The impact of the fish population .on trophic properties of lake water, was experimentally studied in an oligotrophic Swedish forest lake. Biotic changes following fish removal resulted in a development in oligotrophic direction as shown by the drop in limnetic primary production, pH, total phosphorus, total nitrogen and the increased transparency.

No abstract

Aquatic animals directly influence the cycling of phosphorus in lakes through feeding and excretion. Traditionally, animals (zooplankton, benthic invertebrates and fish) have been assigned only minor roles in the process of freshwater phosphorus cycling. They were regarded as consumers without much regulating influence. Today there is growing evidence that animals, predators and herbivores, directly or indirectly can control biomass of primary producers and internal cycling of phosphorus.This paper summarizes different mechanisms of transformation and translocation of phosphorus via different groups of organisms.

Aquatic animals directly influence the cycling of phosphorus in lakes through feeding and excretion. Traditionally, animals (zooplankton, benthic invertebrates and fish) have been assigned only minor roles in the process of freshwater phosphorus cycling. They were regarded as consumers without much regulating influence. Today there is growing evidence that animals, predators and herbivores, directly or indirectly can control biomass of primary producers and internal cycling of phosphorus.This paper summarizes different mechanisms of transformation and translocation of phosphorus via different groups of organisms.

The gelatinous capsule of Holopedium gibberum is variable in size, and seasonal size development shows different courses in different lakes. Reasons for this variation were experimentally studied. Differences in density of invertebrate predators rather than abiotic differences were responsible.In experiments, adaptive change in capsule size took place without any physical contact between Holopedium and the predator Chaoborus obscuripes, suggesting a chemically triggered mechanism. The specific receptor in Holopedium and the key substance released by the predator are not yet known. The experiments showed that the same Holopedium individual that received the stimulus reacted by a direct phenotypical change.The reduction of capsule size in response to reduced predator density suggests a cost in maintaining this antipredator armor. A larger capsule leads to a higher cost for food gathering because of an increased drag in the water. The need for better armor may postpone the start of reproduction in Holopedium.