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. 

Seasonal variability in benthic and water column productivity in a Venezuelan llanos river

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

We have been studying water column and benthic productivity in the Cinaruco River in the llanos at various hydrological stages: low and high water and transition phases. This is a blackwater river ecosystem in the Orinoco River basin that is of great ecological interest because of high fish diversity (>260 species). Benthic chambers were used to examine net production and respiration in shallow regions (<1 m) of the river and lagoons that are connected to the river in the high water period and disconnected during low water. Changes in dissolved oxygen concentrations were monitored in transparent and dark chambers/bottles over a daily period to estimate rates of benthic and water column production and respiration. We assumed respiratory and production quotients of 1 to convert oxygen measurements to carbon equivalents. Mean benthic respiration rates varies from 50-162 mg C m^-2 d^-1 and water column rates varies from 3-168 mg C m^-2 d^-1 with the highest values observed during the low water period. Net production is less variable in the benthos and more variable in the water column. Benthic production varies only from 9-58 mg C m^-2 d^-1 whereas water column production varies from 17-223 mg C m^-2 d^-1 with the highest rates observed in the low water period, likely due to increased nutrient concentrations and higher light levels at that time.  This research is funded by the National Science Foundation.

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.

Phylogeny of Geophagine Cichlids from South America (Perciformes: Labroidei)

Hernán López-Fernández, Rodney L. Honeycutt, Kirk O. Winemiller and Melanie L.J. Stiassny (American Museum)

Phylogenetic relationships among 16 genera and 30 species of Geophaginae were investigated using 136 morphological characters combined with DNA sequences coding for the mitochondrial gene NADH dehydrogenase subunit 4 (ND4) and the nuclear Recombination Activating Gene 2 (RAG2).  Data from previous studies were integrated with the new dataset by incorporating published DNA sequences from the mitochondrial genes cytochrome b and 16S and the microsatellite flanking regions Tmo-M27 and Tmo-4C4.  Combined analysis revealed that Geophaginae is monophyletic and includes eighteen genera grouped into two major clades.  In the first clade, the tribe Acarichthyini (genera Acarichthys and Guianacara) is sister-group to a clade in which Gymnogeophagus, ‘Geophagus’ steindachneri, and Geophagus sensu stricto are sister to ‘Geophagus’ brasiliensis and Mikrogeophagus; all these are in turn sister-group to Biotodoma, Dicrossus and Crenicara.  In the second clade, Satanoperca, Apistogramma (including Apistogrammoides), and Taeniacara are sister to Crenicichla and Biotoecus.  Monophyly and significantly short branches at the base of the phylogeny indicate that genera within Geophaginae differentiated rapidly within a relatively short period.  High morphological, ecological, and behavioral diversity within the subfamily suggest that geophagine divergence is the result of an adaptive radiation.

Fish assemblages of the Casiquiare, zone of biotic interchange between Upper Orinoco and Rio Negro

Kirk O. Winemiller, Hernán López-Fernández, D. Albrey Arrington (Univ. Alabama), Donald C. Taphorn (UNELLEZ), Leo G. Nico (USGS) & Aniello Barbarino Duque (INIA)

The Casiquiare River flows from the Upper Orinoco to the Upper Rio Negro which should permit biotic exhange between these two great river systems, yet several Amazonian faunal elements are absent from the Orinoco Basin (e.g., Osteoglossidae, <i>Symphysodon</i> spp.) and many allopatric sister taxa are divided between the basins (e.g., <i>Pygocentrus cariba, P. nattereri</i>).  The Casiquiare Basin contains diverse aquatic habitats, and water quality varies from clearwater to extreme blackwater conditions.  We examined fish assemblage structure relative to 11 habitat characteristics with dataset containing 215 samples and 454 species from surveys catalogued in the Museo de Ciencias Naturales in Guanare, Venezuela.  We performed canonical correspondence analysis based on species presence/absence using two dataset versions:  one that eliminated sites having <5 species, and species occurring at <5 sites; and another that eliminated sites having <10 species, and species occurring at <10 sites.  Results from both analyses were the same qualitatively.  The dominant environmental axis contrasted sites with blackwater versus clearwater conditions.  Longitudinal position on the mainstem Casiquiare was correlated (r²= 0.30) with CCA axis-1 scores, indicating blackwater conditions nearer the mouth and clearwater nearer the origin.  The second CCA axis was most strongly associated with habitat size and structural complexity.  Species were clustered using the centroid method and pairwise squared Euclidean distances calculated from species loadings on CCA axes 1-4.  Seven ecological groupings were identified:  (1) species normally associated with clearwater in large habitats, (2) species associated with structured habitats in blackwater, (3) species associated with a variety of mostly shallow habitats in blackwater, (4) species associated with channel shoreline areas mostly in clearwaters, and (5-7) three large, relatively ubiquitous species.  High fish species richness in the Casiquaire Basin appears to be a function of historical biogeography, high habitat diversity, and semi-permeable barriers to regional dispersal. 

Habitat structural complexity and morphological diversity of fish assemblages in a Neotropical floodplain river

Stuart C. Willis, Kirk O. Winemiller and Hernán López-Fernández

High species diversity in structurally complex habitats has been hypothesized to be associated with niche partitioning.  To test this idea, relationships between habitat structural complexity in river littoral-zone habitats and ecomorphological diversity of tropical fishes were examined in the Cinaruco River, Venezuela.  Six habitat attributes were quantified in 45 sites spanning a range of structural complexity.  Fishes were collected during day and night to estimate species density and relative abundances at each site.  Twenty-two morphological variables were measured for each species.  Principal components analysis (PCA) of physical habitat data yielded two axes that modeled >80% of variation across sites.  The first two axes from PCA of fish morphological variables modeled >70% of variation.  Species density during both day and night was positively associated with high habitat complexity and low flow velocity.  Similarity of day and night samples from the same site was significantly greater for sites with high habitat complexity and low flow.  In general, mean local assemblage morphological PC scores were not significantly associated with habitat PC scores.  Thus, higher species density in more structurally complex habitats is associated with greater interspecific morphological variation and could indicate species packing via niche compression, expansion of total assemblage niche space, or both.  Average, maximum, and standard deviation of morphological Euclidean distances of local assemblages revealed positive associations with structural complexity and low flow.  These relationships held even when the positive relationship of species density was statistically removed from assemblage morphological data.  Findings suggest that both species niche compression and assemblage niche space increase when habitat complexity is greater and flow velocity is lower. 

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.

Community assembly at the patch scale in a species rich tropical river

D. Albrey Arrington (Univ. Alabama), Kirk O. Winemiller and Craig A. Layman

In tropical floodplain rivers, communities associated with structurally complex habitats are disassembled and reassembled as aquatic organisms repeatedly colonize new areas in response to gradual but continuous changes in water level.  Thus, a neutral model reflecting random colonization and extinction dynamics may be sufficient to predict assemblage patterns at the scale of local habitat patches.  If water level fluctuations and associated patch dynamics are sufficiently predictable, however, community assembly on habitat patches also may be influenced by species-specific responses to habitat features and/or species interactions.  We experimentally manipulated structural complexity and colonization rate of simulated rocky patches in the littoral zone of a tropical lowland river (Rio Cinaruco, Venezuela) and demonstrate significant effects of both factors on species density of fishes and macroinvertebrates.  Interspecific variation in vagility significantly affected assemblage response to habitat complexity.  In a second experiment, created habitat patches were sampled over time intervals ranging from 1 to 36 days to examine temporal dynamics of community assembly.  A null-model test revealed that assemblage structure became increasingly non-random with increasing species density over time.  In structured, shallow-water habitats of this tropical lowland river, species density and non-random assemblage structure are influenced by physical habitat characteristics (e.g. structural complexity, depth), the spatial distribution of habitat patches (e.g. colonization rate), and temporal dynamics of pair-wise species interactions. This project was funded by the National Science Foundation.

Effects of seasonality and migratory prey on body condition of Cichla species in a tropical floodplain river

David J. Hoeinghaus, Kirk O. Winemiller, Craig A. Layman, D. Albrey Arrington and David B. Jepsen

The Cinaruco River of southwestern Venezuela is a temporally dynamic floodplain system with uncommonly high densities of large piscivores of the genus Cichla.  Three Cichla species naturally occur in this system, and are known to exhibit significant spatial and temporal resource partitioning.  Resource availability in the Cinaruco varies in response to seasonal changes in water level.  The prochilodontid Semaprochilodus kneri migrates en mass from the Orinoco floodplain into the Cinaruco system during the falling-water period, and is potentially an important prey of larger-sized piscivores.  In the present study we investigate patterns of variation in body condition of Cichla among species, seasons, and two size-classes.  Morphological characteristics were used to determine the size class division a priori, representing the threshold after which Cichla can consume Semaprochilodus based on gape-limitation.  Significant seasonal differences in the intercepts of the log-transformed length-weight relationships were observed.  Lowest body condition was observed during the rising-water period for all three species, whereas only C. temensis had significantly higher body condition during the falling-water period than low-water period.  Comparison body condition among size classes within species resulted in a single significant difference: large C. temensis had significantly greater condition during the falling-water period compared to smaller individuals.  Our results correspond with previous studies on Cichla ecology in the Cinaruco River, which describe the importance of S. kneri in the diet of large C. temensis only (comprising more than 50% of ingested prey), and numerically characterize the increase in body condition experienced by large C. temensis as a result of this subsidy.  Transfer of whitewater production into nutrient-poor rivers by migratory fishes appears to be a general phenomenon in the major Neotropical river basins, and may partially account for high standing stocks of large piscivores in these unproductive ecosystems.

Comparisons of nuclear and mtDNA genes implicate natural selection and demography as primary determinants of intraspecific genetic variation in prochilodontid fishes of northern South America

Thomas F. Turner (Univ. New Mexico), Gregory R. Moyer (UNM), Megan V. McPhee (UNM) and Kirk O. Winemiller

Previous morphological and molecular studies of the highly migratory and abundant characiform family Prochilodontidae have strongly implicated geological events as important for shaping patterns of genetic diversity in this important group of freshwater fishes.  We conducted a mitochondrial DNA study of four prochilodontid fishes that occur in major drainages of northern South America, and found that the geologically complex landscape of this region explained much of the genetic diversity among species.  However, unusual patterns of mtDNA variation were revealed within major lineages.  Prochilodus mariae and Prochilodus rubrotaeniatus (Rio Negro lineage) harbored little or no variation at mtDNA markers we studied.  This observation was contrary to expectations based on their vast abundance in these systems.  Statistical testing revealed weak evidence for purifying selection on mitochondrial genes in P. mariae, but the absence of selection in P. rubrotaeniatus.  If purifying selection is an important process affecting genetic diversity then an unlinked, selectively-neutral locus should be highly variable in P. mariae.  Characterization of two nuclear gene introns, calmodulin and elongation factor one alpha (EF1-α) revealed high levels of genetic variation in P. mariae, which is consistent with the operation of selection on the mitochondrion.  P. rubrotaeniatus in the Rio Negro was monomorphic for variation at calmodulin and showed little diversity at EF1-α.  This observation is consistent with a genetic bottleneck, perhaps associated with a founder event in the Rio Negro from a source population in the Rio Essequibo.  This research was funded by the National Geographic Society.

Updated Monday February 27, 2006