Abstract
To demonstrate that the effects of higher trophic elements on plankton in laboratory aquaria are not simple top‐down or bottom‐up processes, we measured phosphorus and chlorophyll concentrations in replicated month‐old aquaria undergoing one of five permutations involving three fish species, Daphnia pulex, and algae. Goldfish (Carassius auratus) extirpated the Daphnia and produced the highest observed algal and nutrient levels. Daphnia persisted with armored catfish (Plecostomus hypostomus) but algal levels were still high, even though total P concentrations of the water were lower. Angelfish (Pterophyllum scalare) cropped all the Daphnia, but only moderate levels of algae resulted, apparently because these fish regenerate sedimented nutrients less effectively. Aquaria with neither fish nor Daphnia supported even less algae, although total P concentration was not reduced. Finally, the water in aquaria with Daphnia, but no fish, had the lowest levels of both chlorophyll and total P. These gross changes were accompanied by shifts between the two dominant algal genera, seemingly because higher levels of nutrient regeneration favor Scenedesmus over Monoraphidium. Apparently, higher trophic elements affect lower ones directly, from above by modifying predation pressure and from below by changing the quantity and quality of regenerated nutrients, and indirectly by modulating competition within the plankton and between plankton and periphyton.
Published Version
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