Riverine Productivity Model Research Articles

Large herbivorous wildlife and livestock differentially influence the relative importance of different sources of energy for riverine food webs

In many regions of the world, large populations of native wildlife have declined or been replaced by livestock grazing areas and farmlands, with consequences for terrestrial-aquatic ecosystem connectivity and trophic resources supporting food webs in aquatic ecosystems. The river continuum concept (RCC) and the riverine productivity model (RPM) predict a shift of energy supplying aquatic food webs along rivers: from terrestrial inputs in low-order streams to autochthonous production in mid-sized rivers. In Afromontane-savanna landscapes, the shifting numbers of large mammalian wildlife present a physical continuum whose ecological implications for rivers is not clearly understood. Here, we studied the influence of replacing large wildlife (mainly hippos) with livestock on the fractional contribution of C3 vegetation, C4 grasses and periphyton on macroinvertebrates in the Mara River, which is an African montane-savanna river known to receive large subsidy fluxes of terrestrial organic matter and nutrients mediated by large mammalian herbivores (LMH), both wildlife and livestock, in its middle and lower reaches. Using stable carbon (δ13C) and nitrogen (δ15N) isotopes, we identified spatial patterns in the fractional contribution of allochthonous organic matter from C3 and C4 plants (woody vegetation and grasses, respectively) and autochthonous energy from periphyton for macroinvertebrates at various sites of the Mara River and its tributaries. Potential energy sources and invertebrates were sampled at 80 sites spanning stream orders 1 to 7, various catchment land uses (forest, agriculture and grasslands) and different loading rates of organic matter and nutrients by LMH (livestock and wildlife, i.e., hippopotamus). The fractional contribution of different sources of energy for macroinvertebrates along the river did not follow predictions of the RCC and RPM. First, the fractional contribution of C3 and C4 carbon was not related to river order or location along the fluvial continuum but to the loading of organic matter (dung) by both wildlife and livestock. Notably, C4 carbon was important for macroinvertebrates even in large river sections inhabited by hippos. Second, even in small 1st -3rd order forested streams, periphyton was a major source of energy for macroinvertebrates, and this was fostered by livestock inputs fuelling aquatic primary production throughout the river network. Importantly, our results show that replacing wildlife (hippos) with livestock shifts river systems towards greater reliance on autochthonous sources of energy through an algae-grazer pathway as opposed to reliance on allochthonous inputs of C4 carbon through a detrital pathway.

Longitudinal pattern of resource utilization by aquatic consumers along a disturbed subtropical urban river: Estimating the relative contribution of resources with stable isotope analysis.

The utilization of food resources by aquatic consumers reflects the structure and functioning of river food webs. In lotic water systems, where food availability and predator–prey relationships vary with gradient changes in physical conditions, understanding diet assimilation by local communities is important for ecosystem conservation. In the subtropical Liuxi River, southern China, the relative contribution of basal resources to the diet assimilation of functional feeding groups (FFGs) was determined by stable carbon (13C) and nitrogen (15N) isotope analyses. The output of Bayesian mixing models showed that diatom‐dominated periphyton (epilithic biofilm), aquatic C3 plants (submerged hydrophytes), and suspended particulate organic matter (SPOM) associated with terrestrial C3 plants contributed the most to the diet assimilation of FFGs in the upper, middle, and lower reaches, respectively. The relative contribution of consumer diet assimilation was weighted by the biomass (wet weight, g/m2) of each FFG to reflect resource utilization at the assemblage level. From the upper to the lower reaches, the spatial variation in the diet assimilation of fish and invertebrate assemblages could be summarized as a longitudinal decrease in periphyton (from 57%–76% to <3%) and an increase in SPOM (from <7% to 51%–68%) with a notable midstream increase in aquatic C3 plants (23%–48%). These results indicate that instream consumers in the Liuxi River rely more on autochthonous production (e.g., periphyton and submerged hydrophytes) than on terrestrially derived allochthonous matter (e.g., terrestrial plants). The pattern of resource utilization by consumers in the mid‐upper Liuxi River is consistent with findings from other open subtropical and neotropical rivers and provides evidence for the riverine productivity model. Our study indicates that protecting inherent producers in rivers (e.g., periphyton and submerged hydrophytes) and restoring their associated habitats (e.g., riffles with cobble substrate) are conducive to aquatic ecosystem management.

Spatial and temporal variability in the nutritional quality of basal resources along a temperate river/estuary continuum

The nature and quality of basal resources within aquatic food webs are complex and have the potential to shift over space and time. We used fatty acid analysis to assess variations in the nutritional structure of the suspended and basal pools along an entire river system, and to assess the contributions of vascular plant (i.e., mainly of terrestrial origin, but could include aquatic macrophytes) vs algal (i.e., aquatic origin) sources to the suspended particulate matter (SPM) pools. Samples were collected in a temperate South African river on four occasions between September 2012 and June 2013. We found orderly patterns in the fatty acid composition of the basal resources at the sites during most seasons. Regardless of site or season, the benthic algal pools (epiphyton, epipelon and epilithon) were the most nutritionally rich resources based on essential fatty acid contents and diatom indices. During early and late spring, proportions of essential fatty acids in the epiphyton decreased downstream where increased light was available, consistent with predictions from the light:nutrient hypothesis (but inconsistent with epilithon and epipelon results). There were substantial changes in vascular plant contributions to the SPM pools along the river, but the same patterns were not produced in all seasons. Of all the river models considered, the data were more consistent with the riverine productivity model, which particularly emphasises the importance of autotrophic production in rivers. Our study provides new detail on the complexity of basal resource nutritional quality and how it can shift along a lotic system over time.

Modeling terrestrial carbon sources for juvenile Chinook salmon in the Merced River, California

This study makes inferences about primary carbon sources for juvenile Chinook salmon (Oncorhynchus tshawytscha) in a highly regulated stream by measuring stable isotope carbon ratios in muscle tissues of net-penned hatchery salmon over three rearing periods and four locations. We used a stable isotope two-end member mixing model within an innovative hierarchical nested Bayesian framework to assess the relative contribution of allochthonous sources for juvenile Chinook salmon in the Merced River, California. The analyses revealed that the contribution of terrestrial carbon sources for juvenile fish was highly variable both spatially and temporally, ranging from 3% to 27% across years and sites. The percent of biomass accounted for by allochthonous carbon suggested that autochthonous carbon was the primary source supporting juvenile Chinook salmon. In 2011 (wet year), our findings were fully compatible with River Continuum Concept since juvenile Chinook salmon allochthony decreased downstream. In 2012 and 2013 (dry years), juvenile Chinook salmon allochthony longitudinal patterns were compatible with the Riverine Productivity Model (foodweb driven primarily by local production), indicating the importance of terrestrial organic carbon contributions from local sources downstream. The primary outcome of this research was that juvenile Chinook salmon utilized both aquatic and terrestrial primary carbon sources in this system and this changed longitudinally through time.

Potamoplankton and primary productivity in the River Danube

Autochthonous production of potamoplankton has recently attracted greater interest as it was incorporated into expanded river concepts such as the flow pulse concept or the riverine productivity model (RPM). This review assembles data on primary production from the River Danube to evaluate the importance of productivity in large rivers. Results indicate positive net production in the middle reach of the river and in impoundments. These sections are characterised by favourable conditions for algal growth. Reduction in flow, reduced concentrations of suspended solids and improved under-water light result in significant increase in plankton biomass. Maximum chlorophyll concentrations were below 20 mg m−3 in 2007 but concentrations up to 130 mg m−3 have been recorded in the past. Since nutrients are not limiting, as in most large rivers, net primary production is largely controlled by availability of photosynthetic active radiation under water, chlorophyll-a, water depth and discharge. Hourly carbon uptake rates of 3–130 mg C m−3 h−1 observed in the Danube are well within the range of 0–790 mg C m−3 h−1 for large rivers of the world. Autochthonous autotrophic production must be regarded as an important feature of large rivers supporting the RPM concept.

Environmental factors affecting incorporation of terrestrial material into large river food webs

AbstractThree enduring conceptual models make predictions regarding the basal production sources supporting the upper food web of rivers: the River Continuum Concept, the Flood Pulse Concept, and the Riverine Productivity Model. Sources of primary production supporting metazoan biomass might best be understood by using a pluralistic approach that views basal sources as a dependent variable and key physicochemical and hydrological factors as independent variables. Here, I review studies in which chemical markers (stable-isotope and fatty-acid analyses) were used to estimate dominant primary producers contributing to consumer biomass in large rivers and evaluate associated independent variables (e.g., hydrologic regime, turbidity, concentration of dissolved organic matter, floodplain vegetation, lateral connectivity, and upstream impoundment) that have been hypothesized to contribute to variation in basal production sources. My review shows that C4 grasses rarely support riverine metazoans and that algae ar...

Spatial and temporal patterns of carbon flow in a temperate, large river food web

Using natural abundances of stable carbon (δ13C) and nitrogen (δ15N) isotopes, we quantified spatial and temporal patterns of carbon flow through the main channel food web in the lowland section of New Zealand’s longest river, the Waikato River. The study was undertaken with the objective of determining whether the Waikato River conforms to contemporary theoretical concepts regarding carbon flow in large river food webs. Potential organic carbon sources and invertebrate and fish consumers were sampled from three different hydrogeomorphic zones on six occasions, representing a range of seasonal and flow conditions. In line with the predictions of the riverine productivity model and riverine ecosystem synthesis, autochthonous algae and biofilms were the most important basal carbon source contributing to consumer biomass. These were often supported by C3 aquatic macrophytes and allochthonous C3 riparian plants. The relative importance of organic carbon sources differed between zones and appeared to change depending on season, presumably in response to water temperature and flow, particularly in the unconstrained zone of the lower river. We also demonstrate that to draw robust conclusions, consideration must be given to quantifying the isotopic signatures of organisms lower in the food web, as these can change significantly between sampling times and hydrogeomorphic zones.

Carbon Sources Supporting Large River Food Webs: A Review of Ecological Theories and Evidence from Stable Isotopes

Abstract Quantifying the contributions of carbon sources that support food webs in large rivers is an important and growing field of ecological research with implications for future management and rehabilitation. Here we review theoretical concepts and recent empirical evidence that address carbon flow through aquatic food webs in large rivers. The literature reviewed focuses on studies using stable isotope analysis, which is a tested framework for identifying the origin of carbon sources that are assimilated by primary consumers and subsequently transferred through the food web to support higher consumers. Theoretical concepts addressing carbon flow in large river food webs have tended to stress the importance of organic matter originating from different sources, such as floodplains (Flood Pulse Concept), local riparian and aquatic primary producers (Riverine Productivity Model), or leakage from upstream processing of terrestrial organic matter (River Continuum Concept). Recent empirical evidence from a ...

Early fish growth varies in response to components of the flow regime in a temperate floodplain river

Summary 1. The biological productivity of floodplain rivers is intimately related to their flow regimes and it has been proposed that fish production should be linked to components of the flow regime in productivity models. To assess applicability of existing models of productivity in floodplain rivers, we tested predictions about growth during the early life stages of a common, short-lived fish (Australian smelt Retropinna semoni) in a non-flow-altered, temperate Australian floodplain river. 2. The morphometric condition of larval and juvenile fish measured over a five-year period was positively related to annual discharge, but the highest average seasonal growth rates occurred in two years of contrasting hydrology, one with early spring flooding and the other with predominantly low flows and a late season (within channel) flow pulse. 3. Analysis of daily growth measures indicated that timing, river height, the duration of in-channel flow events and antecedent flood events are all significant factors influencing the early growth of Australian smelt. The flexible manner in which fish growth responds to these factors appears to be an effective early life history strategy for a short-lived species occupying a highly variable environment. 4. Growth rates conformed to some predictions of the Flood Pulse Concept (in particular the Extended Flood Pulse Concept), but specific growth responses suggest that the Riverine Productivity Model and tenets of the Low Flow Recruitment Hypothesis best describe the production of Australian smelt in this system. We suggest that none of the existing conceptual models adequately describes fish productivity in temperate Australian floodplain rivers but that aspects of each are likely to be relevant under different flow conditions.

Allochthonous and autochthonous carbon sources for fish in floodplain lagoons of an Australian dryland river

Dryland rivers associated with arid and semi-arid land areas offer an opportunity to explore food web concepts and models of energy sources in systems that experience unpredictable flooding and long dry spells. This study investigated the sources of energy supporting three species of fish feeding at different trophic levels within floodplain lagoons of the Macintyre River in the headwaters of the Murray-Darling river system, Australia. Stable isotope analyses revealed that fish consumers derived, on average, 46.9% of their biomass from zooplankton, 38.1% from Coarse Particulate Organic Matter (CPOM) and 24.0% from algae. Ambassis agassizii derived on average 57.6% of its biomass carbon from zooplankton and 20.4–27.8% from algae or CPOM. Leiopotherapon unicolor derived most of its carbon from zooplankton and CPOM (38.3–39.5%), with relatively high contributions from algae compared to the other species (33.3%). An average of 48.4% of the biomass of Nematalosa erebi was derived from zooplankton, with CPOM contributing another 38.1%. Zooplankton was the most important source of organic carbon supporting all three fish species in floodplain lagoons. Phytoplankton, and possibly, particulate organic matter in the seston, are the most likely energy sources for the planktonic suspension feeders (zooplankton) and, consequently, the fish that feed on them. These results indicate a stronger dependence of consumers on autochthonous sources and on locally produced organic matter from the riparian zone (i.e., the Riverine Productivity Model), than on other resources.

Landscape-Scale Hydrologic Characteristics Differentiate Patterns of Carbon Flow in Large-River Food Webs

Efforts to conserve, restore, or otherwise manage large rivers and the services they provide are hindered by limited understanding of the functional dynamics of these systems. This shortcoming is especially evident with regard to trophic structure and energy flow. We used natural abundances of carbon and nitrogen isotopes to examine patterns of material flow in ten large-river food webs characterized by different landscape-scale hydrologic characteristics (low-gradient river, high-gradient river, river stretches downstream of reservoirs, and reservoirs), and tested predictions from three ecosystem concepts commonly applied to large-rivers: The River Continuum Concept, The Flood Pulse Concept and the Riverine Productivity Model. Carbon derived from aquatic C3 plants and phytoplankton were the dominant energy sources supporting secondary consumers across the ten large-river food webs examined, but relative contributions differed significantly among landscape types. For low-gradient river food webs, aquatic C3 plants were the principal carbon source, contributing as much as 80% of carbon assimilated by top consumers, with phytoplankton secondarily important. The estimated relative importance of phytoplankton was greatest for food webs of reservoirs and river stretches downriver from impoundments, although aquatic C3 plants contributed similar amounts in both landscape types. Highest 99th percentile source contribution estimates for C4 plants and filamentous algae (both approximately 40%) were observed for high-gradient river food webs. Our results for low-gradient rivers supported predictions of the Flood Pulse Concept, whereas results for the three other landscape types supported the Riverine Productivity Model to varying degrees. Incorporation of landscape-scale hydrologic or geomorphic characteristics, such as river slope or floodplain width, may promote integration of fluvial ecosystem concepts. Expanding these models to include hydrologically impacted landscapes should lead to a more holistic understanding of ecosystem processes in large-river systems.

Patterns of primary and heterotrophic productivity in an arid lowland river

AbstractThree river conceptual models make differing predictions about the major source of primary production in lowland rivers, acknowledging the importance of primary productivity in the ecology and management of lowland rivers. Patterns of primary production in lowland rivers are still an area of considerable uncertainty. The objective of this study was to examine the major sources and transformations of organic matter in an Australian lowland river and compare them to the predictions of existing models. The broad approach adopted was to quantify the contribution from the major ecosystem components and compare these with estimates of system metabolism determined using open water measures of diel oxygen change. Three 4‐km river reaches were selected to represent the extent of variation found along the free‐flowing lowland sections of the Murray River, one of Australia's largest and most regulated rivers. Annual open water gross primary production (GPP) estimates for the Murray R. during this study ranged from 221 to 376 gC m−2 y−1 and were similar to other large rivers. Examination of the net contribution of organic matter to the channel indicates that primary productivity in the Murray R. is derived from a combination of phytoplankton, riparian vegetation and macrophytes, but that the major source varies both spatially and temporally. The present study confirms that the River Continuum Concept (RCC), the Flood Pulse Concept (FPC) and Riverine Productivity Model (RPM) all have some application to Australian lowland rivers, but that synthesis of the models will be difficult until we can incorporate the extent, causes and consequences of primary production variability. This study also highlights the importance of the microbial loop and macrophytes in the ecology of the Murray R. Copyright © 2007 John Wiley &amp; Sons, Ltd.

Regulation and Seasonal Dynamics of Extracellular Enzyme Activities in the Sediments of a Large Lowland River

We tested whether seasonal changes in the sources of organic substances for microbial metabolism were reflected changes in the activities of five extracellular enzymes in the eighth order lowland River Elbe, Germany. Leucine aminopeptidase showed the highest activities in the water column and the sediments, followed by phosphatase > beta-glucosidase > alpha-glucosidase > exo-1,4-beta-glucanase. Individual enzymes exhibited characteristic seasonal dynamics, as indicated by their relative contribution to cumulative enzyme activity. Leucine aminopeptidase was significantly more active in spring and summer. In contrast, the carbohydrate-degrading enzymes peaked in autumn, and beta-glucosidase activity peaked once again in winter. Thus, in sediments, the ratio of leucine aminopeptidase/beta-glucosidase reached significant higher medians in spring and summer (5-cm depth: ratio 7.7; 20-cm depth: ratio 10.1) than in autumn and winter (5-cm depth: ratio 3.7, 20-cm depth: ratio 6.3). The relative activity of phosphatase in the sediments was seasonally related to both the biomass of planktonic algae as well as to the high content of total particulate phosphorus in autumn and winter. Due to temporal shifts in organic matter supply and changes in the storage capacity of sediments, the seasonal peaks of enzyme activities in sediments exhibited a time lag of 2-3 months compared to that in the water column, along with a significant extension of peak width. Hence, our data show that the seasonal pattern of extracellular enzyme activities provides a sensitive approach to infer seasonal or temporary availability of organic matter in rivers from autochthonous and allochthonous sources. From the dynamics of individual enzyme activities, a consistent synoptic pattern of heterotrophic functioning in the studied river ecosystem could be derived. Our data support the revised riverine productivity model predicting that the metabolism of organic matter in high-order rivers is mainly fuelled by autochthonous production occurring in these reaches and riparian inputs.

Cinétique du zooplancton dans un continuum aquatique : de la Marne et son réservoir à l’estuaire de la Seine

A study was carried out within a 700-km river sector, including three types of ecosystems (a reservoir, a river and its estuary) to characterise the major features of zooplankton communities in the Seine Basin. In rivers, zooplankton biomass becomes significant only when the growth rate of the organisms is higher than the dilution rate (4–5th orders rivers, according to River Continuum Concept). Upstream, short residence times favour the development of small species (Rotifers) with low individual body weight and biomass. Conversely, larger species (microcrustaceans) develop more downstream, where increased residence time leads to autochthonous production (Riverine Productivity Model). Such a pattern is greatly modified by human impact. Zooplankton input from the Marne reservoir represents one type of disruption in the general upstream–downstream trend (according to the Serial Discontinuity Concept). This reservoir is a source of microcrustaceans; they rapidly disappear mainly through fish predation, and therefore have little impact on the river phytoplankton. Discontinuities, such as confluences, have a relatively small effect on the stock of zooplankton with regard to the water release from the reservoir, but they persist more downstream, because they have the same lotic origin. A few microhabitats with macrophytes play a small role for this canalised river, but they can modify locally the plankton community structure and composition. As a whole, the flux of zooplankton rises exponentially, whereas discharge increases linearly from upstream (4th order) to downstream (8th order). In the canalised sectors, Dreissena larvae build up an important biomass, adding to that of the zooplankton sensu stricto. Especially abundant in the downstream sector of the Marne and Seine Rivers, the larvae show a widespread colonisation of the benthic substrates by the adult Dreissena. One of the largest mussel colonies in the middle estuary can contribute to a rapid decrease of zooplankton. Estuary ecosystems form a transitional zone between freshwater and seawater, with zooplankton dynamics closely linked to the particular conditions on this part of the river system.

Dominance of autochthonous autotrophic carbon in food webs of heterotrophic rivers

This paper addresses the river heterotrophy paradox, “How can animal biomass within riverine food webs be fueled primarily by autochthonous autotrophic production if the ecosystem as a whole is heterotrophic?”. Reviewed, stable isotope data from tropical, temperate, and arctic rivers provide evidence consistent with the revised riverine productivity model (RPM): “The primary, annual energy source supporting overall metazoan production and species diversity in mid‐ to higher‐trophic levels of most rivers (≥4th order) is autochthonous primary production entering food webs via algal‐grazer and decomposer pathways”. The revised RPM does not conflict with the heterotrophy paradox because: (a) the decomposer (microbial loop) food pathway processes most of the transported, allochthonous and autochthonous carbon and, with algal respiration in some cases, is primarily responsible for a river's heterotrophic state (P/R&lt;1); but (b) biomass production of mid‐ to higher‐trophic levels is principally supported by an algal‐grazer (phytoplankton and benthic microalgae) pathway that is only weakly linked to the decomposer pathway. The reason the algal‐grazer pathway supports the majority of metazoan biomass is that allochthonous carbon is mostly recalcitrant, whereas carbon from autochthonous primary production, though much less plentiful, is commonly more labile (easier to assimilate), contains more energy per unit mass, and is typically preferred by metazoa.

Life in the fast lane: fish and foodweb structure in the main channel of large rivers

We studied the main channel of the lower Illinois River and of the Mississippi River just upstream and downstream of its confluence with the Illinois River to describe the abundance, composition, and/or seasonal appearance of components of the main-channel community. Abundance of fishes in the main channel was high, especially adults. Most adult fishes were present in the main channel for either 3 or 4 seasons/y, indicating that fishes regularly reside in the main channel. We documented abundant zooplankton and benthic invertebrates in the main channel, and the presence of these food types in the diets of channel catfish and freshwater drum. All trophic levels were well represented in the main channel, indicating that the main channel supports a unique food web. The main channel also serves as an important energetic link with other riverine habitats (e.g., floodplains, secondary channels, backwater lakes) because of the mobility of resident fishes and because of the varied energy sources supplying this food web. It may be more realistic to view energy flow in large-river systems as a combination of 3 existing concepts, the river continuum concept (downstream transport), the flood pulse concept (lateral transport to the floodplain), and the riverine productivity model (autochthonous production). We urge additional research to quantify the links between the main channel and other habitat types in large rivers because of the apparent importance of main-channel processes in the overall structure and function of large-river ecosystems.

Lowland rivers: an Australian introduction

An evident trend in river ecosystem research over the last 10 years has been the increased emphasis on larger systems and, in particular, on the importance of interconnections between river channels and their floodplains. The trend has been reflected in a rapidly expanding literature describing the character of large rivers. The 1989 special issue of Canadian Journal of Fisheries and Aquatic Sciences was one of the first publications to deal with these systems. These papers from the symposium on large rivers (Dodge, 1989) had a multidisciplined focus but only 11 of the 49 papers dealt with large floodplain rivers. The geomorphological character of lowland rivers was presented in Lowland Floodplain Ri6ers: Geomorphological Perspecti6es (Carling and Petts, 1992). There the aim was to describe the holistic nature of the river valley, shifting attention away from the river bed per se and considering, instead, the data that are required for a better understanding of channel–floodplain relationships over a range of time scales. More recently, selected papers from the International Conference, Sustaining the Ecological Integrity of Large Floodplain Ri6ers were presented in an issue of this journal. The common theme of the papers was the highly variable physical, chemical and biological natures of large river systems. Large rivers were portrayed as ‘moving targets’ or highly dynamic systems, with the general synthesis that research should focus on the trajectories of these systems rather than their status at points in time and space. Despite the lack of a detailed definition of lowland rivers there are a number of ecosystem models that conceptualize their structure and function. The River Continuum Concept (RCC; Vannote et al., 1980) was initially applied but later criticized (Sedell et al., 1989) because of its inability to evaluate biotic assemblages in large rivers and metabolic interactions in ecosystems and its emphasis on longitudinal linkages rather than lateral transfers between the channel and floodplain. The Flood Pulse Concept (FPC; Junk et al., 1989) addresses this problem and concerns itself with the ecological significance of temporal patterns of flow in lowland rivers. It recognizes the importance of floods in the dynamics of river–floodplain systems. However, this concept may be less applicable to confined lowland systems. The Riverine Productivity Model of Thorp and Delong (1994) concentrated on confined lowland systems and highlighted the significance of instream production and floodplain inputs of carbon in periods not just related to floods. Recently, Walker et al. (1995) have called for the development of a more generic model for dryland–lowland rivers. They suggest a more flexible approach that recognizes the importance of all energy transfers, and propose that a hybrid model could be formulated from the models and concepts already outlined. In a review of the ecology of Australian lotic systems, Lake (1994) notes that contemporary ecosystem theories, developed from research on temperate perennial streams to explain river system functioning, do not apply to Australian river systems. This observation has been echoed by many, including Walker et al. (1995). Moreover, water policies and legislation (Johnson, 1993), and river management and restoration strategies, e.g. Petts and Calow (1996), also share similar origins. Boulton et al. (2000) suggest that the uncritical extrapolation of these theories to any system can prove perilous and even misleading, as observed for Australian systems (Williams, 1988).

Isotopic analysis of three food web theories in constricted and floodplain regions of a large river.

Analyses of stable isotope (δ13C and δ15N) and C:N ratios of food webs within a floodplain and a constricted-channel region of the Ohio River during October 1993 and July 1994 indicate that the increasingly influential flood pulse concept (FPC) does not, for either location, adequately address food web structure for this very large river. Furthermore, results of this study suggest that the riverine productivity model (RPM) is more appropriate than the widely known river continuum concept (RCC) for the constricted region of this river. These␣conclusions are based on stable isotope analyses of potential sources of organic matter (riparian C3 trees, riparian C4 grasses and agricultural crops, submerged macrophytes, benthic filamentous algae, benthic particulate organic matter, and transported organic matter containing detritus and phytoplankton) and various functional feeding groups of invertebrate and fish consumers. The FPC, which stresses the key contribution of organic matter, particularly terrestrial organic matter, originating from the floodplain to riverine food webs, was judged inappropriate for the floodplain region of the Ohio River for hydrodynamic and biotic reasons. The rising limb and peak period of discharge typically occur in November through March when temperatures are low (generally much less than 10°C) and greater than bank-full conditions are relatively unpredictable and short-lived. The major food potentially available to riverine organisms migrating into the floodplain would be decaying vegetation because autotrophic production is temperature and light limited and terrestrial insect production is minimal at that time. It is clear from our data that terrestrial C4 plants contribute little, if anything, to the consumer food web (based on δ13C values), and δ15N values for C3 plants, coarse benthic organic matter, and fine benthic organic matter were too depleted (∼7-12‰ lower than most invertebrate consumer values) for this organic matter to be supporting the food web. The RPM, which emphasizes the primary role of autotrophic production in large rivers, is the most viable of the remaining two ecosystem models for the constricted-channel region of the Ohio based on stable isotope linkage between sources and consumers of organic matter in the food web. The most important form of food web organic matter is apparently transported (suspended) fine (FTOM) and ultra-fine particulate organic matter. We propose that phytoplankton and detritus of an autochthonous origin in the seston would represent a more usable energy source for benthic (bivalve molluscs, hydropsychid caddisflies) and planktonic (microcrustaceans) suspension feeders than the more refractory allochthonous materials derived from upstream processing of terrestrial organic matter. Benthic grazers depend heavily on nonfilamentous benthic algae (based on gut analysis from a separate study), but filamentous benthic algae have no apparent connection to invertebrate consumers (based on δ13C values). Amphipod and crayfish show a strong relationship to aquatic macrophytes (possibly through detrital organic matter rather than living plant tissue). These observations contrast with the prediction of the RCC that food webs in large rivers are based principally on refractory FTOM and dissolved organic matter from upstream inefficiencies in organic-matter processing and the bacteria growing upon these suspended or dissolved detrital compounds. The conclusions drawn here for the Ohio River cannot yet be extended to other floodplain and constricted-channel rivers in temperate and tropical latitudes until more comparable data are available on relatively pristine and moderately regulated rivers.

The Riverine Productivity Model: An Heuristic View of Carbon Sources and Organic Processing in Large River Ecosystems

Our current views of the structure and function of large river ecosystems are based primarily on three influential and still valuable riverine models: (1) the river continuum concept, or RCC (e.g., Vannote et al. 1980); (2) the serial discontinuity concept (Ward and Stanford 1983), which integrated the effects of large dams and reservoirs on the RCC; and (3) the flood pulse concept in river-floodplain systems (Junk et al. 1989) and its relationship to the RCC (Sedell et al. 1989). With regard to food webs in large rivers, these models accentuated the importance of nutrients derived from either headwater streams or seasonal floodplain pulses and downplayed or virtually ignored the role of local instream primary production and riparian litterfall. We believe that the general portrayal of ecosystem function within large rivers needs substantial revision because previous models relied too heavily on data from either lower-order streams, floodplain rivers (thereby exluding large rivers with constricted channels), or main channel habitats with their dominant collector feeding guild (thus de-emphasizing nearshore areas where species in many feeding guilds congregate). We propose an alternative hypothesis the riverine productivity model (RPM) which stresses the varying importance to large river food webs of local autochthonous production and direct organic inputs from the riparian zone. The RPM's portrayal of ecosystem function differs most significantly from that of previous models for rivers characterized by constricted channels, such as the upper two-thirds of the Ohio River.