Experimental demonstrations of seasonal fish effects on benthic ecology of a Neotropical floodplain river 

Kirk O. Winemiller, Jose Vicente Montoya, Craig A. Layman, Daniel L. Roelke and James B. Cotner (University of Minnesota)

The Cinaruco River, a lowland floodplain river in the Venezuelan llanos, has seasonal hydrology, low nutrient availability, and high fish diversity and abundance.  Algae- and detritus-feeding fishes are abundant and may influence benthic ecology.  During the dry season (2002), large-mesh exclosures in the channel accrued significantly more sediment, organic material, and chlorophyll than control cages.  Grazing scars revealed that the sapuara, Semaprochilodus kneri, was a major consumer of organic-rich sediments.  Further experiments were conducted to test the hypothesis that the relative strength of bottom-up (nutrient availability) and top-down (grazer) control of organic matter in sediments varies according to species and hydrologic period.  Sapuara densities are highest (0.28/m²) during the low-water season.  At flooding onset (May), sapuaras migrate to the Orinoco River to reproduce and feed, and their densities are extremely low in the Cinaruco (0.02/m²) during the interval when nutrient inputs from newly flooded plains are greatest.  Experiments conducted during March 2002 (low water) in the channel and floodplain lakes revealed significant treatment effects (large fish exclosure; total fish exclosure; sapuara enclosure; control) for sediment mass, organic material mass, and chlorophyll a concentration on hard substrates, but no treatment effects for sand.  Chlorophyll a concentrations were significantly affected by habitat.  Mean mass of sediments and organic material matched our prediction of grazer control during the low-water season, but due to high within-treatment variance, the only statistically significant mean differences were sapuara enclosure < control and total fish exclosure.  Subsequent experiments of daily sediment accrual revealed that sapuaras infrequently enter 3-sided control cages, but quickly remove sediments from 1 or 2-sided control cages.  Experiments during the early rising-water phase, yielded no significant habitat or treatment effects.  This research is ongoing with funding from the National Science Foundation. 

Food web architecture in natural and impounded rivers of the upper Paraná River basin, Brazil

David J. Hoeinghaus, Kirk O. Winemiller and Ângelo A. Agostinho (Nupélia, Univ. Estadual de Maringá, Brazil)

Food webs provide the framework for many areas of ecology.  Understanding human impacts on food webs is a fundamental problem that has not received attention proportionate to its importance.  Possibly the most dramatic impact humans have on fluvial ecosystems is the damming of rivers.  Using stable isotopes, we are conducting a large-scale comparison of food web architecture in natural and impounded rivers of the upper Paraná River basin, Brazil.  The sampling region is > 500 km and includes the last remaining natural floodplain and unmodified tributaries.  The basin supports a diverse flora and fauna, with many of the fish species historically undergoing long-distance reproductive migrations.  Carbon sources (C3 and C4 aquatic and riparian vegetation, detritus, algae, and seston - phytoplankton and suspended detritus <50 μm), primary consumers (snails, bivalves, filter-feeding zooplankton - Daphnia and Calanoida >125 μm, and detritivorous and herbivorous fishes), and piscivores were collected from impounded rivers (reservoirs) and natural high-gradient and low-gradient rivers for isotopic analysis.  Bivalves and snails will be used to provide a time-integrated baseline of carbon and nitrogen signatures for the pelagic and littoral habitats respectively.  We will explicitly evaluate species realized trophic positions and food chain length among sites.  In addition, we will compare the relative importance of various carbon sources among species and sites.  Comparative food web studies of diverse tropical and subtropical ecosystems are rare, and the proposed study will describe essential features of ecosystem structure and potential impacts of hydrological modifications. 

Aquatic Food-web Structure along the Longitudinal Gradient of the Bladen Monkey River, Belize

Kirk Winemiller, Rodney Honeycutt, Peter Esselman, Will Heyman, Allison Pease, Elizabeth Carrera, Donmale Gbaanador, Josiah Payne, John Putegnat, Gabriela Tamez

This project examines food-web structure and primary production sources supporting the aquatic fauna of the Bladen River and upland tributaries, the Monkey River estuary, and adjacent coastal waters using stable isotope methodology.  Bladen-Monkey River watershed lies within one of the most important conservation regions of Central America.  The watershed contains a variety of terrestrial and aquatic ecosystems that support rich biodiversity, and also influences important and sensitive coastal and coral reef ecosystems.  Ratios of the stable isotopes of carbon are being used to estimate assimilation of carbon from various plant production sources by animals.  To identify the predominant energy source for aquatic consumers, a mixing model is employed with the following basal elements:  periphyton, macroalgae and vascular plants (aquatic macrophytes, grass, tree leaves, seeds, fruits).  Nitrogen isotopic ratios are used as indicators of trophic position (vertical position in the food web) in aquatic systems, where ¹⁵N enrichment increases predictably with trophic level of consumers.  The heavier ¹⁵N accumulates in consumers as nitrogen moves up the food web, and as a result top consumers tend to have higher values for δ¹⁵N than consumers near the base of the web.  In early 2006, we collected tissue samples from several of the most common plants and animals in these aquatic ecosystems for laboratory analysis (mass spectrometry) of ratios of heavy and light stable isotopes of carbon and nitrogen.  Knowledge of food web structure and the energy sources that support faunal elements of the aquatic community will greatly increase understanding of tropical river ecology.  It is anticipated that the proportion of allochthonous basal inputs (plant material of terrestrial origin) to the aquatic food web will decline from upstream sites in the Bladen River to the meso-haline waters near the Monkey River mouth.  We hope that this food-web project will yield scientific information and insights that will assist local efforts in "ridge-to-reef” ecological watershed conservation.

Seasonal Variation in Food Web Composition and Structure in a Temperate Tidal Estuary, Mad Island Marsh, Texas

Kirk Winemiller, Senol Akin & Steve Zeug

Seasonal variation in aquatic food web structure at Mad Island Marsh, Matagorda Bay, Texas was examined using dietary information obtained from the analysis of gut contents from large samples of fish and crustacean specimens.  Unique aspects of this study include the use of large samples of consumer gut contents (N= 6,452), long-term sampling (bimonthly surveys over 18 mo), and standard methods of data collection and analysis facilitating comparisons with other aquatic food webs.  Dietary data were partitioned for analysis into summer and winter seasons.  Most consumers fed low in the food web, with trophic levels ranging from about 2 to 3.5 during both summer and winter.  Vegetative detritus was more important in macroconsumer diets than live algae and macrophytes.  Low trophic levels of consumers reflected the important role of abundant detritivores (e.g., striped mullet, Gulf menhaden, and macroinvertebrates) in linking detritus to top predators via short food chains. 

Most food web properties revealed comparatively little seasonal variation.  The summer food web had more nodes (86), more links (562), a higher density of links as indicated by connectance (0.08), and a slightly higher predator/prey ratio (0.51) compared to the winter food web (75 nodes, 394 links, connectance = 0.07, pred./prey ratio= 0.47).  Proportions of top (0.06-0.07) and intermediate (0.75-0.76), and basal (0.19) species did not vary significantly between seasons, but mean trophic level was higher during summer.  Addition of feeding links based on information from the literature increased connectance to 0.13 during the both seasons, however other web parameters had values similar to those obtained for our directly estimated food webs.  Findings were consistent with earlier studies identifying detritus a major pathway from production sources to estuarine consumers.  Seasonal variation in food web structure was influenced by changes in community composition (e.g. influxes of postlarval estuarine-dependent marine fishes during winter), availability of resources (e.g. more submerged macrophytes amphipods during summer), and size structure and ontogenetic diet shifts of dominant consumer taxa. 

Food web structure and major sources of primary production consumed by metafauna of Mad Island Marsh also were compared using stable isotopes and dietary analysis.  Isotopic results revealed two interlinked subwebs– one with algae as a primary production source, and the other with C₄ saltmarsh grasses as a production source, with the latter apparently consumed in the form of detritus and associated heterotrophic bacteria.  Both methods indicate that terrestrial C₃ plants probably contribute relatively little material to the aquatic food web.  The two subwebs are spatially segregated, with most larger fishes and invertebrates associated with deeper areas and algal production.  Isotopic analysis could not reveal the detailed structure of predator-prey interactions at the species level; greater detail of trophic pathways was revealed by the dietary analysis.  Estimates of vertical web structure (species trophic levels) by the two methods were largely concordant.  The exceptions were zooplanktivorous and detritivorous fish species that had higher trophic levels according to nitrogen isotope ratios.  The isotope method more accurately indexed the number of trophic transfers than the dietary method that depends on accurate dietary estimation for all elements of food chains leading to a consumer, and which assumes equal assimilation efficiencies for elements found in stomach contents. 

Isotopic variation of fishes along a longitudinal salinity gradient in a large subtropical coastal lagoon

Alexandre M. Garcia (FURG), David J. Hoeinghaus, João P. Vieira (FURG) & K.O. Winemiller

We used stable C and N isotope ratios of tissues from 29 fish species from a large subtropical lagoon in southern Brazil to examine: (1) spatial variability in isotopic composition across freshwater and estuarine habitats, and (2) vertical trophic structure and its relationships with body size in the two habitats. Fish assemblages showed a significant shift in their carbon isotopic signatures between freshwater and estuarine sites. Depleted carbon signatures (from -24.7 to -17.8 ‰) were found in freshwater, whereas more enriched signatures (from -19.1 to -12.3 ‰) were obtained within the estuarine zone downstream. These δ¹³C ranges suggested that carbon sources supporting fish production at the freshwater site derived from a mixture of emergent C₃ marsh plants and phytoplankton, and from a mixture of benthic microalgae, C₄ grasses (Spartina) and marine phytoplankton at estuarine sites. Our results corroborate the hypothesis that fish assemblages are generally supported by autochthonous primary production. Freshwater fishes that likely were displaced downstream into the estuary during periods of high freshwater discharge had low δ¹³C values that were characteristic of the upper lagoon. These results suggest that spatial food web subsidies can occur within the lagoon. Nitrogen isotope ratios indicated three consumer trophic levels within the lagoon, with most fishes occupying the secondary consumer level. A significant positive correlation between trophic position and body size was only observed for the estuarine assemblage. Such contrasts observed between freshwater and estuarine fish assemblages could be due to differences in fish assemblage structure and their ecological use of both habitats.  This research is conducted in collaboration with the Brazilian LTER program - Lagoa dos Patos.

Pattern, Process and Scale in the Food Web Paradigm: Moving on the path from Abstraction to Prediction

Kirk O. Winemiller and Craig A. Layman

Like any scientific endeavor, research on food webs advances on three interacting fronts:  description, theory, and model testing (experimentation).  Development of food web theories (models) and their applications are greatly outpacing advances in the descriptive and experimental arenas.  Although this state of affairs is not unexpected in an immature scientific discipline, it results in inefficient development of understanding.  Why have empirical components lagged behind theoretical developments?  We propose that unresolved issues of scale and resolution have hindered empirical research.  Resolution of three basic features of food webs is required.  First, the spatial and temporal boundaries of a community food web are always arbitrary, and it should be recognized that any food web is a module within a larger system.  Objective methods for defining and quantifying nested modules are needed.  Second, great variation is observed in the treatment of food web components, ranging from species life stages to functional groups containing diverse taxa.  In most empirical studies, these components have been invoked a posteriori rather than a priori.  We propose that species populations are the only natural food web components, because populations are evolutionary units with dynamics that are independent of membership in a guild or functional group.  Third, food web links must be accompanied by some measure of interaction strength or magnitude.  Theoretical studies have firmly established that interaction strength and adaptive foraging determine food web dynamics.  Consequently, food webs created with unchanging binary links (present or absent) are irrelevant.  Moreover, empirical research is needed to establish the degree to which direct mutualism (e.g., pollination, seed dispersal) contributes to food web dynamics.   A fourth critical issue is the degree to which food web structure and dynamics are driven by environmental factors, especially climatic and landscape factors.  We briefly discuss four alternative models of food web structure, and contend that available empirical evidence is insufficient to evaluate them.  Despite the fact that a deficient empirical knowledge base is the main hurdle to scientific advancement, pressing natural resource problems require application of existing models to inform management.  We propose a multi-faceted empirical approach for long-term field studies as means to advance understanding of food webs.     Click [here] to view a powerpoint presentation given at the 3rd decadal food web symposium in Giessen, Germany, November 2003.

Body size and prey availability drive predation patterns in a species-rich tropical river food web

Craig A. Layman, Kirk O. Winemiller, D. Albrey Arrington, David B. Jepsen (Oregon State Univ.) and Carmen Montaña (Univ. Western Llanos, Venezuela)

We examined predator-prey dynamics in a species-rich Neotropical river as a function of (1) relative prey and predator body size and (2) seasonal prey availability.  Piscivores consumed a phylogenetically and morphologically diverse group of fishes, reflecting the overall diversity of fish species in this river (>280 species).  There was a nested hierarchy of predator-prey interactions in which smaller prey taxa were consumed by a greater diversity of predators.  Prey/predator body size ratios were relatively low (0.11 – 0.20), and decreased as water level dropped during the annual flood cycle.  Prey availability likely drives this seasonal decline in ratios.  Piscivores consume large-bodied algivore/detritivores during high water, whereas during low-water prey were smaller but had higher average trophic positions (i.e. many prey were invertivores as opposed to algivores).  These data are an example of how body size and prey availability can be used to generalize interactions even in complex, species-rich food webs, and emphasize the importance of physical drivers (e.g., seasonal hydrology) underlying these patterns.  We also used stomach contents and stable isotope data of piscivorous fishes, collected over ten years, to examine body size-trophic position relationships.  Regardless of size, piscivores clearly derive energy and nutrients from short, efficient food chains.  This “compressed” food web structure has important dynamical implications for assessing human impacts in species-rich, tropical ecosystems.  This research was funded by the National Science Foundation.

Comparison of guts contents analysis and stable isotopes analysis in determining food web structure in three tropical streams

Kirk O. Winemiller, Steve C. Zeug and Clint R. Robertson

Recently, the use of stable isotopes to evaluate food web structure has become increasingly commonplace.  Isotopic analysis provides information on longer-term feeding trends based only on assimilated dietary items.  Stomach contents analysis, although only providing a record of very recent feeding activity, provides a more detailed picture of feeding relationships than is generally possible using isotopes in diverse systems.  Winemiller (1990) evaluated food web structure in three streams in Tortuguero and Corcovado National Parks using guts contents analysis.  The present study analyzes food web structure in the same streams using stable isotope analysis, and compares the structure resulting from both analyses.

Effects of hydrologic connectivity on food web structure in the Brazos river-floodplain system

Steven Zeug & Kirk O. Winemiller

Off-channel floodplain habitats such as oxbow lakes are recognized as important sources of biological productivity in river-floodplain systems.  Because the flow regimes of these systems are temporally variable, the influence of river-floodplain connectivity on trophic structure is expected to be pulsed.   In this study we examine the influence of hydrologic connectivity on trophic structure in the Brazos River channel and two oxbow lakes with different connection frequencies using stable isotopes of carbon and nitrogen.  Tissue samples from species representing several trophic guilds were collected monthly for a period of one year.  During the study period, each habitat experienced different levels of hydrologic connectivity that may influence trophic structure by transporting organic material, inorganic nutrients and facilitating faunal exchange among habitats.

Big Bend Oxbow,

Brazos River Floodplain

< before flood

during flood >

Hydrological seasonality and spatio-temporal dynamics of physico-chemical variables of a tropical floodplain river

Jose Vicente Montoya, Daniel L. Roelke, Kirk O. Winemiller and James B. Cotner (Univ. Minnesota)

Rio Cinaruco is a tropical floodplain river in the Orinoco River basin (Venezuela), characterized by relatively high transparency, and low conductivity, pH, and suspended sediment load. The purpose of this study was to evaluate spatio-temporal dynamics of physico-chemical variables of the main channel and floodplain lakes of Rio Cinaruco. We hypothesized higher similarity among all sites during periods with high lateral hydrological connectivity (LHC) and lower similarity during low-water phases. Also, we predicted divergence among floodplain lake sites during the low-water period. Samplings occurred monthly on twelve occasions between 2002 and 2003 at 10 sites in the main channel and lakes. We measured water level, temperature, DO, pH, conductivity, water transparency, and water flow in situ. Water samples were taken superficially for nitrates, nitrites, ammonia, urea, orthophosphates, silica, chlorophyll-a, and phaeophytin, and analyzed using standard methods. Correspondence analyses showed that over 80% of the variation among sites and sampling dates is explained by two axes. During the high-water period, river sites and lakes were similar with low values for conductivity, flow, silica and chl-a concentrations. Strong shifts for these four variables were detected for most sites during transitional periods between high and low LHC. Nevertheless, after these shifts that tended to differentiate sites, the dynamics of physico-chemical variables became similar among lakes. Repeated spatio-temporal trends in the dynamics of physico-chemical variables of Rio Cinaruco and its lakes can be directly attributable to a strong effect of dilution and transitions between lentic and lotic conditions in the floodplain, driven by the annual flood-pulse.

Preliminary food web analysis of Taim Ecological Reserve, Brazil

Alexandre M. Garcia (FURG), David J. Hoeinghaus, David da Motta Marques (Univ. Fed. Rio Grande do Sul), João P. Vieira (FURG), Marlise Bemvenuti (FURG) and Kirk O. Winemiller

Taim Ecological Reserve (referred to in Brazil as the southern Pantanal) is a diverse subtropical freshwater wetland ecosystem in Rio Grande do Sul, southern Brazil.  Although extensive work has been conducted on aspects of hydrology, limnology and macrophytes, comparatively little work has addressed fish ecology and trophic relationships in this heterogeneous system.  In March 2004 we conducted an initial survey of fishes, plants, and invertebrates and collected samples for stable isotope analysis.  This study will provide a baseline for continuing work on aquatic community and landscape ecology of this system.  In the near future we hope to secure funding for broader scale studies of aquatic community structure, species resource use and life-history ecology, trophic interactions and food webs, and fish movement.  This research is conducted in collaboration with the Brazilian LTER program - Taim Hydrological System.

Habitat attributes influence ontogenetic diet shits of gizzard shad (Dorosoma cepedianum) in a large floodplain river

Zeug, S.C., D. Peretti & K.O. Winemiller

Gizzard shad (Dorosoma cepedianum) are recognized as an important trophic link in many aquatic food webs because of their ability to link detrital carbon sources with higher trophic levels (e.g. piscivorous fishes).  Most research on the feeding ecology of gizzard shad concerns populations stocked in reservoirs whereas populations in more dynamic habitats (river-floodplain systems) have been neglected.  Here we examine ontogenetic diet shifts in gizzard shad inhabiting oxbow lakes and channel habitats of the Brazos River, Texas.  Using stable isotopes of nitrogen and stomach contents we found that oxbow populations show an increase in the consumption and assimilation of primary consumers (zooplankton) during ontogeny resulting in elevated trophic positions despite high diet overlap values calculated using volumetric proportions of stomach contents.  Differences in d¹⁵N values indicate that shad greater than 200 mm standard length have a trophic position approximately one level higher than shad less than 200 mm.  Populations in the river channel maintain similar trophic positions during ontogeny indicative of detritivory.  These results challenge previous models of ontogenetic diet shifts in gizzard shad and similar to other dietary studies using multiple methodologies, suggest that material assimilated may not be proportional to the volume of material consumed.

Resource partitioning among piscivorous fishes of the middle Brazos River, Texas

Steve C. Zeug, Clint R. Robertson and Kirk O. Winemiller

Previous research has shown that piscivory may control the ability of some species to colonize floodplain habitats.  In response to these findings, and in conjunction with other on-going projects in the middle Brazos River-floodplain system, we are examining gut contents of all piscivorous fishes collected during monthly sampling in the main river channel and two oxbows that form connections to the channel at different water levels for a period of one year.  This study should allow us to examine the influence of piscivory on colonization rate in these habitats.

Updated August 27, 2008