Zooplankton structure and dynamics in Mediterranean marshes (Empordà Wetlands): a size-based approach

Zooplankton community structure (composition, diversity, dynamics and trophic relationships) of Mediterranian marshes, has been analysed by means of a size based approach. In temporary basins the shape of the biomass-size spectra is related to the hydrological cycle. Linear shape spectra are more frequent in flooding situations when nutrient input causes population growth of small-sized organisms, more than compensating for the effect of competitive interactions. During confinement conditions the scarcity of food would decrease zooplankton growth and increase intra- and interspecific interactions between zooplankton organisms which favour the greatest sizes thus leading to the appearance of curved shape spectra. Temporary and permanent basins have similar taxonomic composition but the latter have higher species diversity, a more simplified temporal pattern and a size distribution dominated mainly by smaller sizes. In permanents basins zooplankton growth is not only conditioned by the availability of resources but by the variable predation of planktivorous fish, so that the temporal variability of the spectra may also be a result of temporal differences in fish predation. Size diversity seems to be a better indicator of the degree of this community structure than species diversity. The tendency of size diversity to increase during succession makes it useful to discriminate between different succession stages, fact that is not achieved by analysing only species diversity since it is low both under large and frequent or small and rare disturbances. Amino acid composition differences found among stages of copepod species indicate a gradual change in diet during the life cycle of these copepods, which provide evidence of food niche partitioning during ontogeny, whereas Daphnia species show a relatively constant amino acid composition. There is a relationship between the degree of trophic niche overlap among stages of the different species and nutrient concentration. Copepods, which have low trophic niche overlap among stages are dominant in food-limited environments, probably because trophic niche partitioning during development allow them to reduce intraspecific competition between adults, juveniles and nauplii. Daphnia species are only dominant in water bodies or periods with high productivity, probably due to the high trophic niche overlap between juveniles and adults. These findings suggest that, in addition to the effect of interspecific competition, predation and abiotic factors, the intraspecific competition might play also an important role in structuring zooplankton assemblages.

Ecological interactions between an invasive fish (Gambusia holbrooki) and native cyprinodonts: the role of salinity

Actualment una de les principals amenaces a la biodiversitat és la introducció d'espècies. Revisant 26 variables de les 69 espècies de peixos continental de la Península Ibèrica concloem que la filogènia, variabilitat i els usos de l'home són necessaris per entendre millor les diferències entres les espècies natives i invasores. Entre les especies més afectades per la introducció de peixos es troben els ciprinodontiformes endèmics del Mediterrani. Aportem les primers dades sobre l'ús d'hàbitats ocasionalment inundats i la selecció de preses del fartet (Aphanius iberus), observant un canvi ontogenètic, clarament relacionat amb el microhàbitat. També demostrem que la salinitat influeix en l'èxit invasor de la gamúsia, afectant la seva densitat i biologia reproductiva. Per altra banda, demostrem experimentalment que amb l'increment de salinitat la gambúsia disminueix la seva agressivitat i captura menys preses, reduint la seva eficàcia competitiva respecte dels ciprinodonts natius.