestration for several days, after the end of senescence of the C3 grasses. On an annual basis, the GPP and NEE were 524 and 49 g C m^?2, respectively, for the dry year, and 1261 and ?190 g C m^?2 for the normal year. Therefore, the grassland was a moderate net source of carbon to the atmosphere, in the dry year, and a considerable net carbon sink, in the normal year. In these 2 years of experiment the total amount of precipitation was the main factor determining the interannual variation in NEE. In terms of relevant controls, GPP and NEE were strongly related to incident photosynthetic photon flux density on short‐term time scales. Changes in LAI explained 84% and 77% of the variation found in GPP and NEE, respectively. Variations in R_eco were mainly controlled by canopy photosynthesis. After each grazing event, the reduction in LAI affected negatively the NEE.     

Response of ecosystem carbon exchange to warming and nitrogen addition during two hydrologically contrasting growing seasons in a temperate steppe

A large remaining source of uncertainty in global model predictions of future climate is how ecosystem carbon (C) cycle feedbacks to climate change. We conducted a field manipulative experiment of warming and nitrogen (N) addition in a temperate steppe in northern China during two contrasting hydrological growing seasons in 2006 [wet with total precipitation 11.2% above the long‐term mean (348 mm)] and 2007 (dry with total precipitation 46.7% below the long‐term mean). Irrespective of strong intra‐ and interannual variations in ecosystem C fluxes, responses of ecosystem C fluxes to warming and N addition did not change between the two growing seasons, suggesting independence of warming and N responses of net ecosystem C exchange (NEE) upon hydrological variations in the temperate steppe. Warming had no effect on NEE or its two components, gross ecosystem productivity (GEP) and ecosystem respiration (ER), whereas N addition stimulated GEP but did not affect ER, leading to positive responses of NEE. Similar responses of NEE between the two growing seasons were due to changes in both biotic and abiotic factors and their impacts on ER and GEP. In the wet growing season, NEE was positively correlated with soil moisture and forb biomass. Negative effects of warming‐induced water depletion could be ameliorated by higher forb biomass in the warmed plots. N addition increased forb biomass but did not affect soil moisture, leading to positive effect on NEE. In the dry growing season, NEE showed positive dependence on grass biomass but negative dependence on forb biomass. No changes in NEE in response to warming could result from water limitation on both GEP and ER as well as little responses of either grass or forb biomass. N addition stimulated grass biomass but reduced forb biomass, leading to the increase in NEE. Our findings highlight the importance of changes in abiotic (soil moisture, N availability) and biotic (growth of different plant functional types) in mediating the responses of NEE to climatic warming and N enrichment in the semiarid temperate steppe in northern China.     

Population dynamics and production of five crustacean zooplankters in a subtropical reservoir during years of contrasting turbidity

SUMMARY 1. The temporal dynamics and demography of Meta-diaptomus meridianus (Van Douwe), Lovenula excellens Kiefer. Daphnia gibba Methuen, D. barbata Weltner and Moina brachiata Jurine were studied for 2 years in a small bay of Lake le Roux (Orange River, South Africa). Total zooplankton biomass and population density were 1.4–3 times higher during the less turbid conditions of 1982/83 (Secchi depth transparency around 35 cm) than they were at around 25 cm Secchi depth during 1981/82, when D. barbata was absent.
2. On average, instantaneous birth rates, rates of population change and death rates varied only slightly between years. Birth and death rates were considerably higher above 15°C than below 15°C. These rates correlated with one another and with zooplankton abundance both inter- and intra-specifically suggesting that competitive interactions were important in population regulation. Mortality rates varied more strongly and consistently in a density-dependent direction than did birth rates. In addition to depressed fecundity, the inferred survival of young was poor and population growth low, possibly because food shortage caused high post-natal mortality.
3. Estimates of annual production derived from finite birth rate values varied consistently with annual differences in biomass, and amounted to between 6 and 10 g m⁻² y⁻¹ dry wt. Annual P/B values varied from around 20 for the daphnids to 55 for the copepods and 75 for Moina. Apart from the latter, whose annual P/B ratio virtually doubled from 45 to 75 following reductions in turbidity, annual differences in P/B ratio were slight.     

Seasonal dynamics of macroinvertebrate communities in a semiarid saline spring stream with contrasting environmental conditions

Semiarid saline streams are rare aquatic ecosystem types. Their constituent biota is expected to have adapted evolutionarily to strong hydrological variability and salinity stress; however, their ecology is not well known. In this study, we quantify the seasonal changes in the structure of the macroinvertebrate community in the Reventón Rambla (south-eastern Spain), a permanent saline spring stream which is included in a drainage system consisting of ephemeral dry channels (so-called “ramblas”). Seasonal patterns of community structure were studied in two reaches with contrasting environmental regimes using univariate and multivariate statistics. The upstream site showed more stable environmental conditions than the downstream site, and both sites also differed with regard to species richness, and structural and functional group attributes. On a seasonal basis, community dissimilarity was high during periods when both sites were isolated during summer droughts but dissimilarity decreased when both sites were connected through surface flow. Furthermore, the communities tended to show cyclical trajectories in multivariate ordination space. Rather than being related to salinity stress, these patterns seemed to track the hydrological disturbance regime of this rambla system. Spates tended to disrupt communities, while signs of recovery were evident during low-flow periods. Results suggest that salinity fluctuation does not pose a severe abiotic constraint to these adapted macroinvertebrate communities. Their suits of functional properties provide them with the necessary traits to recover quickly from natural disturbance. While human-caused salinization of streams severely impacts communities eventually reducing their recovery potential, our results suggest that communities in natural saline streams may show similar responses to hydrological disturbance as communities from non-saline streams.     

The breeding biology of the Lapland Bunting Calcarius lapponicus in West Greenland during two contrasting years

The breeding biology of the Lapland Bunting was studied in an area of central West Greenland during two contrasting seasons. In the delayed thaw of 1984, male birds arrived on the breeding grounds at the same time as in the more typical spring of 1979, but females delayed arrival by two weeks in 1984 compared with 1979. Mean clutch size was depressed in the latter year and declined through the season; breeding productivity was correspondingly reduced in 1984 compared to 1979. Egg and juvenile predation rates were similar for the two years. Nestlings grew as rapidly in 1984 as in 1979 despite an apparent reduction in detected ground-dwelling invertebrates known to form an important constituent of the nestling diet. Because they bred later in 1984, females tended to commence moult later in 1984 than 1979 but completed moult by early September in both years. Males also commenced moult later but completed earlier in 1984.     

Modeling fish dynamics and effects of stress in a hydrologically pulsed ecosystem

Many wetlands undergo seasonal cycles in precipitation and water depth.This environmental seasonality is echoed in patterns of production of fishbiomass, which, in turn, influence the phenology of other components of thefood web, including wading birds. Human activities, such as drainage orother alterations of the hydrology, can exacerbate these natural cycles andresult in detrimental stresses on fish production and the higher trophic levels dependent on this production. In this paper we model theseasonal pattern of fish production in a freshwater marsh, with specialreference to the Everglades/Big Cypress region of southern Florida.The model illustrates the temporal pattern of production through theyear, which can result in very high densities of fish at the end of ahydroperiod (period of flooding), aswell as the importance of ponds and other deep depressions, both as refugia and sinks during dry periods. The model predicts that: (1) there is an effective threshold in the length of the hydroperiod that must beexceeded for high fish-population densities to be produced, (2) large,piscivorous fishes do not appear tohave a major impact on smaller fishes in the marsh habitat, and (3) therecovery of small-fish populations in the marsh following a major droughtmay require up to a year. The last of these results is relevant toassessing anthropogenic impacts on marsh production, as these effectsmay increase the severity and frequency of droughts.     

Microbial dynamics during the decline of a spring diatom bloom in the Northeast Atlantic

The microbial dynamics during a spring diatom bloom declinewas monitored in the Northeast Atlantic during a 5-day Lagrangianstudy (8–12 April 2002). Phytoplankton abundance, compositionand health status were related to viral and bacterial abundance,zooplankton abundance and grazing rates, as well as bacterialproduction. Phytoplankton reached maximum concentration on Day3 (Chl a >5 µg L^–1) and declined on Day 5 (Chla 2 µg L^–1) and was dominated (70% of Chl a) bydiatoms. Bacterial production increased substantially to >20µg C L^–1 day^–1 on Day 3 and concomitantlylarge viruses decreased in number by half to <10 x 10³ mL^–1.This was followed by a 5-fold increase in large viruses on Day5, indicating infection and subsequent lysis on Days 3 and 5,respectively. Micro- and mesozooplankton grazing were not theprincipal cause for the decline of the bloom and pheophorbide-ashowing little variation in concentration from Days 1–4(100 ng L^–1) although doubled on Day 5. The poor physiologicalstatus of the diatoms, indicated by the high chlorophyllide-aconcentrations (50–480 ng L^–1), likely promoteda series of closely interrelated events involving bacteria andviruses leading to the demise of the diatom bloom.     

荒漠草原与典型草原NPP、碳积累对不同降雨年份和利用方式的响应

不同草原利用方式正在影响着内蒙古的草原生态系统,而且在未来降水空间格局变化的背景下,它们共同决定了生态系统植被类型、净初级生产力(NPP)和生态系统碳积累。选取内蒙古中部两个重要的草地类型:荒漠草原和典型草原,研究不同草原利用方式(围栏禁牧、划区轮牧、割草、自由放牧)植物群落在降雨量不同的两个生长季节地上(ANPP)、地下净初级生产力(BNPP)的变化,同时也评估了植物群落的碳积累,研究结果表明:1)在降雨量亏缺年份,与围封相比,荒漠草原自由放牧区ANPP、BNPP及碳积累分别下降了57.1%、51.7%和56.0%,而典型草原自由放牧区分别下降了18.4%、25.1%和17.9%。2)在降雨量充足年份,与围封相比,荒漠草原划区轮牧区ANPP、BNPP以及碳积累分别增加了18.2%、9.8%和21.9%,而典型草原各处理下围封禁牧区ANPP仍是最高;3)两种草地类型下,降雨量对自由放牧的调控作用高于其它草地利用方式;4)荒漠草原ANPP在丰雨年是欠雨年的2倍,而典型草原仅增加了79.0%,降雨量对荒漠草原生产力的季节调控作用远高于典型草原。在未来全球气候变暖和降水格局变化的情况下,荒漠草原降雨量是影响荒漠植物群落NPP和碳积累的主导因子。     

Seasonal dynamics and bioavailability of dissolved organic matter in two contrasting temperate estuaries

Production and bioavailability of dissolved organic matter (DOM) were followed during a year in the nutrient-rich estuary, Roskilde Fjord (RF), and the more oligotrophic strait, Great Belt (GB), in Denmark. Bioavailability of dissolved organic carbon (DOC), nitrogen (DON), and phosphorous (DOP) was determined during incubations over six months. Overall, RF had three to five times larger pools of total nitrogen (TN) and total phosphorous (TP) and five to eight times higher concentrations of inorganic nutrients compared to GB. However, the allocation of carbon, nitrogen, and phosphorous into different pools were remarkably similar between the two systems. DON and DOP contributed with about equal relative fractions in the two systems: 72 ± 13% of total nitrogen and 21 ± 12% of total phosphorous. The average bioavailability of DOM was 25 ± 15, 17 ± 5.5, and 49 ± 29% for carbon, nitrogen, and phosphorous, respectively. The observed release of DIN from degradation of DON amounted to between 0.1 (RF winter) and 14 times (GB summer) the loadings from land and contributed with half of the total input of bioavailable nitrogen during summer. Hence, this study shows that nitrogen in DOM is important for the nitrogen cycling, especially during summer. The sum of inorganic nutrients, particulate organic matter, and bioavailable DOM (the dynamic pools of nutrients) accounted for 42 and 92% of nitrogen, and phosphorous, respectively, and was remarkably similar between the two systems compared to the difference in nutrient richness. It is hypothesized that the pelagic metabolism of nutrients in marine systems dictates a rather uniform distribution of the different fractions of nitrogen and phosphorous containing compounds regardless of eutrophication level.     

Tree species dynamics and disturbance in three Swedish boreal forest stands during the last two thousand years

Abstract. Data from three forest stands for the past 2000 yr show how the shade-intolerant species Pinus sylvestris and Betula pubescens maintain significant populations in the Swedish boreal landscape. Age structure data from a northern stand close to the range limits of Picea abies and Pinus complement a local pollen diagram, and reveal cyclic population fluctuations which can be related to periods of climatic stress and fire. Pollen data from two southern stands show that high fire frequencies in the past prevented the expansion of Picea populations. Reduction of the fire frequency during the last 200 yr has favoured Picea. A long time perspective reveals the population dynamics of long-lived species and indicates the controlling factors. Such knowledge permits assessment of the current status and likely future of forest stands.     

Dinophyceae fluctuations in two alpine lakes of contrasting size during a 10-year fortnightly survey

Colbricon Superiore and Inferiore are two small adjacent high-mountain lakes located in the Paneveggio Natural Park (Italy) that offer the rare opportunity to study two iso-ecologic water environments differing only by area and volume in a ratio of 2:1 and 3:1, respectively. We took advantage of this setting to investigate phytoplankton dynamics, compare variability and productivity differences between the two basins, and assess size-dependent issues. The phytoplankton group of the Dinophyceae was chosen as the indicator organisms of ecological perturbation owing to their high sensitivity to environmental variations, as well as their acknowledged nature of versatile proxy to report global climatic changes. The study was conducted for over 10 years with fortnightly samplings. Results indicated that (a) the Dinophyceae communities in the smaller lake were significantly more resistant to changes exerted by the fluctuation of lakewater transparency and pH; and (b) the smaller lake sustained a consistently higher production with an average Dinophyceae density 1.73 fold higher than that of the larger lake. The coefficients of variation show that the chemical parameters in the smaller lake display higher time-related fluctuation while being spatially homogeneous and that such conditions correlate with a higher stability of the Dinophyceae assemblage. The use of this setting is also proposed as a model to test relationships between ecosystem production and physical stability.     

Diversity and biomass dynamics of unicellular marine fungi during a spring phytoplankton bloom

Microbial communities have important functions during spring phytoplankton blooms, regulating bloom dynamics and processing organic matter. Despite extensive research into such processes, an in-depth assessment of the fungal component is missing, especially for the smaller size fractions. We investigated the dynamics of unicellular mycoplankton during a spring phytoplankton bloom in the North Sea by 18S rRNA gene tag sequencing and a modified CARD-FISH protocol. Visualization and enumeration of dominant taxa revealed unique cell count patterns that varied considerably over short time scales. The Rozellomycota sensu lato (s.l.) reached a maximum of 10⁵ cells L⁻¹, being comparable to freshwater counts. The abundance of Dikarya surpassed previous values by two orders of magnitude (10⁵ cells L⁻¹) and the corresponding biomass (maximum of 8.9 mg C m⁻³) was comparable to one reported for filamentous fungi with assigned ecological importance. Our results show that unicellular fungi are an abundant and, based on high cellular ribosome content and fast dynamics, active part of coastal microbial communities. The known ecology of the visualized taxa and the observed dynamics suggest the existence of different ecological niches that link primary and secondary food chains, highlighting the importance of unicellular fungi in food web structures and carbon transfer.     

Bacterial dynamics and distribution during a spring diatom bloom in the Hudson River plume, USA

Bacterial abundance and [³H]thymidine incorporation rates wereused to characterize bacterial distributions and dynamics duringa spring diatom bloom in the coastal plume of the Hudson Riverduring March, 1981. Bacterial abundance did not decline significantlyaway from the plume or across the continental shelf. However,a pronounced gradient was observed for [³H]thymidine incorporation. Following a northeast gale, a bloom of Skeletonema costatumdeveloped in response to coastal upwelling. As the plume becamesaltier, warmer and larger, bacterial abundance averaged 1.3x 10⁹ cells 1^–1. Bacterial incorporation of [³H]thymidineinto cold 5% TCA insoluble materials, bacterial production andspecific growth rates averaged 80–120 pmol 1^–1 d^–1,1.4–2.0 x 10⁹ cells 1^–1 and 1.2–1.5 d^–1.respectively. The mean density of bacteria in the plume didnot change even though growth rates were higher than expectedlosses from sinking, mixing and export. The abundance and production levels of attached bacteria inthe plume were significantly higher than they were in coastalwater outside the plume. In contrast, free bacterial levelswere similar in all regions of the shelf and plume. Bacterialparameters in the plume were not correlated with phytoplanktonpigment concentrations, while significant positive correlationswere found in shelf waters >33 salinity. Thus bacteria werecoupled to phytoplankton in shelf waters but not in the plumewhere allochthonous dissolved matter was probably supportingthe bacteria. ^*Lamont-Doherty Geological Observatory Contribution No. 3459 ²Present address: Department of Microbiology, University ofGeorgia, Athens, GA 30602, USA.     

Flooding, soil seed bank dynamics and vegetation resilience of a hydrologically variable desert floodplain

1. This paper explores soil seed bank composition and its contribution to the vegetation dynamics of a hydrologically variable desert floodplain in central Australia: the Cooper Creek floodplain. We investigated patterns in soil seed bank composition both temporally, in response to flooding (and drying), and spatially, with relation to flood frequency. Correlations between extant vegetation and soil seed bank composition are explored with respect to flooding. 2. A large and diverse germinable soil seed bank was detected comprising predominantly annual monocot and annual forb species. Soil seed bank composition did not change significantly in response to a major flood event but some spatial patterns were detected along a broad flood frequency gradient. Soil seed bank samples from frequently flooded sites had higher total germinable seed abundance and a greater abundance of annual monocots than less frequently flooded sites. In contrast, germinable seeds of perennial species belonging to the Poaceae family were most abundant in soil seed bank samples from rarely flooded sites. 3. Similarity between the composition of the soil seed bank and extant vegetation increased following flooding and was greatest in more frequently flooded areas of the floodplain, reflecting the establishment of annual species. The results indicate that persistent soil seed banks enable vegetation in this arid floodplain to respond to unpredictable patterns of flooding and drying.     

Carbon and nitrogen dynamics along the decay continuum: Plant litter to soil organic matter

Decay processes in an ecosystem can be thought of as a continuum beginning with the input of plant litter and leading to the formation of soil organic matter. As an example of this continuum, we review a 77-month study of the decay of red pine (Pinus resinosa Ait.) needle litter. We tracked the changes in C chemistry and the N pool in red pine (Pinus resinosa Ait.) needle litter during the 77-month period using standard chemical techniques and stable isotope, analyses of C and N.Mass loss is best described by a two-phase model: an initial phase of constant mass loss and a phase of very slow loss dominated by degradation of lignocellulose (acid soluble sugars plus acid insoluble C compounds). As the decaying litter enters the second phase, the ratio of lignin to lignin and cellulose (the lignocellulose index, LCI) approaches 0.7. Thereafter, the LCI increases only slightly throughout the decay continuum indicating that acid insoluble materials (lignin) dominate decay in the latter part of the continuum.Nitrogen dynamics are also best described by a two-phase model: a phase of N net immobilization followed by a phase of N net mineralization. Small changes in C and N isotopic composition were observed during litter decay. Larger changes were observed with depth in the soil profile.An understanding of factors that control lignin degradation is key to predicting the patterns of mass loss and N dynamics late in decay. The hypothesis that labile C is needed for lignin degradation must be evaluated and the sources of this C must be identified. Also, the hypothesis that the availability of inorganic N slows lignin decay must be evaluated in soil systems.     

Carbon dynamics during flood events in a lowland river: the importance of anabranches

1. Our objective was to measure the influence of hydrological connection with anabranch channels on the availability of major carbon sources in a lowland, anabranching floodplain river landscape. 2. Results show that anabranch channels are sinks for large quantities of sediment‐associated carbon, facilitated by high rates of sediment deposition, and are sources for dissolved organic carbon (DOC), partly via inundation‐stimulated release from surface sediments and leaf litter. This dual role influences ecological pattern and process at multiple spatial and temporal scales, including within‐flow pulse phase differences in carbon availability and anticlockwise hysteresis in the DOC–discharge relationship. 3. Hydrological connection with anabranch channels in riverine landscapes appears to increase the retention, concentration and diversity of carbon sources over both space and time, changing the timing of carbon transport downstream and shortening the carbon spiral at a landscape scale. 4. In contrast to floodplains, anabranches exchange carbon sources with the river ecosystem during flow pulses below bankfull. They are relatively easy to target for management because they have definable commence‐to‐flow levels and require relatively small amounts of water for connection. However, the type and amount of carbon exchanged between the anabranches and the river channel will vary depending on the frequency, magnitude and duration of flow pulses. 5. Managed, periodic connection of anabranch channels via environmental flows should be considered as an option between large flood events.