Small Seasonal Variations Research Articles

Characterization of the mean and seasonal flow regime on Georges Bank from shipboard acoustic Doppler current profiler data

From 1995 through 1999 the Northwest Atlantic GLOBEC program sponsored over 100 oceanographic cruises to Georges Bank and the surrounding waters. About 12,000 hours of shipboard acoustic Doppler current profiler (ADCP) data were collected during the program on more than 80 of these cruises, forming one of the most extensive three‐dimensional current measurement data sets ever collected. These data have been used here to examine the seasonal variability of the bank’s barotropic flow field. The variability has been described using the 5 years of ADCP observations to derive both small area statistics and seasonal least squares fits, and a more flexible description based upon interpolating functions describing the spatial and temporal dependence of the bank‐wide stream functions. Both methods show the seasonal acceleration of the bank gyre in the summer with larger velocities along the north flank and smaller seasonal variations over the southern flank. The stream functions show the seasonal change from a winter condition with on‐bank flow from the northwest through to the acceleration and ecologically critical closure of the anticyclonic Georges Bank gyre during the summer. On the basis of annual and semiannual decompositions of the seasonal progression, the closure of the gyre in the area of the Great South Channel appears to occur by June, first in the shallow waters and then progressively southward, but it does not extend as far as the middle of the southern flank. A series of monthly Lagrangian drift predictions based upon the stream function analysis, suggest that the observed seasonal movement of the larval fish from their spawning grounds over the Northeast Peak in March, through to their late summer concentration over the central bank, can generally be ascribed to the seasonal barotropic flow over the inner portion of the bank. The analysis also showed that those larvae found over the outer part of the bank are, almost certainly, doomed to be swept out of the banks’ ecosystem, to either the west or south.

Daily and estrous rhythmicity of body temperature in domestic cattle

BackgroundRhythmicity in core body temperature has been extensively studied in humans and laboratory animals but much less in farm animals. Extending the study of rhythmicity of body temperature to farm animals is important not only from a comparative perspective but also from an economic perspective, as greater knowledge of this process can lead to improvements in livestock production practices. In this study in cattle, we investigated the maturation of the daily rhythm of body temperature in newborn calves, characterized the parameters of the daily rhythm in young cows, and studied the oscillation in body temperature associated with the estrous cycle in adult cows.ResultsWe found that the daily rhythm of body temperature is absent at birth but matures fully during the first two months of life. The mature rhythm had a mean level of 38.3°C, a range of excursion of 1.4°C, and was more robust than that of any mammalian species previously studied (90% of maximal robustness). Sexually mature cows also exhibited a robust estrous rhythm of body temperature. An elevation of about 1.3°C was observed every 21 days on the day of estrus. Small seasonal variations in this pattern were observed.ConclusionIn conclusion, calves exhibit a very robust daily rhythm of body temperature, although this rhythm is absent at birth and develops during the first two months of life. Adult cows exhibit also 21-day rhythmicity in body temperature reflecting the duration of the estrous cycle.

Strontium:calcium ratios in statoliths of Japanese common squid Todarodes pacificus (Cephalopoda: Ommastrephidae) as indicators of migratory behavior

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 251:169-179 (2003) - doi:10.3354/meps251169 Strontium:calcium ratios in statoliths of Japanese common squid Todarodes pacificus (Cephalopoda: Ommastrephidae) as indicators of migratory behavior Yuzuru Ikeda1,*, Nobuaki Arai2, Hideaki Kidokoro3, Wataru Sakamoto4,5 1Brain Science Institute, RIKEN, Wako, Saitama 351-0198, Japan 2Graduate School of Informatics, Kyoto University, Kyoto 606-8501, Japan 3Japan Sea National Fisheries Research Institute, Niigata 951-8121, Japan 4Graduate School of Agriculture, Kyoto University, Kyoto 606-8502, Japan 5Present address: Fisheries Laboratory of Kinki University, Shirahama, Wakayama 649-2211, Japan *Email: ikeda@brain.riken.go.jp ABSTRACT: Ontogenetic variation in the strontium:calcium (Sr:Ca) ratios was measured in statoliths of the Japanese common squid Todarodes pacificus. Two different geographical groups of T. pacificus inhabit the Sea of Japan; these groups are separated by the Subarctic Front during the summer. Element ratios were measured with a wavelength-dispersive spectrometer for 11 individuals from a cold-water subarctic region, which harbors large squid (Subarctic Group), for 12 individuals from a warm-water region, which harbors smaller squid (Tsushima Group), and for 2 individuals from the Subarctic Front. The squid ranged from 190 to 218 mm in mantle length (ML) and were 191 to 262 d old. Similar measurements were obtained for 12 tagged T. pacificus (187 to 259 mm ML; 214 to 312 d old) that were released and recovered in the Subarctic and the Tsushima Current between June and August 1997 (tagging duration 5 to >40 d). Ontogenetic variation in Sr:Ca ratios from hatchlings to sub-adults differed between the 2 geographical groups. Sr:Ca ratios were high at hatching followed by a drastic decrease up to 10 d of age for both groups. These ratios increased until 40 d of age, continuing at high levels until around 150 d, when they decreased in all specimens of the Tsushima Group. In the Subarctic Group, however, the variations in Sr:Ca ratios differed between individuals. Sr:Ca ratios at the age when squid recruited to either the Subarctic or the Tsushima Current were similar. These differences reflect the different spawning grounds and transport routes of the 2 groups. Thermal history was recorded from the statoliths of tagged individuals, and small fluctuations and seasonal variations in the Sr:Ca ratios were observed. These correspond to diel vertical movements and endogenous rhythms of the squid, and the formation and movements of the cold-water layer in their habitat. KEY WORDS: Japanese common squid · Statoliths · Sr:Ca ratios · Migration · Sea of Japan Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 251. Online publication date: April 11, 2003 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2003 Inter-Research.

Nutrient loads, advection and turnover at the entrance to the Baltic Sea

The monthly advective supply of nutrients and nutrient pool sizes in the Kattegat and the Belt Sea are estimated for the period 1989–1996 from a combination of time-dependent hydrodynamic model results and concentration measurements of dissolved inorganic nitrogen (DIN), phosphate (DIP) and total nitrogen and phosphorous (TN and TP). The net supply of nutrients to these seas is computed as the sum of advection and nutrient loads from atmosphere and land to the Kattegat and the Belt Sea. The average net supplies are 5000 t N per month and 500 t P per month in both areas, corresponding to approximately 10% of the amounts required for the annual primary production in the area. The net supply of N displays a small seasonal variation, while the net supply of P exhibits both a spring maximum and a winter minimum. The seasonal nutrient turnover cycle is estimated from the residual of the net supply and the seasonal variation in nutrient pools. Net release of DIP from the sediment takes place during late summer and winter, while net release of DIN occurs mainly during winter. Thus, the net release from the sediment adds to the pool of nutrients available for the spring bloom in the Baltic entrance the following year. Both advection and the autumn–winter release of P exceed the land load of P. Thus, on a short time-scale, reductions in P load cannot be expected to significantly change the availability of P for biogeochemical processes. However, the monthly N load from land and atmosphere exceeds the advective supply. Reductions in N load thus have a significant influence on availability of N for biogeochemical processes.

Variability of, and the influence of environmental factors on, the recruitment of postlarval and juvenile Penaeus merguiensis in the Matang mangroves of Malaysia

Postlarval and small juvenile Penaeus merguiensis were sampled fortnightly by beam trawls on the ebb tides over 4 years in the Matang mangroves on the west coast of peninsular Malaysia. Larger juveniles were sampled by bag nets on both the ebb and flood tides every fortnight over 5 years. Rainfall is consistently high throughout the year (>100 mm in each month) and the annual rainfall ranges from about 1,800 to 2,600 mm. Water temperatures ranged from 27°C to 36°C and salinity from 14‰ to 32‰ during this study. Fortnightly catches by beam trawls and bag nets were highly variable but showed a single broad period of postlarval and juvenile recruitment extending from December to May and an extended broad period of juvenile emigration from February to July. Beam trawl catches of postlarvae and juvenile prawns were consistently low between June and August. Higher catches of postlarvae and juveniles were recorded at two sites in close proximity to small adjoining creeks than one with no adjacent creek. Catches of juveniles in bag nets were higher during the day than the night and were correlated with the strength of the water flow. Environmental variables generally accounted for only a small proportion of the variation in catches, even for rainfall. The small proportion of variation explained by environmental variables is probably because of their relatively small seasonal variation in the Matang region. The lack of environmental influence on the catches of P. merguiensis implies that managing the mangrove nursery habitats and fishing pressure may be the main considerations for sustaining their stocks in the Matang region.

Seasonal dynamics of coastal ecosystems and export production at high latitudes: A modeling study

Export of organic matter from the surface to deeper waters often shows much smaller seasonal variations than primary production or nitrate‐based new production in mid‐ to high‐latitude marine systems. The mechanisms underlying this pattern remain poorly understood, but seasonal shifts in food web structure and dynamics have been implicated. We report here on an ecosystem modeling analysis of a high‐resolution (biweekly) time series of biomass, production, and export flux (sediment trap) measurements conducted in 1991 in Bonne Bay (Newfoundland). This time series shows the classical pattern of a spring bloom followed by a summer low biomass period, yet export is bimodal, with maxima during spring and late summer. The ecosystem model was forced by diagnostic vertical mixing calculations based on temperature and salinity records taken every 3 d and hourly wind data. The physical analysis indicated that the nitrate flux into the euphotic zone during summer was equivalent to that during the spring and fall seasons and accounted for half of the summer export. Statistical adjustments of the parameters of the ecosystem model indicated that strong production of dissolved organic carbon during the spring bloom, high temperature dependence of microbial activity, and physico‐chemical particle aggregation played key roles in explaining the remainder of the summer export. Seasonal changes in trophic pathways between spring and summer, such as a shift from a herbivorous to a microbial food web, played a comparatively smaller role. Our modeling analysis suggests that physical mixing processes and physico‐chemical aggregation processes are at least as important as shifts in food web trophic pathways in explaining the postbloom export flux in mid‐ to high‐latitude marine systems.

Plasma Homocysteine Is Not Subject to Seasonal Variation

Studies investigating the relationship between plasma total homocysteine (tHcy) and vascular disease usually rely on a single measurement. Little information is available, however, on the seasonal variability of plasma tHcy. The aim of this study was to investigate the seasonal variation in fasting plasma tHcy and related B-vitamin intake and status in a group of people who did not consume fortified foods or take B-vitamin supplements. In this longitudinal study, a group of 22 healthy people were followed for 1 year. A fasting blood sample and dietary information were collected from each individual every 3 months, i.e., at the end of each season. There was no significant seasonal variation in plasma tHcy or in B-vitamin intake or status with the exception of red cell folate (significantly lower in spring compared with autumn or winter) and serum folate (significantly lower in spring compared with the other seasons). Although the between-person variation in plasma tHcy was high (47%), the within-person variation was low (11%). This low variation, combined with the low methodologic imprecision of 3.8%, yielded a high reliability coefficient for plasma tHcy (0.97). Although there was a small seasonal variation in folate status, there was no corresponding seasonal variation in plasma tHcy. The high reliability coefficient for plasma tHcy suggests that a single measurement is reflective of an individual's average plasma tHcy concentration, thus indicating its usefulness as a potential predictor of disease. This, however, needs to be confirmed in different subgroups of the population.

Seasonal Reproduction of a Neotropical Frog, the Cave Coquí (Eleutherodactylus cooki)

The aim of this study was to evaluate factors contributing to seasonal reproductive variation in a Neotropical frog, the Puerto Rican cave coquí (Eleutherodactylus cooki). There was a relatively small seasonal variation in ambient temperature and relative humidity in the caves occupied by E. cooki. Nonetheless, the reproduction of E. cooki was strongly seasonal, with clutches produced from March to November, and no additional clutches deposited from December to February. This seasonal reproductive pattern may reflect a response to small changes in the thermal environment, because the frequency of clutch production and number of clutches deposited per nest site increased when air temperature was warmest (during the middle of the breeding season). Clutches occurred singly (71% of all clutches) or in groups of two (23%), three (6%), or four (< 1%). The median clutch size was 15 (range = 1–29). Significant egg mortality occurred, despite male attendance of nest sites. The average rate of egg mortality was 16% per week. Extrapolating to a hatching period of 3.5 weeks, 44% of produced eggs survived to hatching. It appears that combined effects of weak environmental variation, clutch predation, and male parental care substantially influence the reproductive output of E. cooki.

Regional Distribution of Natural Stream Nitrate in Central Japan

Over 60 natural streams with forested catchments were surveyed for nitrate (NO3−) concentration in central Japan. Stream NO3− concentrations showed a clear zonal gradient with a range nearly as large as 100-fold, where their small seasonal variations (av. CV = 27%) justified respective statuses. Elevated NO3− concentrations (more than 70 µM) described a distribution of N saturation along a course of polluted air advection in the Kanto Plain. The extent of NO3− leaching from forests was not proportional to atmospheric N deposition probably because of ecosystem N retention. A dependency of NO3− level on the watershed altitude and quite constant δ15N of stream NO3− over a wide NO3− range were observed, which could be also related to this cause.

Bibliografía comentada

The large area (560 m2) GRAPES-3 tracking muon telescope has been operating uninterruptedly at Ooty, India since 2001. Every day, it records 4 × 109 muons of ≥ 1 GeV with an angular resolution of ∼ 4°. The variation of atmospheric temperature affects the rate of decay of muons produced by the galactic cosmic rays (GCRs), which in turn modulates the muon intensity. By analyzing the GRAPES-3 data of six years (2005–2010), a small (amplitude ∼ 0.2%) seasonal variation (1 year (Yr) period) in the intensity of muons could be measured. The effective temperature ‘Teff’ of the upper atmosphere also displays a periodic variation with an amplitude of ∼ 1 K which was responsible for the observed seasonal variation in the muon intensity. At GeV energies, the muons detected by the GRAPES-3 are expected to be anti-correlated with Teff. The anti-correlation between the seasonal variation of Teff, and the muon intensity was used to measure the temperature coefficient αT by fast Fourier transform (FFT) technique. The magnitude of αT was found to scale with the assumed attenuation length ‘λ’ of the hadrons in the range λ = 80–180 g cm−2. However, the magnitude of the correction in the muon intensity was found to be almost independent of the value of λ used. For λ = 120 g cm−2 the value of temperature coefficient αT was found to be (−0.17±0.02)% K−1.

INFLUENCE OF ADVECTION OF KUROSHIO WATER ON ABNORMAL CATCH OF FISH IN JUNE 1999, AT THE COASTAL ZONE OF SOMA, NORTHEAST JAPAN

We analyze month-to-month variability of offshore condition using chlorophyll concentration based on in situ and satellite observations and to examine the abnormal catch event around sea area of Soma in June 1999. Time series of vertical distribution of DO and nutrient concentration (nitrogen, phosphorus), wind vector and AVHRR/SST are also used to understand the surrounding environments around Sendai Bay. Sea WiFS-derived concentration of chlorophyll a reveals that Sendai Bay represents small seasonal variation and the coastal region around the bay seems to be not affected directly by offshore condition. It is found that the abnormal catch phenomenon resulted from that the occurrence of advection and upwelling in this area does not appear to be due to the local wind but offshore current.

A Global Photometric Analysis of 2MASS Calibration Data

We present results from the application of a global photometric calibration (GPC) procedure to calibration data from the first 2 years of The Two Micron All Sky Survey (2MASS). The GPC algorithm uses photometry of both primary standards and moderately bright `tracer' stars in 35 2MASS calibration fields. During the first two years of the Survey, each standard was observed on approximately 50 nights, with about 900 individual measurements. Based on the photometry of primary standard stars and secondary tracer stars and under the assumption that the nightly zeropoint drift is linear, GPC ties together all calibration fields and all survey nights simultaneously, producing a globally optimized solution. Calibration solutions for the Northern and Southern hemisphere observatories are found separately, and are tested for global consistency based on common fields near the celestial equator. Several results from the GPC are presented, including establishing candidate secondary standards, monitoring of near-infrared atmospheric extinction coefficients, and verification of global validity of the standards. The solution gives long-term averages of the atmospheric extinction coefficients, A_J=0.096, A_H=0.026, A_{K_s}=0.066 (North) and A_J=0.092, A_H=0.031, A_{K_s}=0.065 (South), with formal error of 0.001. The residuals show small seasonal variations, most likely due to changing atmospheric content of water vapor. Extension of the GPC to approximately 100 field stars in each of the 35 calibration fields yields a catalog of more than two thousand photometric standards ranging from 10th to 14th magnitude, with photometry that is globally consistent to $\sim 1%$.

Ozone and sulfur dioxide dry deposition to forests: Observations and model evaluation

Fluxes and deposition velocities of O3 and SO2 were measured over both a deciduous and a mixed coniferous‐deciduous forest for full growing seasons. Fluxes and deposition velocities of O3 were measured over a coniferous forest for a month. Mean deposition velocities of 0.35 to 0.48 cm/s for O3 and 0.6 to 0.72 cm/s for SO2 were observed during the growing seasons of 1997 and 1998. Weekly averages of O3 deposition velocity ranged from 0.25 cm/s at the beginning and end of the season to 1.25 cm/s in late June. SO2 had a smaller seasonal variation, from 0.75 to 1.5 cm/s between the beginning and peak of the season. Because O3 concentrations are higher, the flux of O3 to forests is considerably greater than the flux of SO2. Daytime deposition velocities are very similar at each site, from 0.75 to 0.79 cm/s for O3, and from 1.01 to 1.04 cm/s for SO2. Diurnal cycles for both gases are discussed, as are the impact of some weather events. The peak time for O3 deposition velocity is in midmorning, while it is near midday for SO2. Surface wetness is usually associated with a small increase in deposition velocity, but for some rain events a major increase was noted. Minimum deposition velocities usually occur at night and increase slowly in the predawn hours before light. Comparisons are made between observations of deposition velocity and predictions made with the Meyers multilayer deposition velocity model. While the model is, on average, unbiased for O3, it tends to underpredict the higher deposition velocity values. The model is slightly biased low (underpredicts) for SO2 deposition velocity. The strengths of the model are noted, as are opportunities for improvement.

Abschätzung der Eignung einer molekularbiologischen Methode zur Charakterisierung der Grundwasserbiozönose

A molecularbiological technique was used to characterise the bacterial community structure of groundwater habitats. This method consists of the isolation of bacterial DNA from the samples, amplification of 16S rDNA by PCR (polymerase chain reaction), and separation of the amplified DNA by DGGE (denaturing gradient gel electrophoresis). By using more specific primer combinations in the PCR instead of universal eubacterial primers, also groups of microorganisms (Proteobacteria, sulfate reducer, Archaea) were determined. The resulting DGGE patterns that reflect the microbial diversity are compared and differences or similarities evaluated. In the present studies, groundwater from different sites (bank filtrate, artificially recharged groundwater, and natural groundwater) and with changing redox milieus (aerobic, anaerobic) were investigated as well as the solid aquifer material. Besides, samples were taken from the different stages of artificial groundwater recharge, i.e., from surface water to the drain tile. Samples from groundwater derived from sites with different hydrogeochemical or hydrological conditions like bank filtrate and recharged groundwater revealed great differences in DGGE patterns indicating a characteristic species composition in these habitats, while samples taken at different times from the same groundwater showed only small seasonal variations. Clearly different patterns were also found for groundwater and the adjacent solid material as well as for anaerobic and aerobic groundwaters. Looking at artificial groundwater recharge, almost identical patterns were found in raw water and samples from gravel and sand filtration. DGGE patterns from the resulting groundwater indicated a total change in community structure during underground passage. By using group specific primers, Desulfovibrionaceae, Desulfobacteriaceae, and Archaea could be detected in anaerobic groundwaters. The molecularbiological approach described here gives an increasingly comprehensive and more precise picture of the microbial population of different environments. It is especially suitable to compare the community structure from different habitats or to analyse changes for example due to environmental stress at the same site.

What are Backshoots Good For? Seasonal Changes in Mineral, Carbohydrate and Water Content of Different Organs of the Epiphytic Orchid, Dimerandra emarginata

The epiphytic orchid, Dimerandra emarginata (G. Meyer) Hoehne, keeps older stems (‘ backshoots’) after the attached foliage dies. I investigated seasonal changes in the water, carbohydrate and nutrient content of these leafless stems along with associated, connected organs for 1 year. Older stems significantly increase the water storage capacity of a plant, allowing it to tolerate long periods of drought. Reduction in total non-structural carbohydrates (TNC) at the beginning of the growing season indicated an important role of carbon stored in older stems for the re-initiation of vegetative growth after the dry season, while the lack of an increase in TNC before fruiting suggests no such role for reproduction. While seasonal fluctuations in water contents and TNC were pronounced, concentrations of nutrients (N, P, K) showed only small seasonal variation in all vegetative organs. Allocation of carbon and nutrients to developing fruits was rather high, exceeding, e.g. 20% of the total plant pool of potassium. This large investment in fruits was derived from all vegetative organs. I conclude that persistent stems serve a crucial function in the water, carbon and nutrient relations of epiphytes such as D. emarginata, which live in habitats where access to water and key nutrients is often severely limited.

The cyclical sensitivity of seasonality in U.S. employment

This paper investigates interactions between seasonal and cyclical movements in U.S. payroll employment. Using a multivariate unobserved components model, we test for such interactions and find that they are statistically significant in a number of industries. Still, most industry-level seasonality appears to be idiosyncratic. The model also identifies an unobserved common cycle that exhibits similar business cycle properties, but smaller seasonal variation, than aggregate payroll employment. The overall industry-level seasonal factors generated by our model do not differ much from univariate X-11 seasonals in sample, but some differences arise in out-of-sample experiments.

Measurements of middle atmospheric water vapor from low latitudes and midlatitudes in the northern hemisphere, 1995–1998

We present middle atmospheric water vapor measurements made at 22 GHz using two Naval Research Laboratory Water Vapor Millimeter‐wave Spectrometers (WVMS2 and WVMS3). We include measurements from an intercomparison campaign at Table Mountain, California (34.4°N, 242.3°E), and measurements obtained since March 1996 from Table Mountain and from Mauna Loa, Hawaii (19.5°N, 204.4°E). The mesospheric data from Mauna Loa show both a mixing ratio peak at a higher altitude and smaller seasonal variations than those from the WVMS instrument at Table Mountain. These differences are qualitatively consistent with the changes in mixing ratio and the increase in mesospheric variability expected with decreasing latitude. The latitudinal variation in the mixing ratio profile observed in the WVMS data is very similar to that observed by the Halogen Occultation Experiment (HALOE) in the upper stratosphere and lower mesosphere. The WVMS measurements generally show higher water vapor mixing ratios than HALOE does in the upper stratosphere and lower mesosphere but smaller mixing ratios in the upper mesosphere. The winter of 1997–1998 shows an unusually large decrease in mesospheric water vapor, a result that is consistent with measurements from HALOE throughout the Northern Hemisphere for this winter. There appears to be a slight overall decrease in water vapor for all seasons over the period of the WVMS observations from Mauna Loa. The relatively small seasonal variations, combined with the small amount of tropospheric water vapor above Mauna Loa, make this an ideal site for the monitoring of multiyear trends in water vapor.

Removal Characteristics of Coliform Bacteria in Biological Reactors at Domestic Wastewater Treatment System.

Total coliforms, fecal coliforms, and pathogenic Escherichia coli O157: H7 were measured in domestic wastewater and biologically treated wastewater from 75 treatment plants serving various types of buildings. The means for the influents of all the plants were 3.7×105 CFU/ml of total coliforms, 1.4×105 CFU/ml of fecal coliforms, and pathogenic E. coli O157 was not detected. The means for the effluents were 4.6×103 CFU/ml of total coliforms, and 1.1×103 CFU/ml of fecal coliforms. Seventy-nine percent of the plants complied with the effluent standards, revealing that efficient treatment can considerably reduce total coliforms even before disinfection. In Johkasous serving multiple dwelling houses, total coliforms in the influents showed relatively small time-serial and seasonal variations, and flow equalization tanks contributed to efficient treatment, resulting in the constant presence of few bacteria in the effluents.A high correlation was obtained between total coliforms and fecal coliforms. A correlation was also found between BOD and total coliforms in the effluents. Total coliforms can be decreased by extending the retention time in a biological reactor, where nitrification can proceed. So operation methods which promote nitrification can reduce total coliforms effectively. The mean removal efficiency of total coliforms was higher using the activated sludge process than the contact aeration process.

Radiation regime and canopy architecture in a boreal aspen forest

This study was part of the Boreal Ecosystem-Atmosphere Study (BOREAS). It took place in a mature aspen forest in Prince Albert National Park, Saskatchewan, Canada. The aspen trees were 21.5 m high with a 2–3 m high hazelnut understory. The objectives were: (1) to compare the radiation regime beneath the overstory before and after leaf emergence; (2) to infer the structural characteristics of the aspen canopy leaf inclination and clumping; (3) to determine the seasonal course of the leaf area index ( L) for both the overstory and understory. Above-stand radiation measurements were made on a 39m walk-up tower, and understory radiation measurements were made on a tram which moved horizontally back and forth at 0.10 m s −1 on a pair of steel cables 65m in length suspended 4 m above the ground. In addition, several LI-COR LAI-2000 Plant Canopy Analyzers were used to determine the effective leaf area index and the zenith angle dependent extinction coefficient (G(θ)) for both the aspen and the hazelnut throughout the growing season. These measurements were supplemented with destructive sampling of the hazelnut at the peak of the growing season. Before leaf emergence, the ratios of below- to above-aspen solar radiation ( S), photosynthetic photon flux density (PPFD) and net radiation ( R n) during most of the day were 0.58, 0.55 and 0.47, respectively. By midsummer, these ratios had fallen to 0.33, 0.26 and 0.26, respectively. The aspen G(θ) was relatively invariant with θ, within ±0.05 of 0.5 throughout the growing season, indicating a spherical distribution of leaf inclination angles (i.e. the leaves were randomly inclined). The hazelnut G(θ) has a cosine response with respect to θ, which was consistent with the generally planophile leaf distribution for hazelnut. Using canopy gap size distribution theories developed by Chen and Black (1992b, Agric. For. Meteorol., 60: 249–266) and Chen and Cihlar (1995a, Appl. Opt., 34: 6211–6222) based on Miller and Norman (1971, Agron. J., 63: 735–738), the foliage clumping index (Ω) of the aspen canopy was derived from high-frequency tram measurements of PPFD. The aspen Ω was fairly constant with θ, but showed a small seasonal variation, with a minimum value of 0.70 in the midsummer. The hazelnut Ω was found to be 0.98 determined using the L from the destructive sampling, indicating no clumping. After corrections for clumping and the wood area index (α), the seasonal course of L near the tower for both aspen and hazelnut was determined, with maximum L of 2.4 and 3.3 for the aspen and hazelnut, respectively.

Comet Lightcurves: Effects of Active Regions and Topography

The lightcurves and derived production rates of volatiles such as water for short-period comets show asymmetries around perihelion. These asymmetries do not follow a specific trend, and short-period comets are as likely to be brighter preperihelion as postperihelion. This and images of the nucleus of Comet P/Halley suggest that the activity of comet nuclei is driven by a small number of active regions. Lightcurve asymmetries may then be understood by changing geometry of the active regions with respect to the Sun. Solutions of the heat diffusion equation for a short-period comet with active regions covering the full range of spin axis orientations show that active regions can produce asymmetries of several orders of magnitude in the sublimation rate of water from the nucleus. These large and sudden swings in activity are reduced if the active zone lies within a crater or topographical depression on the nucleus. Simulation of the sublimation rate of H2O from craters on a comet nucleus show that topography can qualitatively change the dependence of water production on heliocentric distance. The effect of negative relief topography is to slow the loss of heat from the nucleus and maintain higher sublimation rates than from a flat surface active region experiencing winter. In the absence of topography, a single active region can lead to sublimation rates that vary over several orders of magnitude on short time scales. Negative relief topography reduces the magnitude and speed of these seasonal changes in comet activity, with rougher surfaces having smaller seasonal variations. The magnitude of seasonal changes in comet activity can be used with the results of this model to place upper limits on the amount of surface roughness at known active regions.