Direct and Indirect Effects of Consumers on Benthic Algae in Isolated Pools of an Ephemeral Stream

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Abstract
Artificial pools were used to test for interactions among dominant consumer and producer populations that coexist in isolated pools of an ephemeral stream each summer. Nitrogen and/or phosphorus were supplied to one set of pools; herbivorous snails and crayfish and a predaceous centrarchid were added in different combinations to other pools. Algal growth was measured inside and outside wire cages placed in pools to exclude herbivores and/or predators. Algal biomass and the abundance of most algal species were increased by nitrogen enrichment. Algal biomass was also enhanced by addition of consumers. There were few differences in structure between algal assemblages inside and outside cages in any treatment. Under conditions of nitrogen limitation, moderate levels of herbivory can enhance algal growth. Positive effects are greatest at the microsite level and may depend on the ability of algal species to resist digestion by grazers. Predators may affect the algal assemblage indirectly by reducing herbivore survival or activity and directly by converting nutrients stored in herbivore biomass into a form available for algal growth. Because the importance of different trophic connections may vary among ecosystems, experimentation must consider all those that are potentially important.