Phosphorus Availability Alters the Effects of Silver Nanoparticles on Periphyton Growth and Stoichiometry

Exposure to silver nanoparticles (AgNPs) may alter the structure and function of freshwater ecosystems. However, there remains a paucity of studies investigating the effects of AgNP exposure on freshwater communities in the natural environment where interactions with the ambient environment may modify AgNP toxicity. We used nutrient diffusing substrates to determine the interactive effects of AgNP exposure and phosphorus (P) enrichment on natural assemblages of periphyton in three Canadian Shield lakes. The lakes were all phosphorus poor and spanned a gradient of dissolved organic carbon availability. Ag slowly accumulated in the exposed periphyton, which decreased periphyton carbon and chlorophyll a content and increased periphyton C:P and N:P in the carbon rich lakes. We found significant interactions between AgNP and P treatments on periphyton carbon, autotroph standing crop and periphyton stoichiometry in the carbon poor lake such that P enhanced the negative effects of AgNPs on chlorophyll a and lessened the impact of AgNP exposure on periphyton stoichiometry. Our results contrast with those of other studies demonstrating that P addition decreases metal toxicity for phytoplankton, suggesting that benthic and pelagic primary producers may react differently to AgNP exposure and highlighting the importance of in situ assays when assessing potential effects of AgNPs in fresh waters.     

Genotypic differences in phosphorus use physiology in producers (<Emphasis Type="Italic">Chlamydomonas reinhardtii</Emphasis>) and consumers (<Emphasis Type="Italic">Daphnia pulex</Emphasis>) interact to alter primary and secondary production

By considering the relative abundance of elements in trophic interactions, ecological stoichiometry makes predictions about key ecological processes such as biomass production and consumer-driven nutrient recycling. Theoretical and empirical work has focused on interspecific variation in elemental composition, and stoichiometric imbalances between resources and consumers in determining productivity, particularly at the base of foodwebs. Recent work has found considerable intraspecific variation in elemental composition. We know little about the ecological relevance of such variation, and whether predictions of stoichiometric theory hold at the intraspecific level. Here, we used two genotypes of a primary producer Chlamydomonas reinhardtii, and two genotypes of a primary consumer Daphnia pulex, which are already known to vary considerably in their phosphorus (P) use physiology, under conditions of P abundance and limitation, to explore whether such intraspecific differences alter primary as well as secondary production. Specifically, we tested whether there are intraspecific differences in the carbon: phosphorus (C:P) stoichiometry of Chlamydomonas genotypes, whether such differences affect growth and abundance of the two Daphnia genotypes, and whether the two Daphnia genotypes had distinct effects on primary production and growth of the two Chlamydomonas genotypes. We found significant differences in C:P stoichiometry between the two Chlamydomonas genotypes in both P supply conditions. Such intraspecific differences altered the growth of Daphnia genotypes, and affected the outcome of genotypic competition. Finally, Daphnia genotype affected primary production, and interacted with P supply to distinctly affect the growth of the two Chlamydomonas genotypes. Together, our results highlight the potential ecological relevance of intraspecific differences in nutrient use physiology and elemental composition, and the utility of ecological stoichiometry in understanding such consequences.     

Elemental stoichiometry of the key calcifying marine phytoplankton Emiliania huxleyi under ocean climate change: A meta-analysis

The elemental composition of marine microorganisms (their C:N:P ratio, or stoichiometry) is central to understanding the biotic and biogeochemical processes underlying key marine ecosystem functions. Phytoplankton C:N:P is species specific and flexible to changing environmental conditions. However, bulk or fixed phytoplankton stoichiometry is usually assumed in biogeochemical and ecological models because more realistic, environmentally responsive C:N:P ratios have yet to be defined for key functional groups. Here, a comprehensive meta-analysis of experimental laboratory data reveals the variable C:N:P stoichiometry of Emiliania huxleyi, a globally significant calcifying phytoplankton species. Mean C:N:P of E. huxleyi is 124C:16N:1P under control conditions (i.e. growth not limited by one or more environmental stressors) and shows a range of responses to changes in nutrient and light availability, temperature and pCO₂. Macronutrient limitation caused strong shifts in stoichiometry, increasing N:P and C:P under P deficiency (by 305% and 493% respectively) and doubling C:N under N deficiency. Responses to light, temperature and pCO₂ were mixed but typically shifted cellular elemental content and C:N:P stoichiometry by ca. 30% or less. Besides these independent effects, the interactive effects of multiple environmental changes on E. huxleyi stoichiometry under future ocean conditions could be additive, synergistic or antagonistic. To synthesise our meta-analysis results, we explored how the cellular elemental content and C:N:P stoichiometry of E. huxleyi may respond to two hypothetical future ocean scenarios (increased temperature, irradiance and pCO₂ combined with either N deficiency or P deficiency) if an additive effect is assumed. Both future scenarios indicate decreased calcification (which is predominantly sensitive to elevated pCO₂), increased C:N, and up to fourfold shifts in C:P and N:P. Our results strongly suggest that climate change will significantly alter the role of E. huxleyi (and potentially other calcifying phytoplankton species) in marine biogeochemical processes.     

Ecological stoichiometry of Eurasian perch – intraspecific variation due to size,habitat and diet

The turnover and distribution of energy and nutrients in food webs is influenced by consumer stoichiometry. Although the stoichiometry of heterotrophs is generally considered to vary only little, there may be intraspecific variation due to factors such as habitat, resources, ontogeny and size. We examined intraspecific variation in Eurasian perch Perca fluviatilis stoichiometry, a common species that exhibits habitat and resource specialization, ontogenetic niche shifts and a large size range. This study investigated the elemental stoichiometry of a wide size range of perch from littoral and pelagic habitats. The mean C:N:P stoichiometry of whole perch was 37:9:1 (molar ratios). However, %C, %P, C:N, C:P and N:P varied with size, morphology, habitat and diet category. These factors together explained 24–40% of the variation in C:N:P stoichiometry. In contrast, perch stoichiometry was not related to diet stoichiometry, suggesting that the former is homeostatically regulated. The results suggest that the high P content of perch may result in stoichiometric constraints on the growth of non‐piscivorous perch, and that piscivory is an efficient strategy for acquiring P. Resource polymorphism, individual diet specialization and intraspecific size variation are widespread among animals. Thus changes in stoichiometry with size, habitat, morphology and resource use, and therefore also stoichiometric demands, are probably common.     

Parasite and host elemental content and parasite effects on host nutrient excretion and metabolic rate

Ecological stoichiometry uses the mass balance of elements to predict energy and elemental fluxes across different levels of ecological organization. A specific prediction of ecological stoichiometry is the growth rate hypothesis (GRH), which states that organisms with faster growth or reproductive rates will require higher phosphorus content for nucleic acid and protein synthesis. Although parasites are found ubiquitously throughout ecosystems, little is understood about how they affect nutrient imbalances in ecosystems. We (1) tested the GRH by determining the carbon (C), nitrogen (N), and phosphorus (P) content of parasitic trematodes and their intermediate host, the freshwater snail Elimia livescens, and (2) used this framework to determine the trematode effects on host nutrient excretion and metabolism. Snail and parasite tissues were analyzed for elemental content using a CHN analyzer and soluble reactive phosphorus (SRP) methods. Ammonium and SRP assays were used to estimate N and P excretion rates. A respirometer was used to calculate individual snail metabolism. Trematode tissues contained lower C:P and N:P (more P per unit C and N) than the snail tissues. Snail gonadal tissues more closely resembled the elemental content of parasite tissues, although P content was 13% higher in the gonad than the trematode tissues. Despite differences in elemental content, N and P excretion rates of snails were not affected by the presence of parasites. Parasitized snails maintained faster metabolic rates than nonparasitized snails. However, the species of parasite did not affect metabolic rate. Together, this elemental imbalance between parasite and host, and the altered metabolic rate of infected snails may lead to broader parasite effects in stream ecosystems.     

Physiological indicators of nutrient deficiency in phytoplankton in southern Chilean lakes

We assessed the nutrient status of phytoplankton in 28 lakes in southern Chile using two types of physiological indicators: specific alkaline phosphatase activity, and the elemental composition (carbon, nitrogen, and phosphorus) of seston. Alkaline phosphatase activity ranged from 0.001 to 0.11 mol P g chl^–1 h^–1, with P-deficiency indicated in about one-half the study lakes. C:N ranged from 3.9 to 24, C:P ranged from 86 to 919, and N:P ranged from 8.7 to 99. C:P and N:P ratios greater than the Redfield ratio were common, suggesting P deficiency in many of the lakes. C:N ratios were not generally indicative of N deficiency. Previous studies have suggested N may be the primary limiting nutrient in southern Chilean lakes, but our results indicate that P should not be discounted as a limiting nutrient.     

EFFECT OF NUTRIENT AVAILABILITY ON THE BIOCHEMICAL AND ELEMENTAL STOICHIOMETRY IN THE FRESHWATER DIATOM STEPHANODISCUS MINUTULUS (BACILLARIOPHYCEAE)*

The objective of this study was to examine the differences in the biochemical and elemental stoichiometry of a freshwater centric diatom, Stephanodiscus minutulus (Grun.), under various nutrient regimes. Stephanodiscus minutulus was grown at μ_max or 22% of μ_max under limitation by silicon, nitrogen, or phosphorus. Cell sizes for nutrient‐limited cultures were significantly smaller than the non‐limited cell sizes, with N‐limited cells being significantly smaller than all other treatments. Compared with the nutrient‐replete treatment, both carbohydrates and lipids increased in Si‐ and P‐limited cells, whereas carbohydrates increased but proteins decreased in N‐limited cells. All of the growth‐limited cells showed an increase of carbohydrate and triglyceride, and a decrease of cell size and polar lipids as a percentage of total lipids. The non‐limited cells also had a significantly higher chl a concentration and galactolipids as a percentage of total lipids than any of the limited treatments, and the low‐Si and low‐P cells had significantly higher values than the low‐N cells. The particulate C concentrations showed significant differences between treatments, with the Si‐ and P‐limited treatments being significantly higher than the N‐ and non‐limited treatments. Particulate Si did not show a strong relationship with any of the parameters measured, and it was the only parameter with no differences between treatments. The low‐Si cells had a significantly higher P content (about two times more) than any other treatment, presumably owing to the luxury consumption of P, and a correspondingly high phospholipid concentration. The elemental data showed that S. minutulus had a high P demand with low optimum N:P (4) and Si:P (10) ratios and a C:N:P ratio of 109:16:2.3. The particulate C showed a positive relationship with POM (r = 0.93), dry weight (r = 0.88), lipid (r = 0.87) and protein (r = 0.84, all P < 0.0001). Particulate N showed a positive relationship with galactolipids (r = 0.95), protein (r = 0.90), dry weight (r = 0.78), lipid (r = 0.75), and cell volume (r = 0.64, all P < 0.0001). It is evident that nutrient limitation in the freshwater diatom S. minutulus has pronounced effects on its biochemical and elemental stoichiometry.     

Allometric and trait‐based patterns in parasite stoichiometry

We measured the elemental content (%C, N and P) and ratios (C:N, C:P, N:P) of a diverse assemblage of parasitic helminths to ask whether taxonomy or traits were related to stoichiometric variation among species. We sampled 27 macroparasite taxa, spanning four phyla, infecting vertebrate and invertebrate hosts from freshwater ecosystems in New Jersey. Macroparasites varied widely in elemental content, exhibiting 4.7‐fold variation in %N, 4.6‐fold variation in %P, and 11.5‐fold variation in N:P. Across all species, parasite %P scaled negatively and C:P scaled positively with body size. Similar relationships between parasite P content and body size occurred at the phylum level and within individual species. The allometric scaling of P across species supports the growth rate hypothesis, which predicts that smaller taxa require more P to support relatively higher growth rates. Life cycle stage was related to %N and C:N, with non‐reproductive parasite stages lower in %N and higher in C:N than actively reproducing parasites. Parasite phylum, functional feeding group, and trophic level did not explain elemental variation among species. Organismal stoichiometry is linked to ecological function, and wide variation in macroparasite stoichiometry likely generates diverse patterns in host–parasite nutrient dynamics and variable relationships between parasitism and nutrient cycling.     

Carbon,nitrogen and phosphorus ecological stoichiometry of Lateolabrax macultus and Acanthogobius ommaturus in the Estuary of Yangtze River,China

Most stoichiometric models do not consider the importance of ontogenetic changes in body nutrient composition. Among aquatic animals, body elemental ontogenetic changes may be particularly important in fish due to bone formation at larval–juvenile development stage. We investigated the stoichiometric characteristics and ontogenetic variation of carbon (C), nitrogen (N), phosphorous (P) and calcium (Ca) in the whole fish without gut of Lateolabrax macultus and Acanthogobius ommaturus from the Yangtze Estuary. Some differences were found in the whole-fish nutrient content across species as well as fish length and mass groups. Mean body content of L. macultus was 41.2% for C (34.9–44.9%), 11.8% for N (10.3–12.9%), and 3.3% for P (1.4–5.4%) and the molar ratio of C:N:P was 32:8:1. A. ommaturus had the following mean compositions: carbon, 43.4% (40.0–45.9%); nitrogen, 13.2% (11.7–14.1%); phosphorus, 2.9% (1.6–6.0%) and molar ratio of C:N:P was 38:10:1. The variation of C content in of L. macultus was minimum (0.05), followed by N (0.06), and the variation of P content was the highest (0.22). The variation of C content in A. ommaturus was minimum (0.03), followed by N (0.04), and the variation of P content was also the highest (0.27). Both species varied the least in C, moderate in N and the most in P. Comparing the two species, A. ommaturus was significantly higher for C, N, C:P, N:P and significantly lower for P and C:N than L. macultus. The positive correlation between C and N contents was significant (P < 0.01) for both species. In contrast, C and P concentrations showed little correlation (P > 0.05). The contents of C, N in L. macultus showed significant negative correlation with total length and wet mass, respectively, while C:N ratio showed significant positive correlation with them. As for A. ommaturus, the content of C and the ratio of C:N showed significant negative correlation with total length and wet mass, respectively. The two estuarine fishes in this study mainly consumed fish, shrimps and crabs, which could be the reason that the stoichiometry of L. macultus and A. ommaturus are higher than other freshwater fishes that have been reported previously. In both species, the content of calcium was highly correlated with the content of body P, which indicated that the change of the content of P is associated with bone formation in ontogeny. Fish shape, scale type, the ratio of head length/standard length and the degree of ossification of the internal skeleton and outer integument might have caused the results that L. macultus had higher contents of P and Ca than A. ommaturus.     

Endogenous versus exogenous nutrient affects C, N, and P dynamics in decomposing litters in mid-subtropical forests of China

The effect of nutrient availability on litter decomposition has been a major focus of global change ecology. The relative impacts of endogenous (litter) and exogenous (soil) nutrient availability remain unclear. We studied the nutrient dynamics of decomposition in litter from two species with contrasting litter nutrient contents and stoichiometry: Pinus massoniana and Castanopsis sclerophylla. During a 540-day field incubation, we manipulated exogenous nutrient levels by adding microbially available C (+C), N (+N), P (+P), and all three (+CNP) at 90-day intervals. Relative to the no-nutrient control (CK), nutrient additions decreased organic C retention in C. sclerophylla, with the greatest effect observed in +CNP. Nitrogen content in P. massoniana litter similarly increased with nutrient addition, particularly +P and +CNP. The P addition treatments also increased P content in the litter of both species. Nitrogen content in C. sclerophylla and organic C content in P. massoniana were unaffected by nutrient additions. The C/N and C/P ratios in decomposing C. sclerophylla litter were significantly lower in the CK treatment, while those of P. massoniana litter were influenced by the interaction of nutrient addition and decomposition time. Increased availability of C, N, and P individually and collectively alters nutrient release dynamics in decomposing foliar litter. Litter quality, as determined by source species, is a key determinant of the impact of exogenous nutrient inputs. A stronger effect of P addition than N addition indicates a relatively N-rich and P-poor ecosystem.     

Organismal stoichiometry at the temporal scale: Seasonal variability shapes interspecific differences in fish

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Plant diversity effects on pollinating and herbivorous insects can be linked to plant stoichiometry

Changes in plant diversity have consequences for higher trophic levels, e.g., higher plant diversity can enhance the reproduction and fitness of plant-associated insects. This response of higher trophic levels potentially depends on diversity-related changes in both resource quantity (abundance) and quality (nutritional content). The availability of elemental nutrients in plant resources is one aspect of nutritional quality, but has rarely been addressed as a pathway relating plant diversity to associated insects. Using the experimental plant diversity gradient of a large biodiversity grassland project, the Jena-Experiment, we analysed the %C, %N and %P and the molar ratios of those elements (C:N, C:P and N:P) in a pollinating bee, Chelostoma distinctum, and an herbivorous grasshopper, Chorthippus parallelus, reared on plots of different plant diversity. Insects showed higher content of C, N and P (% dry mass), and lower C:N and C:P ratios than plants. C:N ratios were significantly higher in grasshoppers than in bees and higher in females than in males of both species. Increasing plant species richness increased the C:N ratio of male bees and female grasshoppers. In both groups, stoichiometry was positively related to plant stoichiometry (male bees: C:P and N:P; grasshoppers: C:N and N:P). Path analysis revealed that diversity-driven changes in plant elemental composition can have consequences for abundance and chemical composition of higher trophic levels, with different responses of the two functional groups.     

Nutrient Dynamics of Estuarine Invertebrates Are Shaped by Feeding Guild Rather than Seasonal River Flow

This study aimed to determine the variability of carbon and nitrogen elemental content, stoichiometry and diet proportions of invertebrates in two sub-tropical estuaries in South Africa experiencing seasonal changes in rainfall and river inflow. The elemental ratios and stable isotopes of abiotic sources, zooplankton and macrozoobenthos taxa were analyzed over a dry/wet seasonal cycle. Nutrient content (C, N) and stoichiometry of suspended particulate matter exhibited significant spatio-temporal variations in both estuaries, which were explained by the variability in river inflow. Sediment particulate matter (%C, %N and C:N) was also influenced by the variability in river flow but to a lesser extent. The nutrient content and ratios of the analyzed invertebrates did not significantly vary among seasons with the exception of the copepod Pseudodiaptomus spp. (C:N) and the tanaid Apseudes digitalis (%N, C:N). These changes did not track the seasonal variations of the suspended or sediment particulate matter. Our results suggest that invertebrates managed to maintain their stoichiometry independent of the seasonality in river flow. A significant variability in nitrogen content among estuarine invertebrates was recorded, with highest % N recorded from predators and lowest %N from detritivores. Due to the otherwise general lack of seasonal differences in elemental content and stoichiometry, feeding guild was a major factor shaping the nutrient dynamics of the estuarine invertebrates. The nutrient richer suspended particulate matter was the preferred food source over sediment particulate matter for most invertebrate consumers in many, but not all seasons. The most distinct preference for suspended POM as a food source was apparent from the temporarily open/closed system after the estuary had breached, highlighting the importance of river flow as a driver of invertebrate nutrient dynamics under extreme events conditions. Moreover, our data showed that estuarine invertebrates concentrated C and N between 10–100 fold from trophic level I (POM) to trophic level II (detritivores/deposit feeders) and thus highlighted their importance not only as links to higher trophic level organisms in the food web, but also as providers of a stoichiometrically homeostatic food source for such consumers. As climate change scenarios for the east coast of South Africa predict increased rainfall as a higher number of rainy days and days with higher rainfall, our results suggest that future changes in rainfall and river inflow will have measurable effects on the nutrient content and stoichiometry of food sources and possibly also in estuarine consumers.     

Detritivory and the stoichiometry of nutrient cycling by a dominant fish species in lakes of varying productivity

Little is known about the stoichiometry of nutrient cycling by detritivores. Therefore, we explored stoichiometric relationships in an omnivorous/detritivorous fish (gizzard shad, Dorosoma cepedianum) in three lakes that differed in productivity. Gizzard shad can feed on plankton and sediment detritus, but in all three lakes adult gizzard shad derived >98% of carbon (C) and phosphorus (P), and >90% of nitrogen (N) from sediment detritus, and the remainder from zooplankton.
Gizzard shad selectively consumed detritus with higher C, N and P concentrations than ambient lake sediments. Selective detritivory (i.e. the nutrient content of consumed detritus divided by the nutrient content of ambient detritus) was most pronounced in the lake with the lowest detrital nutrient concentrations. N and P cycling rates per fish were also consistently higher in this lake, in agreement with the prediction of stoichiometry theory that excretion rates should increase with food nutrient content. Among-lake differences in nutrient cycling rates were unrelated to inter-lake variation in fish body nutrient contents, which was minimal. The N:P ratio excreted was near Redfield (∼14:1) in all three lakes.
Stoichiometric analyses showed that the C:N and C:P ratios of sediment detritus were much higher (∼2.8×) than ratios of gizzard shad bodies, revealing substantial N and P imbalances between consumers and their food source. Gizzard shad alleviate N imbalance by selectively feeding on high N detritus (low C:N, high N:P), and apparently alleviate P imbalance by excreting nutrients at a higher N:P than that of their food or their bodies. Thus, this detritivore apparently regulates nutrient acquisition and allocation via both pre-absorption processes (selective feeding) and post-absorptive processes (differential N and P excretion).     

Effects of increased temperature and nutrient enrichment on the stoichiometry of primary producers and consumers in temperate shallow lakes

1. We studied the effects of increased water temperatures (0–4.5 °C) and nutrient enrichment on the stoichiometric composition of different primary producers (macrophytes, epiphytes, seston and sediment biofilm) and invertebrate consumers in 24 mesocosm ecosystems created to mimic shallow pond environments. The nutrient ratios of primary producers were used as indicative of relative nitrogen (N) or phosphorus (P) limitation. We further used carbon stable isotopic composition (δ¹³C) of the different primary producers to elucidate differences in the degree of CO₂ limitation. 2. Epiphytes were the only primary producer with significantly higher δ¹³C in the enriched mesocosms. No temperature effects were observed in δ¹³C composition of any primary producer. Independently of the treatment effects, the four primary producers had different δ¹³C signatures indicative of differences in CO₂ limitation. Seston had signatures indicating negligible or low CO₂ limitation, followed by epiphytes and sediment biofilm, with moderate CO₂ limitation, while macrophytes showed the strongest CO₂ limitation. CO₂ together with biomass of epiphytes were the key variables explaining between 50 and 70% of the variability in δ¹³C of the different primary producers, suggesting that epiphytes play an important role in carbon flow of temperate shallow lakes. 3. The ratio of carbon to chlorophyll a decreased with increasing temperature and enrichment in both epiphytes and seston. The effects of temperature were mainly attributed to changes in algal Chl a content, while the decrease with enrichment was probably a result of a higher proportion of algae in the seston and epiphytes. 4. Macrophytes, epiphytes and seston decreased their C : N with enrichment, probably as an adaptation to the different N availability levels. The C : N of epiphytes and Elodea canadensis decreased with increasing temperature in the control mesocosms. Sediment biofilm was the only primary producer with lower C : P and N : P with enrichment, probably as a result of higher P accumulation in the sediment. 5. Independently of nutrient level and increased temperature effects the four primary producers had significantly different stoichiometric compositions. Macrophytes had higher C : N and C : P and, together with epiphytes, also the highest N : P. Seston had no N or P limitation, while macrophytes and epiphytes may have been P limited in a few mesocosms. Sediment biofilm indicated strong N deficiency. 6. Consumers had strongly homeostatic stoichiometric compositions in comparison to primary producers, with weak or no significant treatment effects in any of the groups (insects, leeches, molluscs and crustaceans). Among consumers, predators had significantly higher N content and lower C : N than grazers.     

Bromeliad growth and stoichiometry: responses to atmospheric nutrient supply in fog-dependent ecosystems of the hyper-arid Atacama Desert, Chile

Carbon, nitrogen, and phosphorus (C, N, P) stoichiometry influences the growth of plants and nutrient cycling within ecosystems. Indeed, elemental ratios are used as an index for functional differences between plants and their responses to natural or anthropogenic variations in nutrient supply. We investigated the variation in growth and elemental content of the rootless terrestrial bromeliad Tillandsia landbeckii, which obtains its moisture, and likely its nutrients, from coastal fogs in the Atacama Desert. We assessed (1) how fog nutrient supply influences plant growth and stoichiometry and (2) the response of plant growth and stoichiometry to variations in nutrient supply by using reciprocal transplants. We hypothesized that T. landbeckii should exhibit physiological and biochemical plastic responses commensurate with nutrient supply from atmospheric deposition. In the case of the Atacama Desert, nutrient supply from fog is variable over space and time, which suggests a relatively high variation in the growth and elemental content of atmospheric bromeliads. We found that the nutrient content of T. landbeckii showed high spatio-temporal variability, driven partially by fog nutrient deposition but also by plant growth rates. Reciprocal transplant experiments showed that transplanted individuals converged to similar nutrient content, growth rates, and leaf production of resident plants at each site, reflecting local nutrient availability. Although plant nutrient content did not exactly match the relative supply of N and P, our results suggest that atmospheric nutrient supply is a dominant driver of plant growth and stoichiometry. In fact, our results indicate that N uptake by T. landbeckii plants depends more on N supplied by fog, whereas P uptake is mainly regulated by within-plant nutrient demand for growth. Overall, these findings indicate that variation in fog nutrient supply exerts a strong control over growth and nutrient dynamics of atmospheric plants, which are ubiquitous across fog-dominated ecosystems.     

Warming and Nitrogen Addition Alter Photosynthetic Pigments,Sugars and Nutrients in a Temperate Meadow Ecosystem

Global warming and nitrogen (N) deposition have an important influence on terrestrial ecosystems; however, the influence of warming and N deposition on plant photosynthetic products and nutrient cycling in plants is not well understood. We examined the effects of 3 years of warming and N addition on the plant photosynthetic products, foliar chemistry and stoichiometric ratios of two dominant species, i.e., Leymus chinensis and Phragmites communis, in a temperate meadow in northeastern China. Warming significantly increased the chlorophyll content and soluble sugars in L. chinensis but had no impact on the carotenoid and fructose contents. N addition caused a significant increase in the carotenoid and fructose contents. Warming and N addition had little impact on the photosynthetic products of P. communis. Warming caused significant decreases in the N and phosphorus (P) concentrations and significantly increased the carbon (C):P and N:P ratios of L. chinensis, but not the C concentration or the C:N ratio. N addition significantly increased the N concentration, C:P and N:P ratios, but significantly reduced the C:N ratio of L. chinensis. Warming significantly increased P. communis C and P concentrations, and the C:N and C:P ratios, whereas N addition increased the C, N and P concentrations but had no impact on the stoichiometric variables. This study suggests that both warming and N addition have direct impacts on plant photosynthates and elemental stoichiometry, which may play a vital role in plant-mediated biogeochemical cycling in temperate meadow ecosystems.     

Periphyton nutrient status in a temperate stream with mixed land-uses: implications for watershed nitrogen storage

We sampled periphyton communities in a highly productive stream to characterize how longitudinal changes in watershed geology and land use affect periphyton nutrient status and elemental composition. Nutrient status was evaluated from measures of periphyton nutrient composition (carbon, nitrogen, and phosphorus), stable isotope signatures (δ¹⁵N and δ¹³C), and the response of periphyton to experimental enrichment with nitrogen. Biomass and nutrient content increased dramatically from the headwaters to downstream, while tissue nutrient ratios (C:P and C:N) were more consistent and did not indicate strong N- or P-limitation. Nitrogen enrichment experiments did not exhibit a consistent response upstream or downstream, and periphyton C:N:P stoichiometry showed no significant response to N-enrichment. Absolute densities of periphyton N were 5- to 90-fold greater than the overlying N concentrations in stream water (159- to 353-fold greater for P), and the δ¹⁵N signal indicates downstream enrichment from likely watershed sources (urban and agriculture land-use). These results suggest that periphyton in Spring Creek are not N-limited and store large quantities of both N and P, which in turn can be transported downstream during high flow events. Handling editor: David Hamilton     

Exploring patterns and mechanisms of interspecific and intraspecific variation in body elemental composition of desert consumers

Key processes such as trophic interactions and nutrient cycling are often influenced by the element content of organisms. Previous analyses have led to some preliminary understanding of the relative importance of evolutionary and ecological factors determining animal stoichiometry. However, to date, the patterns and underlying mechanisms of consumer stoichiometry at interspecific and intraspecific levels within natural ecosystems remain poorly investigated. Here, we examine the association between phylogeny, trophic level, body size, and ontogeny and the elemental composition of 22 arthropod as well as two lizard species from the coastal zone of the Atacama Desert in Chile. We found that, in general, whole‐body P content was more variable than body N content both among and within species. Body P content showed a significant phylogenetic signal; however, phylogeny explained only 4% of the variation in body P content across arthropod species. We also found a significant association between trophic level and the element content of arthropods, with carnivores having 15% greater N and 70% greater P contents than herbivores. Elemental scaling relationships across species were only significant for body P content, and even the P content scaling relationship was not significant after controlling for phylogeny. P content did decrease significantly with body size within most arthropod species, which may reflect the size dependence of RNA content in invertebrates. In contrast, larger lizards had higher P contents and lower N:P ratios than smaller lizards, which may be explained by size‐associated differences in bone and scale investments. Our results suggests that structural differences in material allocation, trophic level and phylogeny can all contribute to variation in the stoichiometry of desert consumers, and they indicate that the elemental composition of animals can be useful information for identifying broad‐scale linkages between nutrient cycling and trophic interactions in terrestrial food webs.     

Nutrient limitation of phytoplankton production in Alaskan Arctic foothill lakes

We used 54 enrichment bioassays to assess nutrient limitation (N, P) of ¹⁴C uptake by natural phytoplankton assemblages in 39 lakes and ponds in the Arctic Foothills region of Alaska. Our purpose was to categorize phytoplankton nutrient status in this under-represented region of North America and to improve our ability to predict the response of primary production to anticipated anthropogenically mediated increases in nutrient loading. Experiments were performed across several watersheds and included assays on terminal lakes and lakes occupying various positions in chains (lakes in series within a watershed and connected by streams). In total, 89% (48 of 54) of the bioassays showed significant stimulation of ¹⁴C primary production by some form of nutrient addition relative to unamended controls. A significant response was observed following enrichment with N and P, N alone and P alone in 83, 35 and 22% of the bioassays, respectively. In experiments where N and P proved stimulatory, the influence of N alone was significantly greater than the influence of P alone. Overall, the data point to a greater importance for N than P in regulating phytoplankton production in this region. The degree of response to N and P enrichment declined as the summer progressed and showed no relationship to irradiance or water temperature, suggesting secondary limitation by some micronutrient such as iron as the summer advanced. Phytoplankton nutrient status was often consistent across lakes within a watershed, suggesting that watershed characteristics influence nutrient availability. Lakes in this region will clearly show increased phytoplankton production in response to anthropogenic activities and anticipated changes in climate that will increase nutrient loading.