Strong Seasonal Changes Research Articles

Gradients of soil salinity and moisture, and plant distribution, in a Mediterranean semiarid saline watershed: a model of soil–plant relationships for contributing to the management

This study reports the soil–plant relationships within a protected landscape in semiarid SE Spain that includes salt marshes and temporary streams and that is affected by agricultural water leaching. The main objective was to establish a conceptual model in order to use vegetation as bioindicator of soil conditions. With this model, environmental changes – based on the observation of vegetation – could be detectable, allowing the prevention of environmental impacts and the improvement of the environmental management of the studied area. Eight sampling stations with a total of 39 plots were established for the sampling of vegetation (floristic composition and species abundance) and soil (moisture, pH, redox potential, electrical conductivity and soluble ions). Multivariate analysis showed that vegetation was closely related to soil moisture and salinity. The soils colonised by steppe grasses (dominated by Lygeum spartum) and halophilus and halonitrophilus shrubs (dominated by Suaeda vera and Limonium spp.) were the driest (moisture <~20%) and least saline (EC <~30dSm−1). Phragmites australis, Sarcocornia fruticosa and Arthrocnemum macrostachyum dominated in the most saline and wettest soils. P. australis reached maximum cover at EC values ~40dS m−1 and soil moisture ~30% and consistently appeared on those soils with lower seasonal contrasts of moisture and salinity. Between 30 and 80dSm−1 of soil salinity, S. fruticosa reached maximum cover (~100%) while A. macrostachyum did not exceed ~80%. Outside this range of salinity S. fruticosa declined (cover <10%), while the other species maintained cover >~40%. In addition, A. macrostachyum grew in soils with stronger seasonal changes of moisture and salinity. Based on the model established, if an expansion of P. australis is detected, an increase in soil moisture and a decrease in soil salinity during the year could be inferred. These changes could be due to an increased entry of effluents of fresh and/or brackish water from agricultural areas. In turn, an increase in the cover of A. macrostachyum would indicate higher soil salinity, which could be a consequence of an increase in the evaporation rates (due to rising temperatures) and a decrease in rainfall (predicted consequences of global warming). The expansion of S. fruticosa would be favoured under relatively high soil salinity conditions (which limit P. australis expansion) jointly with high soil moisture, without strong seasonal changes (which limit A. macrostachyum expansion). Our results support the role of vegetation as bioindicator of disturbances and the use of soil–plant relationships models to improve the environmental management of saline ecosystems.

Enhanced Deep Blue aerosol retrieval algorithm: The second generation

The aerosol products retrieved using the Moderate Resolution Imaging Spectroradiometer (MODIS) collection 5.1 Deep Blue algorithm have provided useful information about aerosol properties over bright‐reflecting land surfaces, such as desert, semiarid, and urban regions. However, many components of the C5.1 retrieval algorithm needed to be improved; for example, the use of a static surface database to estimate surface reflectances. This is particularly important over regions of mixed vegetated and nonvegetated surfaces, which may undergo strong seasonal changes in land cover. In order to address this issue, we develop a hybrid approach, which takes advantage of the combination of precalculated surface reflectance database and normalized difference vegetation index in determining the surface reflectance for aerosol retrievals. As a result, the spatial coverage of aerosol data generated by the enhanced Deep Blue algorithm has been extended from the arid and semiarid regions to the entire land areas. In this paper, the changes made in the enhanced Deep Blue algorithm regarding the surface reflectance estimation, aerosol model selection, and cloud screening schemes for producing the MODIS collection 6 aerosol products are discussed. A similar approach has also been applied to the algorithm that generates the Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS) Deep Blue products. Based upon our preliminary results of comparing the enhanced Deep Blue aerosol products with the Aerosol Robotic Network (AERONET) measurements, the expected error of the Deep Blue aerosol optical thickness (AOT) is estimated to be better than 0.05 + 20%. Using 10 AERONET sites with long‐term time series, 79% of the best quality Deep Blue AOT values are found to fall within this expected error.

Soil moisture overshadows temperature control over soil CO2 efflux in a Pinus canariensis forest at treeline in Tenerife, Canary Islands

Soil CO2 efflux (ES) of a 50 year-old Mediterranean Pinus canariensis Chr. Sm. Ex DC. at treeline in Tenerife (Canary Islands), an ecotone with strong seasonal changes in soil temperature and soil water availability. ES was measured in weekly to monthly intervals between October 2008 and September 2009. Although ES varied markedly throughout the year, the temperature sensitivity of soil CO2 efflux (QS) and ES normalized to 10 °C temperature (ES10) did not vary considerably with respect to season. The Q10 values were 1.92 for the cold and wet season and 1.98 for the warm and dry season, respectively. The corresponding ES10 values were 0.61 and 0.50 μmol m−2 s−1, respectively. ES was largely controlled by soil temperature (Ts) and 76% of the annual variation in ES was explained by this factor. However, during the warm and dry period when soil water potential in 25–30 cm depth (Ψ) was lower than −0.04 MPa and during the cold and wet period when Ψ was higher than −0.02 MPa, Ts control over ES was limited. A model combining soil temperature and soil water availability explained 75% of the annual variation in ES. When integrated over an entire year, ES was 314 g carbon m−2 and the contribution of the cold and wet season (October–May; 214 days) and of the warm and dry season (June–September; 154 days) to annual ES was 48 and 52%, respectively.Our results reveal that soil water availability limits temperature control over ES during the wet and cold season and during the warm and dry period in Mediterranean forest ecosystems at their upper distribution limit.

Assessment of ecosystem CO2 efflux and its components in a Pinus canariensis forest at the treeline

We estimated respiratory fluxes in a treeline-associated Pinus canariensis forest in Tenerife, Canary Islands, an ecotone with strong seasonal changes in soil water availability. CO2 efflux rates from the foliage, above ground woody tissue and the soil were measured by chamber techniques. Site-specific models obtained from these chamber measurements were then combined with half-hourly measurements of canopy, stem and soil temperature as well as with soil water potential, leaf area and stem sapwood volume data for scaling up component-specific CO2 efflux to ecosystem respiration (R ECO). Integrated over an entire year R ECO was 550 g C m−2 ground surface area (average of 2008 and 2009) and comprised the following component fluxes: 57 % from the soil, 10 % from above ground woody tissue and 33 % from the foliage. Between year differences in R ECO and its components were <3 %. R ECO varied markedly throughout an entire year generally following the seasonal trends in temperature during most of the year. During the dry summer, however, R ECO was significantly reduced due to limited soil water availability in the rooting horizon. Thus, in treeline-associated forests under predicted scenarios of increasing aridity in Mediterranean regions, it is likely that there will be a shift in the contribution of CO2 efflux from the soil (R S), the foliage (R F) and above ground woody tissue (R W) to R ECO from predominately below ground to increasingly above ground. Such changes should be taken into account for predicting the response of treeline forests to a changing climate at their upper distribution limit.

Synthesis of primary production in the Arctic Ocean: I. Surface waters, 1954–2007

The spatial and seasonal magnitude and variability of primary production in the Arctic Ocean (AO) is quantified with a pan-arctic approach. We synthesize estimates of primary production (PP), focusing on surface waters (0–5m), using complementary methods that emphasize different spatial and temporal scales. These methods include (1) in situ observations of 14C uptake mostly and possibly some O2 production reported in units of carbon (in situ PP), (2) remotely sensed primary production (sat-PP), and (3) an empirical algorithm giving net PP as a function of in situ chlorophyll a (in situ Chl-PP). The work presented herein examines historical data for PP collected in surface waters only, as they form the majority of the values of a larger ensemble of PP data collected over >50years (ARCSS-PP) by many national and international efforts. This extended set of surface and vertically-resolved data will provide pan-Arctic validation of remotely sensed chlorophyll a and PP, an extremely valuable tool in this environment which is so difficult to sample. To this day, PP data in the AO are scarce and have uneven temporal and spatial coverage which, when added to the AO’s regional heterogeneity, its strong seasonal changes, and limited access, have made and continue to make obtaining a comprehensive picture of PP in the AO difficult.Daily surface in situ PP averaged 70 and 21mgCm−3d−1 for spring and summer, respectively, for the ca. 50year period across the AO. Average daily estimates of in situ PP in surface waters on a pan-Arctic basis were several fold higher with respect to remotely sensed PP (sat-PP) and in situ chlorophyll-derived PP (Chl-PP) in the spring period, likely due to differences in data availability and coverage. Summer daily averages for surface in situ PP and sat-PP were similar and twice as high as in situ Chl-PP. Differences among annual estimates of surface in situ PP, in situ Chl-PP and sat-PP across the Arctic Ocean are presented and discussed. The mode of all three differences was 0mg Cm−3y−1 and the median difference was within ±400mgCm−3y−1. Agreement between annual surface sat-PP and in situ PP was within ±1000mgCm−3y−1 for half of the data. We hope that the entire ARCSS-PP data set, especially if combined with any archiving by the latest IPY effort, will be employed to develop an improved, panarctic-specific PP remote sensing algorithm and to test hypotheses regarding the controls of PP in the AO. Furthermore, the ARCSS-PP is openly available to be used by AO modelers to calibrate, assimilate, validate and compare community ecosystem and biogeochemical numerical models and, when combined with field work, will allow for improved understanding, detection and prediction of long-term biological patterns.

Palaeoecology of well-preserved coral communities in a siliciclastic environment from the Late Pleistocene (MIS 7), Kish Island, Persian Gulf (Iran): the development of low-relief reef frameworks (biostromes) in increasingly restricted environments

Major changes in community structure and depositional relief of high-latitude coral communities in the southern Persian Gulf between marine isotope stage (MIS) 7 and the present day suggest that the area has become increasingly restricted. Corals and bivalves from outcrops on Kish Island, Iran, were identified in order to interpret the Late Pleistocene palaeoenvironmental setting. U/Th disequilibrium dating was used to constrain the ages of the stratigraphic units. During MIS 7, two coral-bearing sequences were deposited on what is now Kish Island. The lower sequence is dated as MIS 7.5 and changes laterally from an assemblage dominated by Cyphastrea sp. and Platygyra daedalea in the west to one characterized by branching Montipora in the east. By contrast, the upper sequence, dated as MIS 7.1, transitions from an assemblage dominated by platy Montipora in the west to a diverse assemblage of Platygyra and other faviids in the east. The assemblages of both sequences are within a marl matrix and bounded by thin lithified mollusc-rich layers. Corals and bivalves indicate that the sequences were deposited on gentle slopes in sheltered environments less than 20 m deep. The MIS 7 deposits may be classified as coral carpets or biostromes that developed a low-relief framework. During MIS 5, coral communities were no longer framework building and are now limited to an Acropora-rich layer of coral rubble that covers large parts of the island, and two small incipient reefs with sparse faviids. Similarities between the MIS 5 and modern nearshore coral communities suggest that the environmental conditions during MIS 5 were comparable to those of today. The late Pleistocene coral carpets and non-framework coral communities of the southern Persian Gulf may serve as models for coral biostromes in the fossil record, which formed under restricted environmental conditions such as elevated terrigenous input, high turbidity, and strong seasonal changes in temperature and/or salinity.

Incorporation of TOPMODEL into land surface model SSiB and numerically testing the effects of the corporation at basin scale

In order to examine and analyze the effects of integration of land surface models with TOPMODEL and different approaches for the integration on the model simulation of water and energy balances, the coupled models have been developed, which incorporate TOPMODEL into the Simplified Biosphere Model (SSiB) with different approaches(one divides a basin into a number of zones according to the distribution of topographic index, and the other only divides the basin into saturated and unsaturated zones). The coupled models are able to (but SSiB is not able to) take into account the impacts of topography variation and vertical heterogeneity of soil saturated hydraulic conductivity on horizontal distribution of soil moisture and in turn on water and energy balances within the basin(or a grid cell). By using the coupled models and SSiB model itself, the daily hydrological components such as runoffs are simulated and final results are analyzed carefully. Simulated daily results of hydrological components produced by both SSiB and coupled models show that (i) There is significant difference between results of soil wetness, its vertical distribution and seasonal variation, water and energy balance, and daily runoff in the basin predicted by SSiB and by the coupled models. The land surface model currently used such as SSiB is likely to misrepresent real feature of water and energy balances in the basin. (ii) Compared with the results for the basin predicted by SSiB, the coupled models predict more strong vertical and seasonal changes in soil wetness, higher evaporation and lower runoff, and improve the base flow simulation obviously. (iii) Comparing the results for the basin predicted by two coupled models with different integration approach and SSiB one by one, the results of daily runoffs and soil wetness predicted by the two coupled models are quite similar. It means, for the coupled models, the approach by dividing a region being considered into more subzones may have limited effects on improving runoff simulation results. The scheme only to divide the region into saturated and unsaturated zones may be a convenient and effective scheme. But then, if the results from the two coupled models for the basin are carefully compared, the simulated results by the coupled model with dividing the basin into more subzones will show higher evaporation and surface runoff but lower subsurface flow, lower total runoff, and lower ground water level averaged for five years.

Responses of diatoms to the Younger Dryas climatic reversal in a South Carpathian mountain lake (Romania)

A high-resolution paleolimnological study from Lake Brazi, a small mountain lake in the Southern Carpathian Mountains, Romania, shows distinct diatom responses to late glacial and early Holocene climate change between ca. 15,750 and 10,000 cal year BP. Loss-on-ignition, titanium, sulphur, phosphorus, biogenic silica content, and diatom assemblage composition were used as proxies for past environmental changes. Total epilimnetic phosphorus (TP) concentrations and lakewater pH were reconstructed quantitatively using diatom-TP and pH transfer functions. The most remarkable changes in the aquatic ecosystem were found at ca. 12,870 and 10,400 cal year BP. Whereas the onset of the Younger Dryas (YD) climatic reversal was conspicuous in our record, the beginning of the Holocene was not well marked. Two diatom assemblage zones characterize the YD in Lake Brazi, suggesting a bipartite division of this climatic oscillation. The diatom responses to the YD cooling were (1) a shift from Staurosira venter to Stauroforma exiguiformis dominance; (2) a decrease in overall diatom diversity; (3) a decrease in lake productivity, inferred from DI-TP, organic matter, and biogenic silica content; and (4) a lowering of the DI-pH. Compositional change of the diatom assemblages suggested a sudden shift towards more acidic lake conditions at 12,870 cal year BP, which is interpreted as a response to prolonged ice cover and thus shorter growing seasons and/or enhanced outwash of humic acids from the catchment. Taking into account the chironomid-based inference of only moderate July mean temperature decrease (<1 °C), together with the pollen-inferred regional opening of the forest cover and expansion of steppe-tundra, our data suggest that ecosystem changes in the Southern Carpathians during the YD were likely determined by strong seasonal changes.

Observations of auroral kilometric radio emission on the Interball-1 satellite in the active Sun period 1999–2000

Results of almost two years (January 1999–October 2000) of continuous observations of auroral kilometric radio emission with the instrument AKR-X onboard the high-apogee satellite of the Earth Interball-1 are presented. The observations were conducted at the growth stage (in 1999) and in the maximum (2000) of solar activity within the 100–1500 kHz frequency band. The results of AKR detection in the vicinity of the maximum of its spectrum at a frequency of 252 kHz are presented. Both similarity (for example, the character of global directivity) and important differences from the AKR emission observed during the solar activity minimum [5] are found. Together with very high sporadicity, strong seasonal changes in the intensity are typical for the emission. It is completely absent in the spring-summer period in the Northern Hemisphere and is strongly suppressed in this period in the Southern Hemisphere. Probable nature of these features of AKR is discussed.

Associations between seasonal variations in day length (photoperiod), sleep timing, sleep quality and mood: a comparison between Ghana (5°) and Norway (69°)

The hypothesis of whether day length (photoperiod) is an important zeitgeber (time-giver) for keeping the circadian rhythm entrained to a 24-hour cycle was examined, as was its association with sleep patterns and mood problems. Data were collected prospectively from a site with very large differences in daylight duration across seasons (Tromsø in Norway, 69°39'N) and a site with very small seasonal differences in daylight duration (Ghana in Accra, 5°32'N). Two hundred subjects were recruited from both sites in January. At the follow-up in August, 180 and 150 subjects in Ghana and Norway participated, respectively. Use of a weekly sleep diary indicated low to moderately strong seasonal changes in rise- and bedtime, sleep efficiency and sleep onset latency only in the northern latitude. No seasonal changes in sleep duration or night awakenings were observed. The self-report measures indicated moderate to strong seasonal differences in insomnia and fatigue, and weaker differences in depressed mood in Norway, but small to non-existing seasonal differences in Ghana. Lack of daylight was related to phase-delayed rise- and bedtimes, increased problems falling asleep, daytime fatigue and depressive mood. However, total sleep duration and sleep quality appeared unaffected.

Lakes an important source of carbon emissions

Around the world, vast detector networks track the transfer of carbon as it flows through the ecosystem, moving among the atmosphere, terrestrial surface, and organic matter. These carbon flux measurements are integral to understanding the role of various ecosystem components in the global carbon cycle and to performing accurate carbon accounting. But when precipitation or other processes pull large volumes of organic matter from the land into nearby lakes, this carbon effectively disappears from the carbon budget if gas fluxes over the lake surface are ignored, which they traditionally have been. Using eddy covariance measurements, Huotari et al. tracked the flow of carbon into and out of Lake Valkea‐Kotinen in southern Finland from the spring of 2003 until the autumn of 2007. They found that the lake was a net source of carbon, emitting between 70 and 100 grams of carbon per square meter per year. When compared against the role of the surrounding forest, which was a net carbon sink, the lake's emissions were enough to offset about 10% of the forest's annual carbon storage. The authors found that the strongest controlling factor on the lake carbon emission rate was the stability of the lake's temperature stratification. Strong winds, heavy precipitation, and seasonal changes that increase water mixing were linked to stronger carbon fluxes. (Geophysical Research Letters, doi:10.1029/2011GL048753, 2011)

Phytoplankton biomass and microbial abundances during the spring upwelling season in the coastal area off Concepción, central-southern Chile: Variability around a time series station

In the coastal system off Concepción, time series observations at a fixed station (St. 18) have shown strong seasonal changes in the oceanographic environment of the upper layer (<35 m depth), accompanied by large increases in phytoplankton biomass during the spring-summer upwelling season. These blooms, dominated by microplanktonic diatoms, have usually overshadowed the relevance of the smaller microbial components during upwelling. This study focuses on the variability of oceanographic conditions and their association with the structure of the planktonic community (size fractionated chlorophyll-a and microbial abundances) in the upper layer during the upwelling season, examining the extent to which St. 18 is representative of the coastal system off Concepción during springtime. For this purpose, data from three consecutive springs (2004, 2005, 2006) were compared, which included cruises for all years (8 stations around St. 18) as well as monthly sampling at St. 18. Most of the spatial (submesoscale) variability in chlorophyll-a and the microbial components was not significant, but data dispersion around mean values was high. Water column structure (temperature and salinity) in the upper layer explained a significant fraction (25–65%) of the spatial variability in most of the planktonic components; their responses to oceanographic variability were linear in some cases and non-linear in others. For the most part, St. 18 appears to adequately represent mean oceanographic conditions and the structure of planktonic communities in the coastal waters off Concepción during springtime, however spatial variability needs to be taken into account in the interpretations of temporal changes at this fixed station as well as in assessments of carbon flow within, and exportation processes from, this upwelling system.

Hypometabolism and basking: the strategies of Alpine ibex to endure harsh over-wintering conditions

Summary 1. The extent to which free-ranging large north-temperate mammals seasonally adjust thermoregulation and their energy expenditure under fully natural conditions are unknown. 2. Therefore, using telemetry we measured the heart rate (as a proxy for metabolic rate), rumen temperature (Tr) and locomotor activity (LA) over 2 years for 20 free-ranging Alpine ibex (Capra ibex ibex) living at high altitudes in the Alps. 3. Ibex showed strong seasonal changes in mean daily heart rate with a winter nadir of about 60% below the summer peak. Only 40% of this variation could be attributed to the changes in daily mean Tr, LA, wind chill, body size and snowfall. The unexplained residual variation in heart rate still showed a strong seasonal pattern. 4. The amplitude of daily rhythms in Tr was twice as high during the winter when compared with summer. This was predominantly due to lower daily minimum Tr. Thus, the substantial down-regulation of endogenous heat production during winter – as indicated by heart rate – had surprisingly small effects on Tr, indicating decreased thermal conductance. 5. Rewarming from the daily Tr minimum during the morning hours was independent of heart rate throughout the year, and occurred phase-delayed to the increase in black bulb temperature (BBT). The effects of BBT and LA on the rate of rewarming were maximized within a small range of BBT around 0 °C. This suggests that the ibex moved at sunrise to the closest sunny spot to facilitate extensive basking. 6. The energetic benefits of basking can explain the strong residual seasonality of heart rate in Alpine ibex. This partially ectothermic strategy – together with metabolic depression – apparently enables a thrifty use of body fat reserves, the major metabolic fuel during winter, and thus survival of extremely harsh winter conditions despite the virtual absence of food. Therefore, hypometabolism and passive rewarming by basking may be of general importance as a strategy for non-hibernating mammals to survive winter in strongly seasonal habitats.

Intra-annual variability of extremely rapid sedimentation onto Gardar Drift in the northern North Atlantic

North Atlantic sediment drifts are valuable archives for paleoceanographic reconstructions spanning various timescales. However, the short-term dynamics of such systems are poorly known, and this impinges on our ability to quantitatively reconstruct past change. Here we describe a high-resolution 319-day time-series of hydrodynamics and near-bottom (4 m) particulate matter flux variability at a 2600 m deep site with an extremely high sediment accumulation rate on the southern Gardar Drift in the North Atlantic. We compare our findings with the actual deposits at the site. The total annual particle flux amounted to ∼360 g m −2 yr −1, varied from ∼0.15 to >5.0 g m −2 day −1 and displayed strong seasonal compositional changes, with the highest proportion of fresh biogenic matter arriving after the spring bloom in June and July. Flux variability also depended on the changing input of lithogenic matter that had been (re)suspended for a longer time (decades). Active focussing of material from both sources is required to account for the composition and the magnitude of the total flux, which exceed observations elsewhere by an order of magnitude. The enhanced focussing or increased delivery appeared to be positively related to current velocity. The intercepted annual particle flux accounted for only 60% of the sediment accumulation rate of 600±20 g m −2 yr −1 (0.20±0.07 cm yr −1), indicating higher intra- and inter-annual variability of both the biogenic and lithogenic fluxes and/or advection of additional sediment closer to the seafloor (i.e. <4 m). This temporal variability in the composition and amount of material deposited highlights intra-annual changes in the flux of lithogenic material, but also underscores the importance of (reworked) sediment focussing and seasonality of the biogenic flux. All should be taken into account in the interpretation of the paleorecord from such depositional settings.

Temporal dynamics of Prochlorococcus ecotypes in the Atlantic and Pacific oceans

To better understand the temporal and spatial dynamics of Prochlorococcus populations, and how these populations co-vary with the physical environment, we followed monthly changes in the abundance of five ecotypes-two high-light adapted and three low-light adapted-over a 5-year period in coordination with the Bermuda Atlantic Time Series (BATS) and Hawaii Ocean Time-series (HOT) programs. Ecotype abundance displayed weak seasonal fluctuations at HOT and strong seasonal fluctuations at BATS. Furthermore, stable 'layered' depth distributions, where different Prochlorococcus ecotypes reached maximum abundance at different depths, were maintained consistently for 5 years at HOT. Layered distributions were also observed at BATS, although winter deep mixing events disrupted these patterns each year and produced large variations in ecotype abundance. Interestingly, the layered ecotype distributions were regularly reestablished each year after deep mixing subsided at BATS. In addition, Prochlorococcus ecotypes each responded differently to the strong seasonal changes in light, temperature and mixing at BATS, resulting in a reproducible annual succession of ecotype blooms. Patterns of ecotype abundance, in combination with physiological assays of cultured isolates, confirmed that the low-light adapted eNATL could be distinguished from other low-light adapted ecotypes based on its ability to withstand temporary exposure to high-intensity light, a characteristic stress of the surface mixed layer. Finally, total Prochlorococcus and Synechococcus dynamics were compared with similar time series data collected a decade earlier at each location. The two data sets were remarkably similar-testimony to the resilience of these complex dynamic systems on decadal time scales.

Phenology and seasonal physiological performance of polar seaweeds

Abstract The phenology of polar seaweeds is strongly tuned to the strong seasonal changes in underwater radiation. Daylength triggers the onset of reproduction and growth during winter in season anticipators, particularly in endemic species. More widely distributed species are often season responders growing predominantly in summer in direct response to light, temperature and nutrient conditions. The physiology of polar seaweeds is strongly linked to the life strategy of the individual species. For season anticipators, photosynthetic rates are often highest in late winter-spring when sunlight penetrates deep into the clear waters. Since Antarctic species are seldom nutrient-limited, even during summer, they mostly incorporate photosynthetically fixed carbon directly into biomass. In contrast, Arctic kelps, such as Laminaria solidungula utilise the carbon fixed during summer for synthesis of storage compounds, which are used to fuel growth during the dark winter period when nutrients are sufficient for new tissue formation. This growth pattern reflects a strategy for species optimally adapted to the seasonal changes of nutrient concentrations, which in the Arctic are low (or nearly undetectable) in summer, and high in winter. This review concludes with a discussion of possible implications of global climate changes on the phenology and productivity of polar seaweed communities.

Global hydrographic variability patterns during 2003–2008

Monthly gridded global temperature and salinity fields from the near‐surface layer down to 2000 m depth based on Argo measurements are used to analyze large‐scale variability patterns on annual to interannual time scales during the years 2003–2008. Previous estimates of global hydrographic fluctuations have been derived using different data sets, partly on the basis of scarce sampling. The substantial advantage of this study includes a detailed summary of global variability patterns based on a single and more uniform database. In the upper 400 m, regions of strong seasonal salinity changes differ from regions of strong seasonal temperature changes, and large amplitudes of seasonal salinity are observed in the upper tropical and subpolar global ocean. Strong interannual and decadal changes superimpose long‐term changes at northern midlatitudes. In the subtropical and tropical basin, interannual fluctuations dominate the upper 500 m depth. At southern midlatitudes, hydrographic changes occur on interannual and decadal time scales, while long‐term changes are predominantly observed in the salinity field. Global mean heat content and steric height changes are clearly associated with a positive trend during the 6 years of measurements. The global 6‐year trend of steric height deduced from in situ measurements explains 40% of the satellite‐derived quantities. The global freshwater content does not show a significant trend and is dominated by interannual variability.

Soil water availability and rooting depth as determinants of hydraulic architecture of Patagonian woody species

Adaptations of species to capture limiting resources is central for understanding structure and function of ecosystems. We studied the water economy of nine woody species differing in rooting depth in a Patagonian shrub steppe from southern Argentina to understand how soil water availability and rooting depth determine their hydraulic architecture. Soil water content and potentials, leaf water potentials (Psi(Leaf)), hydraulic conductivity, wood density (rho(w)), rooting depth, and specific leaf area (SLA) were measured during two summers. Water potentials in the upper soil layers during a summer drought ranged from -2.3 to -3.6 MPa, increasing to -0.05 MPa below 150 cm. Predawn Psi(Leaf) was used as a surrogate of weighted mean soil water potential because no statistical differences in Psi(Leaf) were observed between exposed and covered leaves. Species-specific differences in predawn Psi(Leaf) were consistent with rooting depths. Predawn Psi(Leaf) ranged from -4.0 MPa for shallow rooted shrubs to -1.0 MPa for deep-rooted shrubs, suggesting that the roots of the latter have access to abundant moisture, whereas shallow-rooted shrubs are adapted to use water deposited mainly by small rainfall events. Wood density was a good predictor of hydraulic conductivity and SLA. Overall, we found that shallow rooted species had efficient water transport in terms of high specific and leaf specific hydraulic conductivity, low rho(w), high SLA and a low minimum Psi(Leaf) that exhibited strong seasonal changes, whereas deeply rooted shrubs maintained similar minimum Psi(Leaf) throughout the year, had stems with high rho(w) and low hydraulic conductivity and leaves with low SLA. These two hydraulic syndromes were the extremes of a continuum with several species occupying different portions of a gradient in hydraulic characteristics. It appears that the marginal cost of having an extensive root system (e.g., high rho(w) and root hydraulic resistance) contributes to low growth rates of the deeply rooted species.

Making no Bones About it: Bone Mineral Density Changes Seasonally in a Nonhibernating Alaskan Rodent

High-latitude voles and lemmings undergo strong seasonal changes in their behavior and physiology, which may lead to concurrent changes in bone mineral density (BMD). We tested whether the BMD of northern red-backed voles (Myodes rutilus) in Alaska changed seasonally, and if so, whether these changes in their weight-bearing bones were correlated with seasonal changes in photoperiod (a mediator of activity and concentrations of reproductive hormones in high-latitude voles and lemmings), body mass, body length, or a combination of these. We used dual-energy X-ray absorptiometry to measure the BMD of the femur and humerus of voles collected in different seasons. BMDs increased dramatically from the start of spring to their peak level in early summer, and then decreased gradually to their lowest point in late winter. BMDs were significantly lower in fall and winter than in spring and early summer. BMDs of long bones were significantly correlated with both body mass and photoperiod, which accounted for 46.2% and 45.7% of the variation in the BMDs of femur and humerus, respectively. The strong changes that we observed in BMD are likely to be due, in part, to the combined effects of strong seasonal changes in body mass, activity, and baseline levels of reproductive hormones.

Respiratory fluxes in a Canary Islands pine forest

We estimated component and whole-ecosystem CO(2) efflux (R(ECO)) in a Pinus canariensis Chr. Sm. ex DC stand in Tenerife, Canary Islands, an ecotone with strong seasonal changes in soil water availability. From November 2006 to February 2008, we measured foliage, stem and soil CO(2) efflux by chamber techniques. Site-specific CO(2) efflux models obtained from these chamber measurements were then combined with half-hourly measurements of canopy, stem and soil temperature as well as soil water potential, leaf and stem surface area data for scaling up component-specific CO(2) efflux to R(ECO). Integrated over an entire year, R(ECO) was 938 g of C m(-2) in 2007 and comprised the following component fluxes: 77% from soil, 11% from stems and 12% from foliage. Whole-ecosystem CO(2) efflux varied markedly throughout the year. During the cold and wet season, R(ECO) generally followed the seasonal trends in temperature, and during the warm and dry summer, however, R(ECO) was significantly reduced because of limited soil water availability in the main rooting horizon.