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Analysis of plankton size spectra irregularities in two subtropical shallow lakes (Esteros del Iberá, Argentina)

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Biomass irregularities in the plankton size spectra of two subtropical shallow lakes have been quantified assuming the classical observed generalities of the size spectra. During a seasonal cycle, three main functional size ranges determined the allocation of the spectra irregularities: microbial food web, nanoplankton–microplankton autotrophs, and herbivorous organisms. The structural adjustments within these trophic positions responded to the internal competition between functional guilds, mainly as the result of size-based characteristics related to the ability to eat and the susceptibility to be eaten. Despite the existence of a typical spectrum undulation resulting from self-organization (well-defined trophic positions, limnetic–benthic interaction), the biomass irregularities were an indicator of the main interactions disturbing the steady state. The mechanisms responsible for the irregularities operated jointly at ecosystem and individual levels. Thus, the irregular spectra of the eutrophic Laguna Iberá suggested a strong top-down control through cascade effects. Specific properties of peculiar organisms like filamentous cyanobacteria contributed to hold these stable irregularities. The higher spectrum regularity of the meso-oligotrophic Laguna Galarza emerged from a more balanced flow of biomass along the food chain.

Les irrégularités de la biomasse dans les spectres de tailles du zooplancton de deux lacs subtropicaux peu profonds ont pu être quantifiées en présupposant les généralisations classiques des spectres de tailles. Au cours d'un cycle saisonnier, trois intervalles de tailles fonctionnels principaux sont responsables de l'allocation des irrégularités du spectre: le réseau trophique microbien, les autotrophes du nanoplancton–microplancton et les organismes herbivores. Les ajustements structuraux parmi ces positions trophiques sont des réactions à la compétition interne entre les guildes fonctionnelles, due principalement à des caractéristiques reliées à la taille dont la capacité de manger et la susceptibilité d'être mangé. Malgré l'existence d'une ondulation spectrale typique associée à l'auto-organisation (guildes trophiques bien définies, interactions limnétiques–benthiques), les irrégularités de la biomasse indiquent les principales interactions qui viennent perturber l'état d'équilibre. Les mécanismes responsables des irrégularités agissent conjointement aux niveaux de l'écosystème et de l'individu. Ainsi, les spectres irréguliers de Laguna Iberá, un lac eutrophe, laissent croire à l'existence d'un fort contrôle descendant qui s'exerce au moyen d'effets en cascade. Les caractéristiques spécifiques d'organismes particuliers, comme les cyanobactéries filamenteuses, contribuent au maintien de ces irrégularités stables. La plus grande régularité spectrale à Laguna Galarza, un lac méso-oligotrophe, résulte d'un flux de biomasse plus équilibré dans la chaîne trophique.[Traduit par la Rédaction]

Document Type: Research Article

Publication date: 2003-04-01

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  • Published continuously since 1901 (under various titles), this monthly journal is the primary publishing vehicle for the multidisciplinary field of aquatic sciences. It publishes perspectives (syntheses, critiques, and re-evaluations), discussions (comments and replies), articles, and rapid communications, relating to current research on cells, organisms, populations, ecosystems, or processes that affect aquatic systems. The journal seeks to amplify, modify, question, or redirect accumulated knowledge in the field of fisheries and aquatic science. Occasional supplements are dedicated to single topics or to proceedings of international symposia.
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