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Spatial use of the nekton community in a subtropical shallow lake without piscivorous fish
(Lake Blanca Uruguay)
G. Goyenola1, C. Iglesias1, M. Meerhoff1,2 and N. Mazzeo1
Sección Limnología, Facultad de Ciencias, Universidad de la República. Iguá 4225, CP 11400, Montevideo, Uruguay.
E-mail: [email protected]
2
Department of Freshwater Ecology, National Environmental Research Institute, Denmark.
1
Introduction
Lake Blanca (34º54' S, 54º50' W) is a subtropical shallow eutrophic system without piscivorous fish. Their absence was
caused by an extreme decrease in the water level in 1997-1998. The lake has the typical biological structure of an
exclusion experiment, at an ecosystem scale. The nekton community is composed only by two species of fish: Jenynsia
multidentata and Cnesterodon decemmaculatus (Jenyns, 1842), and a shrimp Palaemonetes argentinus (Nobili, 1901).
The high abundance of these potential planktivores (associated with the absence of piscivorous fish), and the important
nutrient load in the system, could promote the development of phytoplankton bloom and a decrease in water quality.
Fish change their diet and behaviour on a daily, seasonal and ontogenetic scale, which is generally associated to habitat
changes (Persson & Crowder, 1998).
The objective of this work was to determine the pattern of use of space of nekton in Lake Blanca and its variation along
the life cycle of the species present. This understanding is critical from an ecological point of view, and also due to the
potential cascading effects on the water quality.
Methodology
We performed seasonal simultaneous sampling of nekton and plankton communities, and physico-chemical analyses,
for a year (2003-2004). We took nekton samples at the surface and the bottom using minnow traps, and following a
stratified random design with repeated measures. The three strata (habitats) were defined according to the presence and
life-form of the plants (submerged: SP, emergent: EP and open water: OW), taking five replicates from each at day and
night.
Results and Discussion
Palaemonetes argentinus dominated in terms of density, and J. multidentata in terms of biomass. Cnesterodon
decemmaculatus was poorly represented. The different species presented a gregarious distribution, with spatial
segregation between fish and shrimps. We found evidence of daily and seasonal differential use of the space by the
three species.
The use of the space and the trophic role of fish varied with genera and the stage of the life-cycle. Adult fish were
strongly associated with macrophytes. The diurnal distribution of adult J. multidentata, located preferentially in
vegetated and deeper parts, is consistent with the hypothesis of minimization of predation risk, in this lake represented
by piscivorous waterfowl. The females of J. multidentata, larger and more abundant than males, could affect the use of
the space by shrimps and other fish (through competition, aggressive behaviour or cannibalism). The other stages of fish
could be forced to move into less-safe habitats. We found young individuals of this species in OW, the habitat with
greater risk of visual predation by waterfowl, probably as a means to avoid cannibalism.
All organisms displayed greater activity during the night. With lower predation risk, the “refuge” offered by the deeper
parts of the water column becomes less important at night. Consistently, we found a more homogeneous vertical
distribution of fish.
As the patterns of spatial use were not explained by the physico-chemical conditions of the water column (Mantel´
tests), biological interactions, as competition and predation risk, are thought to be the main explanatory factors for the
observed patterns.
References
Persson, L. & L.B. Crowder. 1998. Fish-habitat interactions mediated via ontogenetic niche shifts. In: E. Jeppesen, M. Søndergaard,
M. Søndergaard & K. Christoffersen (ed.) The Structuring Role of Submerged Macrophytes in Lakes, Springer-Verlag, New York.
pp. 3-23.