Mariana Reissig

Daphnia distribution in Andean Patagonian lakes: effect of low food quality and fish predation

Food quality in terms of carbon (C):phosphorus (P) ratio can constrain the success of highly demanding P herbivores as Daphnia. North Andean Patagonian lakes are ultraoligotrophic with low nutrient concentrations and well-developed euphotic zones. We investigated the distribution of the large Daphnia commutata in relation with food quality (sestonic C:P ratio) and predation risk in these lakes. The predation risk was estimated based on the fish species present and their relative eye diameter and transparency of the lake. The C:P ratios in the lakes were high, varying from 350 to >1,200. The lakes with D. commutata had significantly lower C:P ratio than those without these daphnids. On the other hand, those lakes where Daphnia is present have the lower predation risk than those were Daphnia is absent. In addition, we carried out growth experiments with neonates and natural seston of three lakes with different C:P ratio. The growth rates were inversely related with C:P of the food. Food quality and predation risk together determined the success or failure of large Daphnia populations in these Andean clear ultraoligotrophic lakes.

Impact of different zooplankton structures on the microbial food web of a South Andean oligotrophic lake

Abstract In oligotrophic Andean lakes, omnivorous calanoid copepods are the dominant zooplankters and, remarkably, phototrophic nanoflagellates and mixotrophic ciliates are the prevailing primary producers. In Lake Rivadavia (Patagonia, Argentina), the centropagids Boeckella michaelseni and Parabroteas sarsi coexist with the large cladoceran Daphnia middendorffiana. The particular feeding modes of these zooplanktonic species probably impact differentially on the microbial community. To determine the effect of predation on the pelagic microbial food web in this lake, we conducted a series of field experiments manipulating different zooplankton structures in 2 l enclosures. The results showed that the presence of B. michaelseni and rotifers depressed ciliates and nanoflagellates, but did not affect autotrophic picoplankton and total bacteria abundances. In contrast, the presence of Daphnia was decisive in decreasing autotrophic picoplankton abundances. P. sarsi was observed to prey on B. michaelseni copepodites and adults and a weak cascading effect on the microbial fraction could be detected.

Effects of Galaxias maculatus on nutrient dynamics and phytoplankton biomass in a North Patagonian oligotrophic lake

AbstractIn this study we analysed the effects of Galaxias maculatus, a landlocked small fish species, on nutrient dynamics, and the consequent effects on phytoplankton biomass of an oligotrophic North Patagonian lake. We performed field and laboratory experiments in order to explore nutrient release by G. maculatus with increasing fish biomass and body size, and the resulting phytoplankton responses. Our results showed that phytoplankton biomass was strongly enhanced in the presence of fish, and that enhancement was greater with increasing fish biomass. These algal increments were associated with higher nutrient concentrations, due to the excretion/egestion processes of fish. In our two laboratory experiments we did not observe phytoplankton increase, probably due to light conditions, but we did observe significant effects of fish on nutrient concentrations. As was expected, mass-specific nutrient release rates were higher in smaller fish than in larger ones. So, the amount of nutrients supplied to phytoplankton would be influenced by the size structure of fish population. As a consequence of different N and P release rates, an increase in the

Mercury in an ultraoligotrophic North Patagonian Andean lake (Argentina): concentration patterns in different components of the water column.

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Interplay between climate and hydrogeomorphic features and their effect on the seasonal variation of dissolved organic matter in shallow temperate lakes of the Southern Andes (Patagonia, Argentina): a field study based on optical properties

:PTDP ratio was observed in the presence of fish. The fact that G. maculatus is a species that moves in schools would determine spatial heterogeneity in nutrient release, with important effects of reducing nutrient limitation and shifting

Sexual size dimorphism in calanoid copepods (Centropagidae) from Patagonia (Argentina)

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