Significant Diel Variation Research Articles

Diel variation of plankton in the highly impacted freshwater zone of Hooghly estuary in relation to ecological alteration.

Plankton are promising ecological monitoring tool that responds quickly to any sort of aquatic ecological alteration, of which many of them are much susceptible to ecological variations. Therefore, monitoring shifts in plankton composition can indicate changes in water quality and aid to identify potential pollution sources. In the present study, the variation in plankton dynamics in relation to ecological variables were monitored in the freshwater zone of the Hooghly estuary from May 2020 to April 2021. The study was conducted in the interval of every six hours. i.e., at 6 A.M., 12 P.M., 6 P.M., and 12 A.M. The present finding revealed the occurrence of 54 phytoplankton and 20 zooplankton taxa/species. Diel variation revealed that among different time intervals, the highest abundance of phytoplankton was recorded 28,307 cells l-1 at 12 P.M, while the lowest was recorded 10,632 cells l-1 at 6 A.M. However, the highest zooplankton abundance was observed 804 ind l-1 at 6 A.M., and the lowest was recorded 156 ind l-1 at 6 P.M. The ANOVA (p < 0.05) analysis indicated significant diel variation for many planktonic genera. The CCA exhibited that most of the phytoplankton were influenced by multiple water quality variables such as temperature, turbidity, calcium, pH, salinity, DO, and nutrients. However, the majority of the zooplankton were affected by turbidity, total phosphorus, sulphate, calcium and available nitrogen. Significant seasonal variation in plankton composition has also been observed. The present study will help to determine the varying diel pattern of planktons in retort to alterations in the water quality parameters and varying ecological niches.

Day–night discrepancy in soil respiration varies with seasons in a temperate forest

Abstract Soil respiration (Rs) during daytime (Rsday) and nighttime (Rsnight) remains poorly understood owing to the limited availability of high‐resolution temporal measurements of Rs. Using continuous automatic chamber measurements at a half‐hourly interval, we explored Rs and its temperature sensitivity (Q10) during the daytime and nighttime over 10 years and determined the best sampling timeslot in a temperate deciduous broadleaf forest. We demonstrate a higher Rsnight than Rsday in spring and summer, a lower Rsnight than Rsday in autumn, but an insignificant difference between them in winter, contrasting with the consensus of a higher Rsday than Rsnight. Such discrepancy may result from the effect of factors other than temperature (e.g. reallocation of photosynthesis carbon, precipitation and soil moisture). We also reveal significant differences in apparent Q10 between seasons, indicating the highest Q10 in spring and significantly higher Q10 at night than at day in autumn. The Rs measured around midday (9:00–11:00 for autumn and 12:00–15:00 for other seasons) and midnight (20:00–22:00 for autumn and 0:00–2:00 for other seasons) provides the lowest bias to the daily mean Rs, whereas the measurement timeslots around midday are preferred due to practical considerations. Our findings highlight the significant diel variation of Rs and its temperature sensitivity across seasons and the necessity to account for Rsnight for a more accurate estimate of soil carbon budget. Read the free Plain Language Summary for this article on the Journal blog.

Environmentally-Driven Variation in the Physiology of a New Caledonian Reef Coral

Given the widespread threats to coral reefs, scientists have lost the opportunity to understand the basic biology of “pristine” corals whose physiologies have not been markedly perturbed by human activity. For instance, high temperature-induced bleaching has been occurring annually since 2014 in New Caledonia. Because most corals cannot withstand repeated years when bleaching occurs, an analysis was undertaken to showcase coral behavior in a period just before the onset of “annual severe bleaching” (ASB; November 2013) such that future generations might know how these corals functioned in their last bleaching-free year. Pocillopora damicornis colonies were sampled across a variety of environmental gradients, and a subset was sampled during both day and night to understand how their molecular biology changes upon cessation of dinoflagellate photosynthesis. Of the 13 environmental parameters tested, sampling time (i.e., light) most significantly affected coral molecular physiology, and expression levels of a number of both host and Symbiodiniaceae genes demonstrated significant diel variation; endosymbiont mRNA expression was more temporally variable than that of their anthozoan hosts. Furthermore, expression of all stress-targeted genes in both eukaryotic compartments of the holobiont was high, even in isolated, uninhabited, federally protected atolls of the country’s far northwest. Whether this degree of sub-cellular stress reflects cumulative climate change impacts or, instead, a stress-hardened phenotype, will be unveiled through assessing the fates of these corals in the wake of increasingly frequent marine heatwaves.

Submerged macrophytes regulate diurnal nitrous oxide emissions from a shallow eutrophic lake: A case study of Lake Wuliangsuhai in the temperate arid region of China

Submerged macrophytes can increase oxygen concentrations of water and promote diel oxygen fluctuations, and this phenomenon is hypothesized to play a vital role in regulating nitrous oxide (N2O) emissions from eutrophic lakes. However, the effects of submerged macrophytes on N2O emissions in shallow eutrophic lakes remain poorly investigated. In this study, Lake Wuliangsuhai, a typical shallow eutrophic lake, was investigated to study the role of submerged macrophytes in regulating N2O emissions. We measured the N2O fluxes and related parameters through continual 72-h in situ diel monitoring in two sampling sections that covered dense submerged macrophyte areas and open water. In this study, the dissolved oxygen (DO) concentration of the water in the submerged macrophyte area exhibited significant diurnal variations, with significantly higher water oxygen concentrations than the open water area during the daytime. The N2O fluxes of Lake Wuliangsuhai ranged from 0.01 to 0.24 μmol m−2 h−1, with an average value of 0.11 μmol m−2 h−1. Moreover, significant diel variations in the N2O flux and net N2O production were observed in the submerged macrophyte areas, where the maximum N2O flux occurred at midday. The molar ratios of NH4+-N to oxygen (N/O ratio) of the water were responsible for the diel variations in the N2O production in the lake. However, the high oxygen concentration of the water was the major regulator of the N2O flux of Lake Wuliangsuhai. Therefore, submerged macrophyte restoration is significant not only for water quality improvement in shallow eutrophic lakes but also for N2O emission mitigation by increasing the DO concentration of the water.

Effects of stream ecosystem metabolisms on CO2 emissions in two headwater catchments, Southeastern China

Headwater streams, as critical zones due to their proximity to terrestrial sources, emit large quantities of CO2 owing to their higher drainage density of catchments, substantial total areas, and stream lengths. Stream ecosystem metabolism (EM, i.e. photosynthesis and respiration) can significantly affect the in-stream carbon cycle. However, how stream CO2 emissions respond to ecosystem respiration (ER) is still not well known. Here we report in-situ daily observations of headwater stream gross primary production (GPP), ecosystem respiration (ER), isotopic composition of dissolved inorganic carbon (δ13CDIC) and CO2 emissions conducted in five streams in two headwater catchments of the Changjiang River in Southeastern China. The results show that δ13CDIC, pCO2, and CO2 emission rates have significant diel variations. Both the day and night δ13CDIC values were more depleted than the theoretical values in the two catchments, but the nocturnal depletion was greater. The headwater streams have high EM, with an average ER of −1.99 ± 0.82 g C m−2 d−1and GPP of 1.52 ± 0.84 g C m−2 d−1 in the two catchments. ER increases with DOC concentration but decreases with water discharge.The nocturnal pCO2 (3885 ± 2959 μatm) was significantly higher than in daytime (2525 ± 2221 μatm). The higher nocturnal CO2 emission rate (225 ± 165 mg C m−2h−1) was about 42 % higher than in daytime. DOC uptake velocity (Vf) was 0.46 ± 0.14 m d−1, and increased with lower water discharge and lower catchment-scale DOC yield. DOC loss rate (ζ) was 0.96 ± 1.25 d−1 and declined exponentially with higher water discharge and catchment-scale DOC yield. Moreover, the removal fraction of DOC via ER was about 0.32 ± 0.14, contributing to 31 ± 16 % of the CO2 flux in streams. The re-estimated annual CO2 flux was about 14.62 ± 4.99 Tg C yr−1 from the Changjiang headwater streams network. This study emphasizes the necessity of incorporating the effects of EM into the river network's CO2 emissions and the importance of the headwater streams to river carbon cycling.

Diel variations in cell division and biomass production of Emiliania huxleyi—Consequences for the calculation of physiological cell parameters

AbstractCell division of the coccolithophore Emiliania huxleyi and other phytoplankton typically becomes entrained to diel light/dark cycles under laboratory conditions, with division occurring primarily during dark phases and production occurring during light phases. Under these conditions, increases in cell and biomass concentrations deviate from exponential functions on time scales &lt; 24 h. These deviations lead to significant diel variations in common measurements of phytoplankton physiology such as cellular quotas of particulate organic and inorganic carbon (POC, PIC) and their production rates. Being time‐dependent, only the temporal mean of the various values during the day are comparable between experiments. Deviations from exponential growth furthermore imply that increases in cell and biomass concentrations cannot be expressed by the daily growth rate μ24 h (typically determined from daily increments in cell concentrations). Consequently, conventional calculations of production as the product of a cellular quota (e.g., POC quota) and μ24 h are mathematically incorrect. To account for this, we here describe short‐term changes in cell and biomass concentrations of fast‐dividing, dilute‐batch cultures of E. huxleyi grown under a diel light/dark cycle using linear regression. Based on the derived models, we present calculations for daily means of cellular quotas and production rates. Conventional (time‐specific) measurements of cellular quotas and production differ from daily means by up to 65% in our example and, under some circumstances, cause false “effects” of treatments. Intending to reduce errors in ecophysiological studies, we recommend determining daily means—mathematically or by adjusting the experimental setup or sampling times appropriately.

Significant Diel Variation of Soil Respiration Suggests Aboveground and Belowground Controls in a Tropical Moist Forest in Puerto Rico

AbstractSoil respiration in tropical forests represents a major source of carbon dioxide (CO2) to the atmosphere. The magnitude of this large flux is projected to change in response to climate change, with global implications due to the disproportionate role of tropical forests in the carbon cycle. Evaluating diel patterns of soil respiration concomitantly with biophysical drivers is a valuable approach for elucidating the mechanisms controlling soil respiration. We measured hourly soil respiration rates in a tropical moist forest in Puerto Rico over a 3‐year period using automated chambers, as well as soil temperature/moisture, air temperature, relative humidity, and photosynthetically active radiation. Hourly soil respiration exhibited as much as threefold variation on diel time scales (monthly diel amplitude ranged from 1 to 7 μmol CO2 m−2 s−1), and both the magnitude and shape of diel patterns changed significantly from month to month. Soil respiration peaked in the morning and late afternoon with a midday decline that was evident during the warmest summer months. The relationship between soil respiration and soil temperature differed during daytime versus the night, with a nonlinear relationship in the daytime but a significant positive linear relationship at night. These findings suggest factors other than/in addition to temperature could be controlling soil respiration during the day; however, soil respiration did not correlate with any of the other measured biophysical variables. Overall, our results highlight the potential role of aboveground processes as drivers of soil respiration at diel time scales, especially in closed canopy tropical forests with low diel variation in soil temperature/moisture.

Summer at the beach: spatio-temporal patterns of white shark occurrence along the inshore areas of False Bay, South Africa

BackgroundUnderstanding white shark (Carcharodon carcharias) habitat use in coastal areas adjacent to large cities, is an important step when formulating potential solutions to the conservation conflict that exists between humans and large predatory sharks. In this study, we present the findings of a 2.5-year study of white shark occurrence and movement patterns adjacent to the City of Cape Town in False Bay, South Africa, with a focus on spring and summer months. Fifty-one white sharks were monitored annually at three offshore and twelve inshore sites by VR2 acoustic receivers, over 975 days from 1 May 2005 to 31 December 2007.ResultsOccurrence patterns at inshore sites during spring and summer were analysed using a generalized additive mixed model (GAMM) with a spatial term (longitude, latitude), time of day and year included as explanatory variables for site use. We found that sharks occurred more frequently at inshore sites along the northern and northwestern shores, compared to the rest of the bay, and they transitioned most frequently between four adjacent beach sites that encompass the most popular recreational water use areas in Cape Town. There was significant diel variation, with higher shark occurrence around midday, and a peak in shark occurrence in 2005, when human-shark interactions also peaked. However, we found no effect of shark size on occurrence patterns at inshore sites.ConclusionsWhite sharks showed the highest levels of occurrence at specific inshore sites between Muizenberg and Strandfontein beach, and thus inclusion of these sites within False Bay’s marine protected area (MPA) network or recognition as Ecological or Biological Significant Areas (EBSAs) should be a future consideration. These insights into white shark habitat use at inshore sites in False Bay are important for successfully applying the principles of marine spatial planning (MSP) and for making science-based policy decisions. Furthermore, this information can be used to reduce potential shark-human conflict by incorporating it into future shark safety education campaigns.

Light-Mediated Reactive Oxygen Species Generation and Iron Redox Transformations in the Presence of Exudate from the Cyanobacterium Microcystis aeruginosa.

The photochemical properties of the organic exudate secreted by a toxic strain of Microcystis aeruginosa were studied by measuring reactive oxygen species (ROS) generation and redox transformations of iron in the presence of the organic exudate under acidic (pH 4) and alkaline (pH 8) conditions. Our results show that the organic exudate generates nanomolar concentrations of superoxide and hydrogen peroxide on irradiation with simulated sunlight in a manner consistent with that reported for terrigenous natural organic matter. The photo-generated superoxide plays an important role in Fe(III) reduction under alkaline conditions with nearly 45% of the observed Fe(II) generation on Fe(III) reduction occurring via Fe(III) reduction by superoxide while the rest of the Fe(III) reduction occurs via a ligand-to-metal charge transfer (LMCT) pathway. In contrast, under acidic conditions, 100% of the observed photochemical Fe(II) generation on Fe(III) reduction occurs via a LMCT pathway. These results suggest that steady-state dissolved Fe concentrations and hence Fe availability in natural waters will significantly increase in the presence of these algal exudates. Furthermore, significant diel variation in Fe(II) concentration is to be expected, even in acidic waters, since time scales of light-mediated Fe(III) reduction and thermal Fe(III) reduction differ markedly. A kinetic model is developed that adequately describes both the generation of ROS and the photochemical redox transformations of iron in the presence of M. aeruginosa exudate.

Icelandic herring-eating killer whales feed at night

Herring-eating killer whales debilitate herring with underwater tail slaps and likely herd herring into tighter schools using a feeding-specific low-frequency pulsed call (‘herding’ call). Feeding on herring may be dependent upon daylight, as the whales use their white underside to help herd herring; however, feeding at night has not been investigated. The production of feeding-specific sounds provides an opportunity to use passive acoustic monitoring to investigate feeding behaviour at different times of day. We compared the acoustic behaviour of killer whales between day and night, using an autonomous recorder deployed in Iceland during winter. Based upon acoustic detection of underwater tail slaps used to feed upon herring we found that killer whales fed both at night and day: they spent 50% of their time at night and 73% of daytime feeding. Interestingly, there was a significant diel variation in acoustic behaviour. Herding calls were significantly associated with underwater tail slap rate and were recorded significantly more often at night, suggesting that in low-light conditions killer whales rely more on acoustics to herd herring. Communicative sounds were also related to underwater tail slap rate and produced at different rates during day and night. The capability to adapt feeding behaviour to different light conditions may be particularly relevant for predator species occurring in high latitudes during winter, when light availability is limited.

The Integration of Gastrointestinal and Renal Function in Nectar‐Feeding Birds

Nectar‐feeding birds face a constant challenge in terms of water balance, experiencing water loading when feeding and potential dehydration when fasting. A growing collection of studies is revealing how species across different orders deal with this osmoregulatory challenge. To meet their high mass‐specific energy demands these birds must often deal with exceptionally high proportionate water fluxes. Birds that ingest large volumes of water may either eliminate excess water in the kidney or regulate the volume of water absorbed in the gastrointestinal tract. Across species, renal filtration during feeding periods generally does not exceed allometric predictions and is not highly responsive to water loading, but renal fractional water reabsorption varies with water load as expected. Hummingbirds (Apodiformes) absorb a constant, high proportion of ingested water across the gastrointestinal tract, while sunbirds and honeyeaters (Passeriformes) modulate intestinal water absorption, potentially avoiding significant water loads when feeding on dilute diets. Data suggest that these passerine nectarivores regulate transepithelial water flux independently of sugar absorption, opening the door to intriguing questions about how water transport is regulated in the vertebrate gastrointestinal tract. Despite reducing their intestinal water absorption, these birds show linear increases in water flux and fractional body water turnover as water intake increases, suggesting that the modulation of fractional water absorption is not sufficient to completely offset dietary water loads. Species in both orders exhibit significant diel variation in renal filtration, essentially shutting down renal filtration during overnight fasts and thus avoiding dehydration. Convergence in diet has led to the evolution of many similar traits in hummingbirds and sunbirds, but the physiological traits of these two groups that allow the processing of a watery and sugary diet may be very different.

Perchlorate exposure does not modulate temporal variation of whole-body thyroid and androgen hormone content in threespine stickleback

Previously we showed that exposure of threespine stickleback (Gasterosteus aculeatus) to the endocrine disruptor perchlorate results in pronounced structural changes in thyroid and gonad, while surprisingly, whole-body thyroid hormone concentrations remain unaffected. To test for hormone titer variations on a finer scale, we evaluated the interactive effects of time (diel and reproductive season) and perchlorate exposure on whole-body contents of triiodothyronine (T3), thyroxine (T4), and 11-ketotestosterone (11-KT) in captive stickleback. Adult stickleback were exposed to 100ppm perchlorate or control water and sampled at 4-h intervals across the 24-hday and at one time-point (1100h) weekly across the reproductive season (May–July). Neither whole-body T3 nor T4 concentration significantly differed across the day in control or perchlorate treated stickleback. Across the reproductive season, whole-body T3 concentration remained stable while T4 significantly increased. However, neither hormone concentration was significantly affected by perchlorate, verifying our previous studies. The concentration of whole-body 11-KT, a major fish androgen, displayed significant diel variation and also steadily declined across the reproductive season in untreated males; perchlorate exposure did not influence the concentration of 11-KT in either diel or reproductive season schedules. Diel and reproductive season variations in 11-KT content in male stickleback are likely related to reproductive physiology and behavior. The observed increase in T4 content across the reproductive season may be reflective of increased energy investment in reproduction near the end of the life cycle.

Variation in the Vocal Behavior of Common Loons (Gavia immer): Insights from Landscape-level Recordings

Abstract. Animal signals play an important role in mate attraction and territory defense, and animals may benefit by adopting signaling strategies that maximize effective communication in the face of changing environmental conditions. In this study, a custom-designed microphone array was used to collect landscape-scale recordings of the acoustic signaling behavior of Common Loons (Gavia immer) along a 10-km transect spanning three lakes in eastern Ontario, Canada. Recordings were collected during the early part of the breeding season during two consecutive years (2008–2009). Analyses focused on understanding how the vocal output of Common Loons varied with time of day, time of year, and in response to variation in weather. Common Loons showed significant diel variation in vocal output, producing more wail, yodel, and tremolo calls at night than during the day. Common Loons showed significant seasonal variation in vocal output, producing fewer wail, yodel, and tremolo calls as the first month of the breedi...

Experimental evidence for diel δ15N‐patterns in different tissues, xylem and phloem saps of castor bean (Ricinus communis L.)

Nitrogen isotope signatures in plants might give insights in the metabolism and allocation of nitrogen. To obtain a deeper understanding of the modifications of the nitrogen isotope signatures, we determined δ(15)N in transport saps and in different fractions of leaves, axes and roots during a diel course along the plant axis. The most significant diel variations were observed in xylem and phloem saps where δ(15)N was significantly higher during the day compared with during the night. However in xylem saps, this was observed only in the canopy, but not at the hypocotyl positions. In the canopy, δ(15)N was correlated fairly well between phloem and xylem saps. These variations in δ(15)N in transport saps can be attributed to nitrate reduction in leaves during the photoperiod as well as to (15)N-enriched glutamine acting as transport form of N. δ(15)N of the water soluble fraction of roots and leaves partially affected δ(15)N of phloem and xylems saps. δ(15)N patterns are likely the result of a complex set of interactions and N-fluxes between plant organs. Furthermore, the natural nitrogen isotope abundance in plant tissue is not constant during the diel course - a fact that needs to be taken into account when sampling for isotopic studies.

Inference of riverine nitrogen processing from longitudinal and diel variation in dual nitrate isotopes

Longitudinal and diel measurements of dual isotope composition (δ15N and δ18O) in nitrate (NO3‐N) were made in the Ichetucknee River, a large (∼8 m3 s−1), entirely spring‐fed river in North Florida, to determine whether isotopic variation can deconvolve assimilatory and dissimilatory removal. Comparing nitrate concentrations and isotope composition during the day and night we predicted (1) daytime declines in total fractionation due to low assimilatory fractionation and (2) diurnal variation in dual isotope coupling between 1:1 (assimilation) and 2:1 (denitrification). Five daytime longitudinal transects comprising 10 sampling stations showed consistent NO3‐N removal (25–35% of inputs) and modest fractionation (15εtotal between −2 and −6‰, enriching the residual nitrate pool). Lower fractionation (by ∼1‰) during two nighttime transects, suggests higher fractionation due to assimilation than denitrification. Total fractionation was significantly negatively associated with discharge, input [NO3‐N], N mass removal, and fractional water loss. Despite well‐constrained mass balance estimates that denitrification dominated total N removal, isotope coupling was consistently 1:1, both for longitudinal and diel sampling. Hourly samples on two dates at the downstream location showed significant diel variation in concentration ([NO3‐N] amplitude = 60 to 90 μg N L−1) and isotope composition (δ15N amplitude = −0.7‰ to −1.6‰). Total fractionation differed between day and night only on one date but estimated assimilatory fractionation assuming constant denitrification was highly variable and implausibly large (for N, 15ε = −2 to −25‰), suggesting that fractionation and removal due to denitrification is not diurnally constant. Pronounced counterclockwise hysteresis in the relationship between [NO3‐N] andδ15N suggests diel variation in N isotope dynamics. Together, low fractionation, isotope versus concentration hysteresis, and consistent 1:1 isotope coupling suggests that denitrification is controlled by NO3− diffusion into the benthic sediments, the length of which is mediated by riverine oxygen dynamics. While using dual isotope behavior to deconvolve removal pathways was not possible, isotope measurements did yield valuable information about riverine N cycling and transformations.

Minke whale (Balaenoptera acutorostrata) boings detected at the Station ALOHA Cabled Observatory

Minke whales (Balaenoptera acutorostrata) in the tropical North Pacific are elusive and difficult to detect visually. The recent association of a unique sound called the "boing" to North Pacific minke whales has made it possible to use passive acoustics to investigate the occurrence of this species in Hawaiian waters. One year of recordings (17 February 2007-18 February 2008) made at the Station ALOHA Cabled Observatory were examined to investigate the characteristics of boings and temporal patterns in their occurrence at this site, located 100 km north of Oahu. Characteristics of boings exhibited low variability. Pulse repetition rate and duration measurements matched those for "central" or "Hawaii" boing types. Boings were detected from October until May, with a peak in March. Although no boings were detected from June to September, the absence of boings does not necessarily indicate the absence of minke whales. Significant diel variation in boing rate was not observed. The absence of a diel pattern in boing production suggests that day- or night-time acoustic surveys are equally acceptable methods for studying minke whale occurrence. Future research should include efforts to determine what other sounds are produced by minke whales in this area, and which age/sex classes produce boings.

Intersexual differences in the postbreeding foraging behaviour of the Northwest Atlantic hooded seal

Sexual dimorphism and different reproductive strategies lead males and females to forage differently among a wide range of species. We used dive and location data collected from 23 hooded seals Cystophora cristata captured in the Gulf of St Lawrence (Canada) during the period from March to June, 1992 to 2005, as proxies for foraging behaviour. Females spent 12 d longer than males in the Gulf before undertaking their migration to Greenland. Females and males greatly overlapped on a horizontal scale but were segregated on a vertical scale, females diving on average 70 m shallower than males during the few weeks preceding the migration and 40 m deeper than males following the migration. Both sexes spent similar amounts of time diving and showed significant diel variation in dive depth but remained at significant depths at night (>200 m), suggesting that both sexes foraged mostly on benthopelagic prey. The relatively minor differences in foraging behaviour observed between sexes may be explained by similar mass loss during the reproduction and the constraints related to the extensive annual migration.

Assessment of diel chemical and isotopic techniques to investigate biogeochemical cycles in the upper Klamath River, Oregon, USA

The upper Klamath River experiences a cyanobacterial algal bloom and poor water quality during the summer. Diel chemical and isotopic techniques have been employed in order to investigate the rates of biogeochemical processes. Four diel measurements of field parameters (temperature, pH, dissolved oxygen concentrations, and alkalinity) and stable isotope compositions (dissolved oxygen– δ 18O and dissolved inorganic carbon– δ 13C) have been performed between June 2007 and August 2008. Significant diel variations of pH, dissolved oxygen (DO) concentration, and DO– δ 18O were observed, due to varying rates of primary productivity vs. respiration vs. gas exchange with air. Diel cycles are generally similar to those previously observed in river systems, although there are also differences compared to previous studies. In large part, these different diel signatures are the result of the low turbulence of the upper Klamath River. Observed changes in the diel signatures vs. sampling date reflect the evolution of the status of the algal bloom over the course of the summer. Results indicate the potential utility of applying diel chemical and stable isotope techniques to investigate the rates of biogeochemical cycles in slow-moving rivers, lakes, and reservoirs, but also illustrate the increased complexity of stable isotope dynamics in these low-turbulence systems compared to well-mixed aquatic systems.

Diel variation in sea grass dwelling shrimp: when to sample at night?

The present study investigates hourly variation in nocturnal abundance fluctuations in a tropical, Caribbean sea grass bed. As expected, results indicate that significant diel variation is present. In addition all dominant species show an abundance peak shortly after sunset, with numbers gradually declining throughout the night. It is therefore suggested that nocturnal samples are not only incorporated into any sampling regime, but that nocturnal samples are taken within two hours after sunset.

Diel and seasonal variations in the chemical composition of biomass burning aerosol

Abstract. Fine aerosol particles were collected separately during daytime and nighttime at a tropical pasture site in Rondônia, Brazil, during the burning and dry-to-wet transition period in 2002. Total carbon (TC) and water-soluble organic carbon (WSOC) were measured by evolved gas analysis (EGA). Based on the thermochemical properties of the fine aerosol, the relative amounts of the volatile and refractory compounds were estimated. It was found that the thermally refractory (possibly higher molecular weight) compounds dominated the TC composition. Their contribution to TC was higher in the daytime than in the nighttime samples. The relative share of WSOC also showed a statistically significant diel variation as did its refractory fraction. Anhydrosugars and phenolic acids were determined by GC-MS and their diel variation was studied. Based on the decrease of their relative concentrations between the biomass burning and transition periods and their distinctly different diel variations, we suggest that the phenolic acids may undergo chemical transformations in the aerosol phase, possibly towards more refractory compounds (humic-like substances, HULIS), as has been suggested previously. These conclusions are supported by the results of the thermally assisted hydrolysis and methylation gas chromatography-mass spectrometry of the same filter samples.