Mean Light Level Research Articles

Here comes the sun: Thermoregulatory behavior in ectotherms illuminated by light‐level geolocators

AbstractDaily activity patterns of free‐ranging wildlife affect a wide range of ecological and physiological processes and, in turn are affected by anthropogenic disturbances to the environment. However, obtaining a continuous record of activity without disturbing wild animals is logistically challenging. We used commercially available, multi‐purpose light‐level geolocator dataloggers to continuously record light environment and time spent out of water during 5‐months (1 May to 9 September 2021) in an ectothermic freshwater turtle species, the northern map turtle (Graptemys geographica). We used these data to compare time of year and sex differences in thermoregulatory behavior in an ectothermic species in its natural habitat. We recorded >500 000 data points from 17 individual turtles (nine males and eight females). We found no differences in the mean light levels, or proportion of time spent out of the water, between males and females. However, there was a significant effect of both time of year and sex by time of year interaction in both light level, proportion of time spent dry, and number of state changes (i.e., shuttling behavior, wet to dry or dry to wet), suggesting that turtles alter their aerial basking behavior over the course of the season and that the changes in patterns of aerial basking behavior differ between the sexes throughout the year. In general, the proportion of time spent dry decreased over the active season, with an increase during the last week of June and the first week of July, while the number of state changes increased in females during late May/early June before decreasing, while males remained relatively constant. These changes may reflect the different energetic demands associated with reproduction between the two sexes. The overall downward trend in aerial basking likely reflects the role of increasing environmental temperatures, particularly water temperature, in the maintenance of body temperature in this largely aquatic species.

The Three‐Dimensional Light Field Within Sea Ice Ridges

AbstractSea ice pressure ridges have been recognized as important locations for both physical and biological processes. Thus, understanding the associated light‐field is crucial, but their complex structure and internal geometry render them hard to study by field methods. To calculate the in‐ and under‐ridge light field, we combined output from an ice mechanical model with a Monte‐Carlo ray tracing simulation. This results in realistic light fields showing that light levels within the ridge itself are significantly higher than under the surrounding level ice. Light guided through ridge cavities and scattering in between ridge blocks also results in a more isotropic ridge‐internal light field. While the true variability of light transmittance through a ridge can only be represented in ray tracing models, we show that simple parameterizations based on ice thickness and macro‐porosity allow accurate estimation of mean light levels available for photosynthesis underneath ridges in field studies and large‐scale models.

Visual discomfort from flicker: Effects of mean light level and contrast

Uncomfortable images generally have a particular spatial structure, which deviates from a reciprocal relationship between amplitude and spatial frequency (f) in the Fourier domain (1/f). Although flickering patterns with similar temporal structure also appear uncomfortable, the discomfort is affected by not only the amplitude spectrum but also the phase spectrum. Here we examined how discomfort from flicker with differing temporal profiles also varies as a function of the mean light level and luminance contrast of the stimulus. Participants were asked to rate discomfort for a 17° flickering uniform field at different light levels from scotopic to photopic. The flicker waveform was varied with a square wave or random phase spectrum and filtered by modulating the slope of the amplitude spectrum relative to 1/f. At photopic levels, the 1/f square wave flicker appeared most comfortable, whereas the discomfort from the random flicker increased monotonically as the slope of the amplitude spectrum decreased. This special status for the 1/f square wave condition was limited to photopic light levels. At the lower mesopic or scotopic levels, the effect of phase spectrum on the discomfort was diminished, with both phase spectra showing a monotonic change with the slope of the amplitude spectrum. We show that these changes cannot be accounted for by changes in the effective luminance contrast of the stimuli or by the responses from a linear model based on the temporal impulse responses under different light levels. However, discomfort from flicker is robustly correlated with judgments of the perceived naturalness of flicker across different contrasts and mean luminance levels.

Responses of corals to chronic turbidity

Dredging increases suspended sediment concentrations (SSCs), causing elevated water turbidity (cloudiness) and light attenuation. Close to dredging, low light periods can extend over many days, affecting phototrophic epibenthic organisms like corals. To improve the ability to predict and manage dredging impacts, we tested the response of corals to an extended period of elevated turbidity using an automated sediment dosing system that precisely controlled SSCs and adjusted light availability accordingly. Replicates of four common species of corals encompassing different morphologies were exposed to turbidity treatments of 0–100 mg L−1 SSC, corresponding to daily light integrals of 12.6 to 0 mol quanta m−2 d−1, over a period of ∼7 weeks. Symbiotic dinoflagellate density and algal pigment concentration, photosynthetic yields, lipid concentrations and ratios and growth varied among the turbidity treatments, with corals exhibiting photoacclimation within low turbidity treatments. A range of physiological responses were observed within the high turbidity treatments (low light), including bleaching and changes in lipid levels and ratios. Most corals, except P. damicornis, were capable of adjusting to a turbidity treatment involving a mean light level of 2.3 mol photons m−2 d−1 in conjunction with a SSC of 10 mg L−1 over the 7 week period.

Heterogeneous Temporal Contrast Adaptation in Drosophila Direction-Selective Circuits.

In visual systems, neurons adapt both to the mean light level and to the range of light levels, or the contrast. Contrast adaptation has been studied extensively, but it remains unclear how it is distributed among neurons in connected circuits, and how early adaptation affects subsequent computations. Here, we investigated temporal contrast adaptation in neurons across Drosophila's visual motion circuitry. Several ON-pathway neurons showed strong adaptation to changes in contrast over time. One of these neurons, Mi1, showed almost complete adaptation on fast timescales, and experiments ruled out several potential mechanisms for its adaptive properties. When contrast adaptation reduced the gain in ON-pathway cells, it was accompanied by decreased motion responses in downstream direction-selective cells. Simulations show that contrast adaptation can substantially improve motion estimates in natural scenes. The benefits are larger for ON-pathway adaptation, which helps explain the heterogeneous distribution of contrast adaptation in these circuits.

Impact of Light Intensity on Sleep of Patients in the Intensive Care Unit: A Prospective Observational Study.

ABSTRACTAims and objectivesSleep deprivation in the intensive care unit (ICU) has been linked to numerous complications. Light levels might impact the sleep of patients in the ICU. The aim of the study was to measure light levels during sleep-protected time in the ICU and to assess the impact of light intensity on sleep quantity/quality.Materials and methodsThis prospective, observational study was conducted in a 10-bed, mixed surgical/medical ICU. For measuring light levels, a commercially available smartphone application was used. The measurements were performed between 23:30 and 06:15 hours at 15-minute intervals. To assess sleep quantity, we used Patient's Sleep Observation Behavioral Tool and to assess sleep quality, we used Richards-Campbell Sleep Scale.ResultsThe median number of time points at which patients were asleep was 20 (interquartile range, IQR 14–23) out of 25 (5 hours). The median self-reported quality of sleep (overall score) was 49 (IQR 28–71). The median values for individual questions are: question 1 (sleep depth)—54.0 (IQR 37–78), question 2 (sleep latency)—40.5 (IQR 6–90), question 3 (awakenings)—52.5 (IQR 28–76), question 4 (returning to sleep)—25.5 (IQR 11–78), and question 5 (sleep quality)—67.5 (IQR 5–76). No correlation was found between self-reported sleep quality and time spent asleep (p = 0.36). There was no correlation between average light levels during sleep-protected time and sleep quantity (p = 0.42)/sleep quality (p = 0.13). There was a correlation between average (13 ± 5 lux) light levels before sleep-protected time and sleep quality (p = 0.008).ConclusionMean light levels of 11 ± 9 lux during sleep-protected time have no negative impact on quantity and quality of sleep in intensive care unit patients. Light levels up to 18 lux directly before falling asleep improve patients’ self-reported quality of sleep in the ICU.Clinical significanceFinding safe levels of light intensity during sleep-protected time in ICU.How to cite this articleCzempik PF, Jarosińska A, Machlowska K, Pluta M. Impact of Light Intensity on Sleep of Patients in the Intensive Care Unit: A Prospective Observational Study. Indian J Crit Care Med 2020;24(1):33–37.

Does sowing an oats catch crop reduce nitrate leaching from urine deposition following simulated winter forage grazing? - a growth chamber experiment

Growing winter forages, such as brassicas and fodder beets, is a common feed strategy to build body condition for non-lactating cows, prior to calving, in the temperate regions of New Zealand. However, the large quantities of feed grown, ~14-20 tonnes ha-1, and resulting high stocking rates, means large volumes of urine are deposited during grazing, over relatively small areas, on cool, bare and often wet soils. These conditions create a high potential for large N leaching losses. Sowing a catch crop might be one way to mitigate this potential but most catch crops are sown in late-summer/autumn, not late winter/spring, so this study’s objectives were to examine the potential of a cold-tolerant cereal, oats, to establish and take up soil N over this period. A study was undertaken using free-draining repacked soil cores in two climate-controlled growth chambers at 6° and 10°C, and at two lighting intensities, 5 and 10 MJ m-2, reflecting the mean soil temperature (0-100 mm) and light levels experienced in Canterbury, New Zealand, around late winter and mid spring, respectively. The cores all received an application of 15N-labelled urine (350 kg N ha-1) and two weeks later, oats were sown in half the soil cores, with the remainder left fallow. Sowing oats at 6° and 10°C both proved highly effective in reducing nitrate leaching loss by around 33% and 75%, respectively, compared to fallow treatments. Soil temperature, however, was the chief driver of oats development, growing an order of magnitude more rapidly at 10°C and reducing nitrate leaching loss quickly through a combination of increased N uptake and reduced drainage. However, ammonia oxidising bacteria (AOB) abundance in 10°C fallow treatments increased more rapidly after urine application than at 6°C, meaning soil nitrate concentrations rose quickly, leaching twice as much nitrate as the 6°C fallow treatments after 90 days. Greater plant growth (10-times more) at 10°C than at 6°C, 75 days after sowing, was offset to some degree by greater N% and N content in the plant stems and roots at 6°C (2-4 times more). Higher light intensity increased evaporative water loss in both 6° and 10°C chambers, reducing drainage and nitrate leaching, and increasing the rate of oats development at 10°C. The sowing of oats as a catch crop following simulated winter forage grazing conditions shows promise as an effective means to limit nitrate leaching. Further research, however, is needed to show that a catch crop can be sown successfully in most years at farm paddock scale across a range of soil conditions.

Interocular interaction of contrast and luminance signals in human primary visual cortex

Interocular interaction in the visual system occurs under dichoptic conditions when contrast and luminance are imbalanced between the eyes. Human psychophysical investigations suggest that interocular interaction can be explained by a contrast normalization model. However, the neural processes that underlie such interactions are still unresolved. We set out to assess, for the first time, the proposed normalization model of interocular contrast interactions using magnetoencephalography (MEG) and to extend this model to incorporate interactions based on interocular luminance differences. We used MEG to record steady-state visual evoked responses (SSVER), and functional magnetic resonance imaging (fMRI) to obtain individual retinotopic maps that we used in combination with MEG source imaging in healthy participants. Binary noise stimuli were presented in monocular or dichoptic viewing and were frequency-tagged at 4 and 6 Hz. The contrast of the stimuli was modulated in a range between 0 and 32%. Monocularly, we reduced the luminance by placing a 1.5 ND filter over one eye in the maximal contrast condition. This ND filter reduces the mean light level by a factor of 30 without any alteration to the physical contrast.We observed in visual area V1 a monotonic increase in the magnitude of SSVERs with changes in contrast from 0 to 32%. For both eyes, dichoptic masking induced a decrease in SSVER signal power. This power decrease was well explained by the normalization model. Reducing mean luminance delayed monocular processing by approximately 38 ms in V1. The reduced luminance also decreased the masking ability of the eye under the filter. Predictions based on a temporal filtering model for the interocular luminance difference prior to the model's binocular combination stage were incorporated to update the normalization model. Our results demonstrate that the signals resulting from different contrast or luminance stimulation of the two eyes are combined in a way that can be explained by an interocular normalization model.

Linking resource availability and heterogeneity to understorey species diversity through succession in boreal forest of Canada

Abstract Understorey vegetation comprises the majority of species diversity and contributes greatly to ecosystem functioning in boreal forests. Although patterns of understorey abundance, species diversity and composition associated with forest stand development are well researched, mechanisms driving these patterns remain largely speculative. We sampled fire‐origin stands of varying stand ages and overstorey compositions on mesic sites of the boreal forest of Canada and used structural equation modelling (SEM) to link time since fire (stand age), light availability and heterogeneity, substrate heterogeneity and soil nitrogen to understorey vegetation cover and species diversity. The most parsimonious model for total understorey cover showed a positive direct effect of stand age (r = .43) and an indirect effect via mean light level (0.18) and shrub cover (−0.11), with a positive total effect (0.50); the per cent broadleaf canopy had a direct negative effect (−0.22) and an indirect effect via shrub cover (−0.11). The model for total understorey species richness showed an indirect effect of stand age via mean light (0.24), light heterogeneity (0.10) and substrate heterogeneity (0.07), with a positive total effect (0.52); per cent broadleaf canopy had an indirect effect via light heterogeneity (0.09), and substrate heterogeneity (−0.10). Soil nitrogen did not significantly influence either understorey cover or species richness. The models for vascular plants followed similar trends to those for total understorey cover and species richness; however, there was an opposite indirect effect of light heterogeneity for both cover and species richness of non‐vascular plants. Shrub cover had positive direct and negative direct and indirect effects on both vascular and non‐vascular cover and species richness. Synthesis. Our findings indicate that understorey cover and species diversity are driven by time since disturbance, light availability as influenced by overstorey and shrub layers, but with important additional effects mediated by light and substrate heterogeneity. Non‐vascular understorey vegetation is more strongly determined by time since disturbance than vascular vegetation, and negatively affected by broadleaf tree abundance. The overall results highlight the importance of colonization, light availability and heterogeneity, substrate specialization and growth dynamics in determining successional patterns of boreal forest understorey vegetation.

The distribution of vascular epiphytes over gradients of light and humidity in north‐east Australian rainforest

AbstractMicroclimatic conditions have a strong influence on the distribution of vascular epiphytes, among which orchids often occur in sunnier and more drought‐prone situations than ferns. However, very few studies have looked at the distribution of ferns and orchids in Australian tropical rainforests. By using transmitted light measurements at the locations of individual epiphytes and vapour pressure deficit from the canopy and base of host trees, we were able to determine the patterns of light and humidity in the rainforest environment, and the responses of ferns and orchids to variation in the physical environments. We surveyed five sites, ranging from 800 to 1180 m in elevation in the lower montane rainforests of north‐east Australia. Data loggers recorded the vapour pressure deficit (VPD) at the forest floor and canopy of each site. Light was correlated with height within the host tree and VPD differed significantly over position in the host tree and elevation. There was a strong partitioning of taxonomic groups over the light and VPD gradients. Orchids occurred in environments that had higher mean light levels and mean daily maximum VPD (27% and 0.43 kPa, respectively) than ferns (21% and 0.28 kPa). There was also strong microclimatic partitioning of species within taxonomic groups, suggesting that microclimatic factors play an important role in the realized niche spaces of epiphytes within the tropical Australian rainforest. Thus, the tested ecological generalizations made on tropical rainforest epiphytes apply in Australia.

Negative effects of fertilization on grassland species richness are stronger when tall clonal species are present

Fertilization of grasslands commonly leads to species loss and a dramatic shift in species composition, but the mechanisms underlying this pattern remain unclear. One oft-stated hypothesis is that fertilization increases competition for light and/or reduces spatial heterogeneity of resources, thereby reducing the niche dimensionality for species to coexist. Here, we summarize results from two long-term field experiments showing that the negative effects of fertilization on grassland species richness are greater in the presence of species with a spreading clonal growth form, particularly species that are tall (hereafter ‘tall clonals’). The interactive effect of fertilization and the presence of clonal species on the loss of species richness is associated with: (1) a pronounced increase in the relative abundance of tall clonals following fertilization, and (2) a greater reduction in mean light levels and reduced spatial heterogeneity in light when tall clonal species are present. We hypothesize that tall clonals gain a competitive advantage following fertilization as a result of two important traits related to clonality: (1) the ability to translocate resources between ramets in high to those in low resource patches and (2) the ability to spread vegetatively, especially under conditions where low-light levels limit recruitment from seeds. We suggest that efforts to mitigate the negative effects of nutrient enrichment on grassland diversity may be enhanced by controlling dominance by tall clonal grasses and forbs, and that these results have important implications for management efforts to maintain and enhance plant diversity in grasslands.

Seasonal Differences in Light Exposure and the Associations With Health and Well-Being in Older Adults: An Exploratory Study.

Objective:This article reports summer verses winter seasonal variations across a suite of blue light, illuminance levels and health and well-being indicators.Background:The quality of lighting in care homes has been assessed previously, yet seasonal comparisons and the associations with sleep quality are limited. This exploratory study investigates light exposure in two seasons to determine the changes over time and the associations with health and well-being.Methods:In a repeated measures design, 16 older people (aged 72–99 years) living in a care home had their personal light exposure and sleep/wake patterns monitored for 4 days. Cognitive ability, mental well-being, daytime physical activity, and visual function were assessed. Mean light levels at preset times across the day, duration in light exposure over 1,000 lux, and sleep parameters were computed. Statistical investigations included correlations exploring associations and paired means tests to detect the changes between seasons.Results:The mean morning illuminance level in summer was 466 lux and 65 lux in winter. Duration in bright light over 1,000 lux was 46 min in summer and 3 min in winter. Light measures were significantly higher in summer. There was no statistical difference in sleep quality parameters between seasons, but there were significant difference in daytime physical activity level (i.e., this was higher in summer).Conclusion:The findings indicate low level of light exposures experienced in both seasons, with exposure levels being particularly low in winter. This provides new insights into the limited amount of light older people receive independent of season and the possible impacts on sleep and daytime physical activity level.

Monitoring sound and light continuously in an intensive care unit patient room: A pilot study

PurposeTo determine the feasibility of continuous recording of sound and light in the intensive care unit (ICU). Materials and methodsFour 1-hour baseline scenarios in an empty ICU patient room by day and night (doors open or closed and maximal or minimal lighting) and two daytime scenarios simulating a stable and unstable patient (quiet or loud devices and staff) were conducted. Sound and light levels were continuously recorded using a commercially available multisensor monitor and transmitted via the hospital's network to a cloud-based data storage and management system. ResultsThe empty ICU room was loud with similar mean sound levels of 45 to 46 dBA for the day and night simulations. Mean levels for maximal lighting during day and night ranged from 1306 to 1812 lux and mean levels for minimum lighting were 1 to 3 lux. The mean sound levels for the stable and unstable patient simulations were 61 and 81 dBA, respectively. The mean light levels were 349 lux for the stable patient and 1947 lux for the unstable patient. ConclusionsCombined sound and light can be continuously and easily monitored in the ICU setting. Incorporating sound and light monitors in ICU rooms may promote an enhanced patient- and staff-centered healing environment.

Ambient illumination switches contrast preference of specific retinal processing streams.

Contrast, a fundamental feature of visual scenes, is encoded in a distributed manner by ∼ 20 retinal ganglion cell (RGC) types, which stream visual information to the brain. RGC types respond preferentially to positive (ON(pref)) or negative (OFF(pref)) contrast and differ in their sensitivity to preferred contrast and responsiveness to nonpreferred stimuli. Vision operates over an enormous range of mean light levels. The influence of ambient illumination on contrast encoding across RGC types is not well understood. Here, we used large-scale multielectrode array recordings to characterize responses of mouse RGCs under lighting conditions spanning five orders in brightness magnitude. We identify three functional RGC types that switch contrast preference in a luminance-dependent manner (Sw1-, Sw2-, and Sw3-RGCs). As ambient illumination increases, Sw1- and Sw2-RGCs shift from ON(pref) to OFF(pref) and Sw3-RGCs from OFF(pref) to ON(pref). In all cases, transitions in contrast preference are reversible and track light levels. By mapping spatiotemporal receptive fields at different mean light levels, we find that changes in input from ON and OFF pathways in receptive field centers underlie shifts in contrast preference. Sw2-RGCs exhibit direction-selective responses to motion stimuli. Despite changing contrast preference, direction selectivity of Sw2-RGCs and other RGCs as well as orientation-selective responses of RGCs remain stable across light levels.

Epiphyte assemblages respond to host life-form independently of variation in microclimate in lower montane cloud forest in Panama

Abstract:We investigated the effects of host tree on epiphyte diversity, controlling for microclimate. We measured the light profiles of the lower trunks of 20 individuals, each from three host groups (tree ferns, dicots, palms) occupying the understorey in a tropical montane forest in Panama. The per cent cover and species richness of vascular and non-vascular epiphytes were surveyed on the lower trunks of each understorey host. Light varied considerably between trees (5–21% total transmitted light) but mean light level did not vary between types of host. Light was not significant as a covariate with host in any model. Tree ferns had higher covers than dicots and palms of filmy ferns (15%, 0.02% and 0.2%), other ferns (7%, 0% and 0.5%) and other vascular epiphytes (16%, 3% and 3.4%), and greater species richness of vascular epiphytes (filmy ferns: 3, 0.4 and 0.5; other ferns: 2, 0.2 and 0; other vascular: 7, 2 and 2). Dicots had a higher cover of liverworts (53%) than palms (18%) and tree ferns (27%). Palms and tree ferns were the compositional extremes. We conclude that the differences in species composition and cover between the three host groups relate better to physical differences between hosts than differences in light climate.

Forest Community Structure Differs, but Not Ecosystem Processes, 25 Years After Eastern Hemlock Removal in an Accidental Experiment

Abstract The spread of Adelges tsugae (Hemlock Woolly Adelgid) directly threatens the survival of Tsuga canadensis (Eastern Hemlock) and has also triggered pre-emptive and salvage logging. In this study, we took advantage of a 25-year-old accidental experiment involving Eastern Hemlock removal by logging at Smith College's MacLeish Field Station, in western Massachusetts, to investigate how microclimate, ecosystem processes, and forest-floor animal communities might change in the decades following Eastern Hemlock loss. On average, mean understory light levels in summer were 68% higher under young Black Birch (Betula lenta) canopies as compared to adjacent mature Eastern Hemlock forest. Mean daily air temperature, relative humidity, soil temperature, and organic-layer moisture content were similar between young Black Birch and mature Eastern Hemlock plots, although some of these factors were significantly more variable in the former. The soil organic horizon was significantly thicker in Eastern Hemlock plo...

Local and Global Contrast Adaptation in Retinal Ganglion Cells

Retinal ganglion cells react to changes in visual contrast by adjusting their sensitivity and temporal filtering characteristics. This contrast adaptation has primarily been studied under spatially homogeneous stimulation. Yet, ganglion cell receptive fields are often characterized by spatial subfields, providing a substrate for nonlinear spatial processing. This raises the question whether contrast adaptation follows a similar subfield structure or whether it occurs globally over the receptive field even for local stimulation. We therefore recorded ganglion cell activity in isolated salamander retinas while locally changing visual contrast. Ganglion cells showed primarily global adaptation characteristics, with notable exceptions in certain aspects of temporal filtering. Surprisingly, some changes in filtering were most pronounced for locations where contrast did not change. This seemingly paradoxical effect can be explained by a simple computational model, which emphasizes the importance of local nonlinearities in the retina and suggests a reevaluation of previously reported local contrast adaptation.

Photoacclimation and non-photochemical quenching under in situ irradiance in natural phytoplankton assemblages from the Amundsen Sea, Antarctica

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 475:15-34 (2013) - DOI: https://doi.org/10.3354/meps10097 Photoacclimation and non-photochemical quenching under in situ irradiance in natural phytoplankton assemblages from the Amundsen Sea, Antarctica Anne-Carlijn Alderkamp1,2,*, Matthew M. Mills1, Gert L. van Dijken1, Kevin R. Arrigo1 1Department of Environmental Earth System Science, Stanford University, Stanford, California 94019, USA 2Department of Ocean Ecosystems, Energy and Sustainability Research Institute, University of Groningen, 9747 AG, Groningen, The Netherlands *Email: alderkamp@stanford.edu ABSTRACT: Photoacclimation strategies and sensitivity to photoinhibition were determined in natural phytoplankton assemblages during a phytoplankton bloom in the Amundsen Sea (Southern Ocean) in relation to community composition, pigment content, light, and iron (Fe). Non-photochemical quenching (qN) was measured during recovery after surface irradiance exposure (SIE) for 20 min. The qN was separated into slow (qI, photoinhibition through damage of Photosystem II) and fast (qE, xanthophyll cycling) relaxing components. Phytoplankton within the upper mixed layer (UML) showed a higher ratio of photoprotective xanthophyll cycle pigments (diadinoxanthin [DD] + diatoxanthin [DT]) to chlorophyll a (chl a), indicative of acclimation to high light, which resulted in lower photoinhibition after SIE when compared to phytoplankton residing below the UML. Within the UML, we found differences in photoacclimation strategies in assemblages dominated by Antarctic diatoms versus Phaeocystis antarctica (Haptophyta). Diatoms had a higher ratio of (DD + DT)/chl a, and the ratio tracked mean light levels within the UML, whereas this relationship was not apparent in P. antarctica, which had a lower (DD + DT)/chl a ratio. Despite these differences, diatoms and P. antarctica exhibited similar degrees of qN that were dominated by qE with very little qI. Bioassays under high and low Fe concentrations revealed an increase in the (DD + DT)/chl a ratio in Fe-limited populations dominated by diatoms and decreased photoinhibition. In experiments dominated by P. antarctica or with mixed populations, acclimation to low Fe increased the (DD + DT)/chl a ratio in most experiments; however, this did not affect photoinhibition. This study shows that under in situ conditions in the Amundsen Sea (1) phytoplankton photoacclimation efficiently minimizes photoinhibition, (2) photoinhibition does not control the relative abundances of P. antarctica or Antarctic diatoms, and (3) Fe limitation does not increase photoinhibition of either P. antarctica or Antarctic diatoms. KEY WORDS: Amundsen Sea · Diatoms · Iron · Non-photochemical quenching · Phaeocystis antarctica · Photoacclimation · Photoinhibition · Xanthophyll cycle Full text in pdf format Supplementary material PreviousNextCite this article as: Alderkamp AC, Mills MM, van Dijken GL, Arrigo KR (2013) Photoacclimation and non-photochemical quenching under in situ irradiance in natural phytoplankton assemblages from the Amundsen Sea, Antarctica. Mar Ecol Prog Ser 475:15-34. https://doi.org/10.3354/meps10097 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 475. Online publication date: February 14, 2013 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2013 Inter-Research.

Phytoplankton distributions in the Shackleton Fracture Zone/Elephant Island region of the Drake Passage in February–March 2004

The Drake Passage region near Elephant Island in the Southern Ocean displays patchy phytoplankton blooms. To test the hypothesis that natural Fe addition from localized sources promoted phytoplankton growth here, a grid of stations (59°S to 62°S, 59°W to 53°W, as well as four stations in the eastern Bransfield Strait) were occupied from 12 February–24 March 2004. Phytoplankton abundance was measured using shipboard flow cytometry (70 stations), with abundances conservatively converted to biomass, and compared with measurements of dissolved iron (dFe) at a subset of stations (30 stations). Based on T–S property plots, stations were divided into Antarctic Circumpolar Current (ACC), Water On Shelf (WOS), Bransfield Strait (BS), and Mixed water stations, the latter representing locations with T–S properties intermediate between ACC and WOS stations. The highest integrated phytoplankton biomass was found at Mixed water stations, however, the highest integrated abundance was found at WOS stations, demonstrating that abundance and biomass do not necessarily show the same patterns. The distributions of nano- and micro-phytoplankton (<20 and >20μm diameter cells, respectively) were also examined, with nano- and micro-plankton contributing equally to the total biomass at WOS and BS stations, but micro-plankton representing ∼2/3 of the biomass at Mixed and ACC stations. Increased inventories of dFe did not always correspond to increases in phytoplankton biomass – rather stations with lower mean light levels in the mixed layer (<110μEinsteinsm−2s−1) had lower biomass despite higher ambient dFe concentrations. However, where the mean light levels in the mixed layer were >110μEinsteinsm−2s−1, total biomass shows a positive trend with dFe, as does micro-phytoplankton biomass, but neither regression is significant at the 95% level. In contrast, if just nano-phytoplankton biomass is considered as a function of dFe, there is a significant correlation (r2=0.62). These data suggest a dual mechanism for the patterns observed in biomass: an increasing reservoir of dFe allows increased phytoplankton biomass, but biomass can only accumulate where the light levels are relatively high, such that light is not limiting to growth.

Understory light regimes following silvicultural treatments in central hardwood forests in Kentucky, USA

Manipulation of the light regime is a primary goal of many silvicultural treatments, but the specific light conditions created remain poorly documented for many forest types and geographic locations. To help quantify effects of silvicultural treatments on light conditions, measurements of basal area, canopy cover, and photosynthetically active radiation (PAR), measured both instantaneously and across time, were collected in central hardwood forests in Kentucky, USA following silvicultural treatments. These measurements were used to: (1) Investigate the magnitudes of differences in understory percent ambient PAR following implementation of shelterwood with reserves and thinning treatments; (2) document the spatial and temporal distribution and variability of understory percent ambient PAR in shelterwood with reserves treatments (mean residual basal area=5.2m2/ha), thinning treatments (18.5m2/ha), and untreated controls (27.1m2/ha); and (3) examine relationships between: basal area and canopy cover; basal area and measured percent ambient PAR; and canopy cover and measured percent ambient PAR.Mean light levels from instantaneous measurements were 78% of ambient in the shelterwood with reserves, 33% of ambient in the thinning and 9% of ambient in the control. Similarly, only 1.3% of the approximately 140h of PAR measurements in the controls indicated high light conditions (>60% of ambient), 15.9% in the thinning treatment and 65.4% in the shelterwood with reserves treatment. There were only 32 periods of high light found across all plots in the control, 176 periods of high light in the thinning treatment, and 441 periods of high light in the shelterwood with reserves treatment. Indexes of variability in light across time and among sampling locations within a stand did not differ statistically between the shelterwood with reserves and thinning treatments but both treatments were statistically more spatially and temporally variable than the uncut control. Simple linear regression relationships were observed between stand basal area and mean relative PAR (r2=0.8784 for instantaneous measurements, r2=0.9697 for continuous measurements), and basal area and canopy cover (r2=0.8479). Such relationships provide a means for including light management in forest planning and application of silvicultural treatments. The results for the distribution of light also suggest, however, that treated stands may have similar mean light levels, but differ substantially in the spatial and temporal distribution of light.