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.