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. 

Comparison of the 1982-1983 and 1997-1998 El Niño effects on the shallow-water fish assemblage of the Patos Lagoon estuary (Brazil)

Alexandre  M. Garcia (Univ. Fed. Rio Grande, Brazil), Jaoa P.Vieira (FURG), Kirk O.Winemiller and Alice M. Grimm

Meteorological impacts of El Niño events of 1982-83 and 1997-98 were observed in locations throughout the world.  In southern Brazil, El Niño events are associated with increased rainfall and higher freshwater discharge into Patos Lagoon, a large coastal lagoon that empties into the Atlantic Ocean. Based on inter-decadal meteorological and biological data set encompassing the two strongest El Niño events of the last 50 years, we evaluate the hypothesis that El Niño-induced hydrological changes is a major driving force controlling the inter-annual variation in the structure and dynamics of fishes in the Patos Lagoon estuary. High rainfall in the drainage basin of the lagoon coincided with low salinity in the estuarine area during both El Niño episodes. But total rainfall at the drainage basin was higher (767 vs. 711 mm) and near-zero salinity conditions at the estuarine area lasted about three months longer during the 1997-98 El Niño compared with the 1982-83 event. Such hydrological changes trigged by both El Niño events had similar relationships to fish species composition and diversity patterns, but the 1997-98 event appeared to have stronger effects on the fish assemblage. In fact, although shifts in species composition had similar patterns during both El Niño events, distance between El Niño and non-El Niño DFA centroids was six orders of magnitude greater during the 1996-2000 sampling period compared with the 1979-1983 period. We provide a conceptual diagram showing the main mechanisms and process connecting the atmospheric-oceanographic interactions trigged by the El Niño phenomena and its impact on this fish estuarine assemblage.  This research was supported by the Inter-American Institute for Global Change.

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.

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.

Updated Wednesday March 01, 2006