Phosphorus‐rich grasshoppers consume plants high in nitrogen and phosphorus

1. Insect herbivores choose their food according to its protein to carbohydrate ratio, but the reasons why different species have contrasting intake targets remain unclear. According to the growth rate hypothesis, P‐rich insects have higher growth rates. It is therefore expected that P‐rich insects will prefer plants that are high in protein (hence in nitrogen, N) and phosphorus (P). 2. To test this hypothesis, the ecological stoichiometry and the interaction network frameworks were combined. The food preferences of 24 plant species by 23 grasshopper species were determined, and the N and P contents of both trophic levels were measured. The weighted mean P and N contents of the consumed plants, which represent the grasshoppers' feeding niche, were highly correlated, indicating that the grasshoppers' diets are spread along a single functional niche axis. The links between the stoichiometry of the plants and their consumers were then tested with the fourth corner analysis, a multivariate technique combining the plant traits, the insect traits and the interaction network. 3. In line with the earlier hypothesis, P‐rich grasshoppers consumed plants high in N and P, probably because their growth rate is higher. These findings therefore introduce a mechanism that accounts for interspecific differences in diet preference. They also contribute to an understanding of how herbivore communities might respond to P and N limitation in ecosystems, and how complex interaction networks can influence biogeochemical cycles of N and P.     

Relationship Between N:P Ratio and Growth Rate During the Life Cycle of Calanoid Copepods: An in situ Measurement

The mean nitrogen and phosphorus contents of the calanoid copepodMixodiaptomus laciniatus Lilljeborg were measured to the stagelevel throughout the ice-free period of a high mountain lake.Our results indicate large intraspecific variations in calanoidelemental composition. While mean N as dry weight increasedfrom 3.0 ± 1.3 in nauplii to 6.0 ± 2.1 in copepodites,mean P content showed the opposite trend, varying intraspecificallyfrom 0.98 ± 0.26% in nauplii, to 0.87 ± 0.21%in copepodites and 0.51 ± 0.16% in adults. Thus, themean N:P ratio increased ontogenetically from 3.3 in naupliito 13.3 in copepodites and 24.6 in adults. Two ontogenetic parameters,the growth rate and body size, were associated with zooplanktonstoichiometry. Among all 11 copepod stages, growth rate waspositively related to %P and negatively related to %N and N:Pratio. A two-part analysis of these relationships, before andafter metamorphosis, showed that the nauplii growth rate explainednearly all the variance in naupliar P content. A high P contentin nauplii may reflect a high content of RNA, translating intorapid growth rates. Overall, these results tend to support thehypothesis linking specific growth rate with P content for copepods,but these results also suggest that the validity of this hypothesisis robustness when differences in the life history of copepodsas a consequence of metamorphosis are accounted for. We suggestthat the intra-stage variation in P content is associated withpeaks of intensive metabolic activity during the process ofmolting in copepods, and we emphasize the importance of newempirical evidence to examine this hypothesis further.     

Stoichiometry of nutrient excretion by fish: interspecific variation in a hypereutrophic lake

This study investigates how nutrient cycling rates and ratios vary among fish species, with a particular focus on comparing an ecologically dominant detritivore (gizzard shad) to other fishes in a productive lake. We also examined how nutrient cycling rates are mediated by body size (as predicted by allometry theory), and how variation in nutrient cycling is related to body and food nutrient contents (according to predictions of ecological stoichiometry). As predicted by allometry, per capita nitrogen and phosphorus excretion rates increased and mass-specific excretion rates decreased, with increasing mass. Body phosphorus content was correlated with body mass only in one species, bluegill. Contrary to stoichiometric predictions, there was no relationship between body P and mass-normalized P excretion rate, or between body N:P and excreted N:P, when all individuals of all species were considered.
However, at the species level, we observed some support for a body nutrient content effect on excretion as predicted by stoichiometry theory. For example, gizzard shad had lower body P (high body N:P) and also excreted P at higher rates (lower N:P) than bluegill, which had high body P (lower body N:P). We applied the Sterner (1990) homeostatic stoichiometry model to the two most common species in the study – gizzard shad and bluegill and found that food N:P had a greater effect than consumer body N:P on excreted N:P. This indicates that, in terms of variation among these species, nutrient excretion may be more of a function of food nutrient content than the nutrient content of the consumer. These results suggest that stoichiometry can provide a framework for variation among species in nutrient cycling and for evaluating the ecosystem consequences of biodiversity loss.     

Ecology of the red-tide dinoflagellate Ceratium furca: distribution, mixotrophy, and grazing impact on ciliate populations of Chesapeake Bay

Ceratium furca is a primarily photosynthetic dinoflagellate also capable of ingesting other protists. During 1995 and 1996, we documented the abundance of C. furca in Chesapeake Bay and determined grazing rates on prey labeled with fluorescent microspheres. Abundance usually remained below 20 cells ml(-1), although the species was capable of localized late-summer blooms (< or = 478 cells ml(-1)) in the more saline lower to mid-Bay region. Feeding rates ranged from 0 to 0.11 prey dinoflagellate(-1) h(-1) or from 0 to 37 pg C dinoflagellate(-1) h(-1) and were highest at lower salinities. Clearance rates averaged 2.5 +/- 0.35 microl dinoflagellate(-1) h(-1). Impact of C. furca feeding on prey populations was higher in the lower Bay, averaging 67% of Strobilidium spp. removed d(-1). Ingestion rates were positively correlated with prey abundance and dissolved inorganic nitrogen, but negatively with salinity, depth, dissolved inorganic phosphorus, and inorganic P:N ratio. Daily consumption of prey biomass by C. furca averaged 4.6% of body carbon, 6.5% of body nitrogen, and 4.0% of body phosphorus. with maximal values of 36, 51, and 32%, respectively. Thus, the ability to exploit an organic nutrient source when inorganic nutrients are limiting may give C. furca a competitive advantage over purely photosynthetic species.     

Feeding strategies and elemental composition in Ponto‐Caspian peracaridans from contrasting environments: can stoichiometric plasticity promote invasion success?

1. Ponto‐Caspian peracaridans, and mysids and amphipods in particular, are among the most successful aquatic invaders. However, species differ in the trophic‐status range of ecosystems they can invade while establishment rates and impacts can vary substantially between habitats. There is limited knowledge of the environmental factors and species characteristics that drive such variation in invasion success. 2. Here we test how trophic level and body stoichiometry vary among peracaridan species and in relation to body size. The amphipod Pontogammarus robustoides and the mysids Limnomysis benedeni and Paramysis lacustris were investigated in ecosystems differing considerably in productivity and nutrient supply, namely an N‐limited eutrophic lagoon and P‐limited mesotrophic lakes. 3. As revealed by stable isotope (¹⁵N/¹⁴N) analysis, herbivory was inferred to be the main feeding mode of L. benedeni. In contrast, the mysid P. lacustris and the amphipod P. robustoides displayed a higher propensity for predatory feeding at larger body sizes, a pattern that was more pronounced in the eutrophic lagoon than in the mesotrophic lakes. 4. Their mean stoichiometric composition (P. robustoides C:N:P 108:20:1, L. benedeni 92:21:1 and P. lacustris 93:22:1) demonstrates that these peracaridans are rich in nutrients, especially nitrogen. They all exhibited the same ontogenetic pattern of reduced stoichiometric regulation during juvenile stages and stricter homoeostasis at older stages. 5. The higher P content in juveniles of all peracaridan species from the lagoon indicates higher potential somatic and population growth rates than those in the mesotrophic lakes. Such a difference may explain the substantially faster rates of invader establishment observed in the lagoon in comparison with lakes of low trophy. 6. Due to differences in ontogenetic and habitat‐induced variation, the study species differed significantly in stoichiometric variability, which was lowest in L. benedeni and highest in P. robustoides. The ranges of species‐specific variation in stoichiometric ratios corresponded to the trophic (by chlorophyll a) and nutrient stoichiometry (N:P) ranges of lentic waters successfully invaded by these species in Lithuania. 7. Stoichiometric plasticity, which should be associated with flexibility of feeding strategy, may enhance the potential of peracaridan species to successfully invade habitats with differing trophy and nutrient supply. The optimal feeding strategy should be omnivory with a propensity for predatory feeding, which can be adjusted with respect to ontogenetic nutrient demands and resource availability. Invading species may have a stronger effect on the local biota in ecosystems with high P levels, which promote growth, and N limitation that should favour predation.     

Ecosystem development in short‐term postglacial chronosequences: N and P limitation in glacier forelands from Santa Inés Island,Magellan Strait

Glacier foreland moraines provide an ideal model to examine the patterns of ecosystem development and the evolution of nitrogen and phosphorous limitation over successional time. In this paper, we focus on a 400‐year soil chronosequence in the glacier forelands of Santa Inés Island in the Magellan Strait, southern Chile by examining forest development on phosphorus (P)‐poor substrates in a uniquely unpolluted region of the world. Results show a steady increase in tree basal area and a humped trend in tree species richness over four centuries of stand development. The increase in basal area suggests that the late successional tree species were more efficient nutrient users than earlier successional ones. Total contents of carbon (C) and nitrogen (N) in soils increased during the chronosequence, reaching an asymptote in late succession. The net increases in soil C : N, C : P and N : P ratios observed over successional time suggest that nutrient limitation is maximal in 400‐year‐old substrates. Foliar C : N and C : P ratios also increased over time to reach an asymptote in old‐growth stages, following soil stoichiometric relationships; however the foliar N‐to‐P ratio remained constant throughout the chronosequence. Biological N fixation was greater in early postglacial succession, associated with the presence of the symbiotic N‐fixer Gunnera magellanica. Declining trends of δ¹⁵N in surface soils through the 400‐year chronosequence are evidence of decreasing N losses in old‐growth forests. In synthesis, glacier foreland chronosequences at this high South American latitude provide evidence for increasing efficiency of N and P use in the ecosystem, with the replacement of shade‐intolerant pioneers by more efficient, shade‐tolerant tree species. This pattern of ecosystem development produces a constant foliar N : P ratio, regardless of variation in soil N‐to‐P ratio over four centuries.     

C : N : P stoichiometry of dominant riparian trees and arthropods along the Middle Rio Grande

1. We examined the role of flooding on the leaf nutrient content of riparian trees by comparing the carbon : nitrogen : phosphorus (C : N : P) ratio of leaves and litter of Rio Grande cottonwood (Populus deltoides ssp. wislizenii) in flood and non‐flood sites along the Middle Rio Grande, NM, U.S.A. The leaf C : N : P ratio was also examined for two non‐native trees, saltcedar (Tamarix chinensis) and Russian olive (Elaeagnus angustifolia), and six species of dominant riparian arthropods. 2. Living leaves and leaf litter of cottonwoods at flood sites had a significantly lower leaf N : P ratio and higher %P compared with leaves and litter at non‐flood sites. A non‐flood site downstream from wastewater effluent had a significantly lower litter C : N ratio than all other sites, suggesting N fertilisation through ground water. The non‐native trees, saltcedar and Russian olive, had higher mean leaf N content, N : P ratio, and lower C : N ratio compared with cottonwoods across study sites. 3. Riparian arthropods ranged from 5.2 to 7.1 for C : N ratio, 56–216 for C : P ratio, and 8.9–34 for N : P ratio. C content ranged from 25 to 52% of dry mass, N content from 4.7 to 10.8%, and P content from 0.59 to 1.2%. Differences in stoichiometry between high C : nutrient leaf litter and low C : nutrient invertebrates suggests possible food‐quality constraints for detritivores. 4. These results suggest that spatial and temporal variation in the C : N : P ratio of cottonwood leaves and leaf litter is influenced by surface and subsurface hydrologic connection within the floodplain. Reach‐scale variation in the elemental composition of riparian organic matter inputs may have important implications for decomposition, nutrient cycling, and food webs in river floodplain systems.     

Stoichiometric imbalances between detritus and detritivores are related to shifts in ecosystem functioning

How are resource consumption and growth rates of litter‐consuming detritivores affected by imbalances between consumer and litter C:N:P ratios? To address this question, we offered leaf litter as food to three aquatic detritivore species, which represent a gradient of increasing body N:P ratios: a crustacean, a caddisfly and a stonefly. The detritivores were placed in microcosms and submerged in a natural stream. Four contrasting leaf species were offered, both singly and in two‐species mixtures, to obtain different levels of stoichiometric imbalance between the resources and their consumers. The results suggest that detritivore growth was constrained by N rather than C or P, even though 1) the N:P ratios of the consumers’ body tissue was relatively low and 2) microbial leaf conditioning during the experiment reduced the N:P imbalance between detritivores and leaf litter. This surprisingly consistent N limitation may be a consequence of cumulative N‐demand arising from the production of N‐rich chitin in the exoskeletons of all three consumer species, which is lost during regular moults, in addition to N‐demand for silk production by the caddisfly. These N requirements are not commonly quantified in stoichiometric analyses of arthropod consumers. There was no evidence for compensatory feeding, but when offered mixed‐species litter varying in C:N:P ratios, detritivores consumed more of the litter species showing the highest N:P and lowest C:N ratio, accelerating the mass loss of the preferred leaf species in the litter mixture. These results show that imbalances in consumer–resource stoichiometry can have contrasting effects on coupled processes, highlighting a challenge in developing a mechanistic understanding of the role of stoichiometry in regulating ecosystem processes such as leaf litter decomposition.     

Nutrient limitation in species‐rich Calthion grasslands in relation to opportunities for restoration in a peat meadow landscape

Questions: Which nutrient(s) limit(s) vegetation productivity in Calthion grasslands? Is phosphorus release a bottleneck for restoration of species‐rich Calthion grasslands on rewetted dairy meadows? Location: Three species‐rich Calthion grasslands in the Western Peat District in the Netherlands. Methods: We conducted a field fertilization experiment with nitrogen (N), phosphorus (P) and potassium (K) in three existing Calthion grasslands to evaluate the potential for restoration on rewetted dairy meadows. Responses of above‐ground biomass, tissue nutrient concentrations and nutrient ratios were determined after 2 yr of fertilization. Results: Biomass increased with fertilization with N‐only and K‐only but did not react to P‐only additions. Comparisons of tissue nutrient concentrations and nutrient ratios also gave indications of N and K limitation. Conclusions: The strong P release expected after rewetting should not necessarily interfere with restoration of Calthion communities on rewetted dairy meadows. It is concluded that for successful restoration management measures should focus on reducing N and/or K availability. Potassium might be an overlooked bottleneck in the restoration of species‐rich grasslands.     

Dietary phosphorus availability influences female cricket lifetime reproductive effort

1. Recent ecological stoichiometric findings indicate that the relationships among key macronutrient elements [e.g. carbon (C), nitrogen (N), and phosphorus (P) of organisms and their resources] may underlie variation in life‐history traits. The amount of phosphorus in an individual's body is often correlated with its rate of growth, and low‐phosphorus diets are known to reduce growth in a number of insect and crustacean herbivores. 2. These findings suggest that the stoichiometric imbalance between organismal biomass requirements and the relative scarcity of nutrients in nature may also underlie variation in lifetime reproductive success. 3. This study investigated how dietary phosphorus availability during adulthood influenced lifetime reproductive effort, compensatory feeding, lifespan, condition, and stoichiometry of adult European House Cricket, Acheta domesticus. 4. Female crickets fed high amounts of phosphorus during adulthood laid significantly more eggs compared to those fed low amounts of phosphorus. Phosphorus availability did not directly influence lifespan, condition, or body stoichiometry, and crickets did not compensate for low phosphorus diets by eating more food. 5. A stoichiometric perspective may help understand the causes of variation in invertebrate fitness.     

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.     

Ontogeny, diet shifts, and nutrient stoichiometry in fish

Most stoichiometric models do not consider the importance of ontogenetic changes in body nutrient composition and excretion rates. We quantified ontogenetic variation in stoichiometry and diet in gizzard shad, Dorosoma cepedianum , an omnivorous fish with a pronounced ontogenetic diet shift; and zebrafish, Danio rerio, grown in the lab with a constant diet. In both species, body stoichiometry varied considerably along the life cycle. Larval gizzard shad and zebrafish had higher molar C:P and N:P ratios than larger fish. Variation in body nutrient ratios was driven mainly by body P, which increased with size. Gizzard shad body calcium content was highly correlated with P content, indicating that ontogenetic P variation is associated with bone formation. Similar trends in body stoichiometry of zebrafish, grown under constant diet in the laboratory, suggest that ontogeny (e.g. bone formation) and not diet shift is the main factor affecting fish body stoichiometry in larval and juvenile stages. The N:P ratio of nutrient excretion also varied ontogenetically in gizzard shad, but the decline from larvae to juveniles appears to be largely associated with variation in the N:P of alternative food resources (zooplankton vs detritus) rather than by fish body N:P. Furthermore, the N:P ratio of larval gizzard shad excretion appears to be driven more by the N:P ratio at which individuals allocate nutrients to growth, more so than static body N:P, further illustrating the need to consider ontogenetic variation. Our results thus show that fish exhibit considerable ontogenetic variation in body stoichiometry, driven by an inherent increase in the relative allocation of P to bones, whereas ontogenetic variation in excretion N:P ratio of gizzard shad is driven more by variation in food N:P than by body N:P.     

Stoichiometry of actual vs. potential predator–prey interactions: insights into nitrogen limitation for arthropod predators

Literature‐compiled data sets demonstrate wide interspecific variation in nitrogen content among terrestrial arthropods and raise the possibility of nitrogen (N) limitation for predatory species. It remains unclear, however, whether the same disparities between N supply and demand that appear in literature compilations also exist in particular ecological communities. To address this uncertainty, we compared arthropod predator–prey stoichiometries derived from a compiled database with those from a natural Spartina saltmarsh community. Separate assessments of potential N‐limitation were made for arthropod predators feeding on herbivores and for intraguild predators feeding on intraguild prey. Relative to the compiled database, saltmarsh consumer–resource interactions exhibited increased disparity between N‐content of herbivores and N‐demand by predators. The high N content of saltmarsh arachnids relative to predatory insects at large may contribute to the supply‐demand disparity. Whether N‐limitation of terrestrial arthropod predators is widespread in the marsh, and in nature in general, depends sensitively on the predatory species’ gross growth efficiencies for N and carbon. Obtaining hard empirical data for these efficiency parameters should be a research goal.     

Stoichiometric and nutrient resorption characteristics of dominant tree species in subtropical Chinese forests

This study investigated seasonal patterns in stoichiometric ratios, nutrient resorption characteristics, and nutrient use strategies of dominant tree species at three successional stages in subtropical China, which have not been fully understood. Fresh leaf and leaf litterfall samples were collected in growing and nongrowing seasons for determining the concentrations of carbon (C), nitrogen (N), and phosphorus (P). Then, stoichiometric ratios (i.e., C:N, C:P, N:P, and C:N:P) and resorption parameters were calculated. Our results found that there was no consistent variation in leaf C:N and C:P ratios among different species. However, leaf N:P ratios in late‐successional species became significantly higher, indicating that P limitation increases during successional development. Due to the P limitation in this study area, P resorption efficiency and proficiency were higher than corresponding N resorption parameters. Dominant tree species at early‐successional stage adopted “conservative consumption” nutrient use strategy, whereas the species at late‐successional stage inclined to adopt “resource spending” strategy.     

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).     

Prey Detection and Prey Capture in Copepod Nauplii

Copepod nauplii are either ambush feeders that feed on motile prey or they produce a feeding current that entrains prey cells. It is unclear how ambush and feeding-current feeding nauplii perceive and capture prey. Attack jumps in ambush feeding nauplii should not be feasible at low Reynolds numbers due to the thick viscous boundary layer surrounding the attacking nauplius. We use high-speed video to describe the detection and capture of phytoplankton prey by the nauplii of two ambush feeding species (Acartia tonsa and Oithona davisae) and by the nauplii of one feeding-current feeding species (Temora longicornis). We demonstrate that the ambush feeders both detect motile prey remotely. Prey detection elicits an attack jump, but the jump is not directly towards the prey, such as has been described for adult copepods. Rather, the nauplius jumps past the prey and sets up an intermittent feeding current that pulls in the prey from behind towards the mouth. The feeding-current feeding nauplius detects prey arriving in the feeding current but only when the prey is intercepted by the setae on the feeding appendages. This elicits an altered motion pattern of the feeding appendages that draws in the prey.     

Nitrogen and phosphorus content of some temperate and tropical freshwater fishes

The nitrogen and phosphorus content of two temperate fishes, Rutilus rutilus and Perca fluviatilis , and six tropical fishes, Oreochromis niloticus , Cichla monoculus , Serrassalmus rhombeus , Plagioscion squamosissimus , Prochilodus brevis and Hoplias malabaricus , were investigated to test the hypothesis that variation in body P content and N:P ratio is related to body size. Regressions of %P and N:P ratios against fish size (length and mass) confirmed the hypothesis for P. fluviatilis and P. squamosissimus , suggesting that body size is an important factor driving body P content and N:P ratios in some fishes. Moreover, significant increases in %N and N:P ratio with body size was found for H. malabaricus , a common piscivorous fish of the Neotropics. Interspecific variation in %P and N:P ranged two-fold and significant differences ( P < 0·05) were found among the tested species. The mean ± s . d . elemental content across all fishes ( n = 170) was 10·35 ± 1·29% for N and 3·05 ± 0·82% for P, while the N:P ratio was 8·00 ± 2·14. Data on fish body nutrient content and ratio will improve parameterization of bioenergetics and mass balance models and help clarify the role of fishes in nutrient cycles in both temperate and tropical freshwaters.     

Atmospheric nitrogen deposition is associated with elevated phosphorus limitation of lake zooplankton

Here, we present data that for the first time suggests that the effects of atmospheric nitrogen (N) deposition on nutrient limitation extend into the food web. We used a novel and sensitive assay for an enzyme that is over‐expressed in animals growing under dietary phosphorus (P) deficiency (alkaline phosphatase activity, APA) to assess the nutritional status of major crustacean zooplankton taxa in lakes across a gradient of atmospheric N deposition in Norway. Lakes receiving high N deposition had suspended organic matter (seston) with significantly elevated carbon:P and N:P ratios, indicative of amplified phytoplankton P limitation. This P limitation appeared to be transferred up the food chain, as the cosmopolitan seston‐feeding zooplankton taxa Daphnia and Holopedium had significantly increased APA. These results indicate that N deposition can impair the efficiency of trophic interactions by accentuating stoichiometric food quality constraints in lake food webs.     

Stoichiometry of nutrient recycling by vertebrates in a tropical stream: linking species identity and ecosystem processes

Ecological stoichiometry offers a framework for predicting how animal species vary in recycling nutrients, thus providing a mechanism for how animal species identity mediates ecosystem processes. Here we show that variation in the rates and ratios at which 28 vertebrate species (fish, amphibians) recycled nitrogen (N) and phosphorus (P) in a tropical stream supports stoichiometry theory. Mass-specific P excretion rate varied 10-fold among taxa and was negatively related to animal body P content. In addition, the N : P ratio excreted was negatively related to body N : P. Body mass (negatively related to excretion rates) explained additional variance in these excretion parameters. Body P content and P excretion varied much more among taxonomic families than among species within families, suggesting that familial composition may strongly influence ecosystem-wide nutrient cycling. Interspecific variation in nutrient recycling, mediated by phylogenetic constraints on stoichiometry and allometry, illustrates a strong linkage between species identity and ecosystem function.     

Shifts in elemental composition,methylmercury content and δ15N ratio during growth of a High Arctic copepod

1. We examined how ontogenetic development in a calanoid copepod from the High Arctic, Limnocalanus macrurus, influenced its elemental composition (carbon, nitrogen, phosphorus), methylmercury (MeHg) content and stable nitrogen and carbon isotope ratios in populations from nine lakes. 2. Population structure explained 33–83% of among‐lake variation in the C, N and P composition of the biomass. Biomass dominated by early‐stage copepodites had a greater P content, which declined in more mature populations, as indicated by significant changes in % P and the molar N/P ratio. Carbon and lipid contents increased with the proportion of adult biomass. Copepod populations sampled in warmer waters had a greater proportion of adult biomass, and water temperature was the most significant environmental variable explaining elemental composition. 3. A δ¹⁵N enrichment of 3.3 ± 1.0‰ was associated with copepodite development. Gut contents of L. macrurus showed no evidence of animal (invertebrate) prey, indicating no change in trophic position. 4. Unexpectedly, MeHg concentration was negatively correlated with the proportion of adult biomass. However, this trend was not significant after correcting MeHg concentration to non‐lipid dry mass, suggesting a lipid dilution effect in more mature copepods. Lake surface area, rather than ontogeny, best explained MeHg concentrations in L. macrurus. 5. Ontogenetic influences on chemical constituents of this common Arctic copepod, particularly δ¹⁵N ratios and uncorrected MeHg concentrations, highlight the relevance of developmental processes for studies of food webs and mercury in species‐poor High Arctic lakes.