Increasing Nutrient Limitation Research Articles

Microbial carbon use efficiency and soil organic carbon stocks across an elevational gradient in the Peruvian Andes

Soils of mountain ecosystems are one of the most vulnerable ecosystems to climate change, while the ecosystem services they produce are significant and currently at risk. High altitude soils contain high C stocks, but due to difficult access to sites these areas are understudied. Moreover, how the C and N cycling is changing in response to climate change in these ecosystems, is still unclear. Microbial carbon use efficiency (CUE) and its dependency on the environmental constraints along the altitudinal gradients is one important unknown factor. Here we present results from an altitudinal gradient study (3500 to 4500 m a.s.l.) from a Polylepis forest in the Peruvian Andes. We measured the soil organic carbon (SOC) stocks and microbial metabolic CUE by 13C glucose tracing and microbial resource use efficiency (CUEC:N) based on enzyme activity measurements. We expected to find an increase in SOC stock, microbial nutrient limitations, and lower CUE with elevation. SOC stocks depended on soil development and followed a unimodal curve that peaks at 4000 m in two of the three studied valleys. Neither 13CUE nor CUEC:N changed significantly with altitude. Soil C:N ratio, β-glucosidase, chitinase, and phosphatase enzyme activities increased with elevation, but peroxidase activity decreased with elevation. We suggest that more labile organic matter left at high elevation could compensate for the increasing nutrient limitation at high elevation, resulting in no noticeable change in CUE with elevation.

Long-term adaptive response of an oceanic diatom to copper deficiency

Enhanced vertical stratification brought about by warming of the ocean surface is expected to reduce vertical circulation and nutrient input with knock-on effects for phytoplankton. Increased nutrient limitation is one predicted outcome, but how that will impact phytoplankton is uncertain because we do not know how they will adapt. We used copper (Cu) as a model catalytic nutrient to explore the adaptive response of an oceanic diatom to continuous nutrient deprivation in laboratory experiments. Populations of Thalassiosira oceanica maintained under Cu-limiting and sufficient conditions for ~380 generations differed significantly in their abilities to grow in medium containing 1 nM Cu. Continued selection for more than 2000 generations increased Cu use efficiency (CuUE) of a low Cu-adapted (LCuA) population by more than 2-fold compared to the control and ancestral populations. The increase in CuUE resulted from a decrease in the amount of cellular Cu required for growth and an increase in the net carbon assimilation rate. Redistribution of cellular Cu and increased efficiency of photosynthetic reactions are hypothesized to explain the fast rates of maximum electron transport of low Cu-adapted cells despite containing less Cu. The results show that adaptation increased resource use efficiency in phytoplankton, which could reduce the impact of increased nutrient deficiency in the future ocean.

Ericoid shrubs shape fungal communities and suppress organic matter decomposition in boreal forests.

Mycorrhizal fungi associated with boreal trees and ericaceous shrubs are central actors in organic matter (OM) accumulation through their belowground carbon allocation, their potential capacity to mine organic matter for nitrogen (N) and their ability to suppress saprotrophs. Yet, interactions between co-occurring ectomycorrhizal fungi (EMF), ericoid mycorrhizal fungi (ERI), and saprotrophs are poorly understood. We used a long-term (19 yr) plant functional group manipulation experiment with removals of tree roots, ericaceous shrubs and mosses and analysed the responses of different fungal guilds (assessed by metabarcoding) and their interactions in relation to OM quality (assessed by mid-infrared spectroscopy and nuclear magnetic resonance) and decomposition (litter mesh-bags) across a 5000-yr post-fire boreal forest chronosequence. We found that the removal of ericaceous shrubs and associated ERI changed the composition of EMF communities, with larger effects occurring at earlier stages of the chronosequence. Removal of shrubs was associated with enhanced N availability, litter decomposition and enrichment of the recalcitrant OM fraction. We conclude that increasing abundance of slow-growing ericaceous shrubs and the associated fungi contributes to increasing nutrient limitation, impaired decomposition and progressive OM accumulation in boreal forests, particularly towards later successional stages. These results are indicative of the contrasting roles of EMF and ERI in regulating belowground OM storage.

Chain or sphere? Perspectives on colony shapes and sizes in microalgae

Abstract Some microalgal species can increase their collective size by forming colonies; notable examples are chained colonies in diatoms and Scenedesmus sp., and spherical colonies in Phaeocystis globosa. For a given cell-specific growth rate, chain formation increases collective length quickly to fend off ciliates, but not against tube- and pallium-feeding heterotrophic dinoflagellates or metazoan grazers with ability to manipulate chains to aid ingestion. Sphere increases in volume relatively slowly but would be difficult to manipulate even for metazoan grazers. Diffusive nutrient supply to a chained colony would be a fixed proportion of that to solitary cells, regardless of chain length, whereas cells within a spherical colony would experience increasing nutrient limitation with increasing colony size. One hemisphere of a spherical colony would inevitably receive less irradiance, creating an auto-light limitation. Experimental data showed that light decreased substantially as it passed through a P. globosa colony, and the optical density of the colony increased linearly with colony diameter. However, neither in situ nutrient nor light limitation alone can explain an order-of-magnitude difference in colony size between the European and the Asian P. globosa populations. Instead, some evidence of different expression of gene(s) involved in colony formation and enlargement suggests genomic variations among the different populations.

Nutrient limitation in Atlantic salmon rivers and streams: Causes, consequences, and management strategies.

Freshwater catchments can experience nutrient deficits that result in reduced primary and secondary productivity. The most commonly limiting nutrients are nitrogen and phosphorus, either separately or together. This review considers the impact of increasing nutrient limitation in temperate basin stream and river systems, focusing on upland areas that currently or previously supported wild Atlantic salmon ( Salmo salar ) populations.Anthropogenic changes to land use and increases in river barriers have altered upland nutrient dynamics, with particular impacts on salmon and other migratory fish species which may be net importers of nutrients to upland streams. Declining salmon populations may further reduce nutrient sources, reducing ecosystem and fisheries productivity below desired levels.Experimental manipulations of nutrient levels have examined the impacts of this cultural oligotrophication. There is evidence that growth and biomass of juvenile salmon can be increased via appropriate additions of nutrients, offering potential as a conservation tool. However, further research is required to understand the long‐term effects of these additions on salmon populations and stream ecosystems, and to assess the vulnerability of downstream habitats to eutrophication as a result.Although purposeful nutrient addition with the aim of enhancing and conserving salmonid populations may be justified in some cases, it should be undertaken in an adaptive management framework. In addition, nutrient addition should be linked to nutrient retention and processing, and integrated into large‐scale habitat restoration and recovery efforts.Both the scientific and the management community should recognize that the ecological costs and benefits associated with adding nutrients to salmon streams may change in a non‐stationary world.

Ecological Stoichiometry in Pinus massoniana L. Plantation: Increasing Nutrient Limitation in a 48-Year Chronosequence

Stoichiometric ratios of carbon (C), nitrogen (N), and phosphorus (P) are considered indicators of nutrient status and ultimate ecosystem health. A detailed investigation of these elements in the leaves, branches, forest layer vegetation and soil, depending on stand age, was carried out. We investigated the effects of stand age (9-, 18-, 28-, and 48-year) on the aboveground plant parts (leaf, branch, herb, shrub, plant litter) and belowground pools (soil, roots) of P. massoniana plantations. The CNP stoichiometry of trees was affected by stand age. Mean N content in the aboveground parts in the nine-yr stand was greater than the other stands (18-, 28-, 48-yr), which decreased with increasing stand age. As stands aged, the nutrient demands of the plantations increased as well as their N:P ratios in soil. C content in the soil ranged from 30 to 105, the total N was 0.06 to 1.6, and the total P content ranged from 3.3–6.4 g kg−1. Soil C, N and P contents were greatly influenced by both stand age and soil depth, because surface soil sequester C and N more actively compared to deeper horizons, and more nutrients are released to the topsoil by the plant litter layer. Similarly, the ratios of other layers had a similar pattern as CNP because more nutrients were taken up by the plantations, decreasing nutrient supply in the deeper soil horizons. The green leaves N:P ratios (16) indicate limited growth of P. massoniana, as the range for global nutrient limitation for woody plants oscillated between 14–16, indicating N and P limitation. Young stands were observed to have greater P content and P resorption efficiency (56.9%–67.3%), with lower C:P and N:P ratios (704.4; 14.8). We conclude that with stand development, the nutrient demands of the plantations also increase, and soil N:P stoichiometry shows that these improve soil quality.

A quarter-century of nutrient load reduction leads to halving river nutrient fluxes and increasing nutrient limitation in coastal waters of central Japan.

Nutrient load reduction is widely used to improve coastal water quality, but it can lead to oligotrophication. This paper evaluates the current status of river water origin and the water recharge system based on isotope values and dissolved compositions recorded in 2018, and it also assesses the impact of nutrient load reduction efforts on river nutrient fluxes and coastal water quality using 25years of monitoring data. This study focuses on the coast of Toyama Bay as a model area because (1) up to 20% of terrestrially derived nutrient support the growth of coastal primary productivity, and (2) the adjacent land is a typical city in a population-dense area (~ 500 persons/km2), a demographic characteristic that exists in 88% of Japan's total land area and 96% of the total length of the country's coastline. Since the government adopted new wastewater treatment systems in 1993, river nutrient supplies in the study area have been halved, while the total river flow and annual precipitation have remained almost unchanged. The reduction in riverine nutrient supply has increased phosphorus deficiencies in the coastal waters. Most notably, the decline in nutrient concentrations in coastal surface waters and the enlarged nutrient-restricted areas are facing most parts of Japan, suggesting a resulting 25-50% decrease in CO2 uptake by primary production. This study is in agreement with previous studies from various countries in emphasizing the importance of setting appropriate nutrient-management goals for maintaining a sustainable marine environment. This paper recommends the need to accumulate various case studies of different areas and to share in a timely manner scientific evidence on a regional and global scale.

Buried hurricane legacies: increased nutrient limitation and decreased root biomass in coastal wetlands

AbstractPlant identity and cover in coastal wetlands is changing in worldwide, and many subtropical salt marshes dominated by low‐stature herbaceous species are becoming woody mangroves. Yet, how changes affect coastal soil biogeochemical processes and belowground biomass before and after storms is uncertain. We experimentally manipulated the percent mangrove cover (Avicennia germinans) in 3 × 3 m cells embedded in 10 plots (24 × 42 m) comprising a gradient of marsh (e.g., Spartina alterniflora, Batis maritima) and mangrove cover in Texas, USA. Hurricane Harvey made direct landfall over our site on 25 August 2017, providing a unique opportunity to test how plant composition mitigates hurricane effects on surface sediment accretion, soil chemistry (carbon, C; nitrogen, N; phosphorus, P; and sulfur, S), and root biomass. Data were collected before (2013 and 2016), one‐month after (2017), and one‐year after (2018) Hurricane Harvey crossed the area, allowing us to measure stocks before and after the hurricane. The accretion depth was higher in fringe compared with interior cells of plots, more variable in cells dominated by marsh than mangrove, and declined with increasing plot‐scale mangrove cover. The concentrations of P and δ34S in storm‐driven accreted surface sediments, and the concentrations of N, P, S, and δ34S in underlying soils (0–30 cm), decreased post‐hurricane, whereas the C concentrations in both compartments were unchanged. Root biomass in both marsh and mangrove cells was reduced by 80% in 2017 compared with previous dates and remained reduced in 2018. Post‐hurricane loss of root biomass in plots correlated with enhanced nutrient limitation. Total sulfide accumulation as indicated by δ34S, increased nutrient limitation, and decreased root biomass of both marshes and mangroves after hurricanes may affect ecosystem function and increase vulnerability in coastal wetlands to subsequent disturbances. Understanding how changes in plant composition in coastal ecosystems affects responses to hurricane disturbances is needed to assess coastal vulnerability.

Different responses of leaf and root traits to changes in soil nutrient availability do not converge into a community-level plant economics spectrum

Spatial heterogeneity in soil can influence the distribution of plant communities because plants differ in their ability to acquire resources. In nutrient-poor soils, plants should bear traits that prioritize conservation of resources over growth, whereas the opposite is expected in nutrient-rich soils. A coordinated response to soil fertility, in which leaves and roots converge into the same strategy, would lead to a community-level plant economics spectrum (PEScom). Here, we examine how nutrient availability across different soil types shapes the functional structure of desert shrub communities and how much of this variation is explained by species turnover or intraspecific variability (ITV). We measured leaf and root traits of 16 shrub species in four sites with contrasting soil nutrient availability in the southern edge of the Atacama Desert. In leaves, changes in soil nutrient availability were associated with a leaf economics spectrum, shifting from communities with more acquisitive strategies to communities with more conservative strategies with increasing nutrient limitation. No such pattern of co-variation was observed for roots; thus, the relationship between above and below-ground traits did not fit the PEScom. Variation of leaf traits was mainly determined by species turnover, while for root traits it was determined equally by species turnover and ITV. The lack of a PEScom appears to be related to the co-limitation of multiple resources, which promotes different responses in root traits. We highlight the importance of incorporating leaf and root traits to understand how multiple resource limitation shapes plant ecological strategies.

Testing mechanisms underlying responses of plant functional traits to flooding duration gradient in a lakeshore meadow

Flooding duration is a major factor determining wetland plant growth and community composition. While there have been extensive studies on how direct effects of flooding duration on plant functional traits, very few have tested the indirect effects on these traits, including those arising from competition. We hypothesized that there would be a shift in importance of competition ability and flooding tolerance to functional traits along the flooding duration gradient. We investigated plant functional traits along a flooding duration gradient for two-dominant wetland plant species (Carex cinerascens and Phalaris arundinacea) in a lakeshore meadow of Poyang Lake, China. We found that these two species tended to have different strategies to adapt to flooding stress. P. arundinacea tended to adopt escape strategy with high growth rate (e.g. greater height, specific leaf area, and leaf N and P content) at longer flooding duration, while C. cinerascens tended to adopt more quiescence strategy, with only 5 of 13 of its traits showing a significant response. With shorter flooding duration, both species tended to have lower leaf N and P content, indicating greater nutrient limitation. Meanwhile, relative abundance of C. cinerascens with relatively higher belowground nutrient competition ability (i.e. greater root/shoot biomass ratio) decreased with flooding duration. These results suggest that belowground competition intensifies at shorter flooding duration, because of increased nutrient limitation with increased productivity. Therefore, our study indicates that both flooding stress and nutrient competition could affect plant functional traits along a flooding duration gradient.

Arctic Ocean biogeochemistry in the high resolution FESOM 1.4-REcoM2 model

In the rapidly changing Arctic Ocean, marine primary productivity is tightly linked to the balance between light and nutrient limitation. To capture this balance in ocean general circulation biogeochemical models (OGCBMs), a good representation of the physics is important due to the tight bio-physical coupling in the Arctic. Using a horizontal model resolution of a few kilometers makes it possible to resolve an increasing number of small scale processes, that otherwise need to be parameterized in OGCBMs. Such high resolution is, however, commonly not possible due to computational constrains. Utilizing an unstructured mesh approach, we have run the finite element sea-ice ocean model (FESOM 1.4) coupled to the biogeochemical model REcoM2 in a global configuration with an Arctic-wide resolution of 4.5 km. This resolution is so far unprecedented for a global biogeochemical setup, and here we present an analysis of the mean state of the model. FESOM-REcoM2’s integrated Arctic net primary production (NPP) averages 445 Tg C yr−1 for the years 2011 to 2015, a value that is in the middle of the range compared to estimates from the literature. Most production takes place in the inflow regions of the Nordic and Chukchi Seas, and 32% is associated with the sea ice zone, the latter including the marginal ice zone and below-ice productivity. Light limits production to some degree at all latitudes north of 60°N, with growth becoming nutrient limited following the initial spring bloom in most places. The model reproduces the relatively low surface concentration of dissolved inorganic nitrogen (DIN) that has been observed in the central Arctic Ocean, as well as the low surface DIN concentration towards the end of the growth season further to the south, thereby also capturing widespread subsurface chlorophyll maxima (SCM). The SCMs are found in the whole Arctic except for the areas where sea ice concentration is high the whole year. They have a duration of two weeks to five months. The balance between nutrient and light limitations, both in the vertical and horizontal direction, highlights that decreased light limitation in a future ice free Arctic Ocean will not necessarily induce an increase in NPP due to increasing nutrient limitation, and that further studies of the role of the SCMs are required.

Productivity and rainfall drive bacterial metabolism in tropical cascading reservoirs

Tropical reservoirs are main carbon sources to the atmosphere, and bacterial metabolism is a key process in these emissions. Here, we explored the drivers of bacterial metabolism in four tropical cascading reservoirs forming a trophic state gradient, and compared them with those found in the literature (mainly from temperate regions). Bacterial production (BP) and growth efficiency (BGE) responded to trophic state-related variables, while bacterial respiration (BR) was weakly and negatively correlated to dissolved organic carbon (DOC). BP and BGE were higher in reservoirs with higher primary production, while BR (high throughout the whole study period) was greater in less productive reservoirs, where planktonic communities were often limited by phosphorus. The high BR and low BGE observed in less productive downstream reservoirs (i.e., less nutrients and organic matter availability) may be explained by increasing nutrient limitation and proportion of recalcitrant DOC along the cascade. Despite the lower productivity, oligotrophic reservoirs may be more important in terms of carbon biogeochemistry, considering that microbes in those systems mineralize more carbon than upstream productive reservoirs. Moreover, the drivers of bacterial metabolism may act differently according to latitude, as seasonality in the tropics is determined mainly by rainfall rather than temperature.

The effects of stress intensity and stress type on inbreeding depression in Silene vulgaris.

Inbreeding depression (ID) is generally assumed to increase under stressful conditions, but a number of studies have found the opposite pattern, that is that crossed offspring were more capable of exploiting benign conditions. Alternatively, the phenotypic variation hypothesis predicts that not stress intensity, but enhanced phenotypic variation in an environment leads to increased ID. We subjected inbred and crossed offspring of Silene vulgaris to drought, simulated herbivory, copper contamination, and two levels of nutrient deficiency and shade. In contrast to the predominant expectation, most stress treatments decreased inbreeding depression. With increasing nutrient limitation, ID decreased strongly, whereas under increasing shade ID did not change. These differences may be due to purging in the population of origin where conditions are nutrient-poor and dry, but not shaded. In contrast to the greenhouse experiment, ID was higher in a field site than in a more benign common garden. However, the predictions of the phenotypic variation hypothesis were met in both the greenhouse and the field versus garden experiment. The results suggest that there may be no general relationship between ID and stress intensity, but specific effects of stress type and the novelty and variability of the environment.

Spatiotemporal proliferation of human stromal cells adjusts to nutrient availability and leads to stanniocalcin-1 expression in vitro and in vivo

Cells and tissues are intrinsically adapted to molecular gradients and use them to maintain or change their activity. The effect of such gradients is particularly important for cell populations that have an intrinsic capacity to differentiate into multiple cell lineages, such as bone marrow derived mesenchymal stromal cells (MSCs). Our results showed that nutrient gradients prompt the spatiotemporal organization of MSCs in 3D culture. Cells adapted to their 3D environment without significant cell death or cell differentiation. Kinetics data and whole-genome gene expression analysis suggest that a low proliferation activity phenotype predominates in stromal cells cultured in 3D, likely due to increasing nutrient limitation. These differences implied that despite similar surface areas available for cell attachment, higher cell concentrations in 3D reduced MSCs proliferation, while activating hypoxia related-pathways. To further understand the in vivo effects of both proliferation and cell concentrations, we increased cell concentrations in small (1.8 μl) implantable wells. We found that MSCs accumulation and conditioning by nutrient competition in small volumes leads to an ideal threshold of cell-concentration for the induction of blood vessel formation, possibly signaled by the hypoxia-related stanniocalcin-1 gene.

Soil fertility induces coordinated responses of multiple independent functional traits

Summary A central goal of functional ecology is to determine how independent functional traits are selectively filtered by environmental conditions to improve our understanding of the mechanisms of community assembly. Soil fertility clearly influences community composition, but it is unclear which plant functional traits are most strongly associated with gradients of increasing nutrient limitation. We hypothesized that leaf economic traits and stem tissue density would be strongly associated with soil fertility given their direct relationship to soil resource acquisition and use. In contrast, we hypothesized that functional traits that are commonly associated with competition for light (maximum height), shade tolerance (seed mass) and resistance to disturbance (bark thickness) would be unrelated to soil fertility. We measured 13 functional traits from 30 tree species occurring in 40 plots across a soil fertility gradient in a mature warm temperate rain forest in Northland, New Zealand. Principal component analysis was used to assess the dimensionality and independence of the functional traits and the soil properties, and regression was used to determine the relationships between community‐weighted mean traits and the soil fertility gradient. We observed a coordinated response of multiple independent traits to soil fertility. Consistent with our hypothesis, species associated with low‐fertility soils had comparatively ‘slower’ leaves (i.e. low SLA and leaf N and P, and high LDMC and thickness) and higher stem tissue density than species associated with high‐fertility soils. Unexpectedly, we observed that species associated with low‐fertility soils had larger maximum heights, thicker bark and lower seed mass. Tall trees can persist on poor soils. Thick bark may be a defensive strategy for trees growing in resource‐limited sites and large‐seeded shade‐tolerant species can persist in fertile soils where light is more limiting. Synthesis. Species sorting can occur over short distances in ecosystems where topographically driven variation in soil fertility leads to complete compositional turnover. Inferences about species distributions based on single‐trait spectrums can be misleading when environmental gradients sort species by filtering multiple independent traits simultaneously. Identifying the multidimensional trait combinations that promote fitness will advance both theory development and ecological restoration.

Ciliates, microbes and nutrients: interactions in the seasonally mixed Gulf of Aqaba

Experiments were conducted in the Gulf of Aqaba to investigate top-down and bottom-up influences on microbial food webs. Experiments were run during winter mixing, spring bloom and summer stratification, periods with increasing nutrient limitation and with picophytoplankton (.98% of total year-round biomass) dominated by different species. Size fractionation (,64, ,6 and ,0.8 mm) was cross-classified with or without the addition of nutrients (0.3 mM NH4 þ and 0.19 mM PO4, equivalent to winter concentrations), using the same design for the six experiments. All experiments showed minimal top-down and strong bottom-up control, independent of season. Among phototrophs, small eukaryotic algae responded more strongly to nutrient addition within the 24-h experiment duration than did Synechococcus or Prochlorococcus. Ciliates responded most strongly to nutrient addition, becoming markedly more abundant in treatments with added nutrients relative to those without; a trend seen to a more limited degree in ciliate biomass, as smaller species responded faster. Treatments with added nutrients were also more dissimilar to one another than those without added nutrients: which species benefitted from added N and P appeared random. This could explain the high ciliate species diversity in the Gulf of Aqaba, if small nutrient pulses are rapidly transferred to different random collections of ciliate species in the vicinity of the pulse.

Organic nutrient uptake by mycorrhizal fungi enhances ecosystem carbon storage: a model-based assessment

Understanding the factors that drive soil carbon (C) accumulation is of fundamental importance given their potential to mitigate climate change. Much research has focused on the relationship between plant traits and C sequestration, but no studies to date have quantitatively considered traits of their mycorrhizal symbionts. Here, we use a modelling approach to assess the contribution of an important mycorrhizal fungal trait, organic nutrient uptake, to soil C accumulation. We show that organic nutrient uptake can significantly increase soil C storage, and that it has a greater effect under nutrient-limited conditions. The main mechanism behind this was an increase in plant C fixation and subsequent increased C inputs to soil through mycorrhizal fungi. Reduced decomposition due to increased nutrient limitation of saprotrophs also played a role. Our results indicate that direct uptake of nutrients from organic pools by mycorrhizal fungi could have a significant effect on ecosystem C cycling and storage.

Biomass and productivity of fine and coarse roots in five tropical mountain forests stands along an altitudinal transect in southern Ecuador

Background: Data on below-ground production of tropical montane forests along elevation gradients are scarce. Aims: To determine fine, coarse and large root biomass and productivity along a 2000 m elevation transect. Methods: In five south Ecuadorian mountain forests along a transect from 1000 to 3000 m above sea level, fine (< 2 mm diameter), coarse (2–50 mm) and large root biomass (> 50 mm) were analysed by soil coring and excavation of soil pits. Fine root production was estimated synchronously by three different approaches (sequential soil coring, the ingrowth core method, and the mini-rhizotron technique). Coarse and large root production was estimated by recording diameter increment using dendrometer tapes. Results: Fine root biomass increased four-fold between 1000 and 3000 m; coarse and large root biomass doubled. The three approaches for estimating fine root production yielded highly divergent results, with the mini-rhizotron approach giving the most reliable data, and indicating a significant increase in fine root production with elevation. Conclusions: Our results indicate a marked carbon allocation shift from above- to below-ground towards higher elevations, which is probably a consequence of increasing nutrient limitation of tree growth with increasing elevation.

Light Absorption Components in Onondaga Lake, New York, U.S.A.

Features of light absorption are critical to optical aspects of water quality and in regulating the signal available in the emergent flux from the surface. Spectral characteristics, dynamics, and relationships with optically active constituents and common optical metrics, are documented for three light absorbing components in culturally eutrophic Onondaga Lake, New York, U.S.A., based on the results of a 5 year study. The absorption coefficient, a(m -1 ), is partitioned according to the additive components of colored dissolved organic matter (a CDOM ), non-algal particles (a NAP ), phytoplankton (a φ ), and water itself (a w ; known), based on laboratory measurements for near surface samples collected weekly for the spring to fall interval over the 2004-2008 period. Supporting bulk measurements included dissolved organic carbon, total suspended solids (TSS) and its organic and inorganic (FSS) fractions, the concentrations of total phosphorus and chlorophyll-a ([Chl]), downwelling irradiance profiles to determine the diffuse attenuation coefficient (K d ), the beam attenuation coefficient at 660 nm [c(660)], and Secchi disc depth. The average exponential slope values for a CDOM (0.0168 nm -1 ) and a NAP (0.0126 nm -1 ) were very similar to those reported for other case 2 systems. The shapes of a φ spectra, represented by the ratio a φ (440): a φ (676), were found to negatively depend on [Chl]. The largest component of a at 440 nm, the reference wavelength, in each of the 5 years was a CDOM (∼45 to 60 %), the smallest was a NAP (∼15 to 25 %). Progressive decreases in yearly average a φ , particularly over the last three years are documented in response to decreases in [Chl], driven by increased nutrient limitation and Daphnia grazing (2008 only). Wide short-term variations are demonstrated within years for the components of a, particularly in response to the timing of runoff and Daphnia grazing events. Strong positive relationships are reported between a NAP and both TSS and FSS, a φ (at both λ = 440 and 676 nm) and [Chl], and a p (a NAP plus a φ ) and both TSS and c(660). The summation of the absorbing components at 440 nm is demonstrated to be a strong predictor of K d . The utility of the information in supporting a model for K d and SD and advancing monitoring of water quality through measurements of surface reflectance is described.

Effect of ecosystem retrogression on stable nitrogen and carbon isotopes of plants, soils and consumer organisms in boreal forest islands

In the prolonged absence of catastrophic disturbance, ecosystem retrogression occurs, and this involves increased nutrient limitation, and reduced aboveground and belowground ecosystem processes rates. Little is known about how the nitrogen and carbon stable isotope ratios (delta(15)N and delta(13)C) of plants, soils and consumer organisms respond to retrogression in boreal forests. We investigated a 5000 year chronosequence of forested islands in the boreal zone of northern Sweden, for which the time since lightning-induced wildfire increases with decreasing island size, leading to ecosystem retrogression. For this system, tissue delta(15)N of three abundant plant species (Betula pubescens, Vaccinium myrtillus and Pleurozium schreberi) and humus all increased as retrogression proceeded. This is probably due to enhanced ecosystem inputs of N by biological fixation, and greater dependency of the plants on organic N during retrogression. The delta(13)C of B. pubescens and plant-derived humus also increased during retrogression, probably through nutrient limitation increasing plant physiological stress. Unlike the plants, delta(15)N of invertebrates (lycosid spiders and ants) did not increase during retrogression, probably because of their partial dependence on aquatic-derived prey that had a variable delta(15)N signature. The delta(13)C of the invertebrates increased as retrogression proceeded and converged towards that of an aquatic prey source (chironomid flies), suggesting increased dependence on aquatic-derived prey during retrogression. These results show that measurement of delta(15)N and delta(13)C of plants, soils, and consumers across the same environmental gradient can provide insights into environmental factors that drive both the aboveground and belowground subsystems, as well as the linkages between them.