DETERMINISM OF FISH ASSEMBLAGE STRUCTURE IN NEOTROPICAL FLOODPLAIN LAKES: INFLUENCE OF INTERNAL AND LANDSCAPE LAKE CONDITIONS

In the neotropics, seasonal flooding renders the floodplain an open system in which many fish species can disperse freely. Consequently, it has been suggested that fish assemblage structure in floodplain lakes is largely stochastic. However, recent studies concluded that fish assemblage structure could be determined mostly by local environmental factors. The present work tested 1) the predictability of fish assemblage structure in lakes of the Mamoré River floodplain, Bolivia, in relation to environmental conditions and 2) the general prediction that fish assemblages are structured following the piscivory-transparency-morphometry (PTM) model originally developed for the Orinoco River. Fish species abundances were quantified in eight lakes of the Mamoré River floodplain, positioned along a spatial gradient of distance to the main river, through five high243 Pouilly M. Rodríguez M.A. 1 Institut de Recherche pour le Développement (IRD) UMR CNRS 5023, Université Lyon 1 43, Bd du 11 Novembre 1918, F-69622 Villeurbanne Cedex – France E-mail: pouilly@univ-lyon1.fr 2 Département de Chimie-Biologie, Université du Québec à Trois-Rivières C.P. 500, Trois-Rivières, Québec G9A 5H7 – Canada E-mail: marco_rodriguez@uqtr.ca DETERMINISM OF FISH ASSEMBLAGE STRUCTURE IN NEOTROPICAL FLOODPLAIN LAKES: INFLUENCE OF INTERNAL AND LANDSCAPE LAKE CONDITIONS water and four low-water surveys. We found strong relationships between fish assemblage structure and abiotic variables. Spatial variation in fish assemblage structure was stronger than temporal variation. Consistent with predictions of the PTM model, relative abundances of siluriforms and gymnotiforms declined in clearer and deeper water, whereas relative abundances of characiforms and clupeiforms increased, as expected from knowledge on the sensory capabilities of these taxa. Partitioning of variation showed that although internal variables, especially transparency and water depth, play an important role in structuring fish assemblages, landscape variables, specifically temporal variability of water quality and connectivity, also influenced assemblage structure. These results support the notion of hierarchical control of assemblage structure. Landscape variables seem to operate as a primary filter that differentially limits local movement and migration as a function of lake connectivity. A secondary filter reflecting internal processes appears to exert stronger control in well-connected lakes where the migration filter might be weak. At the ordinal level, the distribution of clupeids, gymnotiforms and siluriforms appeared to be shaped by both landscape and internal variables. In contrast, that of characiforms did not seem limited by landscape variables. INTRODUCTION The structure of fish assemblages is influenced by environmental variations at multiple spatial and temporal scales. Assemblage patterns should therefore be evaluated with respect to the relative contribution of small-scale, local and larger-scale, regional, ecological processes (Angermeier and Winston 1998). Specifically, environmental influences acting at different scales can be viewed as hierarchical filters that control species presence or abundance (Tonn et al. 1990). Species should be influenced differentially as a function of their adaptations to abiotic and biotic selective forces. Abiotic conditions may be influential at all spatial scales, although biotic interactions are likely to operate only at the local scale (Tonn et al. 1990; Keddy 1992). Patterns of control in fish assemblages differ among systems. For example, in Mediterranean streams, variation in fish assemblage structure is mostly explained by large-scale factors (stream size and catchment position) rather than by microhabitat and biotic interactions (Magalhães, Batalha and CollaresPereira 2002). In small temperate lakes, piscivory and both local and larger-scale environmental variables (water depth, surface area, isolation) influence the structure of fish assemblages (Tonn et al. 1990). Finally, in neotropical floodplain lakes of the Orinoco River, the piscivory-transparency-morphometry model (PTM) proposed by Rodríguez and Lewis (1997) indicates that species distribution and abundance is tightly linked to lake water transparency, which is in turn controlled by lake morphometry. A similar pattern was found in the Araguaia River floodplain, Brazil (Tejerina-Garro, Fortin and Rodriguez 1998). In most neotropical floodplains, the annual flood cycle of the river is predictable and exhibits marked seasonal fluctuations in water level that constitute one of the main ecological characteristics of floodplain waterbodies (Junk, Bayley and Sparks 1989). Floodplain lakes are mainly isolated during the dry season, but during the wet season, lake interconnections and forest flooding give fish access to a broad range of habitats. Consequently, the alternation of dry and wet seasons produces major change in patterns of 244 Determinism of fish assemblage structure in neotropical floodplain lakes: