Diel Time Scales Research Articles

Coastal low cloudiness and fog enhance crop water use efficiency in a California agricultural system

Impacts of climate change threaten California farmers in a number of ways, most importantly through a decline in freshwater availability, concurrent with a rise in water demand. In coastal California, the growing season of economically important crops, such as strawberries, overlap with the occurrence of summertime coastal fog, which buffers the summer dry season through shading effects and direct water inputs. The impacts of coastal fog on plant physiology have been extensively studied in natural ecosystems. Yet, very few studies have evaluated its direct effects on crop water use and demand, which has potential to curtail groundwater use. We established two sites on large, conventional strawberry farms along a coastal-inland gradient in the Salinas Valley, California, where we monitored variation in microclimate conditions and measured strawberry plant physiological responses to foggy and non-foggy conditions between June–September 2015. Spatial analysis of coastal low clouds and fog from satellite imagery was preformed to quantify and characterize fog events at seasonal and diel time scales. We found strong agreement between field and satellite-derived observations of coastal fog events. Canopy-level conductance and whole-plant carbon uptake were reduced by 60% and 30%, respectively, on foggy compared to clear-sky days. Leaf-level photosynthesis and stomatal conductance were 30% lower on foggy compared to clear-sky days, which was driven by reduced photosynthetically active radiation and cooler temperatures during fog events. Taken together, we found that whole-plant water use efficiency increased significantly during foggy periods, and these patterns were driven by changes in the radiation balance and atmospheric water stress. Our results provide evidence that the shading effect by fog is a primary influence on crop water use efficiency in coastal agricultural fields during summer. The outcome of our research can inform estimates of how much irrigation water may be reduced during foggy periods without sacrificing crop yields on coastal agricultural lands.

Calcium carbonate (CaCO3) sediment dissolution under elevated concentrations of carbon dioxide (CO2) and nitrate (NO3−)

Ocean acidification (OA), attributed to the sequestration of atmospheric carbon dioxide (CO2) into the surface ocean, and coastal eutrophication, attributed in part to land-use change and terrestrial runoff of fertilizers, have received recent attention in an experimental framework examining the effects of each on coral reef net ecosystem calcification (Gnet). However, OA and eutrophication in conjunction have yet to receive attention from the perspective of coral reef sediment dissolution. To address this omission, CO2 and nitrate (NO3−) addition experiments were performed in Mo'orea, French Polynesia. Incubation chambers were used to measure sediment Gnet during the day and night under three different [NO3−] (0, 9.8, and 19.7μM) that were nested within four separate constructed coral reef communities maintained at different PCO2 levels (417, 721, 1030, and 1333μatm, respectively). PCO2 negatively affected sediment Gnet during the day and night, resulting in a shift to diel net dissolution at a PCO2 of 1030μatm. Elevated NO3− alone, and the combination of NO3− and PCO2, both negatively affected sediment Gnet at night. However, the response of Gnet to NO3− was less clear during the day, where diurnal sediment Gnet was enhanced under the combined treatment of elevated NO3− and PCO2, resulting in no net effect of NO3− on sediment Gnet on diel timescales. Overall, these results show that ocean acidification represents a greater threat to the balance of calcification and dissolution in Mo'orea's back reef sediment communities than the potential impact of NO3− enrichment on relatively short timescales.

Two‐stage metabolism inferred from diel oxygen dynamics in aquatic ecosystems

AbstractDynamics of dissolved oxygen in aquatic ecosystems reflect the biological, physical, and chemical processes that regulate ecosystem metabolism. Organic matter that supports ecosystem respiration (ER) is produced both by in situ photosynthesis and via loading from terrestrial ecosystems. Terrestrial‐derived organic matter is relatively recalcitrant and its availability is stable at diel time scales relative to substrates produced through photosynthesis, which are more labile and often show distinct diel changes in availability. Here, we explored whether the contributions of these two sources of organic matter to ecosystem metabolism could be quantified by a process model of photosynthesis and ER fit to high‐frequency observations of oxygen concentration in streams. We found that a two‐stage model of respiration provided a better fit to diel oxygen dynamics in most streams than a model assuming that the substrates supporting ER were one temporally stable pool. Two‐stage models estimated peak daytime respiration rates that were ~2.9× higher on average than nighttime rates, but this increase was variable and ranged from 1.1 to 11.6×. Estimates of gross primary production were 1.35× higher, on average, (range = 1.04–7.91×) compared to estimates generated by a single‐stage model. Streams draining watersheds with less than about 7% gradient exhibited oxygen dynamics that provided comparable statistical support for single‐stage metabolism, likely due to the higher loading of allochthonous organic matter that swamped metabolism based on autochthonous production in these streams. Ecosystem metabolism in streams draining steeper watersheds was best characterized by two‐stage metabolism, reflecting the greater importance of autochthonous contributions to labile organic carbon pools in these ecosystems.

Stream phosphorus dynamics of minimally impacted coastal plain watersheds

AbstractUnderstanding natural variation in stream phosphorus (P) concentrations over space and time is critical for understanding natural drivers of catchment behavior and establishing regulatory standards. Across minimally impacted benchmark streams (n = 81) in Florida, spatial variation in mean total P concentrations was large, indicating the importance of geologic controls on catchment solute dynamics. While this variation was significantly predicted by geographic regions, within regions we observed nearly comparable cross‐site variation, suggesting important finer‐scale heterogeneity in baseline catchment chemistry. Within‐site residual variation (unexplained by region or site) was as large as spatial variation, suggesting temporal variation in response to drivers such as flow may be critically important. To further explore timescales of P export variation, we collected long‐term, high‐frequency (subdaily) measurements of stream discharge (Q) and soluble reactive P (SRP) in 2 forested watersheds. We observed significant variation at annual, event, and diel timescales, all of which arise primarily from corresponding Q‐variation. Over the entire period of record, we generally observed a strong dilution signal, with SRP concentrations declining with increased Q. Despite significant SRP variation, flow variation was far larger and, thus, dominated temporal control on downstream flux. Within‐storm events, we observed strong and consistent clockwise SRP versus Q hysteresis, suggesting mobilization of proximal SRP stores. Diel variation exhibited mid‐afternoon concentration minima, Q‐controlled amplitude, and pronounced seasonal shifts in both magnitude and timing consistent with riparian evapotranspiration‐regulating lateral inputs of P‐rich groundwater. Such high‐resolution temporal signals allow identification of solute sources and provide insights into geologic and hydrologic drivers of solute variation.

Decoupling between bacterial production and primary production over multiple time scales in the North Pacific Subtropical Gyre

We measured rates of 3H-leucine (3H-Leu) incorporation, as a proxy for bacterial production, at Station ALOHA (22°45′N, 158°W) in the oligotrophic North Pacific Subtropical Gyre (NPSG). We report measurements conducted between January 2011 and April 2013, examining variability in 3H-Leu incorporation over diel, daily, and monthly time scales. Rates of 3H-Leu were evaluated in the context of contemporaneous 14C-based primary productivity (14C-PP) to identify potential temporal coupling between these measures of productivity. Throughout the upper ocean (0–125m), rates of 3H-Leu incorporation measured in the light (3H-LeuLight) were stimulated (1.5-fold, on average) relative to measurements in the dark (3H-LeuDark). At monthly scales, rates of 3H-LeuLight and 3H-LeuDark varied 4.9-fold and 3.8-fold, respectively, while rates of 14C-PP varied 1.7-fold. Rates of 14C-PP were often elevated during summer months (May through August) when incident light flux was greatest, while rates of both 3H-LeuLight and 3H-LeuDark often peaked in early fall (August through October) when seawater temperatures were maximal. Near-daily measurements of 3H-Leu incorporation and 14C-PP conducted over a 62-day period in the summer of 2012 revealed that rates of 3H-LeuLight and 3H-LeuDark varied ~2.5 and 2.0-fold, respectively, similar to ~1.8-fold daily variability observed in rates of 14C-PP. Over diel time scales, rates of 3H-LeuLight and 3H-LeuDark demonstrated different patterns, with rates of 3H-LeuLight elevated at mid-day and rates of 3H-LeuDark greatest in the early evening. Together, these results suggest that in this oligotrophic ecosystem, photosynthetic production of organic matter and bacterial production can be temporally uncoupled across daily to seasonal scales.

Juvenile coho salmon track a seasonally shifting thermal mosaic across a river floodplain

Summary Seasonal variation in river water levels creates a shifting mosaic of habitat conditions associated with variables such as water temperature, chemistry and prey availability to consumers. Previous work has shown that fishes can exploit spatial variation in water temperature, but less is known about how they respond to shifts in the spatial arrangement of habitat conditions through time. Juvenile coho salmon (Oncorhynchus kisutch) are the numerically dominant fish species in many southwest Alaskan streams, which exhibit seasonal variation in water level and temperature due to changes in precipitation and snowmelt. We assessed the degree to which juvenile coho salmon exploit the associated shifting mosaic of water temperature by monitoring the spatial distribution of water temperatures and juvenile coho salmon in the lake‐influenced reaches of a southwest Alaskan stream. We also monitored the diets of juvenile coho salmon relative to the spatial distribution of prey taxa. Juvenile coho salmon exhibited two scales of movement to track spatiotemporal variation in habitat conditions. First, over the course of 6 weeks, individuals moved among off‐channel units, tracking shifts in the location of warm water habitat caused by receding water level. Second, individuals moved at diel time scales, foraging on benthic macroinvertebrates in the cold thalweg of the stream at night and then digesting prey in warmer off‐channel habitats during the day. Seasonally asynchronous variation in water temperature among off‐channel habitat units produced portfolio effects in habitat conditions, such that coho salmon had continual access to warm habitat for digestion despite its ephemeral availability at discrete locations. Our study demonstrates that behavioural thermoregulation by juvenile fishes can be important throughout the growing season and is not restricted to ephemeral events such as resource pulses or heat stress. Our results have implications for the conservation of highly connected, heterogeneous landscapes, and their ability to support economically and ecologically important species such as coho salmon.

Diel Surface Temperature Range Scales with Lake Size.

Ecological and biogeochemical processes in lakes are strongly dependent upon water temperature. Long-term surface warming of many lakes is unequivocal, but little is known about the comparative magnitude of temperature variation at diel timescales, due to a lack of appropriately resolved data. Here we quantify the pattern and magnitude of diel temperature variability of surface waters using high-frequency data from 100 lakes. We show that the near-surface diel temperature range can be substantial in summer relative to long-term change and, for lakes smaller than 3 km2, increases sharply and predictably with decreasing lake area. Most small lakes included in this study experience average summer diel ranges in their near-surface temperatures of between 4 and 7°C. Large diel temperature fluctuations in the majority of lakes undoubtedly influence their structure, function and role in biogeochemical cycles, but the full implications remain largely unexplored.

Acoustic and temporal partitioning of cicada assemblages in city and mountain environments.

Comparing adaptations to noisy city environments with those to natural mountain environments on the community level can provide significant insights that allow an understanding of the impact of anthropogenic noise on invertebrates that employ loud calling songs for mate attraction, especially when each species has its distinct song, as in the case of cicadas. In this study, we investigated the partitioning strategy of cicada assemblages in city and mountain environments by comparing the acoustic features and calling activity patterns of each species, recorded using automated digital recording systems. Our comparison of activity patterns of seasonal and diel calling revealed that there was no significant temporal partitioning of cicada assemblages in either environment. In addition, there was no correlation between the acoustic distance based on spectral features and temporal segregation. Heterospecific spectral overlap was low in both city and mountain environments, although city and mountain cicada assemblages were subject to significantly different levels of anthropogenic or interspecific noise. Furthermore, for the common species found in both environments, the calling activity patterns at both seasonal and diel time scales were significantly consistent across sites and across environments. We suggest that the temporal calling activity is constrained by endogenous factors for each species and is less flexible in response to external factors, such as anthropogenic noise. As a result, cicada assemblages in city environments with low species diversity do not demonstrate a more significant temporal partitioning than those in mountain environments with high species diversity.

Soil CO2 efflux and production rates as influenced by evapotranspiration in a dry grassland

Our aim was to study the effect of potential biotic drivers, including evapotranspiration (ET) and gross primary production (GPP), on the soil CO2 production and efflux on the diel time scale. Eddy covariance, soil respiration and soil CO2 gradient systems were used to measure the CO2 and H2O fluxes in a dry, sandy grassland in Hungary. The contribution of CO2 production from three soil layers to plot-scale soil respiration was quantified. CO2 production and efflux residuals after subtracting the effects of the main abiotic and biotic drivers were analysed. Soil CO2 production showed a strong negative correlation with ET rates with a time lag of 0.5 h in the two upper layers, whereas less strong, but still significant time-lagged and positive correlations were found between GPP and soil CO2 production. Our results suggest a rapid negative response of soil CO2 production rates to transpiration changes, and a delayed positive response to GPP. We found evidence for a combined effect of soil temperature and transpiration that influenced the diel changes in soil CO2 production. A possible explanation for this pattern could be that a significant part of CO2 produced in the soil may be transported across soil layers via the xylem.

Hysteresis responses of evapotranspiration to meteorological factors at a diel timescale: patterns and causes.

Evapotranspiration (ET) is an important component of the water cycle in terrestrial ecosystems. Understanding the ways in which ET changes with meteorological factors is central to a better understanding of ecological and hydrological processes. In this study, we used eddy covariance measurements of ET from a typical alpine shrubland meadow ecosystem in China to investigate the hysteresis response of ET to environmental variables including air temperature (T a), vapor pressure deficit (VPD) and net radiation (R n) at a diel timescale. Meanwhile, the simulated ET by Priestly-Taylor equation was used to interpret the measured ET under well-watered conditions. Pronounced hysteresis was observed in both T a and VPD response curves of ET. At a similar T a and VPD, ET was always significantly depressed in the afternoon compared with the morning. But the hysteresis response of ET to R n was not evident. Similar hysteresis patterns were also observed in the T a/VPD response curves of simulated ET. The magnitudes of the measured and simulated hysteresis loops showed similar seasonal variation, with relatively smaller values occurring from May to September, which agreed well with the lifetime of plants and the period of rainy season at this site. About 62% and 23% of changes in the strength of measured ET-T a and ET-VPD loops could be explained by the changes in the strength of simulated loops, respectively. Thus, the time lag between R n and T a/VPD is the most important factor generating and modulating the ET-T a/VPD hysteresis, but plants and water status also contribute to the hysteresis response of ET. Our research confirmed the different hysteresis in the responses of ET to meteorological factors and proved the vital role of R n in driving the diel course of ET.

Using diel movement behavior to infer foraging strategies related to ecological and social factors in elephants.

BackgroundAdaptive movement behaviors allow individuals to respond to fluctuations in resource quality and distribution in order to maintain fitness. Classically, studies of the interaction between ecological conditions and movement behavior have focused on such metrics as travel distance, velocity, home range size or patch occupancy time as the salient metrics of behavior. Driven by the emergence of very regular high frequency data, more recently the importance of interpreting the autocorrelation structure of movement as a behavioral metric has become apparent. Studying movement of a free ranging African savannah elephant population, we evaluated how two movement metrics, diel displacement (DD) and movement predictability (MP - the degree of autocorrelated movement activity at diel time scales), changed in response to variation in resource availability as measured by the Normalized Difference Vegetation Index. We were able to capitalize on long term (multi-year) yet high resolution (hourly) global positioning system tracking datasets, the sample size of which allows robust analysis of complex models. We use optimal foraging theory predictions as a framework to interpret our results, in particular contrasting the behaviors across changes in social rank and resource availability to infer which movement behaviors at diel time scales may be optimal in this highly social species.ResultsBoth DD and MP increased with increasing forage availability, irrespective of rank, reflecting increased energy expenditure and movement predictability during time periods of overall high resource availability. However, significant interactions between forage availability and social rank indicated a stronger response in DD, and a weaker response in MP, with increasing social status.ConclusionsRelative to high ranking individuals, low ranking individuals expended more energy and exhibited less behavioral movement autocorrelation during lower forage availability conditions, likely reflecting sub-optimal movement behavior. Beyond situations of contest competition, rank status appears to influence the extent to which individuals can modify their movement strategies across periods with differing forage availability. Large-scale spatiotemporal resource complexity not only impacts fine scale movement and optimal foraging strategies directly, but likely impacts rates of inter- and intra-specific interactions and competition resulting in socially based movement responses to ecological dynamics.

Evaluation of carbon isotope flux partitioning theory under simplified and controlled environmental conditions

Separation of the photosynthetic (FP) and respiratory (FR) fluxes of net CO2 exchange (FN) remains a necessary step toward understanding the biological and physical controls on carbon cycling between the soil, biomass, and atmosphere. Despite recent advancements in stable carbon isotope partitioning methodology, several potential limitations can cause uncertainty in the partitioned results. Here, we combined an automated chamber system with a tunable diode laser (TDL) to evaluate carbon isotope partitioning under controlled environmental conditions. Experiments were conducted in a climate controlled greenhouse utilizing both soybean (C3 pathway) and corn (C4 pathway) treatments. Under these conditions, net exchange of CO132 and CO122 was obtained with an improved signal to noise ratio. Further, the chamber system was used to estimate soil evaporation (E) and plant transpiration (T), allowing for an improved estimate of the total conductance to CO2 (gc). This study found that the incorporation of short-term and diel variability in the isotope composition of respiration (δR) caused FP to nearly double in the corn system while only slightly increasing in the soybean system. Variability in both gc and the CO2 bundle sheath leakage factor for C4 plants (ϕ) also had a significant influence on FP. In addition, chamber measurements of FN and its isotope composition (δN) indicated that post-illumination processes caused a decrease in plant respiration for up to 3h following light termination. Finally, this study found systematic differences between the isotope and temperature-regression partitioning methods on the diel time scale.

Carbon dioxide exchange of a Sitka spruce plantation in Scotland over five years

Scotland's 17% forested land cover sequesters 10% of Scotland's emissions of greenhouse gases. The expected rise of forested land cover to 25% by the middle of this century make forests an important component of the national carbon budget. This paper presents the carbon exchange measurement methodology, results, and modeling associated with a five-year experiment at a plantation forest in Scotland, UK, as part of a European initiative to develop and verify vegetation carbon-exchange models.We have developed a site specific correction for advective flux loses. While based on ustar corrections, it differs from current approaches in that it includes site topographic effects, corrects existing data instead of replacing it, and applies over the entire diel cycle instead of only during nocturnal periods.Models of soil respiration are compared and reveal the inadequacy of soil state parameterization. A non-rectangular hyperbola model of assimilation is selected to examine the maximum assimilation rate and quantum use efficiency parameters’ responses to environmental temperature, vapour pressure, and sky condition. Intra-annual variability is found to be highest in early autumn, linked to a combination of seasonal changes in radiation and phenological changes in canopy quantum yield. Inter-annual variability was low, with less than %5 variation from year to year, and factors driving this variability were not clearly apparent.Over the 5 years of reported measurements, this forest sequestered about 6 tonnes of C per hectare per annum. Photosynthetically active radiation use efficiency of the forest is about 4.2%. Constancy of radiation use efficiency was observed at both diel and annual scales. This constancy depended on the ecosystem's thermal inertia and assimilation/radiation response curve characteristics at diel time scales but depended on thermal inertia and phenological changes in quantum yield at annual time scales.The net carbon uptake rates are consistent with two other flux sites in the British Isles and suggest that the relatively maritime climatology of these areas is highly suitable for the species of trees routinely planted here.

The diel imprint of leaf metabolism on the δ13C signal of soil respiration under control and drought conditions

Recent (13) CO(2) canopy pulse chase labeling studies revealed that photosynthesis influences the carbon isotopic composition of soil respired CO(2) (δ(13) C(SR)) even on a diel timescale. However, the driving mechanisms underlying these short-term responses remain unclear, in particular under drought conditions. The gas exchange of CO(2) isotopes of canopy and soil was monitored in drought/nondrought-stressed beech (Fagus sylvatica) saplings after (13) CO(2) canopy pulse labeling. A combined canopy/soil chamber system with gas-tight separated soil and canopy compartments was coupled to a laser spectrometer measuring mixing ratios and isotopic composition of CO(2) in air at high temporal resolution. The measured δ(13) C(SR) signal was then explained and substantiated by a mechanistic carbon allocation model. Leaf metabolism had a strong imprint on diel cycles in control plants, as a result of an alternating substrate supply switching between sugar and transient starch. By contrast, diel cycles in drought-stressed plants were determined by the relative contributions of autotrophic and heterotrophic respiration throughout the day. Drought reduced the speed of the link between photosynthesis and soil respiration by a factor of c. 2.5, depending on the photosynthetic rate. Drought slows the coupling between photosynthesis and soil respiration and alters the underlying mechanism causing diel variations of δ(13) C(SR).

Water regulated effects of photosynthetic substrate supply on soil respiration in a semiarid steppe

Soil respiration is an important part of the global carbon (C) cycle and the largest component of C flux from terrestrial ecosystems to the atmosphere. Here, we investigated possible effects of photosynthetic substrate supply on soil respiration in a semiarid ecosystem. A field experiment combining water addition and shading (low and high shading) treatments was conducted to manipulate photosynthetic substrate supply in a temperate semiarid steppe in two growing seasons. Our result showed that water addition and/or low shading significantly increased net primary productivity (ecosystem-level photosynthetic substrate supply) and soil respiration in both two growing seasons. However, the effects of high shading on net primary productivity and soil respiration depended on soil water condition, which were negative in wet year (2008) but positive in dry year (2009). On the diel timescale, soil respiration was out of phase with soil temperature and leaf net photosynthesis, but in phase with leaf sugar and starch contents (leaf-level photosynthetic substrate production). The results indicated that photosynthetic substrate supply was an important factor in regulating soil respiration on both daily and seasonal timescales. Moreover, its effect on soil respiration increased with increasing water availability in this region. The predominant role of C assimilate supply on soil respiration indicates that the predicted positive influence of rising temperature on soil respiration will be simultaneously mediated by substrate supply and water availability in semiarid steppe ecosystems.

Temporal dynamics of seston: A recurring nighttime peak and seasonal shifts in composition in a stream ecosystem

We measured the baseflow concentration and composition of seston and suspended particulate organic carbon (POC) over a 1-yr period in White Clay Creek, a third-order stream in the southeastern Pennsylvania Piedmont, to assess temporal variability in seston concentration and quality at seasonal and diel timescales. Each month, we sampled stream water under baseflow conditions every 1.5 h over a 24-h period and measured seston and POC concentrations, carbon composition, pigment content, and 13C isotopic ratios. Seston and POC concentrations exhibited a strong diel pattern; nighttime concentrations exceeded daytime concentrations by 80% and 43%, respectively. Suspended chlorophyll a concentrations did not exhibit a diel pattern. We attribute the diel pattern of seston concentration to bioturbation by the nocturnal stream community, including crayfish, amphibians, eels, and macroinvertebrates. Seasonally, carbon content of seston increased from 9% throughout most of the year to 15% during November, December, and March, while seston d13C was depleted in the late fall and enriched in early spring months relative to the rest of the year. Chlorophyll a and pheophytin a concentrations in seston peaked during the early spring. Seasonal patterns in seston and POC composition reflect cycles of autumnal leaf litter inputs and vernal algal production. Bioturbation and shifts in organic carbon inputs mediate changes in POC quality and fluxes, which affect the bioavailability of POC and ultimately influence rates of heterotrophic respiration. Particulate organic carbon (POC) provides energy for microorganisms and macroinvertebrates in headwater streams and, when transported in the suspended load (seston), supports heterotrophic metabolism in downstream reaches and rivers (Wipfli and Gregovich 2002; Mayorga et al. 2005). POC transport represents a longitudinal linkage between upstream autochthonous and allochthonous production and downstream ecosystems, where the magnitude of connection is controlled by the quality, quantity, and timing of POC delivery (Mayorga et al. 2005).

Soil respiration in perennial grass and shrub ecosystems: Linking environmental controls with plant and microbial sources on seasonal and diel timescales

A mechanistic understanding of soil respiration is a major impediment to predicting terrestrial C fluxes spatially and temporally. Automated measurements of soil respiration offer the high‐resolution information necessary to observe temporal variation in soil respiration, but spatially these measurements are under‐represented in water‐limited and non‐forested ecosystems. We measured soil respiration with automated chambers over the growing season, at two sites with the same semi‐arid climate, but with different dominant vegetation, perennial grasses and shrubs in the Owens Valley, CA, USA. An isotope mass balance technique was used to partition soil respiration into autotrophic and heterotrophic components at two time points, early and late growing season. Results showed large differences in the magnitude of growing season soil respiration between the two sites (910 versus 126 g C m−2 for grasses and shrubs respectively over 5 months). We attribute this to site differences in soil water availability and belowground allocation and productivity. Diel patterns of soil respiration between the two sites were similar. Temperature explained most of the diel variability in the early growing season, when soil moisture was greatest. As soil moisture declined over the growing season, diel patterns became increasingly decoupled temporally from temperature due to increased water‐limitation on surface heterotrophic sources and hypothesized strong photosynthetic control over soil respiration rates. Partitioning of soil respiration into autotrophic and heterotrophic sources showed the dominance of autotrophic sources across seasons and ecosystems. However, heterotrophic respiration was more dynamic from early to late growing season, declining in the grass ecosystem; and a surprising increase in the shrub ecosystem, attributed to warming of the soil profile enhancing microbial decomposition at depth.

Temporal dynamics of the carbon isotope composition in a Pinus sylvestris stand: from newly assimilated organic carbon to respired carbon dioxide

The (13)C isotopic signature (C stable isotope ratio; delta(13)C) of CO(2) respired from forest ecosystems and their particular compartments are known to be influenced by temporal changes in environmental conditions affecting C isotope fractionation during photosynthesis. Whereas most studies have assessed temporal variation in delta(13)C of ecosystem-respired CO(2) on a day-to-day scale, not much information is available on its diel dynamics. We investigated environmental and physiological controls over potential temporal changes in delta(13)C of respired CO(2) by following the short-term dynamics of the (13)C signature from newly assimilated organic matter pools in the needles, via phloem-transported organic matter in twigs and trunks, to trunk-, soil- and ecosystem-respired CO(2). We found a strong 24-h periodicity in delta(13)C of organic matter in leaf and twig phloem sap, which was strongly dampened as carbohydrates were transported down the trunk. Periodicity reappeared in the delta(13)C of trunk-respired CO(2), which seemed to originate from apparent respiratory fractionation rather than from changes in delta(13)C of the organic substrate. The diel patterns of delta(13)C in soil-respired CO(2) are partly explained by soil temperature and moisture and are probably due to changes in the relative contribution of heterotrophic and autotrophic CO(2) fluxes to total soil efflux in response to environmental conditions. Our study shows that direct relations between delta(13)C of recent assimilates and respired CO(2) may not be present on a diel time scale, and other factors lead to short-term variations in delta(13)C of ecosystem-emitted CO(2). On the one hand, these variations complicate ecosystem CO(2) flux partitioning, but on the other hand they provide new insights into metabolic processes underlying respiratory CO(2) emission.

Influence of in‐stream diel concentration cycles of dissolved trace metals on acute toxicity to one‐year‐old cutthroat trout (Oncorhynchus clarki lewisi)

Extrapolating results of laboratory bioassays to streams is difficult, because conditions such as temperature and dissolved metal concentrations can change substantially on diel time scales. Field bioassays conducted for 96 h in two mining-affected streams compared the survival of hatchery-raised, metal-naïve westslope cutthroat trout (Oncorhynchus clarki lewisi) exposed to dissolved (0.1-microm filtration) metal concentrations that either exhibited the diel variation observed in streams or were controlled at a constant value. Cadmium and Zn concentrations in these streams increased each night by as much as 61 and 125%, respectively, and decreased a corresponding amount the next day, whereas Cu did not display a diel concentration cycle. In High Ore Creek (40 km south of Helena, MT, USA), survival (33%) after exposure to natural diel-fluctuating Zn concentrations (range, 214-634 microg/L; mean, 428 microg/L) was significantly (p = 0.008) higher than survival (14%) after exposure to a controlled, constant Zn concentration (422 microg/L). Similarly, in Dry Fork Belt Creek (70 km southeast of Great Falls, MT, USA), survival (75%) after exposure to diel-fluctuating Zn concentrations (range, 266-522 microg/L; mean, 399 microg/L) was significantly (p = 0.022) higher than survival (50%) in the constant-concentration treatment (392 microg/L). Survival likely was greater in these diel treatments, both because the periods of lower metal concentrations provided some relief for the fish and because toxicity during periods of higher metal concentrations was lessened by the simultaneous occurrence each night of lower water temperatures, which reduce the rate of metal uptake. Based on the present study, current water-quality criteria appear to be protective for streams with diel concentration cycles of Zn (and, perhaps, Cd) for the hydrologic conditions tested.

Seasonal and diel changes of dissolved oxygen in a hypertrophic prairie lake

AbstractHumboldt Lake, a hypertrophic prairie lake typical of many found on the Great Plains of North America, is usually ice‐covered from early November to about mid‐May. The lake is an important recreational fishery, now mainly stocked with walleye. It has a high potential risk of experiencing fish kills because of the very large cyanobacterial blooms that develop in it, the high rates of algal and bacterial production and the high concentrations of ammonia (NH3‐N) and dissolved organic matter. Following the collapse of cyanobacterial blooms, shallow prairie lakes are known to undergo periods of anoxia that can lead to summer fish kills. In some of the lakes, anoxia forms during the long period of ice cover, causing winter fish kills. Two years of seasonal and diel data (total phosphorus, dissolved oxygen (DO), NH3‐N and chlorophyll‐a concentrations, and bacterial production) were analysed in this study to assess why significant fish kills did not occur during this period or during the ≈ 30 years of records from Saskatchewan Environment. Humboldt Lake did not become anaerobic, either following the collapse of the cyanobacterial bloom or under ice cover, indicating that the oxygen (O2) influx (strong mixing) and production processes were greater than the microbial and chemical O2 demands, both over seasonal and diel time scales. Several published risk threshold criteria to predict the probability of summer and/or winter fish kills were applied in this study. The threshold criteria of maximum summer chlorophyll and maximum winter NH3‐N concentrations indicated that a summer fish kill was unlikely to occur in this hypertrophic prairie lake, provided its water quality remained similar to that during this study. Similarly, the threshold criteria of initial DO storage before ice cover and the rate of O2 depletion under ice cover also indicated a winter fish kill was unlikely. However, recent development in the watershed might have resulted in significant water quality deterioration and the winter fish kill that occurred in 2005.