Food Web Analysis Research Articles

Importance of parasites and their life cycle characteristics in determining the structure of a large marine food web

Summary 1Despite their documented effects on trophic interactions and community structure, parasites are rarely included in food web analyses. The transmission routes of most parasitic helminths follow closely the trophic relationships among their successive hosts and are thus embedded in food webs, in a way that may influence energy flow and the structure of the web. 2We investigated the impact of parasitism on the food web structure of a New Zealand intertidal mudflat community. Different versions of the food web were analysed, one with no parasites, one with all parasite species and several other versions, each including a single parasite species. We measured key food web metrics such as food chain length, linkage density and proportions of top, intermediate and basal species. 3The inclusion of all parasite species in the food web resulted in greatly increased mean and maximum food chain length, but had little impact on linkage density and realized connectance. The main change caused by introduction of parasites was the relegation of a number of species from top predators to intermediate status, although the addition of parasites as top predators left the actual ratio of predators to prey relatively unchanged. 4When individual parasites were added to the food web, their effect on food web properties was generally minimal. However, one trematode species that affected several host species, because of its complex life cycle and low host specificity, produced food web properties similar to those in the web version including all parasite species. 5The respective effect of individual parasite species was roughly proportional to the number of host species they affected, and thus the life cycle characteristics of parasites determine to a large extent their impact on food web structure. The next step would be to quantify how they affect energy flow through the web.

A study of equal node models for food webs

It is argued to study better non-binary (weighted) food webs where links are measured quantitatively, instead of binary (unweighted) food webs where links are just present or absent. Binary webs can simply be deduced from the more general non-binary ones: if a flux is larger than a critical flux value (below denoted by “cut”) a link is drawn, otherwise it is not. However, the amount of a flux between two nodes critically depends on the biomass of these nodes. Unfortunately, a clear definition of what to subsume into one node is missing up to now. The simplest (and probably most important) statistical description of non-binary food webs is given by the flux distribution function. However, this function may strongly be changed by just merging two nodes into one (as we have shown). In order to solve this decisive problem we propose to setup food webs with all nodes containing the same amount of biomass: equal-node-webs. A simple merging of just two nodes is forbidden here, because such a merged node would contain more biomass. The best basis for such a new food web theory would be provided by empirical equal-node-webs, established by field ecologists. However, such data has not been available up to now. Nevertheless, we demonstrate some advantages of studying equal-node-webs. For that purpose, first we show—by means of two example food webs—how equal-node-webs can be deduced from already published non-binary webs. Second, we describe the deduced equal-node-webs by their flux distribution functions as well as by different statistical measures. It is shown that equal-node-webs are more scale invariant than common non-binary food webs. Therefore, equal-node-webs provide a crucial, trustworthy basis for sound statistical descriptions and comparative analyses of food webs. Moreover, the article contains a new detailed comparison between a highly productive lake and a low productive one. It is shown that the food web of the latter has a higher complexity than that of the former.

ELEVATED δ15N IN STREAM BIOTA IN AREAS WITH SEPTIC TANK SYSTEMS IN AN URBAN WATERSHED

Anthropogenic inputs of nitrogen from human sewage are a central concern in urban watershed management. However, identifying the locations of these inputs, whether from improperly functioning or ill‐maintained septic tanks or from leaking sewer lines, is difficult. We used nitrogen stable‐isotope analysis of aquatic food webs as indicators of sewage‐derived nitrogen in Valley Creek watershed. Stable nitrogen isotope analysis revealed elevated δ15N in all trophic levels at stations located downstream of the divide between sewered and nonsewered (septic tanks) neighborhoods in the watershed. Stations not located in the septic system area or on the other branch of the creek did not show this elevated level of δ15N. Allochthonus inputs, which do not derive nitrogen from aquatic sources, such as detrital leaf material, showed no difference in δ15N between Valley Creek and Little Valley Creek. Particular fish species that are found throughout both branches, such as blacknose dace and creek chub, had as much as a 10‰ enrichment in δ15N in septic vs. sewered areas. These results indicate that stable‐isotope analysis can be a useful tool for determining the location of inputs of human sewage and may have broad applications in watershed planning and management, especially in urban systems.

Internal variations in nutrient concentrations and the C and N stable isotope ratios in leaves of the seagrass Enhalus acoroides

We analyzed the content of tissue nutrients (carbon, nitrogen, and phosphorus), the C:N:P ratio, the carbon stable isotope ratio ( δ 13 C ) and the nitrogen stable isotope ratio ( δ 15 N ) of leaves of Enhalus acoroides (L. f.) Royle growing in two submerged beds in southwest Thailand. The N and P concentrations and δ 15 N decreased with increasing distance along the leaf. The maximum ranges in one leaf were 1.80–4.05% for N, 2.85–6.64 mg g −1 for P, and 1.28–2.93‰ for δ 15 N . The C:N, C:P, and N:P ratios increased as the distance along the leaf increased. The maximum ranges in one leaf were 12.5–24.3 for C:N, 162–622 for C:P, and 14.0–28.9 for N:P. In contrast, C concentration and δ 13 C fluctuated with distance along the leaf, and the longest leaves from each beds showed similar patterns of fluctuation. The variation of the chemical signature within leaves suggests that special attention is needed to apply these data to the evaluation of nutrient conditions during growth in food-web analyses involving seagrass beds.

PREDICTING OCCURRENCES AND IMPACTS OF SMALLMOUTH BASS INTRODUCTIONS IN NORTH TEMPERATE LAKES

Smallmouth bass and other warmwater littoral piscivores are presently expanding their geographic range northward into lakes across southern Canada. Smallmouth bass introduction can dramatically reduce minnow abundances, causing native lake trout to shift to low quality, invertebrate-based diets. Here we develop models to predict future occurrences and impacts of smallmouth bass in central Ontario, with the goal of identifying “vulnerable” lakes in order to better guide prevention efforts. Using local and regional environmental variables for 3046 central Ontario lakes, an artificial neural network was used to predict lakes that are likely to be invaded by bass. Smallmouth bass can significantly influence the occurrence and abundance of small-bodied fishes (mainly minnows), and stable isotope analysis of food webs in 18 lakes revealed that lake trout are buffered from impacts of bass on minnows in lakes containing pelagic prey fishes. In the absence of pelagic prey fishes, the trophic niche of lake trout depends on the presence of bass; lake trout feed primarily on zooplankton in the presence of bass, and minnows in the absence of bass. Of the 3046 lakes, the 788 lake trout lakes in central Ontario were classified according to their vulnerability to bass invasion based on the predictability of bass occurrence and their subsequent impacts, and mapped in a Geographic Information System (GIS). Only 48 lake trout lakes (6%) were classified as “high vulnerability”—predicted to be invaded and impacted by bass. Another 301 lakes had a sensitive food web structure but were not predicted to support a bass population. Based on this information, efforts to prevent further impacts can be optimized by focusing on this vulnerable subset of lakes.

Sulfur stable isotopes separate producers in marine food-web analysis.

Ecological applications of stable isotope analysis rely on different producers having distinct isotopic ratios to trace energy and nutrient transfer to consumers. Carbon (C) and nitrogen (N) are the usual elements analysed. We tested the hypothesis that producers unable to be separated using C and N would be separated by sulphur (S), by reviewing estuarine and marine food web studies using all three elements (total of 836 pairwise comparisons between producers). S had a wider range of values across all producers than C and N (S: 34.4, C: 23.3, N: 18.7 per thousand ), and a higher mean difference among producers (S: 9.3, C: 6.5, N: 3.3 per thousand ). We varied from 1 to 10 per thousand the distance producers must be apart to be considered separate. For each of these gap distances, S-separated producers tied on C and N in 40% or more of cases. Comparing the three elements individually, S had fewer tied pairs of producers for any gap distance than C or N. However, S also has higher within-producer variability. Statistical tests on simulated data showed that this higher variability caused S to be less effective than C for analysing differences among mean producer values, yet mixing models showed that S had the smallest confidence intervals around mean estimates of source contributions to consumers. We also examined the spatial and temporal scales over which S isotope signatures of the saltmarsh plant Spartina alterniflora varied. Differences between samples taken within tens of metres were smallest, but between samples hundreds of metres apart were as different as samples thousands of kilometres apart. The time between samples being taken did not influence S signatures. Overall, the use of S is recommended because it has a high probability of distinguishing the contribution of different producers to aquatic food webs. When two elements are employed, the combination of S and C separates more producers than any other combination.

Benthos as the basis for arctic lake food webs

Plankton have traditionally been viewed as the basis for limnetic food webs, with zooplankton acting as a gateway for energy passing between phytoplanktonic primary producers and fish. Often, benthic production has been considered to be important primarily in shallow systems or as a subsidy to planktonic food web pathways. Stable isotope food web analyses of two arctic lakes (NE14 and I minus) in the Toolik Lake region of Alaska indicate that benthos are the primary source of carbon for adults of all species of benthic and pelagic fish present. We found no effect of turbidity, which may suppress benthic algae by shading, on food web structure. Even though Secchi transparency varied from 10.2 m in NE14 to 0.55–2.6 m in I minus, food webs in both lakes were based upon benthos, had four trophic levels, and culminated with omnivorous lake trout. We suggest that the importance of benthos in the food webs of these lakes is due to their extreme oligotrophy, resulting in planktonic resources that are insufficient for the support of planktivorous consumers.

What is hidden behind the concept of ecosystem efficiency in energy transformation?

The number of energy transformation levels in trophic webs is usually below five, but can be extended up to ten when parasites and hyper-parasites are included. Research on the structure and function of food webs is relevant to the complexity–stability–productivity debate. The aim of this theoretical analysis is to link energetic and connectional aspects of ecosystems with information theory. Updating an energetic model reported by Ricklefs [Ecologia, Zanichelli Editore S.p.A., Bologna, Italy, 1993, p. 896], our approach is integrated with a static analysis of food webs. The length of food webs is theoretically associated with the average ecological efficiency which can be empirically correlated with the effective connectance between species. Furthermore, the advantage of greater complexity when applied to a signalling network is qualitatively addressed. The overall efficiency of energy transformation into biomass throughout a trophic web, in an ecosystem with a given number of species, is the resultant of the various ecological efficiencies, η, at the transitions between the trophic levels. However, we propose that an increment in effective connectance and interspecies connectivity based on a superimposed signalling web may increase the η values, despite the fact that signalling per se has an energetic cost. According to this hypothesis, ecosystem stability would not be necessarily reduced by increasing the number of trophic levels, N, whenever stability in terms of persistence is improved by a cost-efficient regulatory network.

Nitrogen loading to Pleasant Bay, Cape Cod: application of models and stable isotopes to detect incipient nutrient enrichment of estuaries

To test and refine methods to detect nutrient enrichment and resulting eutrophication, we applied the Waquoit Bay nitrogen loading model (NLM) and Estuarine loading model (ELM) to estuaries of Pleasant Bay that receive increasing but low N loads (25–199 kg N ha −1 yr −1) from land. Contributions of wastewater to these estuaries increased from 7% to 63% as N loads increased, and modeled estimates of dissolved inorganic nitrogen in the water were within ∼27% of measured values. N isotopic signatures in suspended and benthic organic matter and in tissue of quahogs increased as wastewater contributions to N loads increased, with clams ∼4‰ heavier than organic matter, indicating that even at these low N loads, N from land-derived sources moved detectably up the food web. These results extend the application of NLM and ELM to detect incipient levels of N enrichment and demonstrate that these models can be used in conjunction with isotope measurements as the basis for food web analyses in a system exposed to relatively lower N loads than previously studied.

Combined stable isotope and gut contents analysis of food webs in plant‐dominated, shallow lakes

Summary 1. To determine feeding links between primary producers, invertebrates and fish, stable isotope analyses and gut content analyses of fish were conducted on the components of four shallow, eutrophic to hypertrophic, plant‐dominated lakes.2. Although separation of basal resources was possible, the diets of both fish and invertebrates were broad, comprising food from different compartments (planktonic, epiphytic/benthic), as well as from different trophic levels.3. Mixing models were used to determine the extent to which periphyton production supported higher trophic levels. Only one species of invertebrate relied upon periphyton production exclusively.4. Fish density affected the diets of invertebrates. The response was different for planktonic and epiphytic/benthic invertebrates. The proportion of periphyton production in the diets of zooplankton appeared to increase with fish density, whilst it decreased for other invertebrates.5. As all zooplankton samples were collected in the open water at dusk, these results are further evidence for the diurnal horizontal migration of zooplankton. Although not conclusive, they are consistent with a behavioural response by invertebrates and zooplankton in the presence of fish.

Analysis of δ 13C, δ 15N, and δ 34S in organic matter from the biominerals of modern and fossil Mercenaria spp.

The response of primary producers to changes in environmental and sea-level conditions is an important topic in the study of estuarine ecosystems. Currently, sea levels are rising, and North American estuaries are in declining ecological health (USEPA, 2002 National Coastal Condition Report. USEPA, Washington, DC). This investigation examines the base of the estuarine food web by evaluating the diets of modern and fossil Mercenaria spp. These clams are infaunal, primary consumers that feed on suspended organic matter. Proteinaceous materials secreted within the biominerals of shell-building organisms are derived from the same diet sources as soft tissue and, therefore, are also capable of recording diet information. We have developed an empirical relationship between the isotopic composition of shell organic matter and soft tissue, which allows food web analyses in fossils to proceed in the same manner as modern food web studies. The Δ tissue-shell for δ 13C, δ 15N, and δ 34S is 0.1, 0.7, and 1.8‰. Isotopic analyses of modern Mercenaria spp. shells from coastal Virginia (δ 13C: −12.8 to −15.9‰, δ 15N: 11.3 to 13.7‰), and the Gulf of Mexico coast of Florida (δ 13C: −19.4 to −21.0‰, δ 15N: 7.2–9.9‰) demonstrate that diets are derived from phytoplankton and local salt marsh plants. Shell organic-matter δ 13C varies from −13.1 to −27.5‰ and δ 15N varies from 2.4 to 9.8‰ in Quaternary Mercenaria spp. from coastal Virginia, North and South Carolina, and Georgia. These data demonstrate that past diet sources have changed in space and time. The fact that past sea-level changes have been accompanied by shifts in local primary production is evidence that significant changes might also occur in association with presently rising sea levels.

Stable isotope food web analysis of a large subtropical lake: alternative explanations for 15N enrichment of pelagic vs. littoral fisheries.

The food webs of littoral, pelagic, and littoral-pelagic ecotone (interface) regions of a large subtropical lake were investigated using stable isotope ratio methods, expanding the focus of a previous fish-only study to include other food web components such as primary producers and invertebrates. In these food webs, δC increased ~4o/oo and δN increased ~10o/oo from primary producers to fish. The δN of fish was ~9o/oo in the littoral zone, ~10 o/oo in the ecotone, and ~12o/oo in the pelagic zone. The cross-habitat enrichment in fish N corresponded with both an increase in the size of fish and an increase in the δN of primary consumers (mollusks). Despite larger body size in the pelagic zone, fish in all three habitats appear to occur at the same average trophic level (TL = 4), assuming an enrichment factor of 3.4o/oo per trophic level, and normalizing to the δN of primary consumers.

IMPACTS ON STREAM FOOD WEBS OF NATIVE AND EXOTIC FOREST: AN INTERCONTINENTAL COMPARISON

Native pine-forest streams from Maine and North Carolina, USA, and exotic pine-forest streams from New Zealand were compared to assess the effects of geographic location on three aspects of community structure: (1) taxonomic composition, (2) trophic structure (summarized in terms of functional feeding groups), and (3) food web structure (as connectivity food webs). In addition, pine-forest assemblages in New Zealand were compared to assemblages from New Zealand native forest and grassland streams. Taxonomic similarity was, as expected, low for invertebrates, but there were strong similarities in the algal assemblages in different geographic locations. Trophic structure analysis was unable to distinguish either geographic or land-use effects. Food web analysis revealed structural similarities between the pine-forest streams, regardless of location, but there were clear differences among land uses in New Zealand. Pine-forest streams were typified by food webs with few algal species, low internal connectance and a relatively square shape. Grassland food webs were more triangular in shape and exhibited high internal connectance, while native forest food webs had intermediate characteristics. The results show that the native stream biota, despite a distinct species composition, can adapt to a novel riparian vegetation type and produce trophic and food web structures that are difficult to distinguish from those in the country of origin. Corresponding Editor: S. L. Kohler.

In situ identification by fluorescently labeled oligonucleotide probes of morphologically similar, closely related ciliate species.

Ciliate protozoa are important members of microbial communities in which they play specific ecological roles. The determination of single species distribution is fundamental for food web analysis, but species recognition, which is mainly based on morphological characters, is often difficult between closely related species. The use of species-specific, purposely designed, fluorescently labeled probes for in situ hybridization is here presented as an easy and fast identification method for three closely related species belonging to the widespread genus Euplotes, namely E. crassus, E. vannus, and E. minuta, that in spite of their remarkable morphological similarity have significant metabolic and ecological differences. These three species can be detected simultaneously, provided the probes employed are bound to different fluorescent dyes: in this way their relative abundance and their population dynamics in the natural environment can be evaluated. As more ciliate sequences become available in databases, species-specific probes can be designed for other ciliates, thus rendering the application of the method of more general importance. The probes used in this study may also provide a tool to prevent erroneous species identification in future studies.

Topological approaches to food web analyses: a few modifications may improve our insights

The topological analysis of food webs is a recently reinvigorated and rapidly growing area of inquiry in community ecology (Williams and Martinez 2000, Sole and Montoya 2001, Camacho et al. 2002, Montoya and Sole 2002). Topological studies assess system properties using the number and distribution of connections among nodes in an interconnected network. Analysis of a food web as a network of links (feeding relationships) and nodes (species) is not a new approach (MacArthur 1955, Gardner and Ashby 1970, Pimm 1979); however recent advances by physicists in the study of complex networks have revived this area of theoretical community ecology. Here we discuss a few characteristics of food webs that may cause them to respond differently to node loss than other types of networks. We also suggest ways in which empiricists can provide data to test predictions derived from complex network theory. Complex network analysis has shown that the sensitivity of a network to node loss depends on the frequency distribution of connections among nodes (Albert et al. 2000). Current theoretical developments suggest that networks can be classified into two broad categories based on the frequency distribution of links: exponential or scale-free. Each node in an exponential network has a similar number of links to other nodes. The frequency distribution of the number of links per node in this type of network has an exponential decay (Albert et al. 2000). Because nodes in an exponential network have similar numbers of links, the loss of any given node from this type of network causes a monotonic increase in the number of links required to connect any two nodes in the network. In contrast, a scale-free network has a few nodes with a large number of links and many nodes with only a few links. Although connectivity in scale-free networks does not decrease with the random loss of nodes as in an exponential network, scale-free networks are extremely sensitive to the loss of highly connected hub nodes (Albert et al. 2000). In recent work, researchers have used frequency distribution models to describe the properties of a broad array of complex networks such as social networks (Watts and Strogatz 1998), the World Wide Web (Albert et al. 1999), transportation networks (Banavar et al. 1999), and enzymatic pathways (Jeong et al. 2000). The obvious extension of this theory to food web dynamics has renewed interest in the topology of trophic webs. Williams and Martinez (2000) show that a simple topological model can reproduce properties of complex food webs. But when we examine empirical webs, does the frequency distribution of trophic connections show a general pattern? If so, is the structure of food webs generally exponential or scale-free? A recent assessment of several food webs suggests that they share the scalefree properties of many other complex networks, implying that their structure should be resistant to random attacks but quite sensitive to loss of hub species (Sole and Montoya 2001). The general applicability of recent findings in complex network theory to food web studies depends on whether topological descriptions capture key aspects of communities and ecosystems such as the flow of energy or factors regulating populations. An increase in biological realism may be merged with suitable data in future food web analyses to better understand processes structuring communities. Further work in this area will allow us to determine the appropriate role of complex network theory in the modeling and conservation of ecological communities.

Interpreting stable isotopes from macroinvertebrate foodwebs in saline wetlands

We compared stable‐isotope (δ13C and δ15N) and gut‐content analyses of macroinvertebrate foodwebs in saline wetlands of the Laramie Basin, Wyoming, USA. Standard assumptions of stable‐isotope fractionation between trophic levels (<1‰ for δ13C, mean of 3.4‰ for δ15N) suggested that zygopteran (damselfly) larvae consumed mainly amphipods. However, the guts of zygopterans contained no amphipods but rather a mix of chironomid larvae and zooplankton. In all wetlands the gut contents of zygopterans indicated that they were secondary consumers (trophic level 3), but enrichment of δ15N between zygopterans and their prey (Δδ15N) varied from 1 to 3.4‰ between wetlands. In other studies, such variation in Δδ15N has been interpreted to mean that food‐chain length differed between aquatic systems. We review alternative interpretations of variable 15N enrichment, namely, varying C:N ratios in food, differential enrichment between consumer species, and habitat‐specific variation of δ15N at the base of foodwebs. We also suggest that variation in the timing and rates of nitrogen cycling can affect measured Δδ15N both within and between foodwebs. For aquatic macroinvertebrates, we urge that stable isotopes be supplemented with independent observations to avoid incorrect conclusions about trophic pathways, trophic levels, and food‐chain lengths in different ecosystems.

FEMME, a flexible environment for mathematically modelling the environment

A new, fortran-based, simulation environment called FEMME (Flexible Environment for Mathematically Modelling the Environment), designed for implementing, solving and analysing mathematical models in ecology is presented. Three separate phases in ecological modelling are distinguished: (1) the model formulation i.e. the choice and implementation of the constitutive relations of the mathematical model, (2) the choice of a numerical solution scheme and (3) the model application i.e. a particular run or set of runs of the model. The basic characteristic of FEMME is to keep these three phases independent. The software is structured in a highly modular fashion, allowing unlimited combination of each. The modeller can restrict attention mostly to the model formulation, while the numerical solution and model application can be specified at run time without programming effort. The object-oriented design strongly reduces redundancy in the code: the same set of solution procedures is linked to all models, and one model implementation can be used without extra coding in a variety of applications. FEMME contains many functional units, such as a diversity of integration routines, steady-state solvers, fitting routines, input and output facilities and allows running Monte Carlo or sensitivity analyses or performing food web analyses. The ease of interfacing model applications with external data facilitates running different scenarios or fitting the model to observations.

Comparative analysis of food webs based on flow networks: effects of nutrient supply on structure and function of coastal plankton communities

The objective of COMWEB was to develop efficient analytical, numerical and experimental methods for assessing and predicting the effects of nutrient (N, P, Si) supply on the stability and persistence of pelagic food web structure and function in coastal waters. The experimental comparative work included a geographic gradient covering Baltic, Mediterranean, and NE Atlantic waters and a NE Atlantic gradient in state of eutrophication. COMWEB has been an experimental approach to coastal eutrophication, studying effects of enhanced nutrient supply on components and flows of the entire lower pelagic food web. Flow network representations of pelagic food webs has been a framework of data reduction and flows were established by sophisticated inverse modelling. Fundamental information on physiological properties of functional key species in the pelagic food web was used to constrain flow estimations. A main conclusion derived from the flow networks was that very little energy and materials were transferred from the microbial food web to the main food chain. The lower food web could therefore be described as two parallel food chains with relatively limited interaction between heterotrophic groups. Short-term effects of nutrient perturbations were examined in mesocosms along the geographic gradient. The response was comparable in all systems, with a stronger effect on the activity and biomass of autotrophic groups than those of heterotrophic ones. Mediterranean waters showed much lower autotrophic biomass response than Baltic and NE Atlantic waters, which responded almost equally. The response of primary production was, however, more comparable. High phytoplankton lysis rate explained this low accumulation of biomass in Mediterranean waters. The study of Atlantic coastal waters of different eutrophic states revealed that the ecological response was higher in the closed nutrient perturbed mesocosms than in open systems exposed for >4 summer months (summer/autumn season). The Atlantic lagoon evolved gradually from the natural oligotrophic situation towards the more eutrophicated North Sea during fertilisation. The responses observed on seasonal and long-term scale (>10 years) may therefore be equal. The differences between short-term (weeks) and intermediate-term (seasonal) responses is most likely a result of the different time scales of perturbation and observation and the variable exchange rates with surrounding waters (water dilution rate). The analysis of pelagic flow networks provided a framework of diagnostic criteria for state and quality assessment of coastal waters. The nutrient loading rates related better to estimates of biotic fluxes than to concentrations of biotic compartments and total nutrients. On the contrary, the concentration of biotic compartments, or the biomasses, related better to total nutrient concentrations. Primary production, mesozooplankton grazing and growth, fraction of primary production consumed by grazers, bacterial production relative to primary production, cycling indices, and path lengths were all well related to nutrient loading rate. Autotrophic biomass, ratio of autotrophic to heterotrophic biomass, and fraction of pico-cyanobacteria of total autotrophic biomass were all related to total nutrients. Some of these variables, which responded equally in all systems, have the potential of becoming unified response functions in a management model for European coastal waters. COMWEB has provided further insight into the mechanisms behind coastal eutrophication. A main achievement is the conceptual framework for unified response functions, important components of management models for nutrient emission to coastal waters.

The Conflicting Values and Differing Priorities of Biological Control

The Conflicting Values and Differing Priorities of Biological Control

Foodweb analysis of the Orinoco floodplain based on production estimates and stable isotope data

AbstractStable isotope data and indirect methods for estimating consumer production were used in a foodweb analysis for the Orinoco River floodplain, Venezuela. The ratio of annual production to mean annual biomass (Pa/B) was estimated from mass at maturity for all major categories of consumers. Field data on B then were used to estimate Pa for each major category. Carbon sources for all categories of consumers were identified through δ13C analysis, and trophic shifts in δ15N were used in assigning trophic levels to consumers. The ultimate C source for both invertebrates and fish was algae (phytoplankton and periphyton), even though macrophytes and litterfall from the floodplain forest composed 98% of potentially available C. Production of invertebrate consumers (benthos, rhizofauna, and zooplankton), which occurred almost entirely through 1st-level consumption, was estimated as 14 g C m−2 y−1. Fish, which produced 11 g C m−2 y−1, showed a mean δ15N trophic shift of 1.8 units above the algal C source, s...