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The study of the trophic position (TP) of organisms is crucial to understand the trophodynamics of ecosystems. The European sardine from the western Mediterranean has been highly exploited during the last decades and has shown an important population decline. To determine the TP, we quantified seasonal nitrogen stable isotope ([delta][sup.15]N) values of compound-specific stable isotope analysis of amino acids (CSIA-AA), as well as bulk [delta][sup.15]N (SIA-bulk) values in muscle from three areas of the north-western Mediterranean. Our results highlight that, despite observing latitudinal changes in nitrogen SIA- bulk, the TP of sardine remains similar between sites. These results suggest that changes in nitrogen SIA-bulk are mainly driven by changes in the isotopic ratio of the baseline. Furthermore, the seasonal nitrogen variation in SIA-bulk is linked to differences in the isotopic baseline of the pelagic food web observed in the source AAs. Using CSIA-AA allowed us to decouple the variation in the baseline from the trophic variation producing robust TP estimates. Without using CSIA-AA, the interpretation of nitrogen SIA-bulk was hampered by the lack of a proper regional baseline of the primary producers. Keywords: trophic position, compound-specific stable isotopes, Sardina pilchardus, latitudinal variation, stable isotopes, Introduction Trophic position (TP) is an important concept in ecology to describe the ecological role of species in the ecosystem (Odum and Heald 1975). It is widely used to describe [...]

Climate change is leading to shifts in not only the average timing of phenological events, but also their variance and predictability. Increasing phenological variability creates a stochastic environment that is critically understudied, particularly in aquatic ecosystems. We provide a perspective on the possible implications for increasingly unpredictable aquatic habitats, including more frequent trophic asynchronies and altered hydrologic regimes, focusing on ice-off phenology in lakes. Increasingly frequent phenological extremes may limit the ability of organisms to optimize traits required to adapt to a warming environment. Using a unique, long-term ecological dataset on Escanaba Lake, WI, USA, as a case study, we show that the average date of ice-off is shifting earlier and becoming more variable, thus altering limnological conditions and yielding uncoupled food web responses with ramifications for fish spawn timing and recruitment success. A genes-to-ecosystems understanding of the responses of aquatic communities to increasingly variable phenology is needed. Our perspective suggests that management for diversity, at the intra- and interspecific levels, will become paramount for conserving resilient aquatic ecosystems. Les changements climatiques entrainent des modifications non seulement de la date moyenne d'etapes phenologiques, mais egalement de leur variance et de leur previsibilite. Une plus grande variabilite phenologique cree un environnement stochastique dangereusement sous-etudie, particulierement dans les ecosystemes aquatiques. Nous presentons une perspective sur les consequences possibles pour des habitats aquatiques de plus en plus imprevisibles, dont la hausse de la frequence d'asynchronies trophiques et la modification des regimes hydrologiques, en mettant l'accent sur la phenologie du depart de la glace dans les lacs. Des extremes phenologiques de plus en plus frequents pourraient limiter la capacite d'organismes a optimiser des caracteres necessaires a l'adaptation a un milieu en rechauffement. En utilisant un ensemble de donnees a long terme singulier pour le lac Escanaba (Wisconsin, Etats-Unis) comme cas de figure, nous demontrons que la date moyenne du depart de la glace est de plus en plus hative et variable, modifiant ainsi les conditions limnologiques et causant des reactions trophiques non couplees qui ont des repercussions sur le moment du frai et le succes de recrutement des poissons. Une comprehension \\\ du gene a l'ecosysteme >> des reactions des communautes aquatiques a une phenologie de plus en plus variable est necessaire. Notre perspective donne a penser que la conservation d'ecosystemes aquatiques resilients necessitera une gestion axee sur la diversite tant intraspecifique qu'interspecifique. [Traduit par la Redaction], Climate change and phenology Phenology is the annual timing of biological events, or the temporal dimension of natural history (Forrest and Miller-Rushing 2010). Because phenology captures the important role the [...]

Inner Bay of Fundy Atlantic salmon (Salmo salar) are endangered anadromous fish that have the potential to provide marine-derived nutrients (MDNs) to freshwater ecosystems depending on their population abundance. Salmon have been reintroduced to the Upper Salmon River, but not to the adjacent Point Wolfe River, in Fundy National Park, New Brunswick, Canada. This study determined whether stocking of adult salmon increased the productivity of the river. To examine the incorporation of MDNs, biofilm, leaf litter, Perlidae, Heptageniidae and brook trout (Salvelinus fontinalis) were sampled pre- and post-spawning in 2015-2017 from down- and upstream of a natural barrier in both rivers and analyzed for carbon ([delta][sup.13]C) and nitrogen ([delta][sup.15]N) isotopes. After salmon spawning in each year, all organisms at the downstream site of the stocked river increased in [delta][sup.13]C and [delta][sup.15]N, with the greatest enrichment in brook trout ([delta][sup.13]C: -23.97 [per thousand] to -21.10 [per thousand], [delta][sup.15]N: +6.36 [per thousand] to +10.73 [per thousand]). The proportion of MDNs in brook trout after salmon spawning (2015: 23.4%, 2016: 40.7%, 2017: 37.4%) also increased with higher numbers of released adult salmon. Results support the importance of salmon restoration for increasing the proportion of MDNs and productivity in rivers in Atlantic Canada. Les saumons atlantiques (Salmo salar) de la haute baie de Fundy sont des poissons anadromes en voie de disparition qui ont le potentiel de fournir des nutriments d'origine marine (NOM) a des ecosystemes d'eau douce en fonction de l'abondance de leur population. Des saumons ont ete reintroduits dans la riviere Upper Salmon, mais non dans la riviere Point Wolfe attenante, dans le parc national Fundy (Nouveau-Brunswick, Canada). La presente etude avait pour but de determiner si l'ensemencement de saumons adultes rehaussait la productivite de la riviere. Afin d'examiner l'incorporation de NOM, des biofilms, de la litiere de feuilles, des perlides, des heptageniides et des ombles de fontaine (Salvelinus fontinalis) ont ete preleves avant et apres le frai de 2015 a 2017 en aval et en amont d'une barriere naturelle dans les deux rivieres, et leurs isotopes de carbone ([delta][sup.13]C) et d'azote ([delta][sup.15]N) ont ete analyses. Chaque annee apres le frai des saumons, tous les organismes au site aval de la riviere ensemencee presentaient une augmentation des valeurs de [delta][sup.13]C et [delta][sup.15]N, l'enrichissement le plus important etant observe dans les ombles de fontaine ([delta][sup.13]C : de -23,97 [per thousand] a -21,10 [per thousand], [delta][sup.15]N : de +6,36 [per thousand] a +10,73 [per thousand]). La proportion de NOM dans les ombles de fontaine apres le frai des saumons (2015 : 23,4 %, 2016 : 40,7 %, 2017 : 37,4 %) a aussi augmente parallelement au nombre de saumons adultes relaches. Les resultats soulignent l'importance du retablissement du saumon pour accroitre la proportion de NOM et la productivite des rivieres au Canada atlantique. [Traduit par la Redaction], Introduction As anadromous fishes, salmon transfer nutrients from marine to freshwater ecosystems (Naiman et al. 2002) via their migration, spawning, and death (Moore et al. 2007). Increased nutrient levels in [...]

Trophic cascade studies often rely on linear food chains instead of complex food webs and are typically measured as biomass averages, not as biomass variation. We study trophic cascades propagating across a complex food web including a measure of biomass variation in addition to biomass average. We examined whether different fishing strategies induce trophic cascades and whether the cascades differ from each other. We utilized an allometric trophic network (ATN) model to mechanistically study fishing-induced changes in food web dynamics. Different fishing strategies did not trigger traditional, reciprocal trophic cascades, as measured in biomass averages. Instead, fishing triggered a variation cascade that propagated across the food web, including fish, zooplankton and phytoplankton species. In fisheries that removed a large amount of top-predatory and cannibalistic fish, the biomass oscillations started to decrease after fishing was started. In fisheries that mainly targeted large planktivorous fish, the biomass oscillations did not dampen but slightly increased over time. Removing species with specific ecological functions might alter the food web dynamics and potentially affect the ecological resilience of aquatic ecosystems. Les etudes de cascades trophiques reposent souvent sur des chaines alimentaires lineaires plutot que sur des reseaux trophiques complexes, ainsi que sur des mesures de la biomasse moyenne, plutot que des variations de la biomasse. Nous etudions la propagation de cascades trophiques a travers un reseau trophique complexe en integrant une mesure de la variation de la biomasse en plus de la moyenne de cette derniere. Nous tentons de determiner si differentes strategies de peche induisent des cascades trophiques et si ces dernieres different les unes des autres. Nous utilisons un modele de reseau trophique allometrique (RTA) pour etudier de maniere mecaniste les modifications induites par la peche de la dynamique du reseau trophique. Differentes strategies de peche ne declenchent pas de cascades trophiques reciproques traditionnelles, telles que mesurees par la biomasse moyenne. La peche declenche plutot une cascade de variations qui se propage dans tout le reseau trophique, incluant des especes de poissons, de zooplancton et de phytoplancton. Dans les peches qui prelevent une grande quantite de poissons cannibales et predateurs de niveau superieur, les oscillations de la biomasse commencent a diminuer apres le debut de la peche. Dans les peches qui ciblent principalement de grands poissons planctonivores, les oscillations de la biomasse ne s'attenuent pas, mais augmentent plutot legerement au fil du temps. Le retrait d'especes ayant des fonctions ecologiques precises pourrait modifier la dynamique des reseaux trophiques et avoir une incidence sur la resilience ecologique des ecosystemes aquatiques. [Traduit par la Redaction], Introduction The widespread importance of trophic cascades ('indirect species interactions that originate with predators and spread downward through food webs' sensu Ripple et al. 2016) has become increasingly recognized since [...]

Euphausiids are a keystone species in marine food webs of the Northeast Pacific Ocean, important for forage fish, commercial fish species and marine predators such as seabirds and whales. Ecosystem-based fisheries management relies on realistic lower-trophic-level information for forecasting under future changing climate scenarios. We synthesised information from quantitative modelling studies analysing the relationship between euphausiids and their environment for two species of euphausiids that dominate assemblages in this region--Euphausia pacifica and Thysanoessa spinifera. Studies analysed suggest that variables reflecting the physical and biological environment in situ, features reflecting the geomorphic marine landscape, and large-scale climate indices all significantly affect euphausiid biomass and distribution. Temperature was the most tested predictor variable in the euphausiid models reviewed; however, it was significant in fewer models than other variables tested. We review and compare model structures, predictor variable selection and temporal lag phases to develop recommendations for species distribution modelling of euphausiids in the Northeast Pacific Ocean. We believe the results from this review will be applicable globally across regions with similar climates where euphausiids are numerous and can be adapted for different species and environments. Les euphausiaces sont des especes cles dans les reseaux trophiques marins du nord-est de l'ocean Pacifique, d'importance pour des poissons-fourrages, des especes commerciales de poissons et des predateurs marins, dont des oiseaux marins et des baleines. La gestion ecosystemique des peches requiert de l'information sur les niveaux trophiques inferieurs pour etablir des previsions associees a des scenarios de changements climatiques futurs. Nous presentons une synthese de l'information issue d'etudes de modelisation quantitatives de la relation entre les euphausiaces et leur milieu dans le nord-est du Pacifique pour deux especes d'euphausiaces dominants dans les assemblages--Euphausia pacifica et Thysanoessa spinifera. Les etudes analysees indiqueraient que des variables qui refletent le milieu physique et biologique en place, des caracteristiques du paysage geomorphologique marin et des indices climatiques a grande echelle ont tous une incidence significative sur la biomasse et la repartition des euphausiaces. Si la temperature est la variable predictive la plus souvent analysee dans les modeles d'euphausiaces examines, elle s'avere toutefois significative dans un moins grand nombre de modeles que d'autres variables analysees. Nous passons en revue et comparons les structures de modeles, la selection de variables predictives et les phases de decalage temporel afin de formuler des recommandations pour la modelisation de la repartition d'especes d'euphausiaces dans le nord-est de l'ocean Pacifique. Nous croyons que les resultats de la presente synthese pourront etre appliques dans differentes regions du monde aux climats semblables ou les euphausiaces sont nombreux et qu'ils peuvent etre adaptes aux conditions oceanographiques propres a differentes especes. [Traduit par la Redaction], 1. Introduction Coastal seas are regions of rich biomass and high biodiversity. Active management of marine resources is often concentrated around the coast where anthropogenic and economic needs most often [...]

Emerging evidence suggests that zooplankton production is affected by physiological and nutritional constraints due to climate change and eutrophication, which in turn could have broad implications for food-web dynamics and fisheries production. In this study, we developed a resource-based zooplankton production dynamics model that causally links freshwater cladoceran and copepod daily production-to-biomass (P/B) ratios with water temperature, phytoplankton biomass and community composition, and zooplankton feeding selectivity. This model was used to evaluate constraints on zooplankton growth under four hypothetical scenarios: involving natural plankton community seasonal succession; lake fertilization to enhance fisheries production; eutrophication; and climatic warming. Our novel modeling approach predicts zooplankton production is strongly dependent on seasonal variation in resource availability and quality, which results in more complex zooplankton dynamics than predicted by simpler temperature-dependent models. For mesotrophic and hypereutrophic lakes, our study suggests that the ultimate control over zooplankton P/B ratios shifts from physiological control during colder periods to strong resource control during warmer periods. Our resource-based model provides important insights into the nature of biophysical control of zooplankton under a changing climate that has crucial implications for food web energy transfer and fisheries production. De nouvelles observations semblent indiquer que des contraintes physiologiques et nutritives decoulant des changements climatiques et de l'eutrophisation ont une incidence sur la production de zooplancton, ce qui pourrait avoir de vastes repercussions sur la dynamique des reseaux trophiques et la production des peches. Nous avons developpe un modele de dynamique de la production de zooplancton base sur les ressources qui relie causalement les rapports quotidiens de la production et de la biomasse (P/B) de cladoceres et de copepodes d'eau douce a la temperature de l'eau, la biomasse et la composition de la communaute du phytoplancton et la selectivite de l'alimentation du zooplancton. Le modele est utilise pour evaluer les contraintes a la croissance du zooplancton dans quatre scenarios hypothetiques, a savoir : une succession saisonniere naturelle de la communaute de plancton, la fertilisation du lac pour accroitre la production des peches, l'eutrophisation et le rechauffement climatique. Notre nouvelle approche de modelisation predit que la production de zooplancton depend fortement des variations saisonnieres de la disponibilite et de la qualite des ressources, ce qui se traduit par une dynamique du zooplancton plus complexe que ce que predisent les modeles plus simples dependants de la temperature. Pour les lacs mesotrophes et hypereutrophes, l'etude indiquerait que les rapports P/B du zooplancton sont ultimement controles par la physiologie durant les periodes plus froides et par les ressources durant les periodes plus chaudes. Notre modele base sur les ressources fournit d'importants renseignements sur la nature du controle biophysique du zooplancton dans des conditions de climat changeant qui a d'importantes repercussions sur les transferts energetiques dans les reseaux trophiques et la production des peches. [Traduit par la Redaction], Introduction Zooplankton represent a biochemical filter in lakes wherein primary producer organic matter is converted to animal matter. Due to the large disparity in phytoplankton and zooplankton biochemical composition, energetic [...]

While recent research has shed insight into how bottom hypoxia affects pelagic food webs in coastal marine ecosystems and natural lakes, its effects on man-made lake (reservoir) food webs remains more incomplete. To address this gap, we conducted a study in two midwestern USA reservoirs to examine how the spatial overlap and vertical distributions of dominant zooplanktivores (i.e., pelagic fish, bentho-pelagic Chaoborus spp.) and their prey vary between periods of normoxia and hypoxia. Surprisingly, we found high levels of spatial overlap between zooplankton and both intermediate consumers (pelagic fish and Chaoborus) during both normoxia and hypoxia, though the extent of spatial overlap was higher during hypoxia at night relative to day. As expected, pelagic fish and zooplankton avoided hypoxic waters, and Chaoborus moved from hypoxic waters during the day to the well-oxygenated surface waters at night. Using our findings, we discuss the potential influence of bottom hypoxia and Chaoborus on the function and structure of north-temperate reservoir food webs. Si des travaux de recherche recents ont ameliore la comprehension de l'incidence de l'hypoxie de fond sur les reseaux trophiques pelagiques dans les ecosystemes marins cotiers et les lacs naturels, la comprehension des effets de cette hypoxie sur les reseaux trophiques de lacs artificiels (reservoirs) demeure moins complete. Pour combler cette lacune, nous avons mene une etude dans deux reservoirs du Midwest americain afin d'examiner les variations du chevauchement dans l'espace et de la repartition verticale de zooplanctivores dominants (c.-a-d., poissons pelagiques et benthopelagiques du genre Chaoborus) et de leurs proies entre des periodes de normoxie et d'hypoxie. Etonnement, nous avons releve des degres eleves de chevauchement dans l'espace entre le zooplancton et les deux types de consommateurs intermediaires (poissons pelagiques et Chaoborus) tant durant des conditions de normoxie que d'hypoxie, l'ampleur de ce chevauchement etant toutefois plus grande durant l'hypoxie la nuit que le jour. Comme prevu, les poissons pelagiques et le zooplancton evitent les eaux hypoxiques, et les Chaoborus se deplacent entre des eaux hypoxiques durant le jour et des eaux de surface bien oxygenees la nuit. A la lumiere de nos observations, nous discutons de l'influence potentielle de l'hypoxie de fond et des Chaoborus sur la fonction et la structure des reseaux trophiques de reservoirs de regions nordiques temperees. [Traduit par la Redaction], Introduction Bottom hypoxia ( In general, organisms have been shown to move vertically (up) or horizontally (nearshore) to avoid bottom hypoxia, which in turn could alter their spatial overlap and [...]

Predator populations are vulnerable to changes in prey distribution or availability. With warming temperatures, lake ecosystems in the Arctic are predicted to change in terms of hydrologic flow, water levels, and connectivity with other lakes. We surveyed lakes in northern Alaska to understand how shifts in the distribution or availability of fish may affect the occupancy and breeding success of Pacific (Gavia pacifica) and Yellow-billed Loons (G. adamsii). We then modeled the influence of the presence and abundance of five fish species and the physical characteristics of lakes (e.g., hydrologic connectivity) on loon lake occupancy and chick production. The presence of Alaska blackfish (Dallia pectoralis) had a positive influence on Pacific Loon occupancy and chick production, which suggests that small-bodied fish species provide important prey for loon chicks. No characteristics of fish species abundance affected Yellow-billed Loon lake occupancy. Instead, Yellow-billed Loon occupancy was influenced by the physical characteristics of lakes that contribute to persistent fish populations, such as the size of the lake and the proportion of the lake that remained unfrozen over winter. Neither of these variables, however, influenced chick production. The probability of an unoccupied territory becoming occupied in a subsequent year by Yellow-billed Loons was low, and no loon chicks were successfully raised in territories that were previously unoccupied. In contrast, unoccupied territories had a much higher probability of becoming occupied by Pacific Loons, which suggests that Yellow-billed Loons have strict habitat requirements and suitable breeding lakes may be limited. Territories that were occupied had high probabilities of remaining occupied for both loon species. Key words: Arctic Coastal Plain; Arctic lakes; bottom-up process; fish community; food web; Gavia adamsii; Gavia pacifica; lake habitat; occupancy modeling; territory occupancy Les populations de predateurs sont vulnerables aux changements de repartition ou de disponibilite des proies. En raison du rechauffement des temperatures, on prevoit que les ecosystemes lacustres de l'Arctique changeront pour ce qui est du regime hydrologique, des niveaux d'eau et de la connectivite avec d'autres lacs. Nous avons examine des lacs du nord de l'Alaska pour comprendre comment les changements en matiere de repartition ou de disponibilite des poissons peuvent avoir des incidences sur le taux d'occupation et sur le succes de reproduction du huart du Pacifique (Gaviapacifica) et du huart a bec blanc (G. adamsii). Ensuite, nous avons modelise l'influence de la presence et de l'abondance de cinq especes de poissons de meme que les caracteristiques physiques de lacs (comme la connectivite hydrologique) par rapport au taux d'occupation lacustre des huarts et a la production d'oisillons. La presence du dallia (Dallia pectoralis) avait une influence positive sur l'occupation et la production d'oisillons chez le huart du Pacifique, ce qui suggere que les especes de poissons au petit corps constituent une proie importante pour les oisillons. Aucune caracteristique de l'abondance des especes de poissons n'a eu d'influence sur l'occupation lacustre du huart a bec blanc. L'occupation du huart a bec blanc a plutot ete influencee par les caracteristiques physiques des lacs qui contribuent aux populations de poissons persistantes, comme la taille du lac et la proportion du lac qui ne gelait pas en hiver. Toutefois, aucune de ces variables n'a exerce d'influence sur la production d'oisillons. La probabilite qu'un territoire inoccupe devienne occupe par le huart au bec blanc au cours d'une annee subsequente etait faible, et aucun oisillon huart n'a ete eleve avec succes dans des territoires d'oisillons anciennement inoccupes. En revanche, les territoires inoccupes avaient une beaucoup plus grande probabilite de devenir occupes par les huarts du Pacifique, ce qui suggere que les huarts a bec blanc ont des exigences strictes en matiere d'habitat et que le nombre de lacs convenant a la reproduction risque d'etre limite. Les territoires qui etaient occupes avaient de fortes probabilites de rester occupes par les deux especes de huarts. Mots cles : Plaine cotiere de l'Arctique; lacs de l'Arctique; processus ascendant; communaute de poissons; reseau trophique; Gavia adamsii; Gavia pacifica; habitat de lac; modelisation de l'occupation; occupation du territoire Traduit pour la revue Arctic par Nicole Giguere., INTRODUCTION Understanding the relationship between animals and the habitats they select is a fundamental issue in ecology. In general, organisms select the highest-quality habitat available to maximize fitness. Many factors [...]

Kelp forests dominated by Macrocystis pyrifera are widely distributed in coastal waters from boreal, temperate and subantarctic regions. This widespread distribution may result in regional differences in food web structure and functioning. In temperate northern regions, where most studies on kelp forest benthic food webs have been conducted, kelp grazing is a well-documented phenomenon and can lead to the overgrazing of M. pyrifera by sea urchins when their predators (e.g., sea otters) are absent. In contrast, little is known about their counterparts in subantarctic areas. The present study aimed to reconstruct the benthic food web of a kelp forest dominated by M. pyrifera in a subantarctic environment using stable isotope analysis. Stable carbon ([delta].sup.13C), nitrogen ([delta].sup.15N) and sulfur ([delta].sup.34S) isotope ratios were measured from food sources (macrophytobenthos, suspended particulate organic matter SPOM, and sediment) and consumers (sponges, bivalves, gastropods, sedentary and mobile polychaetes, arthropods and echinoderms) which were sampled in a kelp forest of the Kerguelen archipelago. Mixing models highlighted two interconnected trophic pathways which were either supported by SPOM and resuspended macrophytobenthos detritus (bentho-pelagic), or by live micro/macrophytobenthos (phytobenthos-based). No major prey were highlighted for several consumers, indicating the existence of potential supplementary trophic pathways. No consumer relying primarily on living M. pyrifera was highlighted by the mixing models. The investigated kelp forest is hence a complex ecosystem supporting multiple trophic pathways, and direct consumption of M. pyrifera is limited. Nonetheless, M. pyrifera and other macrophytobenthos species may constitute a pool of detritus supporting several trophic pathways., Author(s): Baptiste Le Bourg [sup.1] [sup.2] , Thomas Saucède [sup.3] , Anouk Charpentier [sup.1] , Gilles Lepoint [sup.1] , Loïc N. Michel [sup.1] [sup.4] Author Affiliations: (1) grid.4861.b, 0000 0001 [...]

We investigated differences in resource use between two sympatric sea snake species from the west coast of India, Hydrophis curtus and H. schistosus, and described the impacts of fishing on the interactions of these species. We compared habitat use, diet, and isotopic niche width between species to determine resource overlap. We then compared trophic overlap of each species with the fisheries in the region and tested the effect of fishing intensity on their isotopic niche width. Hydrophis curtus used deeper habitats than H. schistosus, resulting in increased spatial overlap with fisheries. The two species also had distinct trophic niches and H. curtus prey formed a larger proportion of fishery catch on average than H. schistosus. This greater overlap could make H. curtus more vulnerable to the effects of fisheries. Both species exhibited expansion in short-term and long-term isotopic niche width along a gradient of fishing intensity which may indicate behavioural changes associated with the presence of fisheries. Hydrophis curtus is a trophic generalist, competes with syntopic species and is dominant in most assemblages. However, H. schistosus exhibits higher plasticity in resource use and may have an advantage over H. curtus. Sea snakes play an important role as mesopredators and as intermediate links in coastal marine food webs. Thus, fishing could alter the relative abundance of these mesopredators with cascading effects through coastal food webs., Author(s): Shawn Dsouza [sup.1] [sup.2] , Chetan Rao [sup.1] , Muralidharan Manoharakrishnan [sup.1] , Kartik Shanker [sup.1] [sup.2] Author Affiliations: (1) Dakshin Foundation, , Bangalore, India (2) grid.34980.36, 0000 0001 [...]

Seagrass beds are important habitats in coastal areas but increasingly decline in area and quality, thus conservation measures are urgently needed. Quantitative food webs, describing the biomass distribution and energy fluxes among trophic groups, reveal structural and functional aspects of ecosystems. Their knowledge can improve ecological conservation. For the recently discovered large warm-temperate seagrass (Zostera japonica) habitat in China's Yellow River Delta wetland, we used [delta].sup.13C and [delta].sup.15N measurements and a Bayesian isotope mixing model to construct its food web diagram with quantitative estimations of consumer diet compositions, comprising detritus and 14 living trophic groups from primary producers to fish. We then estimated the quantitative food web fluxes based on biomass measurements and calculated corresponding ecosystem functions. Pelagic producers were significantly .sup.13C-depleted compared to benthic sources. Consumers (except zooplankton) were increasingly .sup.13C-depleted with increasing trophic positions even though the consumed benthic production surpassed the pelagic one. Bivalves dominated consumer biomasses and fluxes and were the first to connect the pelagic and benthic pathways, whereas zooplankton and gastropods were specialized on the two pathways, respectively. We found flat biomass and production pyramids indicating low trophic transfer efficiencies. Generally, the energetic structure of the quantitative food web was consistent with the stable isotope analysis, and the estimated net primary production and most estimated production to biomass ratios of the trophic groups fell within literature ranges. This study provides a systematical understanding of the quantitative trophic ecology of a seagrass bed and facilitates synergistic knowledge on management, conservation, and restoration., Author(s): Xiaoxiao Li [sup.1] [sup.2] , Wei Yang [sup.1] [sup.3] , Tao Sun [sup.1] [sup.3] , Ursula Gaedke [sup.2] Author Affiliations: (1) grid.20513.35, 0000 0004 1789 9964, State Key Laboratory [...]

Climate change will alter marine ecosystems; however, the complexity of the food webs, combined with chronic undersampling, constrains efforts to predict their future and to optimally manage and protect marine resources. Sustained observations at the West Antarctic Peninsula show that in this region, rapid environmental change has coincided with shifts in the food web, from its base up to apex predators. New strategies will be required to gain further insight into how the marine climate system has influenced such changes and how it will do so in the future. Robotic networks, satellites, ships, and instruments mounted on animals and ice will collect data needed to improve numerical models that can then be used to study the future of polar ecosystems as climate change progresses. doi: 10.1126/science.1185779

Many studies have aimed to understand food webs by investigating components such as trophic links (one consumer taxon eats one resource taxon), tritrophic interactions (one consumer eats an intermediate taxon, which eats a resource), or longer chains of links. We show here that none of these components (links, tritrophic interactions, and longer chains), individually or as an ensemble, accounts fully for the properties of the next higher level of organization. As a cell is more than its molecules, as an organ is more than its cells, and as an organism is more than its organs, in a food web, new structure emerges at every organizational level up to and including the whole web. We demonstrate the emergence of properties at progressively higher levels of structure by using all of the directly observed, appropriately organized, publicly available food web datasets with relatively complete trophic link data and with average body mass and population density data for each taxon. There are only three such webs, those of Tuesday Lake, Michigan, in 1984 and 1986, and Ythan Estuary, Scotland. We make the data freely available online with this report. Differences in web patterns between Tuesday Lake and Ythan Estuary, and similarities of Tuesday Lake in 1984 and 1986 despite 50% turnover of species, suggest that the patterns we describe respond to major differences between ecosystem types. body mass | community ecology | food chain | population density | trophic link doi/10.1073/pnas.0910582106

We used (1) water temperature (a proxy for water movement), (2) chlorophyll (a proxy for phytoplankton), and (3) zooplankton (bulk, small, and large size classes) to investigate the relationship between changing wind conditions and spatial patterns along linear transects (n = 150) in two basins, South Arm and Annie Bay, of Lake Opeongo (Ontario, Canada). The basins have similar biological characteristics, but South Arm is larger and is oriented along the prevailing westerly wind direction. Large-scale patterns (>1 kin) were described with an accumulation index, and wavelet analysis was used to describe small-scale patterns (< 1 kin). Spatial descriptors were correlated with five descriptors of wind conditions: (1) wind force, (2) scalar wind speed, (3) vector wind speed, (4) wind persistence, and (5) wind direction. Persistent westerly winds in South Arm resulted in more downwind accumulation of warm water and total and large-bodied zooplankton than in Annie Bay, while chlorophyll and small zooplankton did not show consistent downwind accumulation. The predominance of small-scale variability, particularly in large zooplankton, increased in the South Arm as the persistence and strength of westerly winds blowing parallel to the sampling transects increased. Only temperature showed such a pattern in the smaller Annie Bay. These patterns were not related to winds blowing at the time of sampling but rather to those blowing up to 12 h before sampling. Our observations provide a basis for future consideration of how simple surface winds may actually shape the nature of trophic interactions in lake ecosystems.

The pattern of predator-prey interactions is thought to be a key determinant of ecosystem processes and stability. Complex eco-logical networks are characterized by distributions of interaction strengths that are highly skewed, with many weak and few strong interactors present. Theory suggests that this pattern promotes stability as weak interactors dampen the destabilizing potential of strong interactors. Here, we present an experimental test of this hypothesis and provide empirical evidence that the loss of weak interactors can destabilize communities in nature. We ranked 10 marine consumer species by the strength of their trophic interactions. We removed the strongest and weakest of these interactors from experimental food webs containing >100 species. Extinction of strong interactors produced a dramatic trophic cascade and reduced the temporal stability of key ecosystem process rates, community diversity and resistance to changes in community composition. Loss of weak interactors also proved damaging for our experimental ecosystems, leading to reductions in the temporal and spatial stability of ecosystem process rates, community diversity, and resistance. These results highlight the importance of conserving species to maintain the stabilizing pattern of trophic interactions in nature, even if they are perceived to have weak effects in the system. biodiversity and ecosystem functioning | dynamic index | interaction strength | predator-prey interactions | temporal and spatial variability

How environmental change affects species abundances depends on both the food web within which species interact and their potential to evolve. Using field experiments, we investigated both ecological and evolutionary responses of pea aphids (Acyrthosiphon pisum), a common agricultural pest, to increased frequency of episodic heat shocks. One predator species ameliorated the decrease in aphid population growth with increasing heat shocks, whereas a second predator did not, with this contrast caused by behavioral differences between predators. We also compared aphid strains with stably inherited differences in heat tolerance caused by bacterial endosymbionts and showed the potential for rapid evolution for heat-shock tolerance. Our results illustrate how ecological and evolutionary complexities should be incorporated into predictions of the consequences of environmental change for species' populations.

As part of the Tropical Amphibian Declines in Streams (TADS) project, we estimated macroinvertebrate production and constructed quantitative food webs for four headwater stream reaches in the Panamanian uplands: two that had experienced massive amphibian declines and two with unaffected amphibian populations. As expected for forested headwaters, allochthonous materials were the dominant energy source. Total macroinvertebrate biomass and production ranged from 231 to 360 mg ash-free dry mass [m.sup.-2] and from 3.1 to 4.4 g ash-free dry mass [m.sup.-2] [yr.sup.-1], respectively, and did not appear influenced by the presence or absence of amphibians. However, macroinvertebrate functional structure differed between pre- and post-decline sites, with shredder production significantly higher in pre-decline sites and scrapers significantly higher in post-decline sites. Taxonomic differences between pre- and post-decline sites were also evident. There was a shift in scrapers from smaller-bodied taxa (e.g., Psephenus) in pre-decline sites to larger-bodied groups (e.g., Petrophila) in post-decline sites. Detrital pathways were dominant in these systems, with shredders and collectors accounting for most energy flow. However, scrapers were well-represented and they were food-limited in these systems, particularly in the presence of larval amphibians at pre-decline sites. Ecological effects of catastrophic amphibian declines ranged from subtle shifts in taxonomic composition and functional structure of remaining consumers to changes in the availability and relative importance of autochthonous energy sources.

Eighteen years of field observations and five summer field experiments in a coastal California river suggest that hydrologic regimes influence algal blooms and,the impacts of fish on algae, cyanobacteria, invertebrates, and small vertebrates. In this Mediterranean climate, rainy winters precede the biologically active summer low-flow season. Cladophora glomerata, the filamentous green alga that dominates primary producer biomass during summer, reaches peak biomass during late spring or early summer. Cladophora blooms are larger if floods during the preceding winter attained or exceeded 'bankfull discharge' (sufficient to mobilize much of the river bed, estimated at 120 [m.sub.3]/s). In 9 out of 12 summers preceded by large bed-scouring floods, the average peak height of attached Cladophora turfs equaled or exceeded 50 cm. In five out of six years when flows remained below bankfull, Cladophora biomass peaked at lower levels. Flood effects on algae were partially mediated through impacts on consumers in food webs. In three experiments that followed scouring winter floods, juvenile steelhead (Oncorhynchus mykiss) and roach (Lavinia (Hesperoleucas) symmetricus) suppressed certain insects and young-of-the-year fish fry, affecting persistence or accrual of algae positively or negatively, depending on the predator-specific vulnerabilities of primary consumers capable of suppressing algae during a given year. During two post-flood years, these grazers were more vulnerable to small predators (odonates and fish fry, which stocked steelhead always suppressed) than to experimentally manipulated, larger fish, which had adverse effects on algae in those years. During one post-flood year, all enclosed grazers capable of suppressing algae were consumed by steelhead, which therefore had positive effects on algae. During drought years, when no bed-scouring winter flows occurred, large armored caddis flies (Dicosmoecus gilvipes) were more abundant during the subsequent summer. In drought-year experiments, stocked fish had little or no influence on algal standing crops, which increased only when Dicosmoecus were removed from enclosures. Flood scour, by suppressing invulnerable grazers, set the stage for fish mediated effects on algae in this river food web. Whether these effects were positive or negative depended on the predator-specific vulnerabilities of primary consumers that dominated during a given summer. Key words: algal blooms; Cladophora glomerata; context dependency; effect sizes; flood scour; food chain length; interaction strength; Lavinia (Hesperoleucas) symmetricus; long-term studies; Mediterranean hydrologic regimes; Oncorhynchus mykiss; predator impacts.

As the theoretical bases for the dynamics of spatially subsidized communities emerge, ecologists question whether spatially subsidized communities exhibit similar structure or dynamics to communities that receive strongly pulsed resources. In both cases, communities may be structured by responses to resources that are potentially absent at any given point in time (pulsed communities) or space (subsidized communities), even if pulsed resources are part of the in situ productivity of the system or the subsidies arrive as a relatively constant input from a nearby system. The potential for significant spatial or temporal resource limitation, therefore, may be a key factor influencing in similar ways the persistence of populations, the structure and dynamics of communities, and the evolution of specific life history traits. In most complex systems, however, multiple resources may arrive for various trophic entities at various points in time and from various points in space, and thus it may be difficult to separate or compare the dynamics of spatially subsidized and pulsed systems. In this paper, we explore the effects of interactions between pulses and subsidies in plant and animal populations and communities on highly pulsed and variably subsidized islands in the Gulf of California. While many of the plant and animal communities on the unsubsidized islands in this system respond to pulses of rain in classic ways, responses to these rain pulses on islands subsidized by seabird guano or other marine resources are quite different and variable, and depend on a combination of life history characteristics, physiology, competitive interactions, and trophic relationships. These variable responses to rain pulses then translate into large differences in dynamics and community structure of subsidized vs. unsubsidized islands. Indeed, most systems experience both temporal pulses and spatial subsidies. When considered in tandem, complementary or synergistic effects of the multiple, temporally and spatially variable resources may emerge that help explain complex food web structure and dynamics. Key words: community dynamics,' deserts; Gulf of California: islands: lizards: nutrient subsidies: plant physiology: population dynamics; primary productivity' rodents: spiders.

Inputs of fresh water and grazing both can control aquatic food webs, but little is known about the relative strengths of and interactions between these controls. We use longterm data on the food web of the freshwater Hudson River estuary to investigate the importance of, and interactions between, inputs of fresh water and grazing by the invasive zebra mussel (Dreissena polymorpha). Both freshwater inputs and zebra mussel grazing have strong, pervasive effects on the Hudson River food web. High flow tended to reduce population size in most parts of the food web. High grazing also reduced populations in the planktonic food web, but increased populations in the littoral food web, probably as a result of increases in water clarity. The influences of flow and zebra mussel grazing were roughly equal (i.e., within a factor of 2) for many variables over the period of our study. Zebra mussel grazing made phytoplankton less sensitive to freshwater inputs, but water clarity and the littoral food web more sensitive to freshwater inputs, showing that interactions between these two controlling factors can be strong and varied. Key words: alien species; anthropogenic change; bivalve; Dreissena polymorpha; estuary; food webs; freshwater flow; herbivory; invasive species; long-term study; regime shift; zebra mussel.

The size structure of phytoplankton assemblages strongly influences energy transfer through the food web and carbon cycling in the ocean. We determined the macroevolutionary trajectory in the median size of dinoflagellate cysts to compare with the macroevolutionary size change in other plankton groups. We found the median size of the dinoflagellate cysts generally decreases through the Cenozoic. Diatoms exhibit an extremely similar pattern in their median size over time, even though species diversity of the two groups has opposing trends, indicating that the macroevolutionary size change is an active response to selection pressure rather than a passive response to changes in diversity. The changes in the median size of dinoflagellate cysts are highly correlated with both deep ocean temperatures and the thermal gradient between the surface and deep waters, indicating the magnitude and frequency of nutrient availability may have acted as a selective factor in the macroevolution of cell size in the plankton. Our results suggest that climate, because it affects stratification in the ocean, is a universal abiotic driver that has been responsible for macroevolutionary changes in the size structure of marine planktonic communities over the past 65 million years of Earth's history. cell size | climate change | dinoflagellates | evolution | food webs

Recent findings indicate that fishes from lakes in partially burned catchments contain greater mercury (Hg) concentrations than fishes from reference catchments. Increased methyl Hg (MeHg) concentrations in fishes can result in serious health problems for consumers. Here we show that a forest fire caused a 5-fold increase in whole-body Hg accumulation by rainbow trout (Oncorhynchus mykiss) and smaller Hg increases in muscle of several fish species in a mountain lake. The enhanced Hg accumulation was caused primarily by increased nutrient concentrations in the lake, which enhanced productivity and restructured the food web through increased piscivory and consumption of Mysis. This restructuring resulted in increases to the trophic positions and Hg concentrations of fishes. Forest fire also caused a large short-term release of total Hg (THg) and MeHg to streams and the lake. This release initiated a small pulse of MeHg in invertebrates that contributed to enhanced Hg accumulation by fishes. Climate change and prescribed burning to compensate for past fire suppression are predicted to increase future forest fire occurrence in North America, and increased Hg accumulation by fishes may be an unexpected consequence. productivity | stable isotopes | methyl mercury | food chain length | climate change

Abstract: Anadromous Pacific salmon (Oncorhynchus spp.) subsidize terrestrial food webs with their nutrients and carcasses, a process driven largely by selective foraging by bears (Ursus spp.). We quantify wildlife transfer [...]

Indirect effects propagated through intervening species in a food web have important effects on community properties. Traditionally, these indirect effects have been conceptualized as mediated through density changes of the intervening species, but it is becoming increasingly apparent that those mediated through trait (phenotypic) responses also can be very important. Because density- and trait-mediated indirect effects have different properties, it is critical that we understand the mechanisms of transmission in order to predict how they will interact, and when or where they will be important. In this study, we examined the mechanisms and consequences of the lethal (density-mediated) and nonlethal (trait-mediated) effects of a larval odonate predator on a guild of four herbivore species (a larval anuran and three species of snails) and their resources. We also manipulated system productivity in order to explore the effects of environmental context on the transmission of these two types of indirect effects. We show that trait-mediated effects arising from the predator can be very strong relative to density-mediated effects on both the competing herbivores and the species composition and production of their resources. A number of these indirect effects are shown to be contingent on productivity of the system. We further present evidence that trait- and density-mediated indirect effects originating from a predator may be transmitted independently through different routes in a food web, particularly when spatial responses of the transmitting prey are involved. Finally, effects on prey growth due to trait responses to the predator varied from negative to positive in predictable ways as a function of time and indirect effects on the larger food web. These results indicate the important role that trait-mediated indirect effects can play in trophic cascades and keystone predator interactions, and we discuss how the mechanisms involved can be incorporated in theory. Key words: anuran larvae; food web; indirect effects; nonlethal effect; periphyton; predator; snails; trait-mediated effects; trophic cascades.

We studied the seasonal dynamics of phytoplankton, bacterioplankton, heterotrophic nanoflagellates, ciliates, and metazoan plankton in the highly eutrophic polymictic Lake Kastoria (Greece), which has a history of toxic cyano-bacterial blooms. An acute increase in the flushing rate of the lake during spring inhibited cyanobacterial biomass accumulation. During this transient oligotrophic period, which was characterized by abundant lake snow particles, the plankton food web was an inverted biomass pyramid (low autotrophic biomass and high heterotrophic biomass). Prokaryotes played a key role in these changes (cyanobacteria during periods of autotrophy and bacteria during periods of heterotrophy). In summer and autumn, toxic cyanobacterial blooms developed, and the microbial loop was weak. The microbial loop was weak because the heterotrophic nanoflagellates and nanociliates decreased to undetectable densities during the summer, when larger bacterivores (rotifers and small cladocera) were abundant. Toxic blooms may have a dual effect on heterotrophic nanoplankton: negative during the first bloom and postbloom period and positive during a following toxic bloom. Different species (Cylindrospermopsis raciborskii, Aphanizomenon spp., and Microcystis aeruginosa) and succession phases of toxic blooms may differentially affect the microbial food web structure.

A new numerical technique is presented that allows the tracking of any chemical element from any source in a simulated foodweb, for instance assessing the proportion of these sources (river loadings, sea entrances, point sources) in the algal diet for the limiting nutrient. An application is shown for nitrogen in an Ulva bloom occurring in a shallow embayment connected with a strongly tidally stirred ecosystem with various sources of inorganic nitrogen, the Bay of Brest, Brittany, France. In a first step, a biogeochemical three-dimensional model was developed to simulate growth and erosion--transport--leposition of free-floating ulvae; this model was able to converge on a realistic distribution of Ulva deposits after a few months, even though it was initialized with a strongly unrealistic distribution of settled ulvae. In a second step, and unfortunately for recovery plans, the tracking technique, applied in this model to all the nitrogen sources entering the bay, revealed that the small, nitrate-polluted rivers flowing directly into the eutrophicated area had a negligible effect, whereas more distant but stronger sources, a big river and a big urban sewage plant, even after dilution, accounted for about 50% and 20%, respectively, of the algal nitrogen content during summer. Despite its high N flux, open ocean contributes only 15% to Ulva growth. The suppression of only one of the main nitrogen sources would not significantly decrease the Ulva bloom, because of the high nitrogen surplus present in the site. The remaining sources would still saturate the needs of the maximum Ulva biomass the site is able to produce. The tracking technique, however, shows that the N turnover in Ulva is only 4 months. Thus, improvements would occur within a year following large N reductions.

We quantify, compare, and generalize responses of experimental nutrient loadings ([L.sub.N]) on planktonic community structure and function in coastal waters. Data were derived from three mesocosm experiments undertaken in Baltic (BAL), Mediterranean (MED), and Norwegian (NOR) coastal waters. A planktonic model with seven functional compartments and 30-32 different carbon flows fit to all three experiments was used as a framework for flow-rate estimation and comparison. Flows were estimated on the basis of time series of measured biomass, some measured flows, and inverse modeling. Biomass and gross uptake rate of carbon of most groups increased linearly with increasing [L.sub.N] in the nutrient input range of 0-1 [micro]tool N [L.sup.-1] [d.sup.-1] at all locations. The fate of the gross primary production (GPP) was similar in all systems. Autotrophic biomass varied by two orders of magnitude among locations, with the lowest biomass and response to nutrient addition in MED waters. The variation of GPP among sites was less than one order of magnitude. Mesozooplankton dominated by doliolids (Tunicata), but not those dominated by copepods, presumably exerted efficient control of the autotrophic biomass, thereby buffering responses of autotrophs to high nutrient input. Among the many factors that can modify the responses of autotrophs to nutrients, the time scale over which the enrichment is made and the precise mode of nutrient enrichment are important. We suggest a general concept that may contribute to a scientific basis for understanding and managing coastal eutrophication.

We conducted a study with cadmium (Cd) and copper (Cu) in the delta of San Francisco Bay, using nitrogen and carbon stable isotopes to identify trophic position and food web structure. Cadmium is progressively enriched among trophic levels in discrete epiphyte-based food webs composed of macrophyte-dwelling invertebrates (the first link being epiphytic algae) and fishes (the first link being gobies). Cadmium concentrations were biomagnified 15 times within the scope of two trophic links in both food webs. Trophic enrichment in invertebrates was twice that of fishes. No tendency toward trophic-level enrichment was observed for Cu, regardless of whether organisms were sorted by food web or treated on a taxonomic basis within discrete food webs. The greatest toxic effects of Cd are likely to occur with increasing trophic positions, where animals are ingesting Cd-rich prey (or food). In Franks Tract this occurs within discrete food chains composed of macrophyte-dwelling invertebrates or fishes inhabiting submerged aquatic vegetation. Unraveling ecosystem complexity is necessary before species most exposed and at risk can be identified.

Elucidating the relative strength of top-down and bottom-up forces in communities of phytophagus insects has been a major historical focus. Current consensus is that both forces play a role, but it is poorly known if these forces act differently on herbivores in the same assemblage and what factors underlie this variation. Using manipulative experiments with an assemblage of sap-feeding phytophagous insects (six species of planthoppers, leafhoppers, and heteropteran bugs) inhabiting intertidal Spartina marshes, we examined the association between herbivore behavior, risk of predation, and ultimately the relative impact of top-down (wolf spider predation) and bottom-up (host-plant nutrition) factors on the population density of each sap-feeding herbivore. A factorial experiment on open Spartina islets in the field (two levels of plant nutrition crossed with two levels of spider predation) showed that bottom-up and top-down manipulations differentially affected the various sap-feeders. Overall, bottom-up effects dominated in this sap-feeder community, whereby the density of all six sap-feeders increased when the nitrogen content of Spartina was elevated. By contrast, wolf-spider addition significantly suppressed populations of only the Prokelisia species and had little impact on the other four sap-feeder species in the community. Functional-response experiments and behavioral studies revealed that certain species (Prokelisia planthoppers) were at much higher risk of attack by wolf spiders than other sap-feeders in the assemblage and that risk of predation was associated with a species' particular 'escape/defensive behavior.' Moreover, risk of spider predation was roughly linked to the strength of top-down impacts in the field, because species with ineffective escape behaviors and a high risk of spider attack (Prokelisia planthoppers) were the only sap-feeders whose populations were suppressed by spider predation in the field. Thus, specific behavioral characteristics of the sap-feeders on Spartina influenced risk of predation and the relative strength of top-down and bottom-up impacts on their population dynamics. Notably, all herbivores in this system were positively influenced by elevated plant nutrition, only the common sap-feeder species (Prokelisia planthoppers) were adversely affected by spider predation, and it was the rarer sap-feeders in the assemblage that were least impacted by predation. These results call into question the overall pervasiveness of top-down forces and underscore the primacy of basal resources in structuring this community of phytophagous insects. Key words: bottom-up vs. top-down impact; phytophagous insect community; planthopper; plant nutrition; population suppression; predation; Prokelisia; risk of predation; sap-feeding insects; Spartina alterniflora; spider; trophic cascades.

We explore two aspects of how predation within the rhizosphere influences nutrient availability, plant productivity, and aboveground community dynamics. First, plant roots and soil microbes have a long history of interaction that is centered on the reciprocal acquisition of carbon by microbes and nitrogen by plants. Predators within the rhizosphere alter these interactions in ways that benefit plants but also influence the dynamics of other species within ecosystems and processes that are important to ecosystem function and stability. These same predators regulate their prey in a traditional 'top-down' manner but in doing so alter the release of nutrients that may limit plant productivity and thereby affect plant growth in a 'bottom-up' fashion as well. Second, much attention has been given to the importance of specific interactions, or, as presented within this series, genes and/ or gene products as critical control points. We suggest that control should be viewed within the framework of interactivity. The interdependence between the aboveground and belowground realms can be explained in terms of the patterning of trophic interactions within the rhizosphere and the influence of these interactions on the supply of nutrients and rates of nutrient uptake by plants. While specific interactions may be important, it is the patterning of these interactions into assemblages of species that share similar growth rates and habitats that is the salient feature of the rhizosphere that confers stability, affects nutrient retention, and regulates aboveground and belowground dynamics. Key words: nutrient dynamics; rhizosphere; soil food webs; stability; trophic structure.