Increased Light Availability Research Articles

The influence of sea ice and snow cover and nutrient availability on the formation of massive under-ice phytoplankton blooms in the Chukchi Sea

A coupled biophysical model is used to examine the impact of changes in sea ice and snow cover and nutrient availability on the formation of massive under-ice phytoplankton blooms (MUPBs) in the Chukchi Sea of the Arctic Ocean over the period 1988–2013. The model is able to reproduce the basic features of the ICESCAPE (Impacts of Climate on EcoSystems and Chemistry of the Arctic Pacific Environment) observed MUPB during July 2011. The simulated MUPBs occur every year during 1988–2013, mainly in between mid-June and mid-July. While the simulated under-ice blooms of moderate magnitude are widespread in the Chukchi Sea, MUPBs are less so. On average, the area fraction of MUPBs in the ice-covered areas of the Chukchi Sea during June and July is about 8%, which has been increasing at a rate of 2%yr–1 over 1988–2013. The simulated increase in the area fraction as well as primary productivity and chlorophyll a biomass is linked to an increase in light availability, in response to a decrease in sea ice and snow cover, and an increase in nutrient availability in the upper 100m of the ocean, in conjunction with an intensification of ocean circulation. Simulated MUPBs are temporally sporadic and spatially patchy because of strong spatiotemporal variations of light and nutrient availability. However, as observed during ICESCAPE, there is a high likelihood that MUPBs may form at the shelf break, where the model simulates enhanced nutrient concentration that is seldom depleted between mid-June and mid-July because of generally robust shelf-break upwelling and other dynamic ocean processes. The occurrence of MUPBs at the shelf break is more frequent in the past decade than in the earlier period because of elevated light availability there. It may be even more frequent in the future if the sea ice and snow cover continues to decline such that light is more available at the shelf break to further boost the formation of MUPBs there.

Photoacclimation of Caulerpa cylindracea: Light as a limiting factor in the invasion of native Mediterranean seagrass meadows

Reduction in light availability caused by the canopy of the Mediterranean seagrass Posidonia oceanica has been suggested as a critical mechanism to resist the invasion of the exotic macroalga Caulerpa cylindracea. We experimentally evaluated the role of light as a limiting factor on the capacity of colonization and spread of this invasive seaweed in P. oceanica meadows by assessing photoacclimation responses and productivity and growth capacity of C. cylindracea in mesocosm and in situ light manipulation experiments. Despite the high photoacclimative plasticity developed by the alga, the light regime within the seagrass meadow during the study period was close to the minimum light requirements for growth, restricting the development capacity of this species. In addition, while increases in light availability resulting from canopy alteration also enhanced the productive capacity of the invasive seaweed in the field, such increase was not followed by gains in biomass production. Our results thus support the hypothesis that light availability has a major role in the underlying resistance of seagrass meadows to the invasion by C. cylindracea, but also indicate that there are additional factors related to the canopy of P. oceanica that further hinder the growth and colonization capacity of the alga.

Contrasting traits, contrasting environments, and considerations on population dynamics under a changing climate: an ecophysiological field study of two co‐dominant tree species

AbstractThe increase in light availability resulting from canopy changes or opening is not always beneficial and can inhibit photosynthesis of tree seedlings already under other environmental stress. Tree seedlings' responses to compounded abiotic stress depend on their life‐history traits, and understanding the variations of such responses is important for understanding population dynamics under a changing climate. In this study we investigate how the photosynthesis of juveniles of two canopy tree species with different life‐history traits, Abies sachalinensis and Betula ermanii, differs in two contrasting sites at a sub‐boreal forest in northern Japan—one under a deciduous canopy (Closed site) and the other at a wide canopy opening (Open site). Seedlings at the Open site had low Fv/Fm (quantum yield of photosystem II) for a longer period than those at the Closed site. Abies sachalinensis at the Closed site showed lower Fv/Fm in spring than those at the Open site, but recovered after the canopy's new leaves flushed, indicating its acclimation to the shaded condition. Mean Pmax (light‐saturated photosynthetic rate at ambient CO2 levels) of A. sachalinensis seedlings was affected by site and air temperature, while B. ermanii seedlings were also affected by precipitation. Only B. ermanii's seedlings presented growth in the period studied, in spite of observed mid‐day drops to Fv/Fm attributed to water‐deficit‐related photoprotection. Results suggest that the climate change predicted for the Hokkaido area may increase the competitive advantage of broad‐leaved deciduous species, such as B. ermanii, in relation to evergreen conifers like A. sachalinensis.

Multi-taxon alpha diversity following bark beetle disturbance: Evaluating multi-decade persistence of a diverse early-seral phase

Early-successional forest ecosystems developing after natural disturbances, such as fire, windthrow or insect outbreaks, can support high diversity of habitat structures, species and processes. However, the specific structural and multi-taxon responses that best define a distinct early-seral pre-forest phase, and the longevity of that phase, remain important research questions. To address these questions, we assessed stand structural heterogeneity and species density of various taxa across three biological kingdoms in the initial early-seral period (∼3years after severe bark beetle outbreak), advanced early-seral period (∼17–25years after severe bark beetle outbreak) and mature spruce forests in unmanaged montane/high-montane ecosystems in Southeastern Germany. We evaluated the hypothesis that changes in structural heterogeneity and increases in diversity would peak in the initial stage and attenuate toward mature forest conditions by 17–25years as the tree canopy closed.We found a clear change in forest structural heterogeneity following the outbreak – most prominently in reduced cover and more clustered patterning of live trees, increased light availability, increased cover of shrubs and herbs, and high volume of dead wood. Most of these structural changes were not ephemeral after outbreaks, but remained high or even increased after multiple decades, suggesting persistence of early-seral heterogeneity well into succession.Biodiversity as measured by species density and rarefaction curves showed variable responses to early-seral conditions depending on taxon and functional group. While some groups either showed no significant change with disturbance (e.g., most epigeics associated with the ground surface), or initially peaked after disturbance before declining quickly (e.g., saproxylic beetles specializing on fresh dead wood), several key groups showed maximum diversity in the advanced early-seral stage (e.g., herbs, herbivores, pollinators) – indicating that the timeframe over which increases occurred tended to be on the order of decades rather than years.Our findings suggest that in unmanaged forests after bark beetle attack, a structurally complex phase prior to tree canopy closure can last several decades, and that many aspects of early-seral biodiversity and ecosystem function only fully develop given this extended time period. Where management of montane forests includes objectives for sustaining biodiversity, accommodating the protracted early-seral stage is important to supporting the full range of organisms and functions associated with canopy-opening disturbances.

Light limitation helps stabilize the phytoplankton assemblage steady-state in a temperate and highly turbid, hypertrophic shallow lake (Laguna Chascomús, Argentina)

We performed a 30-month field study in a highly turbid, hypertrophic shallow lake from the Pampa Plain (Argentina) in order to investigate the occurrence of steady-state phases of phytoplankton, with an emphasis on the effect of subtle, but measurable, increase in light availability. The results revealed that a steady-state condition prevailed during the first 14 months, during which Aphanocapsa cf. delicatissima represented, on average, 91% of the total phytoplankton biomass. The statistical analyses provide support for the hypothesis that low light availability in the water column regulates the structure of the algal assemblage in this lake. Our results agree with predictions of the light-limited growth theory (i.e. the observed increase in light supply promoted an increase in algal density) and are also consistent with the notion that environmental constraints influence the dominant morphological traits in phytoplankton assemblages, particularly, that small organisms and/or attenuated forms are favoured under very poor light conditions. Nevertheless, further work is needed to get a full understanding of the processes controlling phytoplankton dynamics in this lake, including careful studies of the ecophysiology of the colonies comprising pico-sized cells of Aphanocapsa cf. delicatissima that dominated the assemblage during all the studied period.

Influence of variable water depth and turbidity on microalgae production in a shallow estuarine lake system — A modelling study

Strongly varying water levels and turbidities are typical characteristics of the large shallow estuarine lake system of St. Lucia, one of the largest on the African continent. This theoretical study investigated the combined effects of variable water depth and turbidity on seasonal pelagic and benthic microalgae production using a mathematical model, in order to ascertain productivity levels during variable and extreme conditions. Simulated pelagic and benthic net production varied between 0.3 and 180 g C m−2 year−1 and 0 and 220 g C m−2 year−1, respectively, dependent on depth, turbidity, and variability in turbidity. Although not surprising production and biomass decreased with increasing turbidity and depth. A high variability in turbidity, i.e. an alteration of calm and windy days, could reduce or enhance the seasonal pelagic and benthic production by more than 30% compared to a low variability. The day-to-day variability in wind-induced turbidity therefore influences production in the long term. On the other hand, varying water depth within a year did not significantly influence the seasonal production for turbidities representative of Lake St. Lucia. Reduced lake area and volume as observed during dry periods in Lake St. Lucia did not reduce primary production of the entire system since desiccation resulted in lower water depth and thus increased light availability. This agrees with field observations suggesting little light limitation and high areal microalgal biomass during a period with below average rainfall (2005–2011). Thus, microalgae potentially fulfil their function in the lake food-web even under extreme drought conditions. We believe that these results are of general interest to shallow aquatic ecosystems that are sensitive to drought periods due to either human or natural causes.

Deep water formation in the North Pacific and deglacial CO2rise

Deep water formation in the North Atlantic and Southern Ocean is widely thought to influence deglacial CO_2 rise and climate change; here we suggest that deep water formation in the North Pacific may also play an important role. We present paired radiocarbon and boron isotope data from foraminifera from sediment core MD02-2489 at 3640 m in the North East Pacific. These show a pronounced excursion during Heinrich Stadial 1, with benthic-planktic radiocarbon offsets dropping to ~350 years, accompanied by a decrease in benthic δ^(11)B. We suggest that this is driven by the onset of deep convection in the North Pacific, which mixes young shallow waters to depth, old deep waters to the surface, and low-pH water from intermediate depths into the deep ocean. This deep water formation event was likely driven by an increase in surface salinity, due to subdued atmospheric/monsoonal freshwater flux during Heinrich Stadial 1. The ability of North Pacific Deep Water (NPDW) formation to explain the excursions seen in our data is demonstrated in a series of experiments with an intermediate complexity Earth system model. These experiments also show that breakdown of stratification in the North Pacific leads to a rapid ~30 ppm increase in atmospheric CO_2, along with decreases in atmospheric δ^(13)C and Δ^(14)C, consistent with observations of the early deglaciation. Our inference of deep water formation is based mainly on results from a single sediment core, and our boron isotope data are unavoidably sparse in the key HS1 interval, so this hypothesis merits further testing. However, we note that there is independent support for breakdown of stratification in shallower waters during this period, including a minimum in δ^(15)N, younging in intermediate water ^(14)C, and regional warming. We also re-evaluate deglacial changes in North Pacific productivity and carbonate preservation in light of our new data and suggest that the regional pulse of export production observed during the Bolling-Allerod is promoted by relatively stratified conditions, with increased light availability and a shallow, potent nutricline. Overall, our work highlights the potential of NPDW formation to play a significant and hitherto unrealized role in deglacial climate change and CO_2 rise.

Effects of Native Herbs and Light on Garlic Mustard (Alliaria petiolata) Invasion

AbstractThe degree to which invasive species drive or respond to environmental change has important implications for conservation and invasion management. Often characterized as a driver of change in North American woodlands, the invasive herb garlic mustard may instead respond to declines in native plant cover and diversity. We tested effects of native herb cover, richness, and light availability on garlic mustard invasion in a Minnesota oak woodland. We planted 50 garlic mustard seeds into plots previously planted with 0 to 10 native herb species. We measured garlic mustard seedling establishment, survival to rosette and adult stages, and average (per plant) and total (per plot) biomass and silique production. With the use of structural equation models, we analyzed direct, indirect, and net effects of native cover, richness, and light on successive garlic mustard life stages. Native plant cover had a significant negative effect on all life stages. Species richness had a significant positive effect on native cover, resulting in indirect negative effects on all garlic mustard stages, and net negative effects on adult numbers, total biomass, and silique production. Light had a strong negative effect on garlic mustard seedling establishment and a positive effect on native herb cover, resulting in significant negative net effects on garlic mustard rosette and adult numbers. However, light's net effect on total garlic mustard biomass and silique production was positive; reproductive output was high even in low-light/high-cover conditions. Combined effects of cover, richness, and light suggest that native herbs provide biotic resistance to invasion by responding to increased light availability and suppressing garlic mustard responses, although this resistance may be overwhelmed by high propagule pressure. Garlic mustard invasion may occur, in part, in response to native plant decline. Restoring native herbs and controlling garlic mustard seed production may effectively reduce garlic mustard spread and restore woodland diversity.

Variability in under‐ice export fluxes of biogenic matter in the Arctic Ocean

AbstractA critical question regarding the organic carbon cycle in the Arctic Ocean is whether the decline in ice extent and thickness and the associated increase in solar irradiance in the upper ocean will result in increased primary production and particulate organic carbon (POC) export. To assess spatial and temporal variability in POC export, under‐ice export fluxes were measured with short‐term sediment traps in the northern Laptev Sea in July‐August‐September 1995, north of the Fram Strait in July 1997, and in the Central Arctic in August–September 2012. Sediment traps were deployed at 2–5 m and 20–25 m under ice for periods ranging from 8.5 to 71 h. In addition to POC fluxes, total particulate matter, chlorophyll a, biogenic particulate silica, phytoplankton, and zooplankton fecal pellet fluxes were measured to evaluate the amount and composition of the material exported in the upper Arctic Ocean. Whereas elevated export fluxes observed on and near the Laptev Sea shelf were likely the combined result of high primary production, resuspension, and release of particulate matter from melting ice, low export fluxes above the central basins despite increased light availability during the record minimum ice extent of 2012 suggest that POC export was limited by nutrient supply during summer. These results suggest that the ongoing decline in ice cover affects export fluxes differently on Arctic shelves and over the deep Arctic Ocean and that POC export is likely to remain low above the central basins unless additional nutrients are supplied to surface waters.

Spatial and temporal variation of net community production and its regulating factors in the Amundsen Sea, Antarctica

We observed ΔO2/Ar in the surface waters of the Amundsen Sea, Antarctica, during the austral summers in 2011 and 2012 to investigate the variability of net community production (NCP). Corresponding to the typical peak phytoplankton bloom period, the ΔO2/Ar of the Amundsen Sea Polynya (ASP) reached 30% in early January 2011 and had a strong positive correlation with the sea surface temperature (SST) and chlorophyll-a (Chl-a). In contrast, ΔO2/Ar decreased to −10% in the sea ice zone (SIZ), which was likely associated with either net O2 consumption in the unlit area or the entrainment of deep water containing low dissolved oxygen. Near the terminal stage of the phytoplankton bloom in late February 2012, we observed the same contrasting ΔO2/Ar features between the ASP and SIZ. However, the ΔO2/Ar in the ASP was not >10%, which corresponded with the overall reduction in Chl-a, solar radiation, and SST compared with the corresponding values in 2011. The average net community production in the ASP was 119 ± 79 mmol O2 m−2 d−1 in January 2011, and 23 ± 14 mmol O2 m−2 d−1 in February 2012. The strong correlations of NCP with SST and mixed layer depth (MLD) indicate that the ASP phytoplankton bloom is likely initiated by a combination of increased light availability and SST in early summer. Low SST and variable fluorescence to maximum florescence ratio (Fv/Fm) in February indicate that decreased solar radiation and Fe availability are likely responsible for the phytoplankton bloom demise.

Effects of light and prey availability on Arctic freshwater protist communities examined by high-throughput DNA and RNA sequencing

Protists in high-latitude lakes are constrained by cold temperatures, low inorganic nutrient supply and low light availability for much of the year due to ice cover and polar darkness. The lengthening ice-free periods in these freshwater ecosystems due to a warming climate results in increased light availability, but the overall impacts on phytoplankton and other protists are unknown. We experimentally investigated protist community responses to changes in light and prey availability in a dilution series in Ward Hunt Lake (latitude 83°05'N), in the Canadian High Arctic. The communities at the end of the experiment were characterized using high-throughput pyrosequencing of the V4 region of the 18S rRNA gene as a measure of taxonomic presence, and of 18S rRNA (from RNA converted to cDNA) as a taxon-specific indicator of community response. At the end of the experiment under low irradiance, cDNA reads were dominated by photosynthetic dinoflagellate genera, except at the greatest dilution where Cercozoa were most abundant. In contrast, the cDNA reads in the high light treatments were dominated by chrysophytes. Given the known trophic differences among dinoflagellates, cercozoans and chrysophytes, this apparent environmental selection implies that the rise in underwater irradiance associated with increasing ice-free conditions may affect microbial food web structure and function in polar lakes.

Evaluating Restoration Baselines for Historically Fire-Protected Woodlands within a Northeastern Illinois Prairie Peninsula Landscape

The forests and woodlands of the Prairie Peninsula region in the Midwestern USA have been heavily impacted by human influences over the past ∼150 years. Current composition, structure, and dynamics in forest communities across the region lie outside the historical range of variability. However, areas along major waterways were afforded greater fire protection historically than the landscape as a whole (and are common locations for modern natural areas), and historical and modern conditions may be more analogous in these locations. This study assessed composition and structure of woodlands in a series of natural areas along the Des Plaines River in Lake County, Illinois, and related current conditions to historical baselines for the locality and region. Modern composition and structure in even these fire-protected habitats appear to lie outside the historical range of variability. High canopy cover, stem density, and dominance by sugar maple (Acer saccharum Marsh.) and European buckthorn (Rhamnus cathartica L.) are especially inconsistent with historical conditions. Current structure suggests a continued trajectory toward homogenized, closed-canopy ecosystems with increased dominance by mesophytic and invasive species and decreased importance of historically-dominant oak (Quercus spp.) species. Community-specific management strategies focused on modifying canopy structure and composition will be necessary to shift these communities toward conditions of increased light availability, structural complexity, and biological diversity. Strategies to achieve these goals are currently not well established, especially those that could be applied in urban natural areas. Implementation of multiple approaches in an adaptive management framework would aid in developing best management practices for wooded ecosystems in the region.

Soil fertility alters the nature of plant–resource interactions in invaded grassland communities

Resource competition theory suggests that the nature of diversity–resource–invasibility interactions will vary along fertility gradients, concurrent with changes in the relative availability of limiting above- versus below-ground resources. Experimental support for this contingency is lacking. Here, we manipulated resident diversity, baseline fertility, and the availabilities of light and soil nitrogen in grassland communities invaded by two functionally distinct non-native plant species (Lolium arundinaceum and Melilotus alba). We tested the hypotheses that increased resident diversity reduces community invasibility and dampens the effects of light and soil nitrogen pulses, and that the relative effects of light versus soil nitrogen additions on diversity–invasibility relationships depend on the baseline fertility of the study system. Our results reveal an overall weak negative effect of resident diversity on Lolium performance, but in contrast to our expectations, this diversity effect did not vary with light or soil nitrogen additions or with baseline fertility. However, the relative effects of above- versus below-ground resource additions on invader performance varied with baseline fertility as expected: Lolium responded most strongly to soil nitrogen additions in low-fertility mesocosms and most strongly to increased light availability in high-fertility mesocosms. In contrast to Lolium, nitrogen-fixing Melilotus was overall less responsive to diversity and resource manipulations. Together, these patterns do not lend support for the dependence of diversity–resource–invasibility relationships on either baseline fertility or invasive species identity, but they do highlight the dominant role of resources over diversity in determining invader performance, as well as the manner in which fertility alters the relative importance of above- versus below-ground resource pulses in promoting invasions.

Herbivores and nutrients control grassland plant diversity via light limitation

Human alterations to nutrient cycles and herbivore communities are affecting global biodiversity dramatically. Ecological theory predicts these changes should be strongly counteractive: nutrient addition drives plant species loss through intensified competition for light, whereas herbivores prevent competitive exclusion by increasing ground-level light, particularly in productive systems. Here we use experimental data spanning a globally relevant range of conditions to test the hypothesis that herbaceous plant species losses caused by eutrophication may be offset by increased light availability due to herbivory. This experiment, replicated in 40 grasslands on 6 continents, demonstrates that nutrients and herbivores can serve as counteracting forces to control local plant diversity through light limitation, independent of site productivity, soil nitrogen, herbivore type and climate. Nutrient addition consistently reduced local diversity through light limitation, and herbivory rescued diversity at sites where it alleviated light limitation. Thus, species loss from anthropogenic eutrophication can be ameliorated in grasslands where herbivory increases ground-level light.

Pathogen induced disturbance and succession in temperate forests: Evidence from a 100-year data set in southern Sweden

Major tree species are declining in many temperate forests due to changing disturbance regimes, including invasive pests and pathogens. We examined the interaction of secondary succession and Dutch elm disease in the Swedish temperate forest reserve Dalby Söderskog, based on five tree surveys made between 1909 and 2011. The forest is characterized by the coexistence of four major European tree species: wych elm (Ulmus glabra), European ash (Fraxinus excelsior), European beech (Fagus sylvatica) and pedunculate oak (Quercus robur). After protection of the forest in 1918, lack of disturbance mainly favoured elm, while the oak population declined due to mortality of old oaks and lack of regeneration. Dutch elm disease has caused high and continuous elm mortality since 1988. As a result, increased light availability at the forest floor favoured abundant regeneration of ash, beech, and lately also oak. The recent arrival of an invasive fungal pathogen causing ash dieback may once again change the course of succession. Open space emerging from loss of elm and ash in forest reserves may be used by reserve managers to favour oak regeneration and biodiversity of semi-open woodlands once lost during succession to closed forest. We conclude that winners and losers change places as an effect of invasive pathogens, resulting in unexpected successions and both losses and gains in valuable ecological niches and habitat structures in temperate broadleaf forests.

Understanding causes of tree growth response to gap formation: ∆13C-values in tree rings reveal a predominant effect of light

Carbon isotope ratios in growth rings of a tropical tree species show that treefall gaps stimulate diameter growth mainly through changes in the availability of light and not water. The formation of treefall gaps in closed canopy forests usually entails considerable increases in light and nutrient availability for remaining trees, as well as altered plant water availability, and is considered to play a key role in tree demography. The effects of gaps on tree growth are highly variable and while usually stimulatory they may also include growth reductions. In most studies, the causes of changes in tree growth rates after gap formation remain unknown. We used changes in carbon isotope 13C discrimination (Δ13C) in annual growth rings to understand growth responses after gap formation of Peltogyne cf. heterophylla, in a moist forest of Northern Bolivia. We compared growth and Δ13C of the 7 years before and after gap formation. Forty-two trees of different sizes were studied, half of which grew close (<10 m) to single treefall gaps (gap trees), the other half more than 40 m away from gaps (controls). We found variable responses among gap trees in growth and Δ13C. Increased growth was mainly associated with decreased Δ13C, suggesting that the growth response was driven by increased light availability, possibly in combination with improved nutrient availability. Most trees showing zero or negative growth change after gap formation had increased Δ13C, suggesting that increased water stress did not play a role, but rather that light conditions had not changed much or nutrient availability was insufficient to support increased growth. Combining growth rates with Δ13C proved to be a valuable tool to identify the causes of temporal variation in tree growth.

Fire drives the reproductive responses of herbaceous plants in a Neotropical swamp

Palm swamps (veredas) are unique and diverse plant communities associated with the headwaters of streams in central Brazil, and they are frequently subjected to fires. We evaluated the effect of fire and the role of different fire-related cues on inducing reproduction by palm swamp vegetation. We compared the responses of species in burned plots, in plots in which the aboveground vegetation was clipped and then removed, and in unburned and unclipped control plots. Both the number of reproductive species and the total number of flowers/fruits produced by all species monthly were significantly greater in the burned than in the clipped and control plots, and greater in the clipped than in the control plots. For 34 of the 48 individual species analyzed the number of flowers/fruits produced per m2/month was greater in the burned than in the control plots, whereas the clipping treatment significantly increased the reproductive rate of only six species. This indicates that increased light availability was not the only factor inducing plant reproduction. Most likely, plant reproduction was also stimulated by the availability of soil nutrients whose concentrations increased significantly after burning. Although our results indicate that most plant species that occur in palm swamps are fire-recruiters, care must be taken in using fire as management tool, especially as the frequency of human-induced fires in palm swamps have increased dramatically in recent years.

Ecosystem metabolism and nutrient uptake in an urban, piped headwater stream

Piped streams, or streams that run underground, are often associated with urbanization. Despite the fact that they are ubiquitous in many urban watersheds, there is little empirical evidence regarding the ecological structure and function of piped stream reaches. This study measured ecosystem metabolism, nutrient uptake, and related characteristics of Pettee Brook—an urban stream that flows through several piped sections in Durham, New Hampshire, USA. Pettee Brook had high chloride and nutrient concentrations, low benthic biomass, and low rates of gross primary productivity (GPP), ecosystem respiration (ER), and nutrient uptake along its entire length during summer. Spring was a period of elevated biological activity, as increased light availability in the un-piped sections of the stream led to substantially higher GPP, ER, NH4 uptake, and PO4 uptake in these open reaches. Piped reaches of Pettee Brook were similar to open reaches in terms of water quality, dissolved O2 concentration, temperature, and discharge. Piped reaches did, however, have significantly less light, shallower sediments, and no debris dams. The absence of light inhibited autotrophic activity in piped reaches, resulting in the complete loss of GPP as well as a significant reduction in benthic AFDM and chlorophyll a biomass. Heterotrophic activity in piped reaches was not impaired to the same extent as autotrophic activity. Reduced ER was observed in piped reaches during the summer, but we failed to find significantly lower DOC or nutrient uptake rates in piped reaches than in open reaches. Carbon consumption in piped reaches, which do not have significant autochthonous or allochthonous carbon replenishment, must rely primarily on upstream inputs of organic matter. These results suggest that although ecological conditions in piped streams may be degraded beyond the extent of other urban stream reaches, piped reaches may still sustain some measurable ecosystem function.

Increasing cloudiness in Arctic damps the increase in phytoplankton primary production due to sea ice receding

Abstract. The Arctic Ocean and its marginal seas are among the marine regions most affected by climate change. Here we present the results of a diagnostic model used to assess the primary production (PP) trends over the 1998–2010 period at pan-Arctic, regional and local (i.e. 9.28 km resolution) scales. Photosynthetically active radiation (PAR) above and below the sea surface was estimated using precomputed look-up tables of spectral irradiance, taking as input satellite-derived cloud optical thickness and cloud fraction parameters from the International Satellite Cloud Climatology Project (ISCCP) and sea ice concentration from passive microwaves data. A spectrally resolved PP model, designed for optically complex waters, was then used to assess the PP trends at high spatial resolution. Results show that PP is rising at a rate of +2.8 TgC yr−1 (or +14% decade−1) in the circum-Arctic and +5.1 TgC yr−1 when sub-Arctic seas are considered. In contrast, incident PAR above the sea surface (PAR(0+)) has significantly decreased over the whole Arctic and sub-Arctic Seas, except over the perennially sea-ice covered waters of the Central Arctic Ocean. This fading of PAR(0+) (−8% decade−1) was caused by increasing cloudiness during summer. Meanwhile, PAR penetrating the ocean (PAR(0−)) increased only along the sea ice margin over the large Arctic continental shelf where sea ice concentration declined sharply since 1998. Overall, PAR(0−) slightly increased in the circum-Arctic (+3.4% decade−1), while it decreased when considering both Arctic and sub-Arctic Seas (−3% decade−1). We showed that rising phytoplankton biomass (i.e. chlorophyll a) normalized by the diffuse attenuation of photosynthetically usable radiation (PUR), accounted for a larger proportion of the rise in PP than did the increase in light availability due to sea-ice loss in several sectors, and particularly in perennially and seasonally open waters. Against a general backdrop of rising productivity over Arctic shelves, significant negative PP trends and the timing of phytoplankton spring-summer bloom were observed in regions known for their great biological importance such as the coastal polynyas of northern Greenland.

Competition Among Surface Roots in a Selectively-Logged, Semi-Deciduous Forest in Southeastern Mexico — Effects on Seedlings of Two Species of Contrasting Shade Tolerance

Experimental manipulations of root competition on naturally established seedlings were conducted across canopy openness and soil depth gradients in a selectively-logged, semideciduous forest on limestone-derived soils in southeastern Mexico. We studied the relatively shade intolerant mahogany (Swietenia macrophylla, Meliaceae) and shade tolerant ramon (Brosimum alicastrum, Moraceae). Both species are ecologically and economically important canopy trees that respond differently to selective logging, hurricanes, and fires on the Yucatan Peninsula. The canopy openness gradient used in the experiment was created by low-intensity selective logging operations. The responses of naturally occurring seedlings of both species growing within 20–30 cm deep circular trenches were examined over a 19-month experimental period. Trenching resulted in increased relative growth in diameter, stem length, and number of leaves of Swietenia compared with controls, but had no effect on Brosimum relative growth or on seedling survival of either species. There was no significant interaction between increased light availability and trenching on Swietenia seedlings, perhaps because of their larger size in gaps. Trenching effects on Swietenia were not greater during the dry season, suggesting that surface trenching affected growth during transitions between seasons. Contrasting responses of Swietenia and Brosimum seedlings to changes in soil and light environments point to the need for diversity in silvicultural practices in seasonally-dry tropical forests such as the community-managed forest examined in this study.